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  • 1. Acke, B.
    et al.
    Min, M.
    Dominik, C.
    Vandenbussche, B.
    Sibthorpe, B.
    Waelkens, C.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Degroote, P.
    Smolders, K.
    Pantin, E.
    Barlow, M. J.
    Blommaert, J. A. D. L.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    De Meester, W.
    Dent, W. R. F.
    Exter, K.
    Di Francesco, J.
    Fridlund, M.
    Gear, W. K.
    Glauser, A. M.
    Greaves, J. S.
    Harvey, P. M.
    Henning, Th
    Hogerheijde, M. R.
    Holland, W. S.
    Huygen, R.
    Ivison, R. J.
    Jean, C.
    Liseau, R.
    Naylor, D. A.
    Pilbratt, G. L.
    Polehampton, E. T.
    Regibo, S.
    Royer, P.
    Sicilia-Aguilar, A.
    Swinyard, B. M.
    Herschel images of Fomalhaut An extrasolar Kuiper belt at the height of its dynamical activity2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 540, p. A125-Article in journal (Refereed)
    Abstract [en]

    Context. Fomalhaut is a young (2 +/- 1 x 10(8) years), nearby (7.7 pc), 2 M-circle dot star that is suspected to harbor an infant planetary system, interspersed with one or more belts of dusty debris. Aims. We present far-infrared images obtained with the Herschel Space Observatory with an angular resolution between 5.7 '' and 36.7 '' at wavelengths between 70 mu m and 500 mu m. The images show the main debris belt in great detail. Even at high spatial resolution, the belt appears smooth. The region in between the belt and the central star is not devoid of material; thermal emission is observed here as well. Also at the location of the star, excess emission is detected. We aim to construct a consistent image of the Fomalhaut system. Methods. We use a dynamical model together with radiative-transfer tools to derive the parameters of the debris disk. We include detailed models of the interaction of the dust grains with radiation, for both the radiation pressure and the temperature determination. Comparing these models to the spatially resolved temperature information contained in the images allows us to place strong constraints on the presence of grains that will be blown out of the system by radiation pressure. We use this to derive the dynamical parameters of the system. Results. The appearance of the belt points toward a remarkably active system in which dust grains are produced at a very high rate by a collisional cascade in a narrow region filled with dynamically excited planetesimals. Dust particles with sizes below the blow-out size are abundantly present. The equivalent of 2000 one-km-sized comets are destroyed every day, out of a cometary reservoir amounting to 110 Earth masses. From comparison of their scattering and thermal properties, we find evidence that the dust grains are fluffy aggregates, which indicates a cometary origin. The excess emission at the location of the star may be produced by hot dust with a range of temperatures, but may also be due to gaseous free-free emission from a stellar wind.

  • 2. Ahmic, Mirza
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars2007Manuscript (preprint) (Other academic)
  • 3. Ahmic, Mirza
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars2007Conference paper (Other (popular science, discussion, etc.))
  • 4. Ahmic, Mirza
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Astronomy.
    Multiplicity among Young Brown Dwarfs and Very Low Mass Stars2007In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, The Astrophysical JournalArticle in journal (Refereed)
  • 5. Andreoni, I.
    et al.
    Ackley, K.
    Cooke, J.
    Acharyya, A.
    Allison, J. R.
    Anderson, G. E.
    Ashley, M. C. B.
    Baade, D.
    Bailes, M.
    Bannister, K.
    Beardsley, A.
    Bessell, M. S.
    Bian, F.
    Bland, P. A.
    Boer, M.
    Booler, T.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brown, I. S.
    Buckley, D. A. H.
    Chang, S. -W.
    Coward, D. M.
    Crawford, S.
    Crisp, H.
    Crosse, B.
    Cucchiara, A.
    Cupak, M.
    de Gois, J. S.
    Deller, A.
    Devillepoix, H. A. R.
    Dobie, D.
    Elmer, E.
    Emrich, D.
    Farah, W.
    Farrell, T. J.
    Franzen, T.
    Gaensler, B. M.
    Galloway, D. K.
    Gendre, B.
    Giblin, T.
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Green, J.
    Hancock, P. J.
    Hartig, B. A. D.
    Howell, E. J.
    Horsley, L.
    Hotan, A.
    Howie, R. M.
    Hu, L.
    Hu, Y.
    James, C. W.
    Johnston, S.
    Johnston-Hollitt, M.
    Kaplan, D. L.
    Kasliwal, M.
    Keane, E. F.
    Kenney, D.
    Klotz, A.
    Lau, R.
    Laugier, R.
    Lenc, E.
    Li, X.
    Liang, E.
    Lidman, C.
    Luvaul, L. C.
    Lynch, C.
    Ma, B.
    Macpherson, D.
    Mao, J.
    McClelland, D. E.
    McCully, C.
    Moller, A.
    Morales, M. F.
    Morris, D.
    Murphy, T.
    Noysena, K.
    Onken, C. A.
    Orange, N. B.
    Oslowski, S.
    Pallot, D.
    Paxman, J.
    Potter, S. B.
    Pritchard, T.
    Raja, W.
    Ridden-Harper, R.
    Romero-Colmenero, E.
    Sadler, E. M.
    Sansom, E. K.
    Scalzo, R. A.
    Schmidt, B. P.
    Scott, S. M.
    Seghouani, N.
    Shang, Z.
    Shannon, R. M.
    Shao, L.
    Shara, M. M.
    Sharp, R.
    Sokolowski, M.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy.
    Staff, J.
    Steele, K.
    Sun, T.
    Suntzeff, N. B.
    Tao, C.
    Tingay, S.
    Towner, M. C.
    Thierry, P.
    Trott, C.
    Tucker, B. E.
    Vaisanen, P.
    Krishnan, V. Venkatraman
    Walker, M.
    Wang, L.
    Wang, X.
    Wayth, R.
    Whiting, M.
    Williams, A.
    Williams, T.
    Wolf, C.
    Wu, C.
    Wu, X.
    Yang, J.
    Yuan, X.
    Zhang, H.
    Zhou, J.
    Zovaro, H.
    Follow Up of GW170817 and Its Electromagnetic Counterpart by Australian-Led Observing Programmes2017In: Publications Astronomical Society of Australia, ISSN 1323-3580, E-ISSN 1448-6083, Vol. 34, article id e069Article, review/survey (Refereed)
    Abstract [en]

    The discovery of the first electromagnetic counterpart to a gravitational wave signal has generated follow-up observations by over 50 facilities world-wide, ushering in the new era of multi-messenger astronomy. In this paper, we present follow-up observations of the gravitational wave event GW170817 and its electromagnetic counterpart SSS17a/DLT17ck (IAU label AT2017gfo) by 14 Australian telescopes and partner observatories as part of Australian-based and Australian-led research programs. We report early- to late-time multi-wavelength observations, including optical imaging and spectroscopy, mid-infrared imaging, radio imaging, and searches for fast radio bursts. Our optical spectra reveal that the transient source emission cooled from approximately 6 400 K to 2 100 K over a 7-d period and produced no significant optical emission lines. The spectral profiles, cooling rate, and photometric light curves are consistent with the expected outburst and subsequent processes of a binary neutron star merger. Star formation in the host galaxy probably ceased at least a Gyr ago, although there is evidence for a galaxy merger. Binary pulsars with short (100 Myr) decay times are therefore unlikely progenitors, but pulsars like PSR B1534+12 with its 2.7 Gyr coalescence time could produce such a merger. The displacement (similar to 2.2 kpc) of the binary star system from the centre of the main galaxy is not unusual for stars in the host galaxy or stars originating in the merging galaxy, and therefore any constraints on the kick velocity imparted to the progenitor are poor.

  • 6.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    EXPOSING THE GAS BRAKING MECHANISM OF THE beta PICTORIS DISK2011In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 729, no 2, p. 122-Article in journal (Refereed)
    Abstract [en]

    Ever since the discovery of the edge-on circumstellar (CS) disk around beta Pictoris, a standing question has been why the gas observed against the star in absorption is not rapidly expelled by the strong radiation pressure from the star. A solution to the puzzle has been suggested to be that the neutral elements that experience the radiation force also are rapidly ionized, and so are only able to accelerate to an average limiting velocity v(ion). Once ionized, the elements are rapidly braked by C II, which is observed to be at least 20x overabundant in the disk with respect to other species. A prediction from this scenario is that different neutral elements should reach different vion, depending on the ionization thresholds and strengths of driving line transitions. In particular, neutral Fe and Na are predicted to reach the radial velocities 0.5 and 3.3 km s(-1), respectively, before being ionized. In this paper, we study the absorption profiles of Fe and Na from the CS gas disk around beta Pic, as obtained by HARPS at the ESO 3.6 m telescope. We find that the Fe and Na velocity profiles are indeed shifted with respect to each other, confirming the model. The absence of an extended blue wing in the profile of Na, however, indicates that there must be some additional braking on the neutrals. We explore the possibility that the ion gas (dominated by C II) can brake the neutrals and conclude that about 2-5x more C than previously estimated is needed for the predicted line profile to be consistent with the observed one.

  • 7.
    Brandeker, Alexis
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Cataldi, Gianni
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Vandenbussche, B.
    Acke, B.
    Barlow, M. J.
    Blommaert, J. A. D. L.
    Cohen, M.
    Dent, W. R. F.
    Dominik, C.
    Di Francesco, J.
    Fridlund, M.
    Gear, W. K.
    Glauser, A. M.
    Greaves, J. S.
    Harvey, P. M.
    Heras, A. M.
    Hogerheijde, M. R.
    Holland, W. S.
    Huygen, R.
    Ivison, R. J.
    Leeks, S. J.
    Lim, T. L.
    Liseau, R.
    Matthews, B. C.
    Pantin, E.
    Pilbratt, G. L.
    Royer, P.
    Sibthorpe, B.
    Waelkens, C.
    Walker, H. J.
    Herschel detects oxygen in the beta Pictoris debris disk2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 591, article id A27Article in journal (Refereed)
    Abstract [en]

    The young star beta Pictoris is well known for its dusty debris disk produced through collisional grinding of planetesimals, kilometre-sized bodies in orbit around the star. In addition to dust, small amounts of gas are also known to orbit the star; this gas is likely the result of vaporisation of violently colliding dust grains. The disk is seen edge on and from previous absorption spectroscopy we know that the gas is very rich in carbon relative to other elements. The oxygen content has been more difficult to assess, however, with early estimates finding very little oxygen in the gas at a C/O ratio that is 20x higher than the cosmic value. A C/O ratio that high is difficult to explain and would have far-reaching consequences for planet formation. Here we report on observations by the far-infrared space telescope Herschel, using PACS, of emission lines from ionised carbon and neutral oxygen. The detected emission from C+ is consistent with that previously reported observed by the HIFI instrument on Herschel, while the emission from O is hard to explain without assuming a higher density region in the disk, perhaps in the shape of a clump or a dense torus required to sufficiently excite the O atoms. A possible scenario is that the C/O gas is produced by the same process responsible for the CO clump recently observed by the Atacama Large Millimeter/submillimeter Array in the disk and that the redistribution of the gas takes longer than previously assumed. A more detailed estimate of the C/O ratio and the mass of O will have to await better constraints on the C/O gas spatial distribution.

  • 8.
    Brandeker, Alexis
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Cataldi, Gianni
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Vandenbussche, Bart
    Acke, Bram
    Barlow, Michael J.
    Blommaert, Joris A. D. L.
    Cohen, Martin
    Dent, William R. F.
    Dominik, Carsten
    Di Francesco, James
    Fridlund, Malcolm
    Gear, Walter K.
    Glauser, Adrian Michael
    Greaves, Jane S.
    Harvey, Paul M.
    Heras, Ana M.
    Hogerheijde, Michiel R.
    Holland, Wayne S.
    Huygen, Rik
    Ivison, Rob J.
    Leeks, Sarah J.
    Lim, Tanya L.
    Liseau, René
    Matthews, Brenda C.
    Pantin, Eric
    Pilbratt, Göran L.
    Royer, Pierre
    Sibthorpe, Bruce
    Waelkens, Christoffel
    Walker, Helen J.
    Herschel detects oxygen in the β Pictoris debris diskIn: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Refereed)
  • 9.
    Cataldi, Gianni
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Chen, C. H.
    Dent, W. R. F.
    Kamp, I.
    Roberge, A.
    Vandenbussche, B.
    Constraints on the gas content of the Fomalhaut debris belt Can gas-dust interactions explain the belt's morphology?2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 574, article id L1Article in journal (Refereed)
    Abstract [en]

    Context. The 440 Myr old main-sequence A-star Fomalhaut is surrounded by an eccentric debris belt with sharp edges. This sort of a morphology is usually attributed to planetary perturbations, but the orbit of the only planetary candidate detected so far, Fomalhaut b, is too eccentric to efficiently shape the belt. Alternative models that could account for the morphology without invoking a planet are stellar encounters and gas-dust interactions. Aims. We aim to test the possibility of gas-dust interactions as the origin of the observed morphology by putting upper limits on the total gas content of the Fomalhaut belt. Methods. We derive upper limits on the CII 158 mu m and 01 63 pint emission by using non detections from the Photocletector Array Camera and Spectrometer (PACS) onboard the Herschel Space Observatory. Line fluxes are converted into total gas mass using the non-local thermodynamic equilibrium (non-LTE) code RADEX. We consider two different cases for the elemental abundances of the gas: solar abundances and abundances similar to those observed for the gas in the beta Pictoris debris disc. Results. The gas mass is shown to be below the millimetre dust mass by a factor of at least similar to 3 (for solar abundances) respectively similar to 300 (for beta Pic-like abundances). Conclusions. The lack of gas co-spatial with the dust implies that gas-dust interactions cannot efficiently shape the Fomalhaut debris belt. The morphology is therefore more likely due to a yet unseen planet (Fomalhaut c) or stellar encounters.

  • 10.
    Cataldi, Gianni
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, Bengt
    Stockholm University, Faculty of Science, Department of Astronomy.
    Liseau, R.
    Blommaert, J.
    Fridlund, M.
    Ivison, R.
    Pantin, E.
    Sibthorpe, B.
    Vandenbussche, B.
    Wu, Y.
    Herschel/HIFI observations of ionised carbon in the beta Pictoris debris disk2014In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 563, article id A66Article in journal (Refereed)
    Abstract [en]

    Context. The dusty debris disk around the similar to 20 Myr old main-sequence A-star beta Pictoris is known to contain gas. Evidence points towards a secondary origin of the gas as opposed to being a direct remnant from the initial protoplanetary disk, although the dominant gas production mechanism is so far not identified. The origin of the observed overabundance of C and O compared with solar abundances of metallic elements such as Na and Fe is also unclear. Aims. Our goal is to constrain the spatial distribution of C in the disk, and thereby the gas origin and its abundance pattern. Methods. We used the HIFI instrument on board the Herschel Space Observatory to observe and spectrally resolve C II emission at 158 mu m from the beta Pic debris disk. Assuming a disk in Keplerian rotation and a model for the line emission from the disk, we used the spectrally resolved line profile to constrain the spatial distribution of the gas. Results. We detect the C II 158 mu m emission. Modelling the shape of the emission line shows that most of the gas is located at about similar to 100 AU or beyond. We estimate a total C gas mass of 1.3(-0.5)(+1.3) x 10(2) M-circle plus (central 90% confidence interval). The data suggest that more gas is located on the south-west side of the disk than on the north-east side. The shape of the emission line is consistent with the hypothesis of a well mixed gas (constant C/Fe ratio throughout the disk). Assuming instead a spatial profile expected from a simplified accretion disk model, we found it to give a significantly poorer fit to the observations. Conclusions. Since the bulk of the gas is found outside 30 AU, we argue that the cometary objects known as falling evaporating bodies are probably not the dominant source of gas; production from grain-grain collisions or photodesorption seems more likely. The incompatibility of the observations with a simplified accretion disk model might favour a preferential depletion explanation for the overabundance of C and O, although it is unclear how much this conclusion is affected by the simplifications made. More stringent constraints on the spatial distribution will be available from ALMA observations of C I emission at 609 mu m.

  • 11.
    Cataldi, Gianni
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Ahmed, Engy
    de Vries, Bernard L.
    Neubeck, Anna
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Singer, Kelsi
    Searching for biosignatures in exoplanetary impact ejectaIn: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070Article in journal (Refereed)
  • 12.
    Cataldi, Gianni
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Singer, Kelsi
    Ahmed, Engy
    Stockholm University, Faculty of Science, Department of Geological Sciences. Royal Institute of Technology (KTH), Sweden.
    de Vries, Bernard L.
    Stockholm University, Faculty of Science, Department of Astronomy. European Space Research and Technology Centre (ESA/ESTEC), The Netherlands.
    Neubeck, Anna
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Searching for Biosignatures in Exoplanetary Impact Ejecta2017In: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 17, no 8, p. 721-746Article in journal (Refereed)
    Abstract [en]

    With the number of confirmed rocky exoplanets increasing steadily, their characterization and the search for exoplanetary biospheres are becoming increasingly urgent issues in astrobiology. To date, most efforts have concentrated on the study of exoplanetary atmospheres. Instead, we aim to investigate the possibility of characterizing an exoplanet (in terms of habitability, geology, presence of life, etc.) by studying material ejected from the surface during an impact event. For a number of impact scenarios, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica, or ejected microorganisms could be detected. For a 20km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust, depending on the exoplanet's size. The collisional evolution is best modeled by considering two independent dust populations, a spalled population consisting of nonmelted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapor evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust, etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future.

  • 13. Cuong Nguyen, Duy
    et al.
    Jayawardhana, Ray
    van Kerkwijk, Marten H.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scholz, Alexander
    Damjanov, Ivana
    Disk-Braking in Young Stars: Probing Rotation in Chamaeleon I and Taurus-Auriga2009Manuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    We present a comprehensive study of rotation, disk and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We find that both accretors and non-accretors have similar distributions of v sin i. The rotational velocities in both regions show a clear mass dependence, with F--K stars rotating on average about twice as fast as M stars, consistent with results reported for other clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular momentum j varies as M^0.5 for our sample which spans a mass range of ~0.16 to ~3 M_sun. This power law complements previous studies in Orion which estimated j is proportional to M^0.25 for < ~2 Myr stars in the same mass regime, and a sharp decline in j with decreasing mass for older stars (~10 Myr) with M < 2 M_sun. For a subsample of 67 objects with mid-IR photometry, we examine the connection between accretion signatures and dusty disks: in the vast majority of cases (63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar to the lifetime of inner disks.

  • 14. Cuong Nguyen, Duy
    et al.
    Scholz, Alexander
    van Kerkwijk, Marten H.
    Jayawardhana, Ray
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    How Variable is Accretion in Young Stars?2009Manuscript (preprint) (Other (popular science, discussion, etc.))
    Abstract [en]

    We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Tau-Aur and Cha I. For all stars, we obtain typically four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the H-alpha 10% width and the CaII-8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the CaII fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while H-alpha 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that CaII fluxes are a more robust quantitative indicator of accretion than H-alpha 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates towards later spectral types, reflecting the dM/dt vs. M relationship. There is a gap between accretors and non-accretors, pointing to a rapid shutdown of accretion. We conclude that the ~2 orders of magnitude scatter in the dM/dt vs. M relationship is dominated by object-to-object scatter instead of intrinsic source variability.

  • 15. Damjanov, Ivana
    et al.
    Jayawardhana, Ray
    Scholz, Alexander
    Ahmic, Mirza
    Nguyen, Duy C.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Kerkwijk, Marten H.
    A Comprehensive View of Circumstellar Disks in Chamaeleon I: Infrared Excess, Accretion Signatures, and Binarity2007In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 670, no 2, p. 1337-1346Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive study of disks around 81 young, low‐mass stars and brown dwarfs in the nearby 2 Myr old Chamaeleon I star‐forming region. We use mid‐infrared photometry from the Spitzer Space Telescope, supplemented by findings from ground‐based high‐resolution optical spectroscopy and adaptive optics imaging. We derive disk fractions of 52%±6% and 58-7+6% based on 8 and 24 μm color excesses, respectively, consistent with those reported for other clusters of similar age. Within the uncertainties, the disk frequency in our sample of K3–M8 objects in Cha I does not depend on stellar mass. Diskless and disk‐bearing objects have similar spatial distributions. There are no obvious transition disks in our sample, implying a rapid timescale for the inner disk clearing process; however, we find two objects with weak excess at 3–8 μm and substantial excess at 24 μm, which may indicate grain growth and dust settling in the inner disk. For a subsample of 35 objects with high‐resolution spectra, we investigate the connection between accretion signatures and dusty disks: in the vast majority of cases (29/35) the two are well correlated, suggesting that, on average, the timescale for gas dissipation is similar to that for clearing the inner dust disk. The exceptions are six objects for which dust disks appear to persist even though accretion has ceased or dropped below measurable levels. Adaptive optics images of 65 of our targets reveal that 17 have companions at (projected) separations of 10–80 AU. Of the five <20 AU binaries, four lack infrared excess, possibly indicating that a close companion leads to faster disk dispersal. The closest binary with excess is separated by ~20 AU, which sets an upper limit of ~8 AU for the outer disk radius. The overall disk frequency among stars with companions (35-13+15%) is lower than (but still statistically consistent with) the value for the total sample.

  • 16. de Vries, B. L.
    et al.
    Acke, B.
    Blommaert, J. A. D. L.
    Waelkens, C.
    Waters, L. B. F. M.
    Vandenbussche, B.
    Min, M.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Dominik, C.
    Decin, L.
    Barlow, M. J.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Di Francesco, J.
    Glauser, A. M.
    Greaves, J.
    Harvey, P. M.
    Holland, W. S.
    Ivison, R. J.
    Liseau, R.
    Pantin, E. E.
    Pilbratt, G. L.
    Royer, P.
    Sibthorpe, B.
    Comet-like mineralogy of olivine crystals in an extrasolar proto-Kuiper belt2012In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 490, no 7418, p. 74-76Article in journal (Refereed)
    Abstract [en]

    Some planetary systems harbour debris disks containing planetesimals such as asteroids and comets(1). Collisions between such bodies produce small dust particles(2), the spectral features of which reveal their composition and, hence, that of their parent bodies. A measurement of the composition of olivine crystals (Mg2-2xFe2xSiO4) has been done for the protoplanetary disk HD 100546 (refs 3, 4) and for olivine crystals in the warm inner parts of planetary systems. The latter compares well with the iron-rich olivine in asteroids(5,6) (x approximate to 0.29). In the cold outskirts of the beta Pictoris system, an analogue to the young Solar System, olivine crystals were detected(7) but their composition remained undetermined, leaving unknown how the composition of the bulk of Solar System cometary olivine grains compares with that of extrasolar comets(8,9). Here we report the detection of the 69-micrometre-wavelength band of olivine crystals in the spectrum of beta Pictoris. Because the disk is optically thin, we can associate the crystals with an extrasolar proto-Kuiper belt a distance of 15-45 astronomical units from the star (one astronomical unit is the Sun-Earth distance), determine their magnesium-rich composition (x = 0.01 +/- 0.001) and show that they make up 3.6 +/- 1.0 per cent of the total dust mass. These values are strikingly similar to those for the dust emitted by the most primitive comets in the Solar System(8-10), even though beta Pictoris is more massive and more luminous and has a different planetary system architecture.

  • 17. Greaves, J. S.
    et al.
    Sibthorpe, B.
    Acke, B.
    Pantin, E. E.
    Vandenbussche, B.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Dominik, C.
    Barlow, M. J.
    Bendo, G. J.
    Blommaert, J. A. D. L.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    de Vries, Bernard L.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Dent, W. R. F.
    Di Francesco, J.
    Fridlund, M.
    Gear, W. K.
    Harvey, P. M.
    Hogerheijde, M. R.
    Holland, W. S.
    Ivison, R. J.
    Liseau, R.
    Matthews, B. C.
    Pilbratt, G. L.
    Walker, H. J.
    Waelkens, C.
    EXTREME CONDITIONS IN A CLOSE ANALOG TO THE YOUNG SOLAR SYSTEM: HERSCHEL OBSERVATIONS OF is an element of ERIDANI2014In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 791, no 1, p. L11-Article in journal (Refereed)
    Abstract [en]

    Far-infrared Herschel images of the is an element of Eridani system, seen at a fifth of the Sun's present age, resolve two belts of debris emission. Fits to the 160 mu m PACS image yield radial spans for these belts of 12-16 and 54-68 AU. The south end of the outer belt is approximate to 10% brighter than the north end in the PACS+SPIRE images at 160, 250, and 350 mu m, indicating a pericenter glow attributable to a planet c From this asymmetry and an upper bound on the offset of the belt center, this second planet should be mildly eccentric (e(c) approximate to 0.03-0.3). Compared to the asteroid and Kuiper Belts of the young Sun, the is an element of Eri belts are intermediate in brightness and more similar to each other, with up to 20 km sized collisional fragments in the inner belt totaling approximate to 5% of an Earth mass. This reservoir may feed the hot dust close to the star and could send many impactors through the Habitable Zone, especially if it is being perturbed by the suspected planet is an element of Eri b, at semi-major axis approximate to 3 AU.

  • 18.
    Grigorieva, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thebault, Philippe
    Stockholm University, Faculty of Science, Department of Astronomy.
    Artymowicz, Pawel
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Survival of icy grains in debris discs. The role of photosputtering2007Manuscript (preprint) (Other academic)
    Abstract [en]

    We put theoretical constraints on the presence and survival of icy grains in debris discs. Particular attention is paid to UV sputtering of water ice, which has so far not been studied in detail in this context. We present a photosputtering model based on available experimental and theoretical studies. We quantitatively estimate the erosion rate of icy and ice-silicate grains, under the influence of both sublimation and photosputtering, as a function of grain size, composition and distance from the star. The effect of erosion on the grain's location is investigated through numerical simulations coupling the grain size to its dynamical evolution. Our model predicts that photodesorption efficiently destroy ice in optically thin discs, even far beyond the sublimation snow line. For the reference case of beta Pictoris, we find that only > 5mm grains can keep their icy component for the age of the system in the 50-150AU region. When taking into account the collisional reprocessing of grains, we show that the water ice survival on grains improves (grains down to ~ 20 um might be partially icy). However, estimates of the amount of gas photosputtering would produce on such a hypothetical population of big icy grains lead to values for the OI column density that strongly exceed observational constraints for beta Pic, thus ruling out the presence of a significant amount of icy grains in this system. Erosion rates and icy grains survival timescales are also given for a set of 11 other debris disc systems. We show that, with the possible exception of M stars, photosputtering cannot be neglected in calculations of icy grain lifetimes.

  • 19.
    Grigorieva, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Stockholm University, Faculty of Science, Department of Astronomy.
    Artymowicz, Pawel
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Survival of icy grains in debris discs. The role of photosputtering2007In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Astronomy and Astrophysics, ISSN 0004-6361, Vol. 475, no 2, p. 755-764Article in journal (Refereed)
    Abstract [en]

    Aims.We put theoretical constraints on the presence and survival of icy grains in debris discs. Particular attention is paid to UV sputtering of water ice, which has so far not been studied in detail in this context.

    Methods: We present a photosputtering model based on available experimental and theoretical studies. We quantitatively estimate the erosion rate of icy and ice-silicate grains, under the influence of both sublimation and photosputtering, as a function of grain size, composition and distance from the star. The effect of erosion on the grain's location is investigated through numerical simulations coupling the grain size to its dynamical evolution.

    Results: Our model predicts that photodesorption efficiently destroy ice in optically thin discs, even far beyond the sublimation snow line. For the reference case of β Pictoris, we find that only ⪆5 mm grains can keep their icy component for the age of the system in the 50-150 AU region. When taking into account the collisional reprocessing of grains, we show that the water ice survival on grains improves (grains down to ≃20 μm might be partially icy). However, estimates of the amount of gas photosputtering would produce on such a hypothetical population of big icy grains lead to values for the O I column density that strongly exceed observational constraints for β Pic, thus ruling out the presence of a significant amount of icy grains in this system. Erosion rates and icy grains survival timescales are also given for a set of 11 other debris disc systems. We show that, with the possible exception of M stars, photosputtering cannot be neglected in calculations of icy grain lifetimes.

  • 20. Janson, Markus
    et al.
    Jayawardhana, Ray
    Girard, Julien H.
    Lafreniere, David
    Bonavita, Mariangela
    Gizis, John
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    NEW BROWN DWARF COMPANIONS TO YOUNG STARS IN SCORPIUS-CENTAURUS2012In: The Astrophysical Journal Letters, ISSN 2041-8205, Vol. 758, no 1, p. L2-Article in journal (Refereed)
    Abstract [en]

    We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of similar to 40-100 M-jup complements previous work in the same region, reporting detections of similarly wide companions with lower masses, in the range of similar to 10-30 M-jup. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass companions to stars.

  • 21. Janson, Markus
    et al.
    Lafreniere, David
    Jayawardhana, Ray
    Bonavita, Mariangela
    Girard, Julien H.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Gizis, John E.
    A MULTIPLICITY CENSUS OF INTERMEDIATE-MASS STARS IN SCORPIUS-CENTAURUS2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 773, no 2, p. 170-Article in journal (Refereed)
    Abstract [en]

    Stellar multiplicity properties have been studied for the lowest and the highest stellar masses, but intermediate-mass stars from F-type to late A-type have received relatively little attention. Here, we report on a Gemini/NICI snapshot imaging survey of 138 such stars in the young Scorpius-Centaurus (Sco-Cen) region, for the purpose of studying multiplicity with sensitivity down to planetary masses at wide separations. In addition to two brown dwarfs and a companion straddling the hydrogen-burning limit which we reported previously, here we present 26 new stellar companions and determine a multiplicity fraction within 0 ''.1-5 ''.0 of 21% +/- 4%. Depending on the adopted semimajor axis distribution, our results imply a total multiplicity in the range of similar to 60%-80%, which further supports the known trend of a smooth continuous increase in the multiplicity fraction as a function of primary stellar mass. A surprising feature in the sample is a distinct lack of nearly equal-mass binaries, for which we discuss possible reasons. The survey yielded no additional companions below or near the deuterium-burning limit, implying that their frequency at >200 AU separations is not quite as high as might be inferred from previous detections of such objects within the Sco-Cen region.

  • 22. Krivov, A. V.
    et al.
    Herrmann, F.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Stockholm University, Faculty of Science, Department of Astronomy.
    Can gas in young debris disks be constrained by their radial brightness profiles?2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 507, no 3, p. 1503-1516Article in journal (Refereed)
    Abstract [en]

    Disks around young stars are known to evolve from optically thick, gas-dominated protoplanetary disks to optically thin, almost gas-free debris disks. It is thought that the primordial gas is largely removed at ages of ~10 Myr and indeed, only little amounts of gas have been deduced from observations for debris disks at ages of ⪆10 Myr. However, gas detections are difficult and often indirect, not allowing one to discern the true gas densities. This suggests using dynamical arguments: it has been argued that gas, if present with higher densities, would lead to flatter radial profiles of the dust density and brightness than those actually observed. In this paper, we systematically study the influence of gas on the radial profiles of brightness. We assume that dust is replenished by planetesimals orbiting in a “birth ring” and model the dust distribution and scattered-light brightness profile in the outer part of the disk exterior to the birth ring, under different assumptions about the gas component. Our numerical simulations, supported with an analytic model, show that the radial profile of dust density and the surface brightness are surprisingly insensitive to variation of the parameters of a central star, location of the dust-producing planetesimal belt, dustiness of the disk and - most importantly - the parameters of the ambient gas. The radial brightness slopes in the outer disks are all typically in the range -3...-4. This result holds for a wide range of gas densities (three orders of magnitude), for different radial profiles of the gas temperature, both for gas of solar composition and gas of strongly non-solar composition. The slopes of -3...-4 we find are the same that were theoretically found for gas-free debris disks, and they are the same as actually retrieved from observations of many debris disks. Our specific results for three young (10-30 Myr old), spatially resolved, edge-on debris disks (β Pic, HD 32297, and AU Mic) show that the observed radial profiles of the surface brightness do not pose any stringent constraints on the gas component of the disk. We cannot exclude that outer parts of the systems may have retained substantial amounts of primordial gas which is not evident in the gas observations (e.g. as much as 50 Earth masses for β Pic). However, the possibility that gas, most likely secondary, is only present in little to moderate amounts, as deduced from gas detections (e.g. ~0.05 Earth masses in the β Pic disk or even less), remains open, too.

  • 23. Lafreniere, David
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    A Multiplicity Census of Young Stars in Chamaeleon I2008Manuscript (preprint) (Other academic)
    Abstract [en]

    We present the results of a multiplicity survey of 126 stars spanning ~0.1-3 solar masses in the ~2-Myr-old Chamaeleon I star-forming region, based on adaptive optics imaging with the ESO Very Large Telescope. Our observations have revealed 30 binaries and 6 triples, of which 19 and 4, respectively, are new discoveries. The overall multiplicity fraction we find for Cha I (~30%) is similar to those reported for other dispersed young associations, but significantly higher than seen in denser clusters and the field, for comparable samples. Both the frequency and the maximum separation of Cha I binaries decline with decreasing mass, while the mass ratios approach unity; conversely, tighter pairs are more likely to be equal mass. We confirm that brown dwarf companions to stars are rare, even at young ages at wide separations. Based on follow-up spectroscopy of two low-mass substellar companion candidates, we conclude that both are likely background stars. The overall multiplicity fraction in Cha I is in rough agreement with numerical simulations of cloud collapse and fragmentation, but its observed mass dependence is less steep than predicted. The paucity of higher-order multiples, in particular, provides a stringent constraint on the simulations, and seems to indicate a low level of turbulence in the prestellar cores in Cha I.

  • 24. Lafreniere, David
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    et al.,
    A Multiplicity Census of Young Stars in Chamaeleon I2008In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, The Astrophysical Journal, Vol. 683, no 2, p. 844-861Article in journal (Refereed)
    Abstract [en]

    We present the results of a multiplicity survey of 126 stars spanning ~0.1-3 Msolar in the ~2 Myr old Chamaeleon I star-forming region, based on adaptive optics imaging with the ESO Very Large Telescope. Our observations have revealed 30 binaries and six triples, of which 19 and four, respectively, are new discoveries. The overall multiplicity fraction we find for Cha I (~30%) is similar to those reported for other dispersed young associations, but significantly higher than seen in denser clusters and the field, for comparable samples. Both the frequency and the maximum separation of Cha I binaries decline with decreasing mass, while the mass ratios approach unity; conversely, tighter pairs are more likely to be equal mass. We confirm that brown dwarf companions to stars are rare, even at young ages at wide separations. Based on follow-up spectroscopy of two low-mass substellar companion candidates, we conclude that both are likely background stars. The overall multiplicity fraction in Cha I is in rough agreement with numerical simulations of cloud collapse and fragmentation, but its observed mass dependence is less steep than predicted. The paucity of higher order multiples, in particular, provides a stringent constraint on the simulations, and seems to indicate a low level of turbulence in the prestellar cores in Cha I.

  • 25. Lafreniere, David
    et al.
    Jayawardhana, Ray
    van Kerkwijk, Marten H.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Janson, Markus
    AN ADAPTIVE OPTICS MULTIPLICITY CENSUS OF YOUNG STARS IN UPPER SCORPIUS2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 785, no 1, p. 47-Article in journal (Refereed)
    Abstract [en]

    We present the results of amultiplicity survey of 91 stars spanning masses of similar to 0.2-10M(circle dot) in the Upper Scorpius star-forming region, based on adaptive optics imaging with the Gemini North telescope. Our observations identified 29 binaries, 5 triples, and no higher order multiples. The corresponding raw multiplicity frequency is 0.37 +/- 0.05. In the regime where our observations are complete-companion separations of 0 ''.1-5 '' (similar to 15-800 AU) with magnitude limits ranging from K < 9.3 at 0 ''.1 to K < 15.8 at 5 '' -the multiplicity frequency is 0.27(-0.04.)(+ 0.05) For similar separations, the multiplicity frequency in Upper Scorpius is comparable to that in other dispersed star-forming regions, but is a factor of two to three higher than in denser star-forming regions or in the field. Our sample displays a constant multiplicity frequency as a function of stellar mass. Among our sample of binaries, we find that both wider (> 100 AU) and higher-mass systems tend to have companions with lower companion-to-primary mass ratios. Three of the companions identified in our survey are unambiguously substellar and have estimated masses below 0.04M(circle dot) (two of them are new discoveries from this survey-1RXS J160929.1-210524b and HIP 78530B-although we have reported them separately in earlier papers). These three companions have projected orbital separations of 300-900 AU. Based on a statistical analysis factoring in sensitivity limits, we calculate an occurrence rate of 5-40 M-Jup companions of similar to 4.0% for orbital separations of 250-1000 AU, compared to < 1.8% at smaller separations, suggesting that such companions are more frequent on wider orbits.

  • 26.
    Liseau, R.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Larsson, B.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, A.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Bergman, P.
    Bernath, P.
    Black, J. H.
    Booth, R.
    Buat, V.
    Curry, C.
    Encrenaz, P.
    Falgarone, E.
    Feldman, P.
    Fich, M.
    Florén, H.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Frisk, U.
    Gerin, M.
    Gregersen, E.
    Harju, J.
    Hasegawa, T.
    Hjalmarson, Å.
    Johansson, L.
    Kwok, S.
    Lecacheux, A.
    Liljeström, T.
    Mattila, K.
    Mitchell, G.
    Nordh, L.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olberg, M.
    Olofsson, G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pagani, L.
    Plume, R.
    Ristorcelli, I.
    Sandqvist, Aa.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Schéele, F. v.
    Serra, G.
    Tothill, N.
    Volk, K.
    Wilson, C.
    First detection of NH3 (10 -> 00) from a low mass cloud core. On the low ammonia abundance of the rho Oph A core2003In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 402, p. L73-L76Article in journal (Refereed)
    Abstract [en]

    Odin has successfully observed the molecular core rho Oph A in the 572.5 GHz rotational ground state line of ammonia, NH3 (JK = 10 -> 00). The interpretation of this result makes use of complementary molecular line data obtained from the ground (C17O and CH3OH) as part of the Odin preparatory work. Comparison of these observations with theoretical model calculations of line excitation and transfer yields a quite ordinary abundance of methanol, X(CH3OH)= 3 x 10-9. Unless NH3 is not entirely segregated from C17O and CH3OH, ammonia is found to be significantly underabundant with respect to typical dense core values, viz. X(NH3) = 8 x 10-10. Based on observations with Odin, a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes) and Centre National d'Études Spatiales (CNES). The Swedish Space Corporation has been the industrial prime contractor. and based on observations collected with the Swedish ESO Submillimeter Telescope, SEST, in La Silla, Chile.

  • 27. Liseau, Rene
    et al.
    Risacher, Christophe
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Eiroa, Carlos
    Fridlund, Malcolm
    Nilsson, Ricky
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pilbratt, Göran L.
    Thébault, Philippe
    q1 Eridani: a solar-type star with a planet and a dust belt2008In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 480, no 3, p. L47-L50Article in journal (Refereed)
    Abstract [en]

    Context. Far-infrared excess emission from main-sequence stars is due to dust produced by orbiting minor bodies. In these disks, larger bodies, such as planets, may also be present and the understanding of their incidence and influence currently presents a challenge.

    Aims. Only very few solar-type stars exhibiting an infrared excess and harbouring planets are known to date. Indeed, merely a single case of a star-planet-disk system has previously been detected at submillimeter (submm) wavelengths. Consequently, one of our aims is to understand the reasons for these poor statistics, i.e., whether these results reflected the composition and/or the physics of the planetary disks or were simply due to observational bias and selection effects. Finding more examples would be very significant.

    Methods. The selected target, q(1) Eri, is a solar-type star, which was known to possess a planet, q(1) Eri b, and to exhibit excess emission at IRAS wavelengths, but had remained undetected in the millimeter regime. Therefore, submm flux densities would be needed to better constrain the physical characteristics of the planetary disk. Consequently, we performed submm imaging observations of q(1) Eri.

    Results. The detected dust toward q(1) Eri at 870 mu m exhibits the remarkable fact that the entire SED, from the IR to mm-wavelengths, is fit by a single-temperature blackbody function (60 K). This would imply that the emitting regions are confined to a narrow region (ring) at radial distances much larger than the orbital distance of q(1) Eri b, and that the emitting particles are considerably larger than some hundred micron. However, the 870 mu m source is extended, with a full-width-half-maximum of roughly 600AU. Therefore, a physically more compelling model also invokes a belt of cold dust (17 K), located at 300AU from the star and about 60AU wide.

    Conclusions. The minimum mass of 0.04 M-circle plus (3 M-Moon) of 1 mm-size icy ring-particles is considerable, given the stellar age of >= 1Gyr. These big grains form an inner edge at about 25 AU, which may suggest the presence of an unseen outer planet (q(1) Eri c).

  • 28. Matthews, Brenda C.
    et al.
    Kennedy, Grant
    Sibthorpe, Bruce
    Holland, Wayne
    Booth, Mark
    Kalas, Paul
    MacGregor, Meredith
    Wilner, David
    Vandenbussche, Bart
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Blommaert, Joris
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Dent, W. R. F.
    de Vries, Bernard L.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Di Francesco, James
    Fridlund, Malcolm
    Graham, James R.
    Greaves, Jane
    Heras, Ana M.
    Hogerheijde, Michiel
    Ivison, R. J.
    Pantin, Eric
    Pilbratt, Goran L.
    THE AU MIC DEBRIS DISK: FAR-INFRARED AND SUBMILLIMETER RESOLVED IMAGING2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 811, no 2, article id 100Article in journal (Refereed)
    Abstract [en]

    We present far-infrared and submillimeter maps from the Herschel Space Observatory and the James Clerk Maxwell Telescope of the debris disk host star AU Microscopii. Disk emission is detected at 70, 160, 250, 350, 450, 500, and 850 mu m. The disk is resolved at 70, 160, and 450 mu m. In addition to the planetesimal belt, we detect thermal emission from AU Mic's halo for the first time. In contrast to the scattered light images, no asymmetries are evident in the disk. The fractional luminosity of the disk is 3.9 x 10(-4) and its milimeter-grain dust mass is 0.01 M-circle dot (+/- 20%). We create a simple spatial model that reconciles the disk spectral energy distribution as a blackbody of 53 +/- 2K (a composite of 39 and 50 K components) and the presence of small (non-blackbody) grains which populate the extended halo. The best-fit model is consistent with the birth ring model explored in earlier works, i.e., an edge-on dust belt extending from 8.8 to 40 AU, but with an additional halo component with an r(-1.5) surface density profile extending to the limits of sensitivity (140 AU). We confirm that AU Mic does not exert enough radiation force to blow out grains. For stellar mass-loss rates of 10-100 times solar, compact (zero porosity) grains can only be removed if they are very small; consistently with previous work, if the porosity is 0.9, then grains approaching 0.1 mu m can be removed via corpuscular forces (i.e., the stellar wind).

  • 29. Mentuch, Erin
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    et al.,
    Lithium Depletion of Nearby Young Stellar Associations2008Manuscript (preprint) (Other academic)
    Abstract [en]

    We estimate cluster ages from lithium depletion in five pre-main-sequence groups found within 100 pc of the Sun: TW Hydrae Association, Eta Chamaeleontis Cluster, Beta Pictoris Moving Group, Tucanae-Horologium Association and AB Doradus Moving Group. We determine surface gravities, effective temperatures and lithium abundances for over 900 spectra through least squares fitting to model-atmosphere spectra. For each group, we compare the dependence of lithium abundance on temperature with isochrones from pre-main-sequence evolutionary tracks to obtain model dependent ages. We find that the Eta Chamaelontis Cluster and the TW Hydrae Association are the youngest, with ages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the Beta Pictoris Moving Group at 21+/-9 Myr, the Tucanae-Horologium Association at 27+/-11 Myr, and the AB Doradus Moving Group at an age of at least 45 Myr (where we can only set a lower limit since the models -- unlike real stars -- do not show much lithium depletion beyond this age). Here, the ordering is robust, but the precise ages depend on our choice of both atmospheric and evolutionary models. As a result, while our ages are consistent with estimates based on Hertzsprung-Russell isochrone fitting and dynamical expansion, they are not yet more precise. Our observations do show that with improved models, much stronger constraints should be feasible: the intrinsic uncertainties, as measured from the scatter between measurements from different spectra of the same star, are very low: around 10 K in effective temperature, 0.05 dex in surface gravity, and 0.03 dex in lithium abundance.

  • 30. Mentuch, Erin
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Kerkwijk, Marten H.
    Jayawardhana, Ray
    Hauschildt, Peter H.
    Lithium Depletion of Nearby Young Stellar Associations2008In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 689, p. 1127-1140Article in journal (Refereed)
    Abstract [en]

    We estimate cluster ages from lithium depletion in five pre-main-sequence groups found within 100 pc of the Sun: the TW Hydrae association, η Chamaeleontis cluster, β Pictoris moving group, Tucanae-Horologium association, and AB Doradus moving group. We determine surface gravities, effective temperatures, and lithium abundances for over 900 spectra through least-squares fitting to model-atmosphere spectra. For each group, we compare the dependence of lithium abundance on temperature with isochrones from pre-main-sequence evolutionary tracks to obtain model-dependent ages. We find that the η Cha cluster and the TW Hydrae association are the youngest, with ages of 12+/-6 Myr and 12+/-8 Myr, respectively, followed by the β Pic moving group at 21+/-9 Myr, the Tucanae-Horologium association at 27+/-11 Myr, and the AB Dor moving group at an age of at least 45 Myr (whereby we can only set a lower limit, since the models-unlike real stars-do not show much lithium depletion beyond this age). Here the ordering is robust, but the precise ages depend on our choice of both atmospheric and evolutionary models. As a result, while our ages are consistent with estimates based on Hertzsprung-Russell isochrone fitting and dynamical expansion, they are not yet more precise. Our observations do show that with improved models, much stronger constraints should be feasible, as the intrinsic uncertainties, as measured from the scatter between measurements from different spectra of the same star, are very low: around 10 K in effective temperature, 0.05 dex in surface gravity, and 0.03 dex in lithium abundance.

  • 31. Nguyen, Duy
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    A Search for Disk-Locking in the Chamaeleon I Star Forming Region2008In: Precision Spectroscopy in Astrophysics, 2008Conference paper (Other academic)
    Abstract [en]

    We investigated the connection between disks and stellar rotation. Disk-locking theory predicts that accreting stars are preferentially slow rotators compared to their peers. If true, classical T Tauri stars (CTTS), which are accreting based on strong Hα, emission, should have observably lower vsini, values compared to weak-lined T Tauri stars (WTTS), which are not accreting. We present our findings from high-resolution optical spectra taken with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph on the Magellan Clay 6.5-m telescope located at the Las Campanas Observatory of 63 T Tauri stars in the Cha I SFR.

  • 32. Nguyen, Duy Cuong
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Jayawardhana, Ray
    van Kerkwijk, Marten H.
    Scholz, Alexander
    Multiplicity in Star Formation: Close Binaries in Chamaeleon I and Taurus-Auriga2009Conference paper (Other academic)
    Abstract [en]

    A large fraction of stars are members of binary systems. Yet, the formation and early evolution of binary and multiple stars is not well understood theoretically and poorly constrained observationally. We present findings from a comprehensive spectroscopic survey for close binaries among nearly 200 members of the Chamaeleon I and Taurus-Auriga star-forming regions. We find that higher mass stars in our sample have a higher spectroscopic binary frequency than their lower mass counterparts, similar to what is seen in the field and for resolved (wide) binaries in young associations. More intriguingly, our observations also reveal a significantly larger fraction of tight binaries in Tau-Aur compared to Cha I, both for F-K stars and for M dwarfs, implying possible regional variations in the formation of multiple stars.

  • 33. Nguyen, Duy Cuong
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Kerkwijk, Marten H.
    Jayawardhana, Ray
    CLOSE COMPANIONS TO YOUNG STARS. I. A LARGE SPECTROSCOPIC SURVEY IN CHAMAELEON I AND TAURUS-AURIGA2012In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 745, no 2, p. 119-Article in journal (Refereed)
    Abstract [en]

    We present the results of a multiplicity survey of 212 T Tauri stars in the Chamaeleon I and Taurus-Auriga star-forming regions, based on high-resolution spectra from the Magellan Clay 6.5 m telescope. From these data, we achieved a typical radial velocity (RV) precision of similar to 80 m s(-1) with slower rotators yielding better precision, in general. For 174 of these stars, we obtained multi-epoch data with sufficient time baselines to identify binaries based on RV variations. We identified eight close binaries and four close triples, of which three and two, respectively, are new discoveries. The spectroscopic multiplicity fractions we find for Chamaeleon I (7%) and Taurus-Auriga (6%) are similar to each other, and to the results of field star surveys in the same mass and period regime. However, unlike the results from imaging surveys, the frequency of systems with close companions in our sample is not seen to depend on primary mass. Additionally, we do not find a strong correlation between accretion and close multiplicity. This implies that close companions are not likely the main source of the accretion shut down observed in weak-lined T Tauri stars. Our results also suggest that sufficient RV precision can be achieved for at least a subset of slowly rotating young stars to search for hot Jupiter planets.

  • 34. Nguyen, Duy Cuong
    et al.
    Jayawardhana, Ray
    van Kerkwijk, Marten H.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scholz, Alexander
    Damjanov, Ivana
    Disk Braking in young Stars: Probing Rotation in Chamaeleon i and Taurus-Auriga2009In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 695, p. 1648-1656Article in journal (Refereed)
    Abstract [en]

    We present a comprehensive study of rotation, disk, and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star-forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infrared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We find that both accretors and non-accretors have similar distributions of vsin i. This result could be due to different initial conditions, insufficient time for disk braking, or a significant age spread within the regions. The rotational velocities in both regions show a clear mass dependence, with F-K stars rotating on average about twice as fast as M stars, consistent with results reported for other clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular momentum j varies as M 0.5 for our sample which spans a mass range of ~0.16-3 M sun. This power law complements previous studies in Orion which estimated j vprop M 0.25 for lsim2 Myr stars in the same mass regime, and a sharp decline in j with decreasing mass for older stars (~10 Myr) with M < 2 M sun. Furthermore, the overall specific angular momentum of this ~10 Myr population is five times lower than that of non-accretors in our sample, and implies a stellar braking mechanism other than disk braking could be at work. For a subsample of 67 objects with mid-infrared photometry, we examine the connection between accretion signatures and dusty disks: in the vast majority of cases (63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar to the lifetime of inner disks.

  • 35. Nguyen, Duy Cuong
    et al.
    Scholz, Alexander
    van Kerkwijk, Marten H.
    Jayawardhana, Ray
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    How Variable is Accretion in Young Stars?2009In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 694, p. L153-L157Article in journal (Refereed)
    Abstract [en]

    We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Taurus-Auriga and Chamaeleon I. For all stars, we typically obtain four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the Hα 10% width and the Ca II-λ8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the Ca II fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while Hα 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that Ca II fluxes are a more robust quantitative indicator of accretion than Hα 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates toward later spectral types, reflecting the \dot{M} versus M relationship. There is a gap between accretors and nonaccretors, pointing to a rapid shutdown of accretion. We conclude that the ~two orders of magnitude scatter in the \dot{M} versus M relationship is dominated by object-to-object scatter instead of intrinsic source variability.

  • 36. Nilsson, R.
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Fathi, Kambiz
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thebault, Ph.
    Liseau, R.
    VLT imaging of the beta Pictoris gas disk2012In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 544, p. A134-Article in journal (Refereed)
    Abstract [en]

    Context. Circumstellar debris disks older than a few Myr should be largely devoid of primordial gas remaining from the protoplanetary disk phase. Tracing the origin of observed atomic gas in Keplerian rotation in the edge-on debris disk surrounding the similar to 12 Myr old star beta Pictoris requires more detailed information about its spatial distribution than has previously been acquired by limited slit spectroscopy. Especially indications of asymmetries and presence of Ca II gas at high disk latitudes call for additional investigation to exclude or confirm its connection to observed dust structures or suggested cometary bodies on inclined eccentric orbits. Aims. We set out to recover a complete image of the Fe I and Ca II gas emission around beta Pic by spatially resolved, high-resolution spectroscopic observations to better understand the morphology and origin of the gaseous disk component. Methods. The multiple fiber facility FLAMES/GIRAFFE at the Very Large Telescope (VLT), with the large integral-field-unit ARGUS, was used to obtain spatially resolved optical spectra (from 385.9 to 404.8 nm) in four regions covering the northeast and southwest side of the disk. Emission lines from Fe I (at 386.0 nm) and Ca II (at 393.4 and 396.8 nm) were mapped and could be used to fit a parametric function for the disk gas distribution, using a gas-ionisation code for gas-poor debris disks. Results. Both Fe I and Ca II emission are clearly detected, with the former dominating along the disk midplane, and the latter revealing vertically more extended gas. The surface intensity of the Fe I emission is lower but more extended in the northeast (reaching the 210 AU limit of our observations) than in the southwest, while Ca II shows the opposite asymmetry. The modelled Fe gas disk profile shows a linear increase in scale height with radius, and a vertical profile that suggests dynamical interaction with the dust. We also qualitatively demonstrate that the Ca II emission profile can be explained by optical thickness in the disk midplane, and does not require Ca to be spatially separated from Fe.

  • 37.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Fathi, Kambiz
    Stockholm University, Faculty of Science, Department of Astronomy.
    Thébault, Philippe
    Liseau, René
    VLT imaging of the β Pictoris gas diskArticle in journal (Refereed)
  • 38.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Liseau, René
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pilbratt, Göran
    Risacher, Christophe
    Rodmann, Jens
    Augereau, Jean-Charles
    Bergman, Per
    Eiroa, Carlos
    Fridlund, Malcolm
    Thébault, Philippe
    White, Glenn
    Kuiper belts around nearby stars2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 518, p. A40-Article in journal (Refereed)
    Abstract [en]

    Context. The existence of dusty debris disks around a large fraction of solar type main-sequence stars, inferred from excess far-IR and submillimetre emission compared to that expected from stellar photospheres, suggests that leftover planetesimal belts analogous to the asteroid-and comet reservoirs of the solar system are common.

    Aims. Sensitive submillimetre observations are essential to detect and characterise cold extended dust originating from collisions of small bodies in disks, belts, or rings at Kuiper-belt distances (30-50 AU or beyond). Measurements of the flux densities at these wavelengths will extend existing IR photometry and permit more detailed modelling of the Rayleigh-Jeans tail of the disks spectral energy distribution (SED), effectively constraining dust properties and disk extensions. By observing stars spanning from a few up to several hundred Myr, the evolution of debris disks during crucial phases of planet formation can be studied.

    Methods. We observed 22 exo-Kuiper-belt candidates at 870 mu m, as part of a large programme with the LABOCA bolometer at the APEX telescope. Dust masses (or upper limits) were calculated from integrated 870 mu m fluxes, and fits to the SED of detected sources revealed the fractional dust luminosities f(dust), dust temperatures T(dust), and power-law exponents beta of the opacity law.

    Results. A total of 10 detections with at least 3 sigma significance were made, out of which five (HD95086, HD131835, HD161868, HD170773, and HD207129) have previously never been detected at submillimetre wavelengths. Three additional sources are marginally detected with > 2.5 sigma significance. The best-fit beta parameters all lie between 0.1 and 0.8, in agreement with previous results indicating the presence of significantly larger grains than those in the ISM. From our relatively small sample we estimate f(dust) proportional to t(-alpha), with a similar to 0.8-2.0, and identify an evolution of the characteristic radial dust distance R(dust) that is consistent with the t(1/3) increase predicted from models of self-stirred collisions in debris disks.

  • 39.
    Nilsson, Ricky
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Liseau, René
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Risacher, Christophe
    Fridlund, Malcolm
    Pilbratt, Göran
    A submillimetre search for cold extended debris disks in the β Pictoris moving group2009In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 508, no 2, p. 1057-1065Article in journal (Refereed)
    Abstract [en]

    Context. Previous observations with the Infrared Astronomical Satellite and the Infrared Space Observatory, and ongoing observations with Spitzer and AKARI, have led to the discovery of over 200 debris disks, based on detected mid-and far infrared excess emission, indicating warm circumstellar dust. To constrain the properties of these systems, e.g., to more accurately determine the dust mass, temperature and radial extent, follow-up observations in the submillimetre wavelength region are needed.

    Aims. The beta Pictoris moving group is a nearby stellar association of young (similar to 12 Myr) co-moving stars including the classical debris disk star beta Pictoris. Due to their proximity and youth, they are excellent targets when searching for submillimetre emission from cold, extended, dust components produced by collisions in Kuiper-Belt-like disks. They also allow an age independent study of debris disk properties as a function of other stellar parameters.

    Methods. We observed 7 infrared-excess stars in the beta Pictoris moving group with the LABOCA bolometer array, operating at a central wavelength of 870 mu m at the 12-m submillimetre telescope APEX. The main emission at these wavelengths comes from large, cold dust grains, which constitute the main part of the total dust mass, and hence, for an optically thin case, make better estimates on the total dust mass than earlier infrared observations. Fitting the spectral energy distribution with combined optical and infrared photometry gives information on the temperature and radial extent of the disk.

    Results. From our sample, beta Pic, HD181327, and HD172555 were detected with at least 3 sigma certainty, while all others are below 2 sigma and considered non-detections. The image of beta Pic shows an offset flux density peak located near the south-west extension of the disk, similar to the one previously found by SCUBA at the JCMT. We present SED fits for detected sources and give an upper limit on the dust mass for undetected ones.

    Conclusions. We find a mean fractional dust luminosity (f) over bar (dust) = 1.1 x 10(-3) at t approximate to 12 Myr, which together with recent data at 100 Myr suggests an f(dust) proportional to t(-alpha) a decline of the emitting dust, with alpha > 0.8.

  • 40. Rauer, H.
    et al.
    Catala, C.
    Aerts, C.
    Appourchaux, T.
    Benz, W.
    Brandeker, Aexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Christensen-Dalsgaard, J.
    Deleuil, M.
    Gizon, L.
    Goupil, M. -J
    Guedel, M.
    Janot-Pacheco, E.
    Mas-Hesse, M.
    Pagano, I.
    Piotto, G.
    Pollacco, D.
    Santos, N. C.
    Smith, A.
    Suarez, J. -C
    Szabo, R.
    Udry, S.
    Adibekyan, V.
    Alibert, Y.
    Almenara, J. -M
    Maro-Seoane, P. A.
    Ammler-von Eiff, M.
    Asplund, M.
    Antonello, E.
    Barnes, S.
    Baudin, F.
    Belkacem, K.
    Bergemann, M.
    Bihain, G.
    Birch, A. C.
    Bonfils, X.
    Boisse, I.
    Bonomo, A. S.
    Borsa, F.
    Brandao, I. M.
    Brocato, E.
    Brun, S.
    Burleigh, M.
    Burston, R.
    Cabrera, J.
    Cassisi, S.
    Chaplin, W.
    Charpinet, S.
    Chiappini, C.
    Church, R. P.
    Csizmadia, Sz.
    Cunha, M.
    Damasso, M.
    Davies, M. B.
    Deeg, H. J.
    Diaz, R. F.
    Dreizler, S.
    Dreyer, C.
    Eggenberger, P.
    Ehrenreich, D.
    Eigmueller, P.
    Erikson, A.
    Farmer, R.
    Feltzing, S.
    de Oliveira Fialho, F.
    Figueira, P.
    Forveille, T.
    Fridlund, M.
    Garcia, R. A.
    Giommi, P.
    Giuffrida, G.
    Godolt, M.
    Gomes da Silva, J.
    Granzer, T.
    Grenfell, J. L.
    Grotsch-Noels, A.
    Guenther, E.
    Haswell, C. A.
    Hatzes, A. P.
    Hebrard, G.
    Hekker, S.
    Helled, R.
    Heng, K.
    Jenkins, J. M.
    Johansen, A.
    Khodachenko, M. L.
    Kislyakova, K. G.
    Kley, W.
    Kolb, U.
    Krivova, N.
    Kupka, F.
    Lammer, H.
    Lanza, A. F.
    Lebreton, Y.
    Magrin, D.
    Marcos-Arenal, P.
    Marrese, P. M.
    Marques, J. P.
    Martins, J.
    Mathis, S.
    Mathur, S.
    Messina, S.
    Miglio, A.
    Montalban, J.
    Montalto, M.
    Monteiro, M. J. P. F. G.
    Moradi, H.
    Moravveji, E.
    Mordasini, C.
    Morel, T.
    Mortier, A.
    Nascimbeni, V.
    Nelson, R. P.
    Nielsen, M. B.
    Noack, L.
    Norton, A. J.
    Ofir, A.
    Oshagh, M.
    Ouazzani, R. -M
    Papics, P.
    Parro, V. C.
    Petit, P.
    Plez, B.
    Poretti, E.
    Quirrenbach, A.
    Ragazzoni, R.
    Raimondo, G.
    Rainer, M.
    Reese, D. R.
    Redmer, R.
    Reffert, S.
    Rojas-Ayala, B.
    Roxburgh, I. W.
    Salmon, S.
    Santerne, A.
    Schneider, J.
    Schou, J.
    Schuh, S.
    Schunker, H.
    Silva-Valio, A.
    Silvotti, R.
    Skillen, I.
    Snellen, I.
    Sohl, F.
    Sousa, S. G.
    Sozzetti, A.
    Stello, D.
    Strassmeier, K. G.
    Svanda, M.
    Szabo, Gy. M.
    Tkachenko, A.
    Valencia, D.
    Van Grootel, V.
    Vauclair, S. D.
    Ventura, P.
    Wagner, F. W.
    Walton, N. A.
    Weingrill, J.
    Werner, S. C.
    Wheatley, P. J.
    Zwintz, K.
    The PLATO 2.0 mission2014In: Experimental astronomy (Print), ISSN 0922-6435, E-ISSN 1572-9508, Vol. 38, no 1-2, p. 249-330Article in journal (Refereed)
    Abstract [en]

    PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 s readout cadence and 2 with 2.5 s cadence) providing a wide field-of-view (2232 deg(2)) and a large photometric magnitude range (4-16 mag). It focuses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2 %, 4-10 % and 10 % for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50 % of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0. The PLATO 2.0 catalogue allows us to e. g.: - complete our knowledge of planet diversity for low-mass objects, - correlate the planet mean density-orbital distance distribution with predictions from planet formation theories,- constrain the influence of planet migration and scattering on the architecture of multiple systems, and - specify how planet and system parameters change with host star characteristics, such as type, metallicity and age. The catalogue will allow us to study planets and planetary systems at different evolutionary phases. It will further provide a census for small, low-mass planets. This will serve to identify objects which retained their primordial hydrogen atmosphere and in general the typical characteristics of planets in such a low-mass, low-density range. Planets detected by PLATO 2.0 will orbit bright stars and many of them will be targets for future atmosphere spectroscopy exploring their atmospheres. Furthermore, the mission has the potential to detect exomoons, planetary rings, binary and Trojan planets. The planetary science possible with PLATO 2.0 is complemented by its impact on stellar and galactic science via asteroseismology as well as light curves of all kinds of variable stars, together with observations of stellar clusters of different ages. This will allow us to improve stellar models and study stellar activity. A large number of well-known ages from red giant stars will probe the structure and evolution of our Galaxy. Asteroseismic ages of bright stars for different phases of stellar evolution allow calibrating stellar age-rotation relationships. Together with the results of ESA's Gaia mission, the results of PLATO 2.0 will provide a huge legacy to planetary, stellar and galactic science.

  • 41. Scholz, Alexander
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Multiplicity among Young Brown Dwarfs and Very Low Mass Stars2007Manuscript (preprint) (Other academic)
  • 42. Scholz, Alexander
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Rotation and Activity of Pre-Main-Sequence Stars2007In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, The Astrophysical JournalArticle in journal (Refereed)
  • 43. Sibthorpe, B.
    et al.
    Vandenbussche, B.
    Greaves, J. S.
    Pantin, E.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Acke, B.
    Barlow, M. J.
    Blommaert, J. A. D. L.
    Bouwman, J.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Cohen, M.
    De Meester, W.
    Dent, W. R. F.
    Di Francesco, J.
    Dominik, C.
    Fridlund, M.
    Gear, W. K.
    Glauser, A. M.
    Gomez, H. L.
    Hargrave, P. C.
    Harvey, P. M.
    Henning, Th.
    Heras, A. M.
    Hogerheijde, M. R.
    Holland, W. S.
    Ivison, R. J.
    Leeks, S. J.
    Lim, T. L.
    Liseau, R.
    Matthews, B. C.
    Naylor, D. A.
    Pilbratt, G. L.
    Polehampton, E. T.
    Regibo, S.
    Royer, P.
    Sicilia-Aguilar, A.
    Swinyard, B. M.
    Waelkens, C.
    Walker, H. J.
    Wesson, R.
    The Vega debris disc: A view from Herschel2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 518, p. L130-Article in journal (Refereed)
    Abstract [en]

    We present five band imaging of the Vega debris disc obtained using the Herschel Space Observatory. These data span a wavelength range of 70-500 mu m with full-width half-maximum angular resolutions of 5.6-36.9 ''. The disc is well resolved in all bands, with the ring structure visible at 70 and 160 mu m. Radial profiles of the disc surface brightness are produced, and a disc radius of 11 '' (similar to 85AU) is determined. The disc is seen to have a smooth structure thoughout the entire wavelength range, suggesting that the disc is in a steady state, rather than being an ephemeral structure caused by the recent collision of two large planetesimals.

  • 44. Vandenbussche, B.
    et al.
    Sibthorpe, B.
    Acke, B.
    Pantin, E.
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Waelkens, C.
    Dominik, C.
    Barlow, M. J.
    Blommaert, J. A. D. L.
    Bouwman, J.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Cohen, M.
    De Meester, W.
    Dent, W. R. F.
    Exter, K.
    Di Francesco, J.
    Fridlund, M.
    Gear, W. K.
    Glauser, A. M.
    Gomez, H. L.
    Greaves, J. S.
    Hargrave, P. C.
    Harvey, P. M.
    Henning, Th.
    Heras, A. M.
    Hogerheijde, M. R.
    Holland, W. S.
    Huygen, R.
    Ivison, R. J.
    Jean, C.
    Leeks, S. J.
    Lim, T. L.
    Liseau, R.
    Matthews, B. C.
    Naylor, D. A.
    Pilbratt, G. L.
    Polehampton, E. T.
    Regibo, S.
    Royer, P.
    Sicilia-Aguilar, A.
    Swinyard, B. M.
    Walker, H. J.
    Wesson, R.
    The beta Pictoris disk imaged by Herschel PACS and SPIRE2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 518, p. L133-Article in journal (Refereed)
    Abstract [en]

    We obtained Herschel PACS and SPIRE images of the thermal emission of the debris disk around the A5V star beta Pic. The disk is well resolved in the PACS filters at 70, 100, and 160 mu m. The surface brightness profiles between 70 and 160 mu m show no significant asymmetries along the disk, and are compatible with 90% of the emission between 70 and 160 mu m originating in a region closer than 200 AU to the star. Although only marginally resolving the debris disk, the maps obtained in the SPIRE 250-500 mu m filters provide full-disk photometry, completing the SED over a few octaves in wavelength that had been previously inaccessible. The small far-infrared spectral index (beta = 0.34) indicates that the grain size distribution in the inner disk (<200 AU) is inconsistent with a local collisional equilibrium. The size distribution is either modified by non-equilibrium effects, or exhibits a wavy pattern, caused by an under-abundance of impactors which have been removed by radiation pressure.

  • 45. Xie, Ji-Wei
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Wu, Yanqin
    ON THE UNUSUAL GAS COMPOSITION IN THE beta PICTORIS DEBRIS DISK2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 762, no 2, p. 114-Article in journal (Refereed)
    Abstract [en]

    Themetallic gas associated with the beta Pic debris disk is not believed to be primordial, but arises from the destruction of dust grains. Recent observations have shown that carbon and oxygen in this gas are exceptionally overabundant compared to other elements, by some 400 times. We study the origin of this enrichment under two opposing hypotheses: preferential production, where the gas is produced with the observed unusual abundance (as may happen if gas is produced by photodesorption from C/O-rich icy grains), and preferential depletion, where the gas evolves to the observed state from an original solar abundance (if outgassing occurs under high-speed collisions) under a number of dynamical processes. We include in our study the following processes: radiative blowout of metallic elements, dynamical coupling between different species, and viscous accretion onto the star. We find that, if gas viscosity is sufficiently low (the conventional a parameter less than or similar to 10(-3)), differential blowout dominates. While gas accumulates gradually in the disks, metallic elements subject to strong radiation forces, such as Na and Fe, deplete more quickly than C and O, naturally leading to the observed overabundance of C and O. On the other hand, if gas viscosity is high (alpha greater than or similar to 10(-1), as expected for this largely ionized disk), gas is continuously produced and viscously accreted toward the star. This removal process does not discriminate between elements so the observed overabundance of C and O has to be explained by a preferential production that strongly favors C and O to other metallic elements. One such candidate is photodesorption off the grains. We compare our calculation against all observed elements (similar to 10) in the gas disk and find a mild preference for the second scenario, based on the abundance of Si alone. If true, beta Pic should still be accreting at an observable rate, well after its primordial disk has disappeared.

  • 46. Zagorovsky, Kyryl
    et al.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Wu, Yanqin
    GAS EMISSION FROM DEBRIS DISKS AROUND A AND F STARS2010In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 720, no 1, p. 923-939Article in journal (Refereed)
    Abstract [en]

    Gas has been detected in a number of debris disk systems. This gas may have arisen from grain sublimation or grain photodesorption. It interacts with the surrounding dust grains through a number of charge and heat exchanges. Studying the chemical composition and physical state of this gas can therefore reveal much about the dust component in these debris disks. We have produced a new code, ONTARIO, to address gas emission from dusty gas-poor disks around A-F stars. This code computes the gas ionization and thermal balance self-consistently, with particular care taken of heating/cooling mechanisms. Line emission spectra are then produced for each species (up to zinc) by statistical equilibrium calculations of the atomic/ionic energy levels. For parameters that resemble the observed beta Pictoris gas disk, we find that the gas is primarily heated by photoelectric emission from dust grains, and primarily cooled through the CII 157.7 mu m line emission. The gas can be heated to a temperature that is warmer than that of the dust and may in some cases reach temperature for thermal escape. The dominant cooling line, CII 157.7 mu m, should be detectable by Herschel in these disks, while the OI 63.2 mu m line will be too faint. We also study the dependence of the cooling line fluxes on a variety of disk parameters, in light of the much improved sensitivity to thermal line emission in the mid/far-infrared and at submillimeter wavelengths provided by, in particular, Herschel, SOFIA, and ALMA. These new instruments will yield much new information about dusty debris disks.

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