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  • 1.
    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.

  • 2.
    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)
  • 3.
    Cataldi, Gianni
    Stockholm University, Faculty of Science, Department of Astronomy.
    Debris disks and the search for life in the universe2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Circumstellar debris disks are the extrasolar analogues of the asteroid belt and the Kuiper belt. These disks consist of comets and leftover planetesimals that continuously collide to produce copious amounts of circumstellar dust that can be observed as infrared excess or in resolved imaging. As an obvious outcome of the planet formation process, debris disks can help us constrain planet formation theories and learn about the history of our own solar system. Structures in the disks such as gaps or warps can hint at the presence of planets. Thus, the study of debris disks is an important branch of exoplanetary science. In this thesis, some aspects of debris disks are considered in detail.

    A handful of debris disks show observable amounts of gas besides the dust. One such case is the edge-on debris disk around the young A-type star β Pictoris, where the gas is thought to be of secondary origin, i.e. derived from the dust itself. By observing this gas, we can thus learn something about the dust, and therefore about the building blocks of planets. In paper I, spectrally resolved observations of C II emission with Herschel/HIFI are presented. The line profile is used to constrain the spatial distribution of carbon gas in the disk, which helps understanding the gas producing mechanism. In paper II, we analyse C II and O I emission detected with Herschel/PACS and find that the oxygen must be located in a relatively dense region, possibly similar to the CO clump seen by ALMA. An upcoming analysis of our ALMA C I observations will give us a clearer picture of the system.

    Another famous debris disk is found around the nearby, 440 Myr old A-star Fomalhaut. Its morphology is that of an eccentric debris belt with sharp edges, suggesting shaping by a planet. However, gas-dust interactions may result in a similar morphology without the need to invoke planets. We test this possibility in paper III by analysing non-detections of C II and O I emission by Herschel/PACS. We find that there is not enough gas present to efficiently sustain gas-dust interactions, implying that the morphology of the Fomalhaut belt is due to a yet unseen planet or alternatively stellar encounters.

    One of the biggest challenges in exoplanetary research is to answer the question whether there are inhabited worlds other than the Earth. With the number of known rocky exoplanets in the habitable zone increasing rapidly, we might actually be able to answer this question in the coming decades. Different approaches exist to detect the presence of life remotely, for example by studying exoplanetary atmospheres or by analysing light reflected off the surface of an exoplanet. In paper IV, we study whether biosignatures (for example, certain minerals or microorganisms) ejected into a circumstellar debris disk by an impact event could be detected. We consider an impact similar to the Chicxulub event and model the collisional evolution of the ejected debris. Dust from such an event can potentially be detected by current telescopes, but analysis of the debris composition has to wait for future, advanced instruments.

  • 4.
    Cataldi, Gianni
    Stockholm University, Faculty of Science, Department of Astronomy.
    Debris disks from an astronomical and an astrobiological viewpoint2013Licentiate thesis, monograph (Other academic)
    Abstract [en]

    In this licentiate thesis, I consider debris disks from an observational, astronomical viewpoint, but also discuss a potential astrobiological application. Debris disks are essentially disks of dust and rocks around main-sequence stars, analogue to the Kuiper- or the asteroid belt in our solar system. Their observation and theoretical modeling can help to constrain planet formation models and help in the understanding of the history of the solar system. After a general introduction into the field of debris disks and some basic debris disk physics, the thesis concentrates on the observation of gas in debris disks. The possible origins of this gas and its dynamics are discussed and it is considered what it can tell us about the physical conditions in the disk and possibly about the dust composition. In this way, the paper associated with this thesis (dealing with the gas in the β Pic debris disk) is set into context. More in detail, we observed the CII emission originating from the carbon-rich β Pic disk with Herschel HIFI and attempted to constrain the spatial distribution of the gas from the shape of the emission line. This is necessary since the gas production mechanism is currently unknown, but can be constraint by obtaining information about the spatial profile of the gas. The last part of the thesis describes our preliminary studies of the possibility of a debris disk containing biomarkers, created by a giant impact on a life-bearing exoplanet.

  • 5.
    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.

  • 6.
    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.

  • 7.
    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)
  • 8.
    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.

  • 9.
    Cataldi, Gianni
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. National Astronomical Observatory of Japan, USA; Hungarian Academy of Sciences, Hungary.
    Brandeker, Alexis
    Stockholm University, Faculty of Science, Department of Astronomy.
    Wu, Yanqin
    Chen, Christine
    Dents, William
    de Vries, Bernard L.
    Stockholm University, Faculty of Science, Department of Astronomy. European Space Research and Technology Centre (ESA/ESTEC), The Netherlands.
    Kamp, Inga
    Liseau, René
    Olofsson, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Pantin, Eric
    Roberge, Aki
    ALMA Resolves CI Emission from the beta Pictoris Debris Disk2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 861, no 1, article id 72Article in journal (Refereed)
    Abstract [en]

    The debris disk around beta Pictoris is known to contain gas. Previous ALMA observations revealed a CO belt at similar to 85 au with a distinct clump, interpreted as a location of enhanced gas production. Photodissociation converts CO into C and O within similar to 50 a. We resolve C I emission at 492 GHz using ALMA and study its spatial distribution. C I shows the same clump as seen for CO. This is surprising, as C is expected to quickly spread in azimuth. We derive a low C mass (between 5 x 10(-4) and 3.1 x 10(-3) MA(circle plus)), indicating that gas production started only recently (within similar to 5000 a). No evidence is seen for an atomic accretion disk inward of the CO belt, perhaps because the gas did not yet have time to spread radially. The fact that C and CO share the same asymmetry argues against a previously proposed scenario where the clump is due to an outward-migrating planet trapping planetesimals in a resonance, nor can the observations be explained by an eccentric planetesimal belt secularly forced by a planet. Instead, we suggest that the dust and gas disks should be eccentric. Such a configuration, we further speculate, might be produced by a recent tidal disruption event. Assuming that the disrupted body has had a CO mass fraction of 10%, its total mass would be greater than or similar to 3M(Moon).

  • 10.
    Cavallius, Maria
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Cataldi, Gianni
    Stockholm University, Faculty of Science, Department of Astronomy. Hungarian Academy of Sciences, Hungary; National Astronomical Observatory of Japan, Japan; University of Toronto, Canada.
    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.
    Upper limits on the water vapour content of the β Pictoris debris disk2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 628, article id A127Article in journal (Refereed)
    Abstract [en]

    Context. The debris disk surrounding β Pictoris has been observed with ALMA to contain a belt of CO gas with a distinct peak at ~85 au. This CO clump is thought to be the result of a region of enhanced density of solids that collide and release CO through vaporisation. The parent bodies are thought to be comparable to solar system comets, in which CO is trapped inside a water ice matrix

    Aims. Since H2O should be released along with CO, we aim to put an upper limit on the H2O gas mass in the disk of β Pictoris.

    Methods. We used archival data from the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory to study the ortho-H2O 1(10)-1(01) emission line. The line is undetected. Using a python implementation of the radiative transfer code RADEX, we converted upper limits on the line flux to H2O gas masses. The resulting lower limits on the CO/H2O mass ratio are compared to the composition of solar system comets.

    Results. Depending on the assumed gas spatial distribution, we find a 95% upper limit on the ortho-H2O line flux of7.5×10−20W m−2or1.2×10−19W m−2. These translate into an upper limit on the H2O mass of7.4×1016–1.1×1018kg depending on both the electron density and gas kinetic temperature. The range of derived gas-phase CO/H2O ratios is marginally consistent with low-ratio solar system comets.

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