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  • 1. Acke, B.
    et al.
    Min, M.
    Dominik, C.
    Vandenbussche, B.
    Sibthorpe, B.
    Waelkens, C.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Degroote, P.
    Smolders, K.
    Pantin, E.
    Barlow, M. J.
    Blommaert, J. A. D. L.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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 activity2012Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 540, s. A125-Artikkel i tidsskrift (Fagfellevurdert)
    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
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Multiplicity among Young Brown Dwarfs and Very Low Mass Stars2007Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, The Astrophysical JournalArtikkel i tidsskrift (Fagfellevurdert)
  • 3. Ahmic, Mirza
    et al.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars2007Konferansepaper (Annet (populærvitenskap, debatt, mm))
  • 4. Ahmic, Mirza
    et al.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Multiplicity Among Young Brown Dwarfs and Very Low Mass Stars2007Manuskript (preprint) (Annet vitenskapelig)
  • 5. Alonso, R.
    et al.
    Florén, Hans Gustav Axel
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, H.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Walton, N. A.
    No random transits in CHEOPS observations of HD 1391392023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 680, artikkel-id A78Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. The star HD 139139 (a.k.a. 'the Random Transiter') is a star that exhibited enigmatic transit-like features with no apparent periodicity in K2 data. The shallow depth of the events (similar to 200 ppm - equivalent to transiting objects with radii of similar to 1.5 R-circle plus in front of a Sun-like star) and their non-periodicity constitute a challenge for the photometric follow-up of this star. Aims. The goal of this study is to confirm with independent measurements the presence of shallow, non-periodic transit-like features on this object. Methods. We performed observations with CHEOPS for a total accumulated time of 12.75 days, distributed in visits of roughly 20 h in two observing campaigns in years 2021 and 2022. The precision of the data is sufficient to detect 150 ppm features with durations longer than 1.5 h. We used the duration and times of the events seen in the K2 curve to estimate how many events should have been detected in our campaigns, under the assumption that their behaviour during the CHEOPS observations would be the same as in the K2 data of 2017. Results. We do not detect events with depths larger than 150 ppm in our data set. If the frequency, depth, and duration of the events were the same as in the K2 campaign, we estimate the probability of having missed all events due to our limited observing window would be 4.8%. Conclusions. We suggest three different scenarios to explain our results : 1) Our observing window was not long enough, and the events were missed with the estimated 4.8% probability. 2) The events recorded in the K2 observations were time critical, and the mechanism producing them was either not active in the 2021 and 2022 campaigns or created shallower events under our detectability level. 3) The enigmatic events in the K2 data are the result of an unidentified and infrequent instrumental noise in the original data set or its data treatment.

  • 6. 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
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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
    Stockholms universitet, Naturvetenskapliga fakulteten, Fysikum. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (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
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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 Programmes2017Inngår i: Publications Astronomical Society of Australia, ISSN 1323-3580, E-ISSN 1448-6083, Vol. 34, artikkel-id e069Artikkel, forskningsoversikt (Fagfellevurdert)
    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.

  • 7. Barros, S. C. C.
    et al.
    Akinsanmi, B.
    Boue, G.
    Smith, A. M. S.
    Laskar, J.
    Ulmer-Moll, S.
    Lillo-Box, J.
    Queloz, D.
    Collier Cameron, A.
    Sousa, S. G.
    Ehrenreich, D.
    Hooton, M. J.
    Bruno, G.
    Demory, B.-O.
    Correia, A. C. M.
    Demangeon, O. D. S.
    Wilson, T. G.
    Bonfanti, A.
    Hoyer, S.
    Alibert, Y.
    Alonso, R.
    Anglada Escude, G.
    Barbato, D.
    Barczy, T.
    Barrado, D.
    Baumjohann, W.
    Beck, M.
    Beck, T.
    Benz, W.
    Bergomi, M.
    Billot, N.
    Bonfils, X.
    Bouchy, F.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Broeg, C.
    Cabrera, J.
    Cessa, V.
    Charnoz, S.
    Damme, C. C.
    Davies, M. B.
    Deleuil, M.
    Deline, A.
    Delrez, L.
    Erikson, A.
    Fortier, A.
    Fossati, L.
    Fridlund, M.
    Gandolfi, D.
    García Muñoz, A.
    Gillon, M.
    Güdel, M.
    Isaak, K. G.
    Heng, K.
    Kiss, L.
    Lecavelier des Etangs, A.
    Lendl, M.
    Lovis, C.
    Magrin, D.
    Nascimbeni, V.
    Maxted, P. F. L.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ottensamer, R.
    Pagano, I.
    Pallé, E.
    Parviainen, H.
    Peter, G.
    Piotto, G.
    Pollacco, D.
    Ragazzoni, R.
    Rando, N.
    Rauer, H.
    Ribas, I.
    Santos, N. C.
    Scandariato, G.
    Ségransan, D.
    Simon, A. E.
    Steller, M.
    Szabó, Gy. M.
    Thomas, N.
    Udry, S.
    Ulmer, B.
    Van Grootel, V.
    Walton, N. A.
    Detection of the tidal deformation of WASP-103b at 3σ with CHEOPS2022Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 657, artikkel-id A52Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. Ultra-short period planets undergo strong tidal interactions with their host star which lead to planet deformation and orbital tidal decay.

    Aims. WASP-103b is the exoplanet with the highest expected deformation signature in its transit light curve and one of the shortest expected spiral-in times. Measuring the tidal deformation of the planet would allow us to estimate the second degree fluid Love number and gain insight into the planet's internal structure. Moreover, measuring the tidal decay timescale would allow us to estimate the stellar tidal quality factor, which is key to constraining stellar physics.

    Methods. We obtained 12 transit light curves of WASP-103b with the CHaracterising ExOplanet Satellite (CHEOPS) to estimate the tidal deformation and tidal decay of this extreme system. We modelled the high-precision CHEOPS transit light curves together with systematic instrumental noise using multi-dimensional Gaussian process regression informed by a set of instrumental parameters. To model the tidal deformation, we used a parametrisation model which allowed us to determine the second degree fluid Love number of the planet. We combined our light curves with previously observed transits of WASP-103b with the Hubble Space Telescope (HST) and Spitzer to increase the signal-to-noise of the light curve and better distinguish the minute signal expected from the planetary deformation.

    Results. We estimate the radial Love number of WASP-103b to be h(f) = 1.59(-0.53)(+0.45). This is the first time that the tidal deformation is directly detected (at 3 sigma) from the transit light curve of an exoplanet. Combining the transit times derived from CHEOPS, HST, and Spitzer light curves with the other transit times available in the literature, we find no significant orbital period variation for WASP-103b. However, the data show a hint of an orbital period increase instead of a decrease, as is expected for tidal decay. This could be either due to a visual companion star if this star is bound, the Applegate effect, or a statistical artefact.

    Conclusions. The estimated Love number of WASP-103b is similar to Jupiter's. This will allow us to constrain the internal structure and composition of WASP-103b, which could provide clues on the inflation of hot Jupiters. Future observations with James Webb Space Telescope can better constrain the radial Love number of WASP-103b due to their high signal-to-noise and the smaller signature of limb darkening in the infrared. A longer time baseline is needed to constrain the tidal decay in this system.

  • 8. Benz, W.
    et al.
    Broeg, C.
    Fortier, A.
    Rando, N.
    Beck, T.
    Beck, M.
    Queloz, D.
    Ehrenreich, D.
    Maxted, P. F. L.
    Isaak, K. G.
    Billot, N.
    Alibert, Y.
    Alonso, R.
    Antonio, C.
    Asquier, J.
    Bandy, T.
    Barczy, T.
    Barrado, D.
    Barros, S. C. C.
    Baumjohann, W.
    Bekkelien, A.
    Bergomi, M.
    Biondi, F.
    Bonfils, X.
    Borsato, L.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Busch, M.-D.
    Cabrera, J.
    Cessa, V.
    Charnoz, S.
    Chazelas, B.
    Collier Cameron, A.
    Corral Van Damme, C.
    Cortes, D.
    Davies, M. B.
    Deleuil, M.
    Deline, A.
    Delrez, L.
    Demangeon, O.
    Demory, B. O.
    Erikson, A.
    Farinato, J.
    Fossati, L.
    Fridlund, M.
    Futyan, D.
    Gandolfi, D.
    Garcia Munoz, A.
    Gillon, M.
    Guterman, P.
    Gutierrez, A.
    Hasiba, J.
    Heng, K.
    Hernandez, E.
    Hoyer, S.
    Kiss, L. L.
    Kovacs, Z.
    Kuntzer, T.
    Laskar, J.
    Lecavelier des Etangs, A.
    Lendl, M.
    Lopez, A.
    Lora, I.
    Lovis, C.
    Lueftinger, T.
    Magrin, D.
    Malvasio, L.
    Marafatto, L.
    Michaelis, H.
    de Miguel, D.
    Modrego, D.
    Munari, M.
    Nascimbeni, V.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ottacher, H.
    Ottensamer, R.
    Pagano, I.
    Palacios, R.
    Pall, E.
    Peter, G.
    Piazza, D.
    Piotto, G.
    Pizarro, A.
    Pollaco, D.
    Ragazzoni, R.
    Ratti, F.
    Rauer, H.
    Ribas, I.
    Rieder, M.
    Rohlfs, R.
    Safa, F.
    Salatti, M.
    Santos, N. C.
    Scandariato, G.
    Segransan, D.
    Simon, A. E.
    Smith, A. M. S.
    Sordet, M.
    Sousa, S. G.
    Steller, M.
    Szabo, G. M.
    Szoke, J.
    Thomas, N.
    Tschentscher, M.
    Udry, S.
    Van Grootel, V.
    Viotto, V.
    Walter, I.
    Walton, N. A.
    Wildi, F.
    Wolter, D.
    The CHEOPS mission2021Inngår i: Experimental astronomy, ISSN 0922-6435, E-ISSN 1572-9508, Vol. 51, s. 109-151Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The CHaracterising ExOPlanet Satellite (CHEOPS) was selected on October 19, 2012, as the first small mission (S-mission) in the ESA Science Programme and successfully launched on December 18, 2019, as a secondary passenger on a Soyuz-Fregat rocket from Kourou, French Guiana. CHEOPS is a partnership between ESA and Switzerland with important contributions by ten additional ESA Member States. CHEOPS is the first mission dedicated to search for transits of exoplanets using ultrahigh precision photometry on bright stars already known to host planets. As a follow-up mission, CHEOPS is mainly dedicated to improving, whenever possible, existing radii measurements or provide first accurate measurements for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys. The expected photometric precision will also allow CHEOPS to go beyond measuring only transits and to follow phase curves or to search for exo-moons, for example. Finally, by unveiling transiting exoplanets with high potential for in-depth characterisation, CHEOPS will also provide prime targets for future instruments suited to the spectroscopic characterisation of exoplanetary atmospheres. To reach its science objectives, requirements on the photometric precision and stability have been derived for stars with magnitudes ranging from 6 to 12 in the V band. In particular, CHEOPS shall be able to detect Earth-size planets transiting G5 dwarf stars (stellar radius of 0.9R(circle dot)) in the magnitude range 6 <= V <= 9 by achieving a photometric precision of 20 ppm in 6 hours of integration time. In the case of K-type stars (stellar radius of 0.7R(circle dot)) of magnitude in the range 9 <= V <= 12, CHEOPS shall be able to detect transiting Neptune-size planets achieving a photometric precision of 85 ppm in 3 hours of integration time. This precision has to be maintained over continuous periods of observation for up to 48 hours. This precision and stability will be achieved by using a single, frame-transfer, back-illuminated CCD detector at the focal plane assembly of a 33.5 cm diameter, on-axis Ritchey-Chretien telescope. The nearly 275 kg spacecraft is nadir-locked, with a pointing accuracy of about 1 arcsec rms, and will allow for at least 1 Gbit/day downlink. The sun-synchronous dusk-dawn orbit at 700 km altitude enables having the Sun permanently on the backside of the spacecraft thus minimising Earth stray light. A mission duration of 3.5 years in orbit is foreseen to enable the execution of the science programme. During this period, 20% of the observing time is available to the wider community through yearly ESA call for proposals, as well as through discretionary time approved by ESA's Director of Science. At the time of this writing, CHEOPS commissioning has been completed and CHEOPS has been shown to fulfill all its requirements. The mission has now started the execution of its science programme.

  • 9. Boldog, Á.
    et al.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Walton, N. A.
    Glancing through the debris disk: Photometric analysis of DE Boo with CHEOPS star2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 671, artikkel-id A127Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Aims. DE Boo is a unique system, with an edge-on view through the debris disk around the star. The disk, which is analogous to the Kuiper belt in the Solar System, was reported to extend from 74 to 84 AU from the central star. The high photometric precision of the Characterising Exoplanet Satellite (CHEOPS) provided an exceptional opportunity to observe small variations in the light curve due to transiting material in the disk. This is a unique chance to investigate processes in the debris disk.

    Methods. Photometric observations of DE Boo of a total of four days were carried out with CHEOPS. Photometric variations due to spots on the stellar surface were subtracted from the light curves by applying a two-spot model and a fourth-order polynomial. The photometric observations were accompanied by spectroscopic measurements with the 1m RCC telescope at Piszkéstető and with the SOPHIE spectrograph in order to refine the astrophysical parameters of DE Boo.

    Results. We present a detailed analysis of the photometric observation of DE Boo. We report the presence of nonperiodic transient features in the residual light curves with a transit duration of 0.3–0.8 days. We calculated the maximum distance of the material responsible for these variations to be 2.47 AU from the central star, much closer than most of the mass of the debris disk. Furthermore, we report the first observation of flaring events in this system.

    Conclusions. We interpreted the transient features as the result of scattering in an inner debris disk around DE Boo. The processes responsible for these variations were investigated in the context of interactions between planetesimals in the system.

  • 10. Bonfanti, A.
    et al.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Zingales, T.
    Characterising TOI-732 b and c: New insights into the M-dwarf radius and density valley2024Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 682, artikkel-id A66Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context

    TOI-732 is an M dwarf hosting two transiting planets that are located on the two opposite sides of the radius valley. Inferring a reliable demographics for this type of systems is key to understanding their formation and evolution mechanisms.

    Aims

    By doubling the number of available space-based observations and increasing the number of radial velocity (RV) measurements, we aim at refining the parameters of TOI-732 b and c. We also use the results to study the slope of the radius valley and the density valley for a well-characterised sample of M-dwarf exoplanets.

    Methods

    We performed a global Markov chain Monte Carlo analysis by jointly modelling ground-based light curves and CHEOPS and TESS observations, along with RV time series both taken from the literature and obtained with the MAROON-X spectrograph. The slopes of the M-dwarf valleys were quantified via a support vector machine (SVM) procedure.

    Results

    TOI-732b is an ultrashort-period planet (P = 0.76837931-0.00000042+0.0000039 days) with a radius Rb = 1.325-0.058+0.057R, a mass Mb = 2.46 ± 0.19 M, and thus a mean density ρb = 5.8-0.8+1.0 g cm-3, while the outer planet at P = 12.252284 ± 0.000013 days has Rc = 2.39-0.11+0.10RMc = 8.04-0.48+0.50M, and thus ρc = 3.24-0.43+0.55 g cm-3. Even with respect to the most recently reported values, this work yields uncertainties on the transit depths and on the RV semi-amplitudes that are smaller up to a factor of ~1.6 and ~2.4 for TOI-732 b and c, respectively. Our calculations for the interior structure and the location of the planets in the mass-radius diagram lead us to classify TOI-732 b as a super-Earth and TOI-732 c as a mini-Neptune. Following the SVM approach, we quantified d log Rp,valley / d logP = -0.065-0.013+0.024, which is flatter than for Sun-like stars. In line with former analyses, we note that the radius valley for M-dwarf planets is more densely populated, and we further quantify the slope of the density valley as d log ρ^valley / d log P = -0.02-0.04+0.12.

    Conclusions

    Compared to FGK stars, the weaker dependence of the position of the radius valley on the orbital period might indicate that the formation shapes the radius valley around M dwarfs more strongly than the evolution mechanisms.

  • 11. Bonfanti, A.
    et al.
    Delrez, L.
    Hooton, M. J.
    Wilson, T. G.
    Fossati, L.
    Alibert, Y.
    Hoyer, S.
    Mustill, A. J.
    Osborn, H. P.
    Adibekyan, V.
    Gandolfi, D.
    Salmon, S.
    Sousa, S. G.
    Tuson, A.
    Van Grootel, V.
    Cabrera, J.
    Nascimbeni, V.
    Maxted, P. F. L.
    Barros, S. C. C.
    Billot, N.
    Bonfils, X.
    Borsato, L.
    Broeg, C.
    Davies, M. B.
    Deleuil, M.
    Demangeon, O. D. S.
    Fridlund, M.
    Lacedelli, G.
    Lendl, M.
    Persson, C.
    Santos, N. C.
    Scandariato, G.
    Szabo, Gy M.
    Collier Cameron, A.
    Udry, S.
    Benz, W.
    Beck, M.
    Ehrenreich, D.
    Fortier, A.
    Isaak, K. G.
    Queloz, D.
    Alonso, R.
    Asquier, J.
    Bandy, T.
    Barczy, T.
    Barrado, D.
    Barragan, O.
    Baumjohann, W.
    Beck, T.
    Bekkelien, A.
    Bergomi, M.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Busch, M.-D.
    Cessa, V.
    Charnoz, S.
    Chazelas, B.
    Corral Van Damme, C.
    Demory, B.-O.
    Erikson, A.
    Farinato, J.
    Futyan, D.
    Garcia Munoz, A.
    Gillon, M.
    Guedel, M.
    Guterman, P.
    Hasiba, J.
    Heng, K.
    Hernandez, E.
    Kiss, L.
    Kuntzer, T.
    Laskar, J.
    Lecavelier des Etangs, A.
    Lovis, C.
    Magrin, D.
    Malvasio, L.
    Marafatto, L.
    Michaelis, H.
    Munari, M.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ottacher, H.
    Ottensamer, R.
    Pagano, I.
    Palle, E.
    Peter, G.
    Piazza, D.
    Piotto, G.
    Pollacco, D.
    Ragazzoni, R.
    Rando, N.
    Ratti, F.
    Rauer, H.
    Ribas, I.
    Rieder, M.
    Rohlfs, R.
    Safa, F.
    Salatti, M.
    Segransan, D.
    Simon, A. E.
    Smith, A. M. S.
    Sordet, M.
    Steller, M.
    Thomas, N.
    Tschentscher, M.
    Van Eylen, V.
    Viotto, V.
    Walter, I.
    Walton, N. A.
    Wildi, F.
    Wolter, D.
    CHEOPS observations of the HD 108236 planetary system: a fifth planet, improved ephemerides, and planetary radii2021Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 646, artikkel-id A157Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. The detection of a super-Earth and three mini-Neptunes transiting the bright (V = 9.2 mag) star HD 108236 (also known as TOI-1233) was recently reported on the basis of TESS and ground-based light curves.

    Aims. We perform a first characterisation of the HD 108236 planetary system through high-precision CHEOPS photometry and improve the transit ephemerides and system parameters.

    Methods. We characterise the host star through spectroscopic analysis and derive the radius with the infrared flux method. We constrain the stellar mass and age by combining the results obtained from two sets of stellar evolutionary tracks. We analyse the available TESS light curves and one CHEOPS transit light curve for each known planet in the system.

    Results. We find that HD 108236 is a Sun-like star with R-star = 0.877 +/- 0.008 R-circle dot, M-star = 0.869(-0.048)(+0.050)M(circle dot), and an age of 6.7(-5.1)(+4.0) Gyr. We report the serendipitous detection of an additional planet, HD 108236 f, in one of the CHEOPS light curves. For this planet, the combined analysis of the TESS and CHEOPS light curves leads to a tentative orbital period of about 29.5 days. From the light curve analysis, we obtain radii of 1.615 +/- 0.051, 2.071 +/- 0.052, 2.539(-0.065)(+0.062), 3.083 +/- 0.052, and 2.017(-0.057)(+0.052) R-circle plus for planets HD 108236 b to HD 108236 f, respectively. These values are in agreement with previous TESS-based estimates, but with an improved precision of about a factor of two. We perform a stability analysis of the system, concluding that the planetary orbits most likely have eccentricities smaller than 0.1. We also employ a planetary atmospheric evolution framework to constrain the masses of the five planets, concluding that HD 108236 b and HD 108236 c should have an Earth-like density, while the outer planets should host a low mean molecular weight envelope.

    Conclusions. The detection of the fifth planet makes HD 108236 the third system brighter than V = 10 mag to host more than four transiting planets. The longer time span enables us to significantly improve the orbital ephemerides such that the uncertainty on the transit times will be of the order of minutes for the years to come. A comparison of the results obtained from the TESS and CHEOPS light curves indicates that for a V similar to 9 mag solar-like star and a transit signal of similar to 500 ppm, one CHEOPS transit light curve ensures the same level of photometric precision as eight TESS transits combined, although this conclusion depends on the length and position of the gaps in the light curve.

  • 12. Bonfanti, A.
    et al.
    Florén, Hans Gustav
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Walton, N. A.
    TOI-1055 b: Neptunian planet characterised with HARPS, TESS, and CHEOPS2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 671, artikkel-id L8Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. TOI-1055 is a Sun-like star known to host a transiting Neptune-sized planet on a 17.5-day orbit (TOI-1055 b). Radial velocity (RV) analyses carried out by two independent groups using nearly the same set of HARPS spectra have provided measurements of planetary masses that differ by ∼2σ.

    Aims. Our aim in this work is to solve the inconsistency in the published planetary masses by significantly extending the set of HARPS RV measurements and employing a new analysis tool that is able to account and correct for stellar activity. Our further aim was to improve the precision on measurements of the planetary radius by observing two transits of the planet with the CHEOPS space telescope.

    Methods. We fit a skew normal function to each cross correlation function extracted from the HARPS spectra to obtain RV measurements and hyperparameters to be used for the detrending. We evaluated the correlation changes of the hyperparameters along the RV time series using the breakpoint technique. We performed a joint photometric and RV analysis using a Markov chain Monte Carlo scheme to simultaneously detrend the light curves and the RV time series.

    Results. We firmly detected the Keplerian signal of TOI-1055 b, deriving a planetary mass of Mb = 20.4−2.5+2.6 M (∼12%). This value is in agreement with one of the two estimates in the literature, but it is significantly more precise. Thanks to the TESS transit light curves combined with exquisite CHEOPS photometry, we also derived a planetary radius of Rb = 3.490−0.064+0.070 R (∼1.9%). Our mass and radius measurements imply a mean density of ρb = 2.65−0.35+0.37 g cm−3 (∼14%). We further inferred the planetary structure and found that TOI-1055 b is very likely to host a substantial gas envelope with a mass of 0.41−0.20+0.34 M and a thickness of 1.05−0.29+0.30 R.

    Conclusions. Our RV extraction combined with the breakpoint technique has played a key role in the optimal removal of stellar activity from the HARPS time series, enabling us to solve the tension in the planetary mass values published so far for TOI-1055 b.

  • 13. Borsato, L.
    et al.
    Piotto, G.
    Gandolfi, D.
    Nascimbeni, V
    Lacedelli, G.
    Marzari, F.
    Billot, N.
    Maxted, P. F. L.
    Sousa, S.
    Cameron, A. C.
    Bonfanti, A.
    Wilson, T. G.
    Serrano, L. M.
    Garai, Z.
    Alibert, Y.
    Alonso, R.
    Asquier, J.
    Bárczy, T.
    Bandy, T.
    Barrado, D.
    Barros, S. C. C.
    Baumjohann, W.
    Beck, M.
    Beck, T.
    Benz, W.
    Bonfils, X.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Broeg, C.
    Cabrera, J.
    Charnoz, S.
    Csizmadia, S.
    Davies, M. B.
    Deleuil, M.
    Delrez, L.
    Demangeon, O.
    Demory, B-O
    des Etangs, A. L.
    Ehrenreich, D.
    Erikson, A.
    Escudé, G. A.
    Fortier, A.
    Fossati, L.
    Fridlund, M.
    Gillon, M.
    Guedel, M.
    Hasiba, J.
    Heng, K.
    Hoyer, S.
    Isaak, K. G.
    Kiss, L.
    Kopp, E.
    Laskar, J.
    Lendl, M.
    Lovis, C.
    Magrin, D.
    Munari, M.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ottensamer, R.
    Pagano, I
    Pallé, E.
    Peter, G.
    Pollacco, D.
    Queloz, D.
    Ragazzoni, R.
    Rando, N.
    Rauer, H.
    Ribas, I
    Ségransan, D.
    Santos, N. C.
    Scandariato, G.
    Simon, A.
    Smith, A. M. S.
    Steller, M.
    Szabó, G.
    Thomas, N.
    Udry, S.
    Van Grootel, V.
    Walton, N.
    Exploiting timing capabilities of the CHEOPS mission with warm-Jupiter planets2021Inngår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 506, nr 3, s. 3810-3830Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present 17 transit light curves of seven known warm-Jupiters observed with the CHaracterising ExOPlanet Satellite (CHEOPS). The light curves have been collected as part of the CHEOPS Guaranteed Time Observation (GTO) program that searches for transit-timing variation (TTV) of warm-Jupiters induced by a possible external perturber to shed light on the evolution path of such planetary systems. We describe the CHEOPS observation process, from the planning to the data analysis. In this work, we focused on the timing performance of CHEOPS, the impact of the sampling of the transit phases, and the improvement we can obtain by combining multiple transits together. We reached the highest precision on the transit time of about 13–16 s for the brightest target (WASP-38, G = 9.2) in our sample. From the combined analysis of multiple transits of fainter targets with G ≥ 11, we obtained a timing precision of ∼2 min. Additional observations with CHEOPS, covering a longer temporal baseline, will further improve the precision on the transit times and will allow us to detect possible TTV signals induced by an external perturber.

  • 14.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    EXPOSING THE GAS BRAKING MECHANISM OF THE beta PICTORIS DISK2011Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 729, nr 2, s. 122-Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 15.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Cataldi, Gianni
    Contrast sensitivities in the Gaia Data Release 22019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, artikkel-id A86Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The source detection sensitivity of Gaia is reduced near sources. To characterise this contrast sensitivity is important for understanding the completeness of the Gaia data products, in particular when evaluating source confusion in lower resolution surveys such as photometric monitoring for transits. Here, we statistically evaluate the catalogue source density to determine the Gaia Data Release 2 source detection sensitivity as a function of angular separation and brightness ratio from a bright source. The contrast sensitivity from similar to 0.4 '' out to 12 '' ranges in Delta G = 0-14 mag. We find the derived contrast sensitivity to be robust with respect to target brightness, colour, source density, and Gaia scan coverage.

  • 16.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Cataldi, Gianni
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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 disk2016Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 591, artikkel-id A27Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 17.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Cataldi, Gianni
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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 diskInngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Artikkel i tidsskrift (Fagfellevurdert)
  • 18.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Florén, Hans Gustav Axel
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The HD 93963 A transiting system: A 1.04d super-Earth and a 3.65 d sub-Neptune discovered by TESS and CHEOPS2022Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 667, artikkel-id A1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We present the discovery of two small planets transiting HD 93963A (TOI-1797), a GOV star (M* = 1.109 ± 0.043MR* = 1.043 ± 0.009 R) in a visual binary system. We combined TESS and CHEOPS space-borne photometry with MuSCAT 2 ground-based photometry, ‘Alopeke and PHARO high-resolution imaging, TRES and FIES reconnaissance spectroscopy, and SOPHIE radial velocity measurements. We validated and spectroscopically confirmed the outer transiting planet HD 93963 A c, a sub-Neptune with an orbital period of Pc ≈ 3.65 d that was reported to be a TESS object of interest (TOI) shortly after the release of Sector 22 data. HD 93963 A c has amass of Mc = 19.2 ± 4.1 M and a radius of Rc = 3.228 ± 0.059 R, implying a mean density of ρc = 3.1 ± 0.7 g cm-3. The inner object, HD 93963 A b, is a validated 1.04 d ultra-short period (USP) transiting super-Earth that we discovered in the TESS light curve and that was not listed as a TOI, owing to the low significance of its signal (TESS signal-to-noise ratio ≈6.7, TESS + CHEOPS combined transit depth Db = 141.5−8.3+8.5 ppm). We intensively monitored the star with CHEOPS by performing nine transit observations to confirm the presence of the inner planet and validate the system. HD 93963 A b is the first small (Rb = 1.35 ± 0.042 R) USP planet discovered and validated by TESS and CHEOPS. Unlike planet c, HD 93963 Ab is not significantly detected in our radial velocities (Mb = 7.8 ± 3.2 M). The two planets are on either side of the radius valley, implying that they could have undergone completely different evolution processes. We also discovered a linear trend in our Doppler measurements, suggesting the possible presence of a long-period outer planet. With a V-band magnitude of 9.2, HD 93963 A is among the brightest stars known to host a USP planet, making it one of the most favourable targets for precise mass measurement via Doppler spectroscopy and an important laboratory to test formation, evolution, and migration models of planetary systems hosting ultra-short period planets.

  • 19.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Heng, K.
    Lendl, M.
    Patel, Jayshil Ashokkumar
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Morris, B. M.
    Broeg, C.
    Guterman, P.
    Beck, M.
    Maxted, P. F. L.
    Demangeon, O.
    Delrez, L.
    Demory, B-O
    Kitzmann, D.
    Santos, N. C.
    Singh, V
    Alibert, Y.
    Alonso, R.
    Anglada, G.
    Barczy, T.
    Barrado y Navascues, D.
    Barros, S. C. C.
    Baumjohann, W.
    Beck, T.
    Benz, W.
    Billot, N.
    Bonfils, X.
    Bruno, G.
    Cabrera, J.
    Charnoz, S.
    Collier Cameron, A.
    van Damme, C. Corral
    Csizmadia, Sz
    Davies, M. B.
    Deleuil, M.
    Deline, A.
    Ehrenreich, D.
    Erikson, A.
    Farinato, J.
    Fortier, A.
    Fossati, L.
    Fridlund, M.
    Gandolfi, D.
    Gillon, M.
    Guedel, M.
    Hoyer, S.
    Isaak, K. G.
    Kiss, L.
    Laskar, J.
    Lecavelier des Etangs, A.
    Lovis, C.
    Luntzer, A.
    Magrin, D.
    Nascimbeni, V
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ottensamer, R.
    Pagano, I
    Pallé, E.
    Peter, G.
    Piotto, G.
    Pollacco, D.
    Queloz, D.
    Ragazzoni, R.
    Rando, N.
    Rauer, H.
    Ribas, I
    Scandariato, G.
    Ségransan, D.
    Simon, A. E.
    Smith, A. M. S.
    Sousa, S. G.
    Steller, M.
    Szabó, G. M.
    Thomas, N.
    Udry, S.
    Van Grootel, V.
    Walton, N.
    Wolter, D.
    CHEOPS geometric albedo of the hot Jupiter HD 209458 b2022Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 659, artikkel-id L4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We report the detection of the secondary eclipse of the hot Jupiter HD 209458 b in optical/visible light using the CHEOPS space telescope. Our measurement of 20.4−3.3+3.2 parts per million translates into a geometric albedo of Ag = 0.096 ± 0.016. The previously estimated dayside temperature of about 1500 K implies that our geometric albedo measurement consists predominantly of reflected starlight and is largely uncontaminated by thermal emission. This makes the present result one of the most robust measurements of Ag for any exoplanet. Our calculations of the bandpass-integrated geometric albedo demonstrate that the measured value of Ag is consistent with a cloud-free atmosphere, where starlight is reflected via Rayleigh scattering by hydrogen molecules, and the water and sodium abundances are consistent with stellar metallicity. We predict that the bandpass-integrated TESS geometric albedo is too faint to detect and that a phase curve of HD 209458 b observed by CHEOPS would have a distinct shape associated with Rayleigh scattering if the atmosphere is indeed cloud free.

  • 20.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Lendl, M.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Patel, Jayshil Ashokkumar
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    55 Cancri e's occultation captured with CHEOPS2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 669, artikkel-id A64Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Past occultation and phase-curve observations of the ultra-short period super-Earth 55 Cnc e obtained at visible and infrared wavelengths have been challenging to reconcile with a planetary reflection and emission model. In this study, we analyse a set of 41 occultations obtained over a two-year timespan with the CHEOPS satellite. We report the detection of 55 Cnc e’s occultation with an average depth of 12 ± 3 ppm. We derive a corresponding 2σ upper limit on the geometric albedo of Ag < 0.55 once decontaminated from the thermal emission measured by Spitzer at 4.5 µm. CHEOPS’s photometric performance enables, for the first time, the detection of individual occultations of this super-Earth in the visible and identifies short-timescale photometric corrugations likely induced by stellar granulation. We also find a clear 47.3-day sinusoidal pattern in the time-dependent occultation depths that we are unable to relate to stellar noise, nor instrumental systematics, but whose planetary origin could be tested with upcoming JWST occultation observations of this iconic super-Earth.

  • 21.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    A new dynamical modeling of the WASP-47 system with CHEOPS observations star2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 673, artikkel-id A42Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Among the hundreds of known hot Jupiters (HJs), only five have been found to have companions on short-period orbits. Within this rare class of multiple planetary systems, the architecture of WASP-47 is unique, hosting an HJ (planet-b) with both an inner and an outer sub-Neptunian mass companion (-e and -d, respectively) as well as an additional non-transiting, long-period giant (-c). The small period ratio between planets -b and -d boosts the transit time variation (TTV) signal, making it possible to reliably measure the masses of these planets in synergy with the radial velocity (RV) technique. In this paper, we present new space- and ground-based photometric data of WASP-47b and WASP-47-d, including 11 unpublished light curves from the ESA mission CHaracterising ExOPlanet Satellite (CHEOPS). We analyzed the light curves in a homogeneous way together with all the publicly available data to carry out a global N-body dynamical modeling of the TTV and RV signals. We retrieved, among other parameters, a mass and density for planet -d of Md = 15.5 ± 0.8 M and ρd = 1.69 ± 0.22 g cm−3, which is in good agreement with the literature and consistent with a Neptune-like composition. For the inner planet (-e), we found a mass and density of Me = 9.0 ± 0.5 M and ρe = 8.1 ± 0.5 g cm−3, suggesting an Earth-like composition close to other ultra-hot planets at similar irradiation levels. Though this result is in agreement with previous RV plus TTV studies, it is not in agreement with the most recent RV analysis (at 2.8σ), which yielded a lower density compatible with a pure silicate composition. This discrepancy highlights the still unresolved issue of suspected systematic offsets between RV and TTV measurements. In this paper, we also significantly improve the orbital ephemerides of all transiting planets, which will be crucial for any future follow-up.

  • 22.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Connecting photometric and spectroscopic granulation signals with CHEOPS and ESPRESSO2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 670, artikkel-id A24Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. Stellar granulation generates fluctuations in photometric and spectroscopic data whose properties depend on the stellar type, composition, and evolutionary state. Characterizing granulation is key for understanding stellar atmospheres and detecting planets.

    Aims. We aim to detect the signatures of stellar granulation, link spectroscopic and photometric signatures of convection for main-sequence stars, and test predictions from 3D hydrodynamic models.

    Methods. For the first time, we observed two bright stars (Teff = 5833 and 6205 K) with high-precision observations taken simultaneously with CHEOPS and ESPRESSO. We analyzed the properties of the stellar granulation signal in each individual dataset. We compared them to Kepler observations and 3D hydrodynamic models. While isolating the granulation-induced changes by attenuating and filtering the p-mode oscillation signals, we studied the relationship between photometric and spectroscopic observables.

    Results. The signature of stellar granulation is detected and precisely characterized for the hotter F star in the CHEOPS and ESPRESSO observations. For the cooler G star, we obtain a clear detection in the CHEOPS dataset only. The TESS observations are blind to this stellar signal. Based on CHEOPS observations, we show that the inferred properties of stellar granulation are in agreement with both Kepler observations and hydrodynamic models. Comparing their periodograms, we observe a strong link between spectroscopic and photometric observables. Correlations of this stellar signal in the time domain (flux versus radial velocities, RV) and with specific spectroscopic observables (shape of the cross-correlation functions) are however difficult to isolate due to S/N dependent variations.

    Conclusions. In the context of the upcoming PLATO mission and the extreme precision RV surveys, a thorough understanding of the properties of the stellar granulation signal is needed. The CHEOPS and ESPRESSO observations pave the way for detailed analyses of this stellar process.

  • 23.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Refining the properties of the TOI-178 system with CHEOPS and TESS2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 678, artikkel-id A200Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. The TOI-178 system consists of a nearby late K-dwarf transited by six planets in the super-Earth to mini-Neptune regime, with radii ranging from ~1.1 to 2.9 R and orbital periods between 1.9 and 20.7 days. All planets but the innermost one form a chain of Laplace resonances. Mass estimates derived from a preliminary radial velocity (RV) dataset suggest that the planetary densities do not decrease in a monotonic way with the orbital distance to the star, contrary to what one would expect based on simple formation and evolution models.

    Aims. To improve the characterisation of this key system and prepare for future studies (in particular with JWST), we performed a detailed photometric study based on 40 new CHEOPS visits, one new TESS sector, and previously published CHEOPS, TESS, and NGTS data.

    Methods. First we updated the parameters of the host star using the new parallax from Gaia EDR3. We then performed a global analysis of the 100 transits contained in our data to refine the physical and orbital parameters of the six planets and study their transit timing variations (TTVs). We also used our extensive dataset to place constraints on the radii and orbital periods of potential additional transiting planets in the system.

    Results. Our analysis significantly refines the transit parameters of the six planets, most notably their radii, for which we now obtain relative precisions of ≲3%, with the exception of the smallest planet, b, for which the precision is 5.1%. Combined with the RV mass estimates, the measured TTVs allow us to constrain the eccentricities of planets c to g, which are found to be all below 0.02, as expected from stability requirements. Taken alone, the TTVs also suggest a higher mass for planet d than that estimated from the RVs, which had been found to yield a surprisingly low density for this planet. However, the masses derived from the current TTV dataset are very prior-dependent, and further observations, over a longer temporal baseline, are needed to deepen our understanding of this iconic planetary system.

  • 24.
    Brandeker, Alexis
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The stable climate of KELT-9b2022Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 666, artikkel-id A118Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Even among the most irradiated gas giants, so-called ultra-hot Jupiters, KELT-9b stands out as the hottest planet thus far discovered with a dayside temperature of over 4500 K. At these extreme irradiation levels, we expect an increase in heat redistribution efficiency and a low Bond albedo owed to an extended atmosphere with molecular hydrogen dissociation occurring on the planetary dayside. We present new photometric observations of the KELT-9 system throughout 4 full orbits and 9 separate occultations obtained by the 30 cm space telescope CHEOPS. The CHEOPS bandpass, located at optical wavelengths, captures the peak of the thermal emission spectrum of KELT-9b. In this work we simultaneously analyse CHEOPS phase curves along with public phase curves from TESS and Spitzer to infer joint constraints on the phase curve variation, gravity-darkened transits, and occultation depth in three bandpasses, as well as derive 2D temperature maps of the atmosphere at three different depths. We find a day-night heat redistribution efficiency of ~0.3 which confirms expectations of enhanced energy transfer to the planetary nightside due to dissociation and recombination of molecular hydrogen. We also calculate a Bond albedo consistent with zero. We find no evidence of variability of the brightness temperature of the planet, excluding variability greater than 1%

  • 25. Cataldi, Gianni
    et al.
    Aikawa, Yuri
    Iwasaki, Kazunari
    Marino, Sebastian
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Hales, Antonio
    Henning, Thomas
    Higuchi, Aya E.
    Hughes, A. Meredith
    Janson, Markus
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Kral, Quentin
    Matra, Luca
    Moor, Attila
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Naturvetenskapliga fakulteten, Oskar Klein-centrum för kosmopartikelfysik (OKC).
    Redfield, Seth
    Roberge, Aki
    Primordial or Secondary? Testing Models of Debris Disk Gas with ALMA*2023Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 951, nr 2, artikkel-id 111Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The origin and evolution of gas in debris disks are still not well understood. Secondary gas production from cometary material or a primordial origin have been proposed. So far, observations have mostly concentrated on CO, with only a few C observations available. We overview the C and CO content of debris disk gas and test state-of-the-art models. We use new and archival Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO and C i emission, complemented by C ii data from Herschel, for a sample of 14 debris disks. This expands the number of disks with ALMA measurements of both CO and C i by 10 disks. We present new detections of C i emission toward three disks: HD 21997, HD 121191, and HD 121617. We use a simple disk model to derive gas masses and column densities. We find that current state-of-the-art models of secondary gas production overpredict the C-0 content of debris disk gas. This does not rule out a secondary origin, but might indicate that the models require an additional C removal process. Alternatively, the gas might be produced in transient events rather than a steady-state collisional cascade. We also test a primordial gas origin by comparing our results to a simplified thermochemical model. This yields promising results, but more detailed work is required before a conclusion can be reached. Our work demonstrates that the combination of C and CO data is a powerful tool to advance our understanding of debris disk gas.

  • 26.
    Cataldi, Gianni
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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?2015Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 574, artikkel-id L1Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 27.
    Cataldi, Gianni
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Larsson, Bengt
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    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 disk2014Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 563, artikkel-id A66Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 28.
    Cataldi, Gianni
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Thébault, Philippe
    Ahmed, Engy
    de Vries, Bernard L.
    Neubeck, Anna
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Singer, Kelsi
    Searching for biosignatures in exoplanetary impact ejectaInngår i: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070Artikkel i tidsskrift (Fagfellevurdert)
  • 29.
    Cataldi, Gianni
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Thébault, Philippe
    Singer, Kelsi
    Ahmed, Engy
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper. Royal Institute of Technology (KTH), Sweden.
    de Vries, Bernard L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. European Space Research and Technology Centre (ESA/ESTEC), The Netherlands.
    Neubeck, Anna
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för geologiska vetenskaper.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Searching for Biosignatures in Exoplanetary Impact Ejecta2017Inngår i: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 17, nr 8, s. 721-746Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 30.
    Cataldi, Gianni
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. National Astronomical Observatory of Japan, USA; Hungarian Academy of Sciences, Hungary.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Wu, Yanqin
    Chen, Christine
    Dents, William
    de Vries, Bernard L.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. European Space Research and Technology Centre (ESA/ESTEC), The Netherlands.
    Kamp, Inga
    Liseau, René
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pantin, Eric
    Roberge, Aki
    ALMA Resolves CI Emission from the beta Pictoris Debris Disk2018Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 861, nr 1, artikkel-id 72Artikkel i tidsskrift (Fagfellevurdert)
    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).

  • 31. Cataldi, Gianni
    et al.
    Wu, Yanqin
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ohashi, Nagayoshi
    Moór, Attila
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Ábrahám, Péter
    Asensio-Torres, Ruben
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Cavallius, Maria
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Dent, William R. F.
    Grady, Carol
    Henning, Thomas
    Higuchi, Aya E.
    Hughes, A. Meredith
    Janson, Markus
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Kamp, Inga
    Kóspál, Ágnes
    Redfield, Seth
    Roberge, Aki
    Weinberger, Alycia
    Welsh, Barry
    The Surprisingly Low Carbon Mass in the Debris Disk around HD 322972020Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 892, nr 2, artikkel-id 99Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Gas has been detected in a number of debris disks. It is likely secondary, i.e., produced by colliding solids. Here, we report ALMA Band 8 observations of neutral carbon in the CO-rich debris disk around the 15-30 Myr old A-type star HD 32297. We find that C-0 is located in a ring at similar to 110 au with an FWHM of similar to 80 au and has a mass of (3.5 0.2) x 10(-3) M-circle plus. Naively, such a surprisingly small mass can be accumulated from CO photodissociation in a time as short as similar to 10(4) yr. We develop a simple model for gas production and destruction in this system, properly accounting for CO self-shielding and shielding by neutral carbon, and introducing a removal mechanism for carbon gas. We find that the most likely scenario to explain both C-0 and CO observations is one where the carbon gas is rapidly removed on a timescale of order a thousand years and the system maintains a very high CO production rate of similar to 15 M-circle plus Myr(-1), much higher than the rate of dust grind-down. We propose a possible scenario to meet these peculiar conditions: the capture of carbon onto dust grains, followed by rapid CO re-formation and rerelease. In steady state, CO would continuously be recycled, producing a CO-rich gas ring that shows no appreciable spreading over time. This picture might be extended to explain other gas-rich debris disks.

  • 32.
    Cavallius, Maria
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Cataldi, Gianni
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Hungarian Academy of Sciences, Hungary; National Astronomical Observatory of Japan, Japan; University of Toronto, Canada.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Olofsson, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Larsson, Bengt
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Liseau, R.
    Upper limits on the water vapour content of the β Pictoris debris disk2019Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 628, artikkel-id A127Artikkel i tidsskrift (Fagfellevurdert)
    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.

  • 33. Cuong Nguyen, Duy
    et al.
    Jayawardhana, Ray
    van Kerkwijk, Marten H.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Scholz, Alexander
    Damjanov, Ivana
    Disk-Braking in Young Stars: Probing Rotation in Chamaeleon I and Taurus-Auriga2009Manuskript (preprint) (Annet (populærvitenskap, debatt, mm))
    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.

  • 34. Cuong Nguyen, Duy
    et al.
    Scholz, Alexander
    van Kerkwijk, Marten H.
    Jayawardhana, Ray
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    How Variable is Accretion in Young Stars?2009Manuskript (preprint) (Annet (populærvitenskap, debatt, mm))
    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.

  • 35. Damasso, M.
    et al.
    Scandariato, G.
    Nascimbeni, V.
    Nardiello, D.
    Mancini, L.
    Marino, G.
    Bruno, G.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leto, G.
    Marzari, F.
    Lanza, A. F.
    Benatti, S.
    Desidera, S.
    Béjar, V. J. S.
    Biagini, A.
    Borsato, L.
    Cabona, L.
    Claudi, R.
    Lodieu, N.
    Maggio, A.
    Mallorquín, M.
    Messina, S.
    Micela, G.
    Ricci, D.
    Sozzetti, A.
    Mascareño, A. Suárez
    Turrini, D.
    Osorio, M. R. Zapatero
    Photometric follow-up of the 20 Myr old multi-planet host star V1298 Tau with CHEOPS and ground-based telescopes2023Inngår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 680, artikkel-id A8Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Context. The 20 Myr old star V1298 Tau hosts at least four planets. Since its discovery, this system has been a target of intensive photometric and spectroscopic monitoring. To date, the characterisation of its architecture and planets’ fundamental properties has been very challenging.

    Aims. The determination of the orbital ephemeris of the outermost planet V1298 Tau e remains an open question. Only two transits have been detected so far by Kepler/K2 and TESS, allowing for a grid of reference periods to be tested with new observations, without excluding the possibility of transit timing variations. Observing a third transit would allow for better constraints to be set on the orbital period and would also help in determining an accurate radius for V1298 Tau e because the previous transits showed different depths.

    Methods. We observed V1298 Tau with the CHaracterising ExOPlanet Satellite (CHEOPS) to search for a third transit of planet e within observing windows selected to test three of the shortest predicted orbital periods. We also collected ground-based observations to verify the result found with CHEOPS. We reanalysed Kepler/K2 and TESS light curves to test how the results derived from these data are affected by alternative photometric extraction and detrending methods.

    Results. We report the CHEOPS detection of a transit-like signal that could be attributed to V1298 Tau e. If so, that result would imply that the orbital period calculated from fitting a linear ephemeris to the three available transits is close to ~45 days. Results from the ground-based follow-up marginally support this possibility. We found that i) the transit observed by CHEOPS has a longer duration compared to that of the transits observed by Kepler/K2 and TESS; and ii) the transit observed by TESS is >30% deeper than that of Kepler/K2 and CHEOPS, and it is also deeper than the measurement previously reported in the literature, according to our reanalysis.

    Conclusions. If the new transit detected by CHEOPS is found to be due to V1298 Tau e, this would imply that the planet experiences TTVs of a few hours, as deduced from three transits, as well as orbital precession, which would explain the longer duration of the transit compared to the Kepler/K2 and TESS signals. Another and a priori less likely possibility is that the newly detected transit belongs to a fifth planet with a longer orbital period than that of V1298 Tau e. Planning further photometric follow-up to search for additional transits is indeed necessary to solve the conundrum, as well as to pin down the radius of V1298 Tau e.

  • 36. Damjanov, Ivana
    et al.
    Jayawardhana, Ray
    Scholz, Alexander
    Ahmic, Mirza
    Nguyen, Duy C.
    Brandeker, Alexis
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    van Kerkwijk, Marten H.