First light of the VLT planet finder SPHERE IV. Physical and chemical properties of the planets around HR8799

M. Bonnefoy (1) , A. Zurlo (2, 3) , J. L. Baudino (4) , Philippe Lucas (5) , D. Mesa (6) , A.L. Maire (6, 7) , Arthur Vigan (8, 9) , R. Galicher (4) , D. Homeier (10, 11) , F. Marocco (5) , R. Gratton (6) , G. Chauvin (1) , F. Allard (11) , S. Desidera (6) , M. Kasper (11) , C. Moutou (8) , A.M. Lagrange (1) , J. Antichi (6) , A. Baruffolo (6) , J. Baudrand (4) , J.L. Beuzit (1) , A. Boccaletti (4) , F. Cantalloube (1, 12) , M. Carbillet (13) , J. Charton (1) , R.U. Claudi (6) , A. Costille (8) , K. Dohlen (12) , C. Dominik (14) , D. Fantinel (6) , P. Feautrier (1) , M. Feldt (7) , T. Fusco (6, 12) , P. Gigan (4) , J.H. Girard (1, 9) , L. Gluck (1) , C. Gry (8) , T. Hening (7) , M. Janson (7, 15) , M. Langlois (8, 11) , F. Madec (8) , Y. Magnard (1) , D. Maurel (1) , D. Mawet (9, 16) , M.R. Meyer (17) , J. Milli (1, 9) , O. Moeller-Nilsson (7) , D. Mouillet (1) , A. Pavlov (7) , D. Perret (4) , P. Pujet (1) , S.P. Quanz (17) , S. Rochat (1) , G. Rousset (4) , A. Roux (1) , B. Salasnich (6) , G. Salter (8) , J.F. Sauvage (6, 12) , H.M. Schmid (17) , A. Sevin (4) , C. Soenke (18) , E. Stadler (1) , M. Turatto (6) , S. Udry (19) , F. Vakili (13) , Z. Wahhaj (8, 9) , F. Wildi (19)

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
2 UDP - Universidad Diego Portales [Santiago - Chili]
3 Millenium Nucleus Protoplanetary Disks in ALMA Early Science
4 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
5 Centre for Astrophysics Research [Hatfield]
6 OAPD - INAF - Osservatorio Astronomico di Padova
7 MPIA - Max-Planck-Institut für Astronomie
8 LAM - Laboratoire d'Astrophysique de Marseille
9 ESO - European Southern Observatory [Santiago]
10 ZAH - Zentrum für Astronomie der Universität Heidelberg
11 CRAL - Centre de Recherche Astrophysique de Lyon
12 ONERA - The French Aerospace Lab [Châtillon]
13 LAGRANGE - Joseph Louis LAGRANGE
14 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
15 ALBANOVA - AlbaNova University Center
16 CALTECH - California Institute of Technology
17 Institute of Astronomy [ETH Zürich]
18 ESO - European Southern Observatory
19 ObsGE - Observatoire Astronomique de l'Université de Genève

Context. The system of four planets discovered around the intermediate-mass star HR8799 offers a unique opportunity to test planet formation theories at large orbital radii and to probe the physics and chemistry at play in the atmospheres of self-luminous young (similar to 30 Myr) planets. We recently obtained new photometry of the four planets and low-resolution (R similar to 30) spectra of HR8799 d and e with the SPHERE instrument (Paper III). Aims. In this paper (Paper IV), we aim to use these spectra and available photometry to determine how they compare to known objects, what the planet physical properties are, and how their atmospheres work. Methods. We compare the available spectra, photometry, and spectral energy distribution (SED) of the planets to field dwarfs and young companions. In addition, we use the extinction from corundum, silicate (enstatite and forsterite), or iron grains likely to form in the atmosphere of the planets to try to better understand empirically the peculiarity of their spectrophotometric properties. To conclude, we use three sets of atmospheric models (BT-SETTL14, Cloud-AE60, Exo-REM) to determine which ingredients are critically needed in the models to represent the SED of the objects, and to constrain their atmospheric parameters (T-eff, log g, M/H). Results. We find that HR8799d and e properties are well reproduced by those of L6-L8 dusty dwarfs discovered in the field, among which some are candidate members of young nearby associations. No known object reproduces well the properties of planets b and c. Nevertheless, we find that the spectra and WISE photometry of peculiar and/or young early-T dwarfs reddened by submicron grains made of corundum, iron, enstatite, or forsterite successfully reproduce the SED of these planets. Our analysis confirms that only the Exo-REM models with thick clouds fit (within 2 sigma) the whole set of spectrophotometric datapoints available for HR8799 d and e for T-eff = 1200 K, log g in the range 3.0-4.5, and M/H = +0.5. The models still fail to reproduce the SED of HR8799c and b. The determination of the metallicity, log g, and cloud thickness are degenerate. Conclusions. Our empirical analysis and atmospheric modelling show that an enhanced content in dust and decreased CIA of H-2 is certainly responsible for the deviation of the properties of the planet with respect to field dwarfs. The analysis suggests in addition that HR8799c and b have later spectral types than the two other planets, and therefore could both have lower masses.

Domaines: Astrophysique [astro-ph]

M. Bonnefoy, A. Zurlo, J. L. Baudino, Philippe Lucas, D. Mesa, et al.. First light of the VLT planet finder SPHERE IV. Physical and chemical properties of the planets around HR8799. Astronomy and Astrophysics - A&A, 2016, 587, ⟨10.1051/0004-6361/201526906⟩. ⟨hal-01434357⟩