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In-depth study of moderately young but extremely red, very dusty substellar companion HD 206893B

P. Delorme 1 T. Schmidt 2 M. Bonnefoy 1 S. Desidera 3 C. Ginski 4, 5 B. Charnay 2 C. Lazzoni 3 V. Christiaens 6, 7, 8 S. Messina 3, 9 V. d'Orazi 3, 10 J. Milli 11, 1 J. E. Schlieder 12, 13 R. Gratton 3 L. Rodet 1 A.-M. Lagrange 1 O. Absil 14 Arthur Vigan 15 R. Galicher 2 J. Hagelberg 1 M. Bonavita 3, 16 B. Lavie 17, 18 A. Zurlo 3, 19 J. Olofsson 12, 20 A. Boccaletti 2 F. Cantalloube 12, 1, 21 D. Mouillet 1 G. Chauvin 1 F. -J. Hambsch 22 M. Langlois 15 S. Udry 18 T. Henning 12 J. -L. Beuzit 1 C. Mordasini 12, 17 P. Lucas 23 F. Marocco 24 B. Biller 12, 16 J. Carson 12, 25 A. Cheetham 18 E. Covino 26 V. de Caprio 27 A. Delboulbe 1 M. Feldt 12 J. Girard 11 N. Hubin 28 A. -L. Maire 3 A. Pavlov 12 C. Petit 21 D. Rouan 2 R. Roelfsema 29 F. Wildi 18
Abstract : Context. The substellar companion HD 206893b has recently been discovered by direct imaging of its disc-bearing host star with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument.Aims. We investigate the atypical properties of the companion, which has the reddest near-infrared colours among all known substellar objects, either orbiting a star or isolated, and we provide a comprehensive characterisation of the host star-disc-companion system.Methods. We conducted a follow-up of the companion with adaptive optics imaging and spectro-imaging with SPHERE, and a multi-instrument follow-up of its host star. We obtain a R = 30 spectrum from 0.95 to 1.64 μm of the companion and additional photometry at 2.11 and 2.25 μm. We carried out extensive atmosphere model fitting for the companions and the host star in order to derive their age, mass, and metallicity.Results. We found no additional companion in the system in spite of exquisite observing conditions resulting in sensitivity to 6 MJup (2 MJup) at 0.5′′ for an age of 300 Myr (50 Myr). We detect orbital motion over more than one year and characterise the possible Keplerian orbits. We constrain the age of the system to a minimum of 50 Myr and a maximum of 700 Myr, and determine that the host-star metallicity is nearly solar. The comparison of the companion spectrum and photometry to model atmospheres indicates that the companion is an extremely dusty late L dwarf, with an intermediate gravity (log g ~ 4.5–5.0) which is compatible with the independent age estimate of the system.Conclusions. Though our best fit corresponds to a brown dwarf of 15–30 MJup aged 100–300 Myr, our analysis is also compatible with a range of masses and ages going from a 50 Myr 12 MJup planetary-mass object to a 50 MJup Hyades-age brown dwarf. Even though this companion is extremely red, we note that it is more probable that it has an intermediate gravity rather than the very low gravity that is often associated with very red L dwarfs. We also find that the detected companion cannot shape the observed outer debris disc, hinting that one or several additional planetary mass objects in the system might be necessary to explain the position of the disc inner edge.
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P. Delorme, T. Schmidt, M. Bonnefoy, S. Desidera, C. Ginski, et al.. In-depth study of moderately young but extremely red, very dusty substellar companion HD 206893B. Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 608, pp.A79. ⟨10.1051/0004-6361/201731145⟩. ⟨hal-01764682⟩

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