Peering into the formation history of β Pictoris b with VLTI/GRAVITY long-baseline interferometry
Förster Schreiber, N. M.; Genzel, R.; Sturm, E.; Tacconi, L. J.; Henning, T.; Chapron, F.; Collin, C.; Brandner, W.; Kervella, P.; Gendron, E.; Gillessen, S.; van Dishoeck, E. F.; Eisenhauer, F.; Abuter, R.; Lacour, S.; Maire, A. -L.; Lagrange, A. -M.; de Zeeuw, P. T.; Beust, H.; Bonnefoy, M.; Eckart, A.; Charnay, B.; Cantalloube, F.; Rabien, S.; Wang, J.; Ott, T.; Paumard, T.; Perrin, G.; Pfuhl, O.; Scheithauer, S.; Wiezorrek, E.; Wieprecht, E.; Garcia Lopez, R.; Pueyo, L.; Haußmann, F.; Le Bouquin, J. -B.; Nowak, M.; Rau, C.; Shangguan, J.; Straubmeier, C.; Widmann, F.; Woillez, J.; Paladini, C.; Perraut, K.; Duvert, G.; Mollière, P.; Amorim, A.; Rousset, G.; Clénet, Y.; Hippler, S.; Jocou, L.; Léna, P.; Straub, O.; Vincent, F.; Yazici, S.; Gao, F.; Bonnet, H.; Coudé Du Foresto, V.; Dembet, R.; Dexter, J.; Hubert, Z.; Lapeyrère, V.; GRAVITY Collaboration; Berger, J. P.; Buron, A.; Rodríguez-Coira, G.; Fédou, P.; Ziegler, D.
France, United Kingdom, Germany, Netherlands, United States, Portugal, Ireland, Belgium
Abstract
Context. β Pictoris is arguably one of the most studied stellar systems outside of our own. Some 30 yr of observations have revealed a highly-structured circumstellar disk, with rings, belts, and a giant planet: β Pictoris b. However very little is known about how this system came into being.
Aims: Our objective is to estimate the C/O ratio in the atmosphere of β Pictoris b and obtain an estimate of the dynamical mass of the planet, as well as to refine its orbital parameters using high-precision astrometry.
Methods: We used the GRAVITY instrument with the four 8.2 m telescopes of the Very Large Telescope Interferometer to obtain K-band spectro-interferometric data on β Pic b. We extracted a medium resolution (R = 500) K-band spectrum of the planet and a high-precision astrometric position. We estimated the planetary C/O ratio using two different approaches (forward modeling and free retrieval) from two different codes (ExoREM and petitRADTRANS, respectively). Finally, we used a simplified model of two formation scenarios (gravitational collapse and core-accretion) to determine which can best explain the measured C/O ratio.
Results: Our new astrometry disfavors a circular orbit for β Pic b (e = 0.15-0.04+0.05). Combined with previous results and with HIPPARCOS/Gaia measurements, this astrometry points to a planet mass of M = 12.7 ± 2.2 MJup. This value is compatible with the mass derived with the free-retrieval code petitRADTRANS using spectral data only. The forward modeling and free-retrieval approches yield very similar results regarding the atmosphere of β Pic b. In particular, the C/O ratios derived with the two codes are identical (0.43 ± 0.05 vs. 0.43-0.03+0.04). We argue that if the stellar C/O in β Pic is Solar, then this combination of a very high mass and a low C/O ratio for the planet suggests a formation through core-accretion, with strong planetesimal enrichment.