摘要：Context.βPictoris 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:βPictoris 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 ofβPictoris 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 obtainK-band spectro-interferometric data onβPic 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 forβPic b (e= 0.15−0.04+0.05). Combined with previous results and with HIPPARCOS/Gaiameasurements, this astrometry points to a planet mass ofM= 12.7 ± 2.2MJup. 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 ofβPic 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 inβPic 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.
关键词：Key wordsenplanets and satellites: formationplanets and satellites: atmospherestechniques: interferometricstars: individual: β Pictoris
