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.
Highlights
The ever-increasing number of exoplanet detections proves that our instrumental capabilities are getting better and better at discovering these other worlds
We present observations of the giant planet β Pic b obtained with GRAVITY and we investigate the possibility of using this K-band spectro-interferometric data to determine the C/O ratio of the planet
We presented the first Very Large Telescope Interferometer (VLTI)/GRAVITY spectrointerferometric observation of the giant planet β Pictoris b
Summary
The ever-increasing number of exoplanet detections (over 4000, at the time of this writing1) proves that our instrumental capabilities are getting better and better at discovering these other worlds. Konopacky et al (2013) used a different approach in their study of HR 8799 c They obtained K-band spectroscopic observations of the planet with the spectrograph OSIRIS on the Keck II telescope, and were able to extract an estimate of the C/O ratio using model grid fitting. They found a value of C/O = 0.65 ± 0.15. Looking at the same planetary system, Lavie et al (2017) estimated the C/O ratio for four planets (HR 8799 b, c, d, and e), using a retrieval analysis method In their analysis, they notably emphasized the importance of high-quality K-band spectroscopic data, which they found to be critical for a reliable measurement of the C/O and C/H ratios.
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