Volume 633, January 2020
|Number of page(s)||19|
|Section||Planets and planetary systems|
|Published online||20 January 2020|
Peering into the formation history of β Pictoris b with VLTI/GRAVITY long-baseline interferometry★
LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris,
5 place Jules Janssen,
2 European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
3 Max Planck Institute for extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
4 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
5 Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
6 1 st Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
7 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
8 Faculdade de Ciências, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
9 Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
10 Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
11 Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
12 CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
13 Space Telescope Science Institute, Baltimore, MD 21218, USA
14 STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
15 Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
16 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
Accepted: 29 November 2019
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.
Key words: planets and satellites: formation / planets and satellites: atmospheres / techniques: interferometric / stars: individual: β Pictoris
The reduced spectrum is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/633/A110
© GRAVITY Collaboration 2020
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