Peering into the formation history of β Pictoris b with VLTI/GRAVITY long-baseline interferometry^★

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: mathias.nowak@obspm.fr
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

Received: 11 October 2019
Accepted: 29 November 2019

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 M_Jup. 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.