The near-infrared spectral energy distribution of β Pictoris b^⋆,⋆⋆

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: bonnefoy@mpia-hd.mpg.de
² LESIA, Observatoire de Paris, CNRS, University Pierre et Marie Curie Paris 6 and University Denis Diderot Paris 7, 5 place Jules Janssen, 92195 Meudon, France
³ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
⁴ CRAL, UMR 5574, CNRS, Université de Lyon, École Normale Supérieure de Lyon, 46 allée d’Italie, 69364 Lyon Cedex 07, France
⁵ European Southern Observatory, Casilla 19001, Santiago 19, Chile
⁶ Department of Astronomy, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
⁷ Department of Planetary Sciences, The University of Arizona, 1929 E. University Blvd., Tucson, AZ 85721, USA

Received: 3 December 2012
Accepted: 5 February 2013

Abstract

Context. A gas giant planet has previously been directly seen orbiting at 8–10 AU within the debris disk of the ~12 Myr old star β Pictoris. The β Pictoris system offers the rare opportunity of both studying the physical and atmospheric properties of an exoplanet placed on a wide orbit and establishing its formation scenario.

Aims. We aim to build the 1–5 μm spectral energy distribution of the planet for the first time. Our goal is to provide secure and accurate constraints on its physical and chemical properties.

Methods. We obtained J (1.265 μm), H (1.66 μm), and M′ (4.78 μm) band angular differential imaging of the system between 2011 and 2012. We used Markov chain Monte Carlo simulations of the astrometric data to revise constraints on the orbital parameters of the planet. Photometric measurements were compared to those of ultra-cool dwarfs and young companions. They were combined with existing photometry (2.18, 3.80, and 4.05 μm) and compared to predictions from 7 PHOENIX-based atmospheric models in order to derive the atmospheric parameters (T_eff, log g) of β Pictoris b. Predicted properties from (“hot-start”, “cold-start”, and “warm start”) evolutionary models were compared to independent constraints on the mass of β Pictoris b. We used planet-population synthesis models following the core-accretion paradigm to discuss the planet’s possible origin.

Results. We detect the planetary companion in our four-epoch observations. We estimate J = 14.0 ± 0.3, H = 13.5 ± 0.2, and M′ = 11.0 ± 0.3 mag. Our new astrometry consolidates previous semi-major axis (8–10 AU) and excentricity (e ≤ 0.15) estimates of the planet. The location of β Pictoris b in color–magnitude diagrams suggests it has spectroscopic properties similar to L0-L4 dwarfs. This enables one to derive Log₁₀ (L / L_⊙) = − 3.87 ± 0.08 for the companion. The analysis with atmospheric models reveals that the planet has a dusty atmosphere with T_eff = 1700 ± 100K and log  g = 4.0 ± 0.5. “Hot-start” evolutionary models give a new mass of 10_-2⁺³ M_Jup from T_eff and 9_-2⁺³ M_Jup from luminosity. Predictions of “cold-start” models are still inconsistent with independent constraints on the planet mass. “Warm-start” models constrain the mass to M ≥ 6 M_Jup and the initial entropies to values (S_init ≥ 9.3K_b / baryon) midway between those considered for cold/hot-start models, but probably closer to those of hot-start models.