MASSIVE: A Bayesian analysis of giant planet populations around low-mass stars

¹ Univ. Grenoble Alpes, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG, UMR 5274), 38000 Grenoble, France
e-mail: justine.lannier@univ-grenoble-alpes.fr
² Institut de Recherche sur les Exoplanètes (iREx), Département de physique and Observatoire du Mont Mégantic, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7, Canada
³ NASA Ames Research Center, Space Science and Astrobiology Division, MS 245-6, Moffett Field, CA 94035, USA
⁴ Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA
⁵ Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁶ CFHT Corporation, 65-1238 Mamalahoa Hwy, Kamuela, HI 96743, USA
⁷ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile

Received: 2 February 2016
Accepted: 22 June 2016

Abstract

Context. Direct imaging has led to the discovery of several giant planet and brown dwarf companions. These imaged companions populate a mass, separation and age domain (mass >1 M_Jup, orbits > 5 AU, age < 1 Gyr) quite distinct from the one occupied by exoplanets discovered by the radial velocity or transit methods. This distinction could indicate that different formation mechanisms are at play.

Aims. We aim at investigating correlations between the host star’s mass and the presence of wide-orbit giant planets, and at providing new observational constraints on planetary formation models.

Methods. We observed 58 young and nearby M-type dwarfs in L′-band with the VLT/NaCo instrument and used angular differential imaging algorithms to optimize the sensitivity to planetary-mass companions and to derive the best detection limits. We estimate the probability of detecting a planet as a function of its mass and physical separation around each target. We conduct a Bayesian analysis to determine the frequency of substellar companions orbiting low-mass stars, using a homogenous sub-sample of 54 stars.

Results. We derive a frequency of 4.4^+3.2_-1.3 for companions with masses in the range of 2−80 M_Jup, and 2.3^+2.9_-0.7% for planetary mass companions (2−14 M_Jup), at physical separations of 8 to 400 AU for both cases. Comparing our results with a previous survey targeting more massive stars, we find evidence that substellar companions more massive than 1 M_Jup with a low mass ratio Q with respect to their host star (Q < 1%), are less frequent around low-mass stars. This may represent observational evidence that the frequency of imaged wide-orbit substellar companions is correlated with stellar mass, corroborating theoretical expectations. Contrarily, we show statistical evidence that intermediate-mass ratio (1% < Q < 5%) companion with masses >2 M_Jup might be independent from the mass of the host star.