The CORALIE survey for southern extra-solar planets VIII. The very low-mass companions of HD141937, HD162020, HD168443, HD202206: brown dwarfs or superplanets?

TL;DR

This study reports the discovery of four companions near the planet/brown dwarf boundary around solar-type stars, analyzing their orbital properties and host star metallicities to understand their nature and formation.

Contribution

It presents new radial velocity measurements revealing companions with masses near the planet/brown dwarf transition and discusses their possible formation scenarios and orbital characteristics.

Findings

Companions have masses close to the 13-80 M_Jup brown dwarf limit.

Most host stars are metal-rich, supporting core accretion formation.

Massive planets tend to lack short orbital periods.

Abstract

Doppler CORALIE measurements of the solar-type stars HD141937, HD162020, HD168443 and HD202206 show Keplerian radial-velocity variations revealing the presence of 4 new companions with minimum masses close to the planet/brown-dwarf transition, namely with m_2sin(i) = 9.7, 14.4, 16.9, and 17.5 M_Jup, respectively. The orbits present fairly large eccentricities (0.22<e<0.43). Except for HD162020, the parent stars are metal rich compared to the Sun, as are most of the detected extra-solar planet hosts. Considerations of tidal dissipation in the short-period HD162020 system points towards a brown-dwarf nature for the low-mass companion. HD168443 is a multiple system with two low-mass companions being either brown dwarfs or formed simultaneously in the protoplanetary disks as superplanets. For HD202206, the radial velocities show an additional drift revealing a further outer companion, the nature of which is still unknown. Finally, the stellar-host and orbital properties of massive planets are examined in comparison to lighter exoplanets. Observed trends include the need of metal-rich stars to form massive exoplanets and the lack of short periods for massive planets. If confirmed with improved statistics, these features may provide constraints for the migration scenario.