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

S. Udry, M. Mayor+5 more|ArXiv.org|Feb 25, 2002

Stellar, planetary, and galactic studiesPhysics and Astronomy36 references87 citations

TL;DR

This paper presents radial-velocity detections of four very low-mass companions around solar-type stars—HD 141937, HD 162020, HD 168443, and HD 202206—each with minimum masses near the planet/brown-dwarf boundary (9.7–17.5 M_Jup). Using Doppler measurements from the CORALIE spectrograph, the study finds that these companions likely represent a transition population, with tidal analysis suggesting HD 162020's companion is a brown dwarf, while others may be superplanets or brown dwarfs formed in protoplanetary disks.

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

Motivation & Objective

• To determine the nature of four newly detected low-mass companions around solar-type stars with minimum masses near the 13 M_Jup brown-dwarf boundary.
• To assess whether these companions are superplanets or brown dwarfs based on orbital and stellar properties.
• To investigate the role of metallicity and orbital eccentricity in distinguishing formation mechanisms.
• To examine the implications of the observed mass and period distribution for planetary migration scenarios.

Proposed method

• High-precision radial-velocity measurements were conducted using the CORALIE echelle spectrograph on the 1.2-m Euler Swiss telescope at La Silla Observatory.
• Keplerian orbital solutions were fitted to radial-velocity data to derive minimum masses (m₂sin i) and orbital elements.
• Tidal dissipation models were applied to the short-period HD 162020 system to infer the true mass and nature of the companion.
• Stellar metallicity was measured from high-resolution spectra to assess its correlation with companion mass.
• Statistical analysis of orbital eccentricity and period distributions was performed to compare 'light' and 'massive' planets.
• The observed distribution of true masses was estimated via statistical deconvolution of the m₂sin i distribution, accounting for observational bias.

Experimental results

Research questions

• RQ1 Are the companions to HD 141937, HD 162020, HD 168443, and HD 202206 best classified as superplanets or brown dwarfs?
• RQ2 Does tidal dissipation in the HD 162020 system support a brown-dwarf nature for its companion?
• RQ3 What is the role of stellar metallicity in hosting massive planets or brown dwarfs?
• RQ4 Why are massive planets (m₂sin i > 10 M_Jup) absent at short orbital periods?
• RQ5 Do the observed orbital eccentricities and period distributions support distinct formation mechanisms for low- and high-mass planets?

Key findings

• The companions to HD 141937, HD 162020, HD 168443, and HD 202206 have minimum masses of 9.7, 14.4, 16.9, and 17.5 M_Jup, respectively, placing them near the planet/brown-dwarf transition.
• Tidal dissipation analysis of the short-period HD 162020 system suggests its companion is more likely a brown dwarf than a planet.
• HD 168443 hosts two low-mass companions, which may have formed simultaneously in a protoplanetary disk as either superplanets or brown dwarfs.
• HD 202206 exhibits an additional radial-velocity drift, indicating a further outer companion whose nature remains undetermined.
• The sample shows a strong correlation between host star metallicity and the presence of massive planets, with no massive planets detected at [Fe/H] < -0.25.
• Massive planets are notably absent at short orbital periods, suggesting they may not migrate as easily or may be prone to falling into the star.