"Retired" Planet Hosts: Not So Massive, Maybe Just Portly After Lunch

TL;DR

This study reevaluates the mass distribution of exoplanet host giants, suggesting many are less massive than previously thought, which impacts understanding of planet occurrence rates and the apparent deficit of short-period planets.

Contribution

It demonstrates that many giant star hosts are likely lower-mass stars, challenging previous assumptions about planet frequency around high-mass stars and proposing new methods to verify stellar masses.

Findings

Exoplanet hosts are more consistent with late F/early G dwarfs than massive giants.

The apparent deficit of short-period planets may be due to tidal capture effects.

Mass estimates of host stars may be systematically overestimated.

Abstract

Studies of the planet abundance as a function of stellar mass have suggested a strong increase in the frequency of planet occurrence around stars more massive than $1.5 M_\odot$, and that such stars are deficit in short period planets. These planet searches have relied on giant stars for a sample of high mass stars, which are hostile to precision Doppler measurements due to rotation and activity while on the main sequence. This paper considers the observed $v\sin i$ and observationally inferred mass for exoplanet hosting giants with the $v\sin i$ of distribution of field stars, which show discrepancies that can be explained by erroneous mass determinations of some exoplanet host stars. By comparison with an expected mass distribution constructed from integrating isochrones, it is shown that the exoplanet hosts are inconsistent with a population of massive stars. These stars are more likely to have originated from a main sequence population of late F/early G dwarfs with mass 1.0--$1.2 M_\odot$, only slightly more more massive than the typical FGK dwarfs with Doppler detected planets. The deficit of short period planets is most likely explained by tidal capture. The planet abundance difference requires either a steeper increase in planet frequency with mass than previously thought or a high rate of false positives due to signals of stellar origin. The measurement of photospheric Carbon isotope ratios is suggested as a method to discriminate whether this sample of giant stars is significantly more massive than the population of FGK dwarfs with Doppler detected planets.