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

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.Comment: Accepted for publication in Astrophysical Journal Letter