The Mass Distribution of Subgiant Planet Hosts

TL;DR

This paper examines the mass distribution of subgiant stars hosting planets, confirming that previous spectroscopic mass estimates are likely overestimated and highlighting the sensitivity of these estimates to Galactic models.

Contribution

It demonstrates that Malmquist bias does not affect mass distributions and supports the conclusion that spectroscopic mass estimates for subgiants are systematically too high.

Findings

Malmquist bias does not impact subgiant mass distributions.

Mass distribution sensitivity depends on Galaxy model assumptions.

Spectroscopic mass overestimation of subgiants is robust against model variations.

Abstract

High mass stars are hostile to Doppler measurements due to rotation and activity on the main-sequence, so radial velocity searches for planets around massive stars have relied on evolved stars. A large number of planets have been found around evolved stars with M>1.5 Msun. To test the robustness of mass determinations, Lloyd (2011) compared mass distributions of planet hosting subgiants with distributions from integrating isochrones and concluded it is unlikely the subgiant planet hosts are this massive, but rather the mass inferences are systematically in error. The conclusions of Lloyd (2011) have been called in to question by Johnson, Morton & Wright (2013), who show TRILEGAL-based mass distributions disagree with the mass distributions in Lloyd (2011), which they attribute to Malmquist bias. Johnson, Morton & Wright (2013) argue that the very small spectroscopic observational uncertainties favor high masses, and there are a large number of high mass sub giants in RV surveys. However, in this letter, it is shown that Malmquist bias does not impact the mass distributions, but the mass distribution is sensitive to Galaxy model. The relationship needed to reconcile the subgiant planet host masses with any model of the Galactic stellar population is implausible, and the conclusion of Lloyd (2011) that spectroscopic mass determinations of subgiants are likely to have been overestimated is robust.