Weighing in on the masses of retired A stars with asteroseismology: K2 observations of the exoplanet-host star HD 212771

Abstract

Doppler-based planet surveys point to an increasing occurrence rate of giant planets with stellar mass. Such surveys rely on evolved stars for a sample of intermediate-mass stars (so-called retired A stars), which are more amenable to Doppler observations than their main-sequence progenitors. However, it has been hypothesised that the masses of subgiant and low-luminosity red-giant stars targeted by these surveystypically derived from a combination of spectroscopy and isochrone fitting -may be systematically overestimated. Here, we test this hypothesis for the particular case of the exoplanet-host star HD 212771 using K2 asteroseismology. The benchmark asteroseismic mass (1.45 +0.10 −0.09 M ⊙ ) is significantly higher than the value reported in the discovery paper (1.15 ± 0.08 M ⊙ ), which has been used to inform the stellar massplanet occurrence relation. This result, therefore, does not lend support to the above hypothesis. Implications for the fates of planetary systems are sensitively dependent on stellar mass. Based on the derived asteroseismic mass, we predict the post-mainsequence evolution of the Jovian planet orbiting HD 212771 under the effects of tidal forces and stellar mass loss. ⋆ Based on observations collected at La Silla Observatory (ESO, Chile) with the FEROS spectrograph at the 2.2-m telescope under programme 088.C-0892(A).†

The Gemini Planet Imager Exoplanet Survey: Giant Planet and Brown Dwarf Demographics from 10 to 100 au

“…Johnson et al (2007) and Johnson et al (2010a) observed a correlation between RV giant planet occurrence rate and stellar host mass, with more massive stars more likely to host giant planets with a<2.5 au, though there is uncertainty in deriving a main sequence mass for subgiants (Lloyd 2011(Lloyd , 2013Johnson & Wright 2013). Results from asteroseismology suggest spectroscopically derived stellar masses are too high (Johnson et al 2014;Campante et al 2017), but not significantly so (North et al 2017;Stello et al 2017). Thus the underlying correlation appears robust: for RV-detected giant planets the occurrence rate is proportional to M * g , with (Ghezzi et al 2018).…”

KELT-20b: A Giant Planet with a Period of P ∼ 3.5 days Transiting the V ∼ 7.6 Early A Star HD 185603

“…In recent years, several papers have provided multiple lines of evidence showing that the initial spectroscopic estimates of the masses of the "Retired A Stars" were likely generally slightly overestimated and the uncertainties in their masses likely underestimated. Indeed, it appears that the majority of the members of the "Retired A Star" sample that have accurate mass measurements (via astroseismology or transits; Johnson et al 2014;Pepper et al 2017;Campante et al 2017;North et al 2017;Stassun et al 2017;Stello et al 2017) appear to actually be "Retired F Stars," i.e., more massive than the solar-type stars that dominated early RV surveys (and are therefore more massive than the Kraft break), but less massive than a zero-age main-sequence (ZAMS) A star. In retrospect, this fact should not be very surprising, as the average age of the Galactic thin disk (whose stars dominate the local solar neighborhood) is 7.4 8.2 Gyr - (Kilic et al 2017), corresponding to the hydrogen-fusing lifetime of a ZAMS late F star.…”

Asteroseismic masses of retired planet-hosting A-stars using SONG

“…Even more recently, Campante et al (2017) used K2 observations of another retired A-star, HD 212771, to measure its mass from asteroseismology to be 1.45 ± 0.10 M ⊙ . Despite this star being in a very similar evolutionary stage to HD 185351, the seismic mass of HD 212771 was larger than the original spectroscopybased value of 1.15 ± 0.08 M ⊙ (Johnson et al 2010).…”