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

Abstract

To better understand how planets form, it is important to study planet occurrence rates as a function of stellar mass. However, estimating masses of field stars is often difficult. Over the past decade, a controversy has arisen about the inferred occurrence rate of gas-giant planets around evolved intermediate-mass stars -the so-called 'retired A-stars'. The high masses of these red-giant planet hosts, derived using spectroscopic information and stellar evolution models, have been called into question. Here we address the controversy by determining the masses of eight evolved planet-hosting stars using asteroseismology. We compare the masses with spectroscopic-based masses from the Exoplanet Orbit Database that were previously adopted to infer properties of the exoplanets and their hosts. We find a significant one-sided offset between the two sets of masses for stars with spectroscopic masses above roughly 1.6M ⊙ , suggestive of an average 15-20% overestimate of the adopted spectroscopicbased masses. The only star in our sample well below this mass limit is also the only one not showing this offset. Finally, we note that the scatter across literature values of spectroscopicbased masses often exceed their formal uncertainties, making it comparable to the offset we report here.

Asteroseismology of the Hyades red giant and planet host ϵ Tauri

“…Based on a stellar mass of M = 2.7 ± 0.1 M ⊙ , they derived a planetary mass of m 2 sin i = 7.6 ± 0.2 M J . With our slightly lower stellar mass of M = 2.458±0.073 M ⊙ , in agreement with Stello et al (2017), we can redetermine the minimum mass of the planet, using the velocity semi-amplitude, orbital period, and eccentricity presented in Sato et al (2007). We find the new minimum mass of the planet to be m 2 sin i = 7.1 ± 0.2 M J .…”

“…Following the approach of Mosser & Appourchaux (2009), which was also applied to SONG data by Stello et al (2017), the frequency of maximum power (ν max ) was determined from the SONG ǫ Tau data by applying a Gaussian fit combined with a linear trend to the filtered data, in order to take background signals into account. This is shown in the upper panel of Fig.…”

KELT-21b: A Hot Jupiter Transiting the Rapidly Rotating Metal-poor Late-A Primary of a Likely Hierarchical Triple System

“…RV surveys have discovered a large number of giant planets around intermediate-mass subgiant and giant stars (e.g., Johnson et al 2011;Reffert et al 2015), which suggest that the frequency of giant planets around A stars may be higher than that around FGK stars. The issue of whether these "retired A stars" are actually intermediate-mass stars or, rather, lower-mass interlopers, is, however, controversial (e.g., Lloyd 2011;Johnson et al 2013;Schlaufman & Winn 2013;Stello et al 2017).…”

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.…”