The properties of planets around giant stars

Abstract

Context. More than 50 exoplanets have been found around giant stars, revealing different properties when compared to planets orbiting solar-type stars. In particular, they are super-Jupiters and are not found orbiting interior to ∼0.5 AU. Aims. We are conducting a radial velocity study of a sample of 166 giant stars aimed at studying the population of close-in planets orbiting giant stars and how their orbital and physical properties are influenced by the post-MS evolution of the host star. Methods. We have collected multiepoch spectra for all of the targets in our sample. We have computed precision radial velocities from FECH/CHIRON and FEROS spectra, using the I 2 cell technique and the simultaneous calibration method, respectively. Results. We present the discovery of a massive planet around the giant star HIP 105854. The best Keplerian fit to the data leads to an orbital distance of 0.81 ± 0.03 AU, an eccentricity of 0.02 ± 0.03 and a projected mass of 8.2 ± 0.2 M J . With the addition of this new planet discovery, we performed a detailed analysis of the orbital properties and mass distribution of the planets orbiting giant stars. We show that there is an overabundance of planets around giant stars with a ∼ 0.5−0.9 AU, which might be attributed to tidal decay. Additionally, these planets are significantly more massive than those around MS and subgiant stars, suggesting that they grow via accretion either from the stellar wind or by mass transfer from the host star. Finally, we show that planets around evolved stars have lower orbital eccentricities than those orbiting solar-type stars, which suggests that they are either formed in different conditions or that their orbits are efficiently circularized by interactions with the host star.

HAT-P-67b: An Extremely Low Density Saturn Transiting an F-subgiant Confirmed via Doppler Tomography∗

“…Radial velocity searches have found that eccentric planets are rarer around evolved hosts than around dwarfs (e.g., Jones et al 2014). However, tidal interactions are likely weak during the lifetime of HAT-P-67b; the characteristic timescale for orbital decay is on the order of~10 11 years (adopting Equation (2) of Hansen 2012 and assumes an effective stellar dissipation coefficient of  s = -10 8 ).…”

“…Planets orbiting high mass stars are most likely born in high mass protoplanetary disks (e.g., Muzerolle et al 2003;Natta et al 2006), which are environments that may yield higher planet occurrence rates (e.g., Johnson et al 2010;Bowler et al 2010) and higher mass planets (e.g., Jones et al 2014;Lovis & Mayor 2007) than those around solar-type stars. Planets around early-type stars also receive a higher incident flux over their lifetimes, which in turn make them anchor-points in the planet mass-radiusequilibrium temperature relationships (e.g., Béky et al 2011;Enoch et al 2012).…”

“…One successful strategy is to conduct radial velocity surveys of "retired A-stars" (i.e., stars that have evolved off the main sequence and have spun down enough to exhibit sharp spectroscopic lines that enable precise radial velocity measurements). These surveys have been extremely successful and have yielded 122 planetary systems to date 14 (e.g., Johnson et al 2007;Wittenmyer et al 2011;Jones et al 2014). Recently, transit surveys have successfully discovered planets around high mass stars.…”

Stellar parameters and chemical abundances of 223 evolved stars with and without planets

“…The KS test gives a ∼ 90% probability that both planetary mass distributions are drawn from the same population. Recently, Jones et al (2014) arrived to a different conclusion, suggesting that the mass distributions of planets around giants and subgiants are different. These authors considered all the planets around subgiants, including those with M p sin i 0.95 M Jup , which we have excluded in this analysis.…”

Kepler-432: A Red Giant Interacting With One of Its Two Long-Period Giant Planets

“…There is already good evidence that planetary populations around intermediate-mass stars are substantially different from those around their low-mass counterparts. Higher mass stars seem to harbor more Jupiters than do Sun-like stars , and the typical planetary mass correlates with the stellar mass (Lovis & Mayor 2007;Döllinger et al 2009;Bowler et al 2010), but there are not many planets within 1 AU of more massive stars (Johnson et al 2007;Sato et al 2008), and their orbits tend to be less eccentric than Jupiters orbiting low-mass stars (Jones et al 2014). Due to the observational difficulties associated with massive and intermediate-mass main-sequence stars, many of the more massive stars known to host planets have already reached an advanced evolutionary state, and it is not yet clear whether most of the orbital differences can be attributed to mass-dependent formation and migration, or if planetary engulfment and/or tidal evolution as the star swells on the giant branch plays a more important role.…”