PLANET ENGULFMENT BY ∼1.5-3M_☉RED GIANTS

Abstract

Recent radial-velocity surveys for GK clump giants have revealed that planets also exist around ∼1.5-3 M ⊙ stars. However, no planets have been found inside 0.6 AU around clump giants, in contrast to solar-type main-sequence stars, many of which harbor short-period planets such as hot Jupiters. In this study we examine the possibility that planets were engulfed by host stars evolving on the red-giant branch (RGB). We integrate the orbital evolution of planets in the RGB and helium burning (HeB) phases of host stars, including the effects of stellar tide and stellar mass loss. Then we derive the critical semimajor axis (or the survival limit) inside which planets are eventually engulfed by their host stars after tidal decay of their orbits. Especially, we investigate the impact of stellar mass and other stellar parameters on the survival limit in more detail than previous studies. In addition, we make detailed comparison with measured semimajor axes of planets detected so far, which no previous study did. We find that the critical semimajor axis is quite sensitive to stellar mass in the range between 1.7 and 2.1 M ⊙ , which suggests a need for careful comparison between theoretical and observational limits of existence of planets. Our comparison demonstrates that all those planets are beyond the survival limit, which is consistent with the planet-engulfment hypothesis. However, on the high-mass side (> 2.1M ⊙ ), the detected planets are orbiting significantly far from the survival limit, which suggests that engulfment by host stars may not be the main reason for the observed lack of short-period giant planets. To confirm our conclusion, the detection of more planets around clump giants, especially with masses 2.5M ⊙ , is required.

Searching for Li-rich giants in a sample of 12 open clusters

“…If this were true, the engulfment should have happened some time ago since the rotation rate is not very fast and thus the star has had time to slow down. Moreover, a putative engulfed planet should have been at a very short orbit, since for stars with M ∼ 3 M the critical initial semi-major axis for engulfment is ∼0.2-0.3 AU at the RGB phase (Villaver & Livio 2009;Kunitomo et al 2011).…”

“…127 and NGC 2423 No. 3 using the stellar evolution code MESA (Paxton et al 2011) as was done in Kunitomo et al (2011). The evolution starts at zero-age main sequence (ZAMS) and ends before the star experiences the thermal pulses on the AGB (see Fig.…”

The properties of planets around giant stars

“…This observational result has been attributed to be due to the dynamical interaction between the host stars and their planets. Different authors have studied this effect, showing that close-in planets spiral inward due to the loss of orbital angular momentum, which is mainly transferred and dissipated in the stellar convective envelope (e.g., Livio & Soker, 1983;Rasio et al 1996;Sato et al 2008;Schröder & Connon Smith 2008;Villaver & Livio 2009;Kunitomo, et al 2011). As a result of the tidal decay, close-in planets are not expected to be found, at least, around post-RGB stars.…”

“…Moreover, several theoretical studies have shown that the tidal interaction between the planet and the stellar convective envelope leads to the loss of orbital angular momentum. As a result, close-in planets are expected to spiral inward and hence are subsequently engulfed by the host star (e.g., Livio & Soker 1983;Kunitomo et al 2011). However, Johnson et al (2007) showed that close-in planets are also absent around intermediate-mass subgiant stars.…”

The fate of exomoons in white dwarf planetary systems

“…Regarding the first point, the internal composition of the moons may be similar to those of the asteroid families inferred from the polluted debris. For the second point, a chain of large (moon-sized or planet-sized) bodies may help perturb an asteroid (Bonsor & Wyatt 2012) into a target as small as a white dwarf, particularly since the inner few au in white dwarf systems will have been cleared out by the increase in size of the star along the giant branch (Villaver & Livio 2009;Kunitomo et al 2011;Mustill & Villaver 2012;Adams & Bloch 2013;Nordhaus & Spiegel 2013;Villaver et al 2014;Staff et al 2016).…”