Planet Engulfment by ~1.5-3 Solar-Mass Red Giants

TL;DR

This study investigates how stellar evolution affects the survival of planets around 1.5-3 solar-mass red giants, finding that stellar mass significantly influences planetary engulfment and the observed distribution of exoplanets.

Contribution

It provides a detailed analysis of the critical orbital limits for planet survival during stellar evolution, incorporating stellar mass effects and comparing with observed exoplanet data.

Findings

Critical semimajor axis is highly sensitive to stellar mass between 1.7 and 2.1 Msun.

All observed planets are beyond the survival limit, supporting the engulfment hypothesis.

Planets around stars > 2.1 Msun are farther from the survival limit, suggesting other factors influence their distribution.

Abstract

Recent radial-velocity surveys for GK clump giants have revealed that planets also exist around ~1.5-3 Msun 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 Msun, 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 (> 2.1 Msun), 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 > 2.5 Msun, is required.