Possible Implications of the Planet Orbiting the Red Horizontal Branch Star HIP 13044

TL;DR

This paper explores how a planet survived a common envelope phase around a metal-poor red horizontal branch star, suggesting a rapid evolution triggered by the star's core helium flash.

Contribution

It proposes a novel scenario where a planet survives a common envelope phase due to rapid stellar evolution after a core helium flash.

Findings

Planet's orbital period is 16.2 days.

Star retained a substantial envelope of 0.3 solar masses.

Survival of the planet is linked to rapid stellar envelope loss.

Abstract

We propose a scenario to account for the surprising orbital properties of the planet orbiting the metal poor red horizontal branch star HIP 13044. The orbital period of 16.2 days implies that the planet went through a common envelope phase inside the red giant branch (RGB) stellar progenitor of HIP 13044. The present properties of the star imply that the star maintained a substantial envelope mass of 0.3Mo, raising the question of how the planet survived the common envelope before the envelope itself was lost? If such a planet enters the envelope of an RGB star, it is expected to spiral-in to the very inner region within <100yr, and be evaporated or destructed by the core. We speculate that the planet was engulfed by the star as a result of the core helium flash that caused this metal poor star to swell by a factor of ~3-4. The evolution following the core helium flash is very rapid, and some of the envelope is lost due to the interaction with the planet, and the rest of the envelope shrinks within about a hundred years. This is about equal to the spiraling-in time, and the planet survived.