Acoustic Signatures of the Helium Core Flash

TL;DR

This paper demonstrates that space-based asteroseismology can observationally detect stars during the helium core flash phase, revealing unique seismic signatures that distinguish them from other red giants.

Contribution

It shows that the helium core flash phase can be identified through specific seismic signatures using existing space-based photometry data.

Findings

Detection of mixed modes during the core flash phase is feasible.

Stars in the flash phase have distinct period spacings (Delta P_g) of 70-100 seconds.

Approximately 1 in 35 red giants are in the core flash phase at any given time.

Abstract

All evolved stars with masses M <2 solar masses undergo an initiating off-center helium core flash in their 0.48 solar mass He core as they ascend the red giant branch (RGB). This off-center flash is the first of a few successive helium shell subflashes that remove the core electron degeneracy over 2 Myrs, converting the object into a He burning star. Though characterized by Thomas over 40 years ago, this core flash phase has yet to be observationally probed. Using the Modules for Experiments in Stellar Astrophysics (MESA) code, we show that red giant asteroseismology enabled by space-based photometry (i.e. Kepler and CoRoT) can probe these stars during the flash. The rapid (< 100,000 years) contraction of the red giant envelope after the initiating flash dramatically improves the coupling of the p-modes to the core g-modes, making the detection of l=1 mixed modes possible for these 2 Myrs. This duration implies that 1 in 35 stars near the red clump in the HR diagram will be in their core flash phase. During this time, the star has a g-mode period spacing of Delta P_g = 70-100 seconds, lower than the Delta P_g=250 seconds of He burning stars in the red clump, but higher than the RGB stars at the same luminosity. This places them in an underpopulated part of the large frequency spacing (Delta nu) vs. Delta P_g diagram that should ease their identification amongst the thousands of observed red giants.