Dark matter capture and annihilation in stars: Impact on the red giant branch tip

TL;DR

This study investigates how dark matter particles captured by low-mass red giant stars can influence their evolution, potentially causing an earlier end to the RGB phase and lowering the luminosity at the TRGB, especially for certain WIMP masses.

Contribution

It introduces a modified stellar evolution model to analyze dark matter effects on RGB stars, revealing that DM can significantly alter stellar evolution and TRGB luminosity under realistic galactic conditions.

Findings

DM accumulation in RGB stars is constant and largely independent of stellar mass and metallicity.

DM annihilation can trigger early helium burning, shortening the RGB phase.

Light WIMPs can cause observable deviations (~8%) in TRGB luminosity in the Milky Way's core.

Abstract

Context: While stars have often been used as laboratories to study dark matter (DM), red giant branch (RGB) stars and all the rich phenomenology they encompass have frequently been overlooked by such endeavors. Aims: We study the capture, evaporation, and annihilation of weakly interacting massive particle (WIMP) DM in low-mass RGB stars ($M=0.8-1.4~\mathrm{M_{\odot}}$). Methods: We used a modified stellar evolution code to study the effects of DM self-annihilation on the structure and evolution of low-mass RGB stars. Results: We find that the number of DM particles that accumulate inside low-mass RGB stars is not only constant during this phase of evolution, but also mostly independent of the stellar mass and to some extent stellar metallicity. Moreover, we find that the energy injected into the stellar core due to DM annihilation can promote the conditions necessary for helium burning and thus trigger an early end of the RGB phase. The premature end of the RGB, which is most pronounced for DM particles with $m_\chi \simeq 100~\mathrm{GeV}$, is thus achieved at a lower helium core mass, which results in a lower luminosity at the tip of the red giant branch (TRGB). Although in the current WIMP paradigm, these effects are only relevant if the number of DM particles inside the star is extremely large, we find that for light WIMPs ($m_\chi \simeq 4~\mathrm{GeV}$), relevant deviations from the standard TRGB luminosity ($\sim 8\%$) can be achieved with conditions that can be realistic in the inner parsec of the Milky Way.