Stability and pulsation of the first dark stars

TL;DR

This study explores the stability, pulsation, and growth of dark stars powered by dark matter annihilation, revealing they are stable, non-oscillating, and can grow to large masses without significant mass loss.

Contribution

It provides the first detailed analysis of non-adiabatic pulsations in dark stars and improves modeling of dark matter energy injection in stellar evolution.

Findings

Dark stars up to 1000 solar masses are stable and free of dynamical instabilities.

Pulsations are excited in stars below 200 solar masses by ionization mechanisms.

Pulsation-induced mass loss is negligible compared to accretion rates.

Abstract

The first bright objects to form in the Universe might not have been "ordinary" fusion-powered stars, but "Dark Stars" (DSs) powered by the annihilation of dark matter (DM) in the form of Weakly Interacting Massive Particles (WIMPs). If discovered, DSs can provide a unique laboratory to test DM models. DSs are born with a mass of order $M_\odot$ and may grow to a few million solar masses; in this work we investigate the properties of early DSs with masses up to $\sim \! 1000 \, M_\odot$, fueled by WIMPS weighing $100$ GeV. We improve the previous implementation of the DM energy source into the stellar evolution code MESA. We show that the growth of DSs is not limited by astrophysical effects: DSs up to $\sim \! 1000 \, M_\odot$ exhibit no dynamical instabilities; DSs are not subject to mass-loss driven by super-Eddington winds. We test the assumption of previous work that the injected energy per WIMP annihilation is constant throughout the star; relaxing this assumption does not change the properties of the DSs. Furthermore, we study DS pulsations, for the first time investigating non-adiabatic pulsation modes, using the linear pulsation code GYRE. We find that acoustic modes in DSs of masses smaller than $\sim \! 200 \, M_\odot$ are excited by the $\kappa-\gamma$ and $\gamma$ mechanism in layers where hydrogen or helium is (partially) ionized. Moreover, we show that the mass-loss rates potentially induced by pulsations are negligible compared to the accretion rates.