Radial pulsation as a function of hydrogen abundance

TL;DR

This study uses linear non-adiabatic pulsation analysis to examine how hydrogen abundance affects the stability of radial pulsations in stars across a broad range of stellar parameters, revealing new instability regions and shifts in the Cepheid instability strip.

Contribution

It provides a comprehensive analysis of radial-mode stability considering varying hydrogen content, identifying new instability regions and shifts in classical pulsation boundaries.

Findings

Decreased hydrogen shifts the Cepheid instability strip to higher temperatures.

High-order modes are more easily excited at lower hydrogen abundances.

New classes of pulsating variables may exist with reduced hydrogen in their envelopes.

Abstract

Using linear non-adabatic pulsation analysis, we explore the radial-mode (p-mode) stability of stars across a wide range of mass (0.2 <= M <= 50 Msun), composition (0 <= X <= 0.7, Z=0.001, 0.02), effective temperature (3 000 <= T_eff <= 40 000 K), and luminosity (0.01 <= L/M <= 100,000 solar units).We identify the instability boundaries associated with low- to high-order radial oscillations (0 <= n <=16). The instability boundaries are a strong function of both composition and radial order (n). With decreasing hydrogen abundance we find that i) the classical blue edge of the Cepheid instability strip shifts to higher effective temperature and luminosity, and ii) high-order modes are more easily excited and small islands of high radial-order instability develop, some of which correspond with real stars. Driving in all cases is by the classical kappa-mechanism and/or strange modes. We identify regions of parameter space where new classes of pulsating variable may, in future, be discovered. The majority of these are associated with reduced hydrogen abundance in the envelope; one has not been identified previously.