Mode lifetimes of stellar oscillations - Implications for asteroseismology

TL;DR

This paper derives a scaling relation linking the lifetimes of solar-like p modes to stellar parameters, revealing implications for the detectability and quality of asteroseismic measurements across different star types.

Contribution

It introduces a simple $T_{ m eff}^{-4}$ scaling for mode lifetimes and explores its impact on mode height and asteroseismic target selection.

Findings

Mode lifetimes scale as $T_{ m eff}^{-4}$ across stellar types.

Mode height in power spectra scales as $g^{-2}$ in intensity observations.

Cooler stars may be as suitable for asteroseismology as hotter stars.

Abstract

Successful inference from asteroseismology relies on at least two things: that the oscillations in the stars have amplitudes large enough to be clearly observable; and that the oscillations themselves be stable enough to enable precise measurements of mode frequencies and other parameters. Solar-like p modes are damped by convection, and hence the stability of the modes depends on the lifetime. We seek a simple scaling relation between the mean lifetime of the most prominent solar-like p modes in stars, and the fundamental stellar parameters. We base our search for a relation on use of stellar equilibrium and pulsation computations of a grid of stellar models, and the first asteroseismic results on lifetimes of main-sequence, sub-giant and red-giant stars. We find that the mean lifetimes of all three classes of solar-like stars scale like $T_{\rm eff}^{-4}$ (where $T_{\rm eff}$ is the effective temperature). When this relation is combined with the well-known scaling relation of Kjeldsen & Bedding (1995) for mode amplitudes observed in narrow-band intensity observations, we obtain the unexpected result that the height (the maximum power spectral density) of mode peaks in the frequency power spectrum scales as $g^{-2}$ (where $g$ is the surface gravity). As it is the mode height (and not the amplitude) that fixes the S/N at which the modes can be measured, and as $g$ changes only slowly along the main sequence, this suggests that stars cooler than the Sun might be as good targets for asteroseismology as their hotter counterparts. When observations are instead made in Doppler velocity, our results imply that mode height then does increase with increasing effective temperature.