Red horizontal branch stars: An asteroseismic perspective

Abstract

Robust age estimates of red giant stars are now possible thanks to the precise inference of their mass based on asteroseismic constraints. However, there are cases where such age estimates can be highly precise yet very inaccurate. An example is giants that have undergone mass loss or mass transfer events that have significantly altered their mass. In this context, stars with ‘apparent’ ages significantly higher than the age of the Universe are candidates for stripped stars, or stars that have lost more mass than expected, most likely via interactions with a companion star or because of the poorly understood mass-loss mechanism along the red-giant branch. In this work we identify examples of such objects among red giants observed by Kepler, both at low ([Fe/H] ≲ −0.5) and solar metallicity. By modelling their structure and pulsation spectra, we find a consistent picture that confirms that they are indeed low-mass objects consisting of a He core of ≈0.5 M⊙ and an envelope of ≈0.1 − 0.2 M⊙. Moreover, we find that these stars are characterised by a rather extreme coupling (q ≳ 0.4) between the pressure-mode and gravity-mode cavities, one that is much higher than the typical value for red clump stars, thus providing a direct seismic signature of their peculiar structure. The complex pulsation spectra of these objects, if observed with sufficient frequency resolution, hold detailed information about the structural properties of likely products of mass stripping and can hence potentially shed light on their formation mechanism. On the other hand, our tests highlight the difficulties associated with reliably measuring the large frequency separation, especially in shorter datasets, which impacts the reliability of the inferred masses and ages of low-mass red clump stars with, for example, K2 or TESS data.