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 as stripped stars, or stars that have lost more mass than expected, most likely via interaction 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 $\text{Kepler}$, both at low ([Fe/H] $\lesssim -0.5$) and solar metallicity. By modelling their structure and pulsation spectra, we find a consistent picture confirming that these are indeed low-mass objects consisting of a He core of $\approx 0.5M_{\odot }$ and an envelope of $\approx 0.1-0.2M_{\odot }$. Moreover, we find that these stars are characterised by a rather extreme coupling ($q\gtrsim 0.4$) between the pressure-mode and gravity-mode cavities, i.e. much higher than the typical value for red clump stars, providing thus 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, hence can potentially shed light on their formation mechanism. On the other hand, our tests highlight the difficulties associated with measuring reliably the large frequency separation, especially in shorter datasets, with impact on the reliability of the inferred masses and ages of low-mass Red Clump stars with e.g. K2 or TESS data.

Accepted for publication in A&A Letters

References in corpus (18)

• Modules for Experiments in Stellar Astrophysics (MESA)
• A grid of MARCS model atmospheres for late-type stars I. Methods and general properties
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• The K2 Mission: Characterization and Early results
• Synthetic Stellar Photometry - I. General considerations and new transformations for broad-band systems
• Angular momentum transport by heat-driven g-modes in slowly pulsating B stars
• NGC 6819: testing the asteroseismic mass scale, mass loss, and evidence for products of non-standard evolution
• The Contour Method: a new approach to finding modes of non-adiabatic stellar pulsations
• Mass loss along the red giant branch in 46 Globular Clusters and their multiple populations
• Period spacings in red giants; III. Coupling factors of mixed modes
• Discovery of post-mass-transfer helium-burning red giants using asteroseismology
• Semi-analytic Expressions for the Isolation and Coupling of Mixed Modes
• Integrated Mass Loss of Evolved Stars in M4 using Asteroseismology
• Probing the mid-layer structure of red giants I. Mixed-mode coupling factor as a seismic diagnosis
• A Semi-Empirical Study of the Mass Distribution of Horizontal Branch Stars in M3 (NGC 5272)
• Variations of the mixing character of dipolar mixed modes in red giant stars
• Asteroseismology of overmassive, undermassive, and potential past members of the open cluster NGC6791
• Multi-cavity gravito-acoustic modes in stars: A general analytical resonance condition