Integrated mass-loss of evolved stars in M4 using asteroseismology

Abstract

ABSTRACT Mass-loss remains a major uncertainty in stellar modelling. In low-mass stars, mass-loss is most significant on the red giant branch (RGB), and will impact the star’s evolutionary path and final stellar remnant. Directly measuring the mass difference of stars in various phases of evolution represents one of the best ways to quantify integrated mass-loss. Globular clusters (GCs) are ideal objects for this. M4 is currently the only GC for which asteroseismic data exist for stars in multiple phases of evolution. Using K2 photometry, we report asteroseismic masses for 75 red giants in M4, the largest seismic sample in a GC to date. We find an integrated RGB mass-loss of $\Delta \overline{M} = 0.17 \pm 0.01 ~\mathrm{M}_{\odot }$, equivalent to a Reimers’ mass-loss coefficient of ηR = 0.39. Our results for initial mass, horizontal branch mass, ηR, and integrated RGB mass-loss show remarkable agreement with previous studies, but with higher precision using asteroseismology. We also report the first detections of solar-like oscillations in early asymptotic giant branch (EAGB) stars in GCs. We find an average mass of $\overline{M}_{\text{EAGB}}=0.54 \pm 0.01 ~\mathrm{M}_{\odot }$, significantly lower than predicted by models. This suggests larger-than-expected mass-loss on the horizontal branch. Alternatively, it could indicate unknown systematics in the scaling relations for the EAGB. We discover a tentative mass bimodality in the RGB sample, possibly due to the multiple populations. In our red horizontal branch sample, we find a mass distribution consistent with a single value. We emphasize the importance of seismic studies of GCs since they could potentially resolve major uncertainties in stellar theory.