Asteroseismology of the multiple stellar populations in the Globular Cluster M4

TL;DR

This study uses asteroseismology from K2 data to analyze stellar populations in the globular cluster M4, confirming mass estimates and revealing a mass-temperature correlation in horizontal branch stars.

Contribution

It provides the largest asteroseismic sample in a globular cluster, verifies asteroseismology in low metallicity regimes, and reports the first direct observation of a predicted mass-temperature correlation.

Findings

Detected solar-like oscillations in 37 stars, the largest sample in a GC.

Estimated mass loss along the RGB consistent with previous estimates.

First direct observation of a mass-temperature gradient in rHB stars.

Abstract

We present a new asteroseismic analysis of the stars in the Globular Cluster (GC) M4 based on the data collected by the K2 mission. We report the detection of solar-like oscillation in 37 stars, 32 red giant branch (RGB) and 6 red horizontal branch (rHB) stars, the largest sample for this kind of study in GC up to date. Combining information from asteroseismology and multi-band photometry we estimate both the masses and the radii of our targets. Our estimates are in agreement with independent sources, serving as a crucial verification of asteroseismology in the low metallicity regime. As M4 is an old GC, it hosts multiple stellar populations differing in light-element abundances and in helium mass fraction. This generates a mass difference between the populations along the RGB, which in the case of M4 is estimated to be $0.017 M_\odot$. With this wealth of information we can assign population membership and estimate the average mass of the stellar populations, but the current uncertainties do not allow us to resolve this mass difference. The population membership and the seismic data of RGB and HB stars, allow us, however, to assess the integrated mass loss along the RGB of the first generation stars in the cluster. We obtain $\rm \Delta M=0.227 \pm0.028 M_\odot$, in good agreement with independent estimates. Finally, we observe the presence of a statistically significant mass-temperature gradient in the rHB stars. This represents the first direct, model-independent observation of the colour-temperature-mass correlation predicted by the theory.