Letter to the Editor

Asteroseismology of the multiple stellar populations in the globular cluster M4

¹ Dipartimento di Fisica e Astronomia Augusto Righi, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
e-mail: marco.tailo@unibo.it; mrctailo@gmail.com
² INAF – Osservatorio Astrofisico di Catania, Via S. Sofia, 78, 95123 Catania, Italy
³ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁴ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁶ School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK

Received: 5 April 2022
Accepted: 12 May 2022

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 six red horizontal branch (rHB) stars, which is the largest sample for this kind of study in GCs 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_⊙. With this wealth of information, we can assign population membership and estimate the average mass of the stellar populations; however, 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 ΔM = 0.227 ± 0.028 M_⊙, which is 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.