Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission

TL;DR

This study detects solar-like oscillations in K giants within the globular cluster M4 using K2 data, validating asteroseismic mass estimates in a metal-poor environment and confirming their consistency with independent methods.

Contribution

First detection of solar-like oscillations in M4 K giants, demonstrating the reliability of asteroseismic mass estimates in metal-poor stars through combined seismic and non-seismic data.

Findings

Asteroseismic masses agree with isochrone-based masses within 4-10%.

Correction to the Delta nu scaling improves mass estimates.

Results support the use of asteroseismology for metal-poor stellar populations.

Abstract

Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens of thousands of field stars. Tests against independent estimates of these properties are however scarce, especially in the metal-poor regime. Here, we report the detection of solar-like oscillations in 8 stars belonging to the red-giant branch and red-horizontal branch of the globular cluster M4. The detections were made in photometric observations from the K2 Mission during its Campaign 2. Making use of independent constraints on the distance, we estimate masses of the 8 stars by utilising different combinations of seismic and non-seismic inputs. When introducing a correction to the Delta nu scaling relation as suggested by stellar models, for RGB stars we find excellent agreement with the expected masses from isochrone fitting, and with a distance modulus derived using independent methods. The offset with respect to independent masses is lower, or comparable with, the uncertainties on the average RGB mass (4-10%, depending on the combination of constraints used). Our results lend confidence to asteroseismic masses in the metal poor regime. We note that a larger sample will be needed to allow more stringent tests to be made of systematic uncertainties in all the observables (both seismic and non-seismic), and to explore the properties of RHB stars, and of different populations in the cluster.