Discovery of long-period variable stars in the very-metal-poor globular cluster M15

TL;DR

This study identifies and analyzes long-period variable stars in the extremely metal-poor globular cluster M15, revealing unique variability patterns and raising questions about dust production at low metallicity.

Contribution

First detection of long-period variable stars in M15, the most metal-poor globular cluster, with detailed analysis of their variability and implications for stellar evolution.

Findings

K757 and K825 show variability over nearly a year.

K825 is on the long secondary period sequence.

No dust production detected despite gaseous mass loss.

Abstract

We present a search for long-period variable (LPV) stars among giant branch stars in M15 which, at [Fe/H] ~ -2.3, is one of the most metal-poor Galactic globular clusters. We use multi-colour optical photometry from the 0.6-m Keele Thornton and 2-m Liverpool Telescopes. Variability of delta-V ~ 0.15 mag is detected in K757 and K825 over unusually-long timescales of nearly a year, making them the most metal-poor LPVs found in a Galactic globular cluster. K825 is placed on the long secondary period sequence, identified for metal-rich LPVs, though no primary period is detectable. We discuss this variability in the context of dust production and stellar evolution at low metallicity, using additional spectra from the 6.5-m Magellan (Las Campanas) telescope. A lack of dust production, despite the presence of gaseous mass loss raises questions about the production of dust and the intra-cluster medium of this cluster.