Multiple populations in low mass globular clusters: Palomar 13

TL;DR

This study investigates multiple stellar populations in the low-mass globular cluster Palomar 13 by analyzing spectral indices, revealing nitrogen variation and suggesting it may be the lowest mass cluster with multiple populations.

Contribution

First spectroscopic evidence of multiple populations in Palomar 13, a very low mass globular cluster, using CN and CH spectral indices.

Findings

Detected nitrogen variation among red giant stars.

Palomar 13 may be the lowest mass globular cluster with multiple populations.

Confirmed membership of stars using Gaia DR2 data.

Abstract

Since the discovery of chemically peculiar stars in globular clusters in the last century, the study of multiple populations has become increasingly important, given that chemical inhomogeneity is found in almost all globular clusters. Despite various proposed theories attempting to explain this phenomenon, fitting all the observational evidence in globular clusters with one single theory remains notoriously difficult and currently unsuccessful. In order to improve existing models and motivate new ones, we are observing globular clusters at critical conditions, e.g., metal-rich end, metal-poor end, and low mass end. In this paper, we present our first attempt to investigate multiple populations in low mass globular clusters. We obtained low-resolution spectra around 4000 A of 30 members of the globular cluster Palomar 13 using OSIRIS/Multi-object spectrograph mounted at the Gran Telescopio Canarias. The membership of red giant branch stars is confirmed by the latest proper motions from Gaia DR2 and literature velocities. After comparing the measured CN and CH spectral indices with those of the stellar models, we found a clear sign of nitrogen variation among the red giant branch stars. Palomar 13 may be the lowest mass globular cluster showing multiple populations.