WIYN Open Cluster Study LXXXI. Caught in the Act? The Peculiar Red Giant NGC 2243-W2135

TL;DR

This study analyzes high-resolution spectra of stars in NGC 2243, revealing a peculiar red giant with unusual properties such as high lithium abundance, variability, and rapid rotation, suggesting a possible recent mass transfer event.

Contribution

It reports the discovery and detailed analysis of a unique red giant star with anomalous properties in an old open cluster, highlighting potential mass transfer as its cause.

Findings

The star exhibits high lithium abundance unusual for its type.

It shows variability and rapid rotation, indicating recent activity.

The star's properties suggest a recent mass transfer event.

Abstract

High-dispersion spectra for giants through turnoff stars in the Li 6708 Angstrom region have been obtained and analyzed in the old, metal-deficient open cluster, NGC 2243. When combined with high dispersion data from other surveys, the cluster is found to contain a uniquely peculiar star at the luminosity level of the red clump. The giant is the reddest star at its luminosity, exhibits variability at a minimum 0.1 mag level on a timescale of days, is a single-lined, radial-velocity variable, and has V_sin(i) between 35 and 40 km/sec. In sharp contrast with the majority of the red giant cluster members, the star has a detectable Li abundance, potentially as high or higher than other giants observed to date while at or just below the boundary normally adopted for Li-rich giants. The observed anomalies may be indicators of the underlying process by which the giant has achieved its unusual Li abundance, with a recent mass transfer episode being the most probable within the currently limited constraints.