The Metallicity of the Open Cluster Tombaugh 2

TL;DR

This study uses high-resolution spectra and photometry to analyze the chemical composition of the open cluster Tombaugh 2, finding no evidence of intrinsic metallicity dispersion and clarifying previous conflicting results.

Contribution

It provides a detailed chemical abundance analysis of Tombaugh 2, refuting prior claims of metallicity spread and confirming a uniform composition consistent with Galactic disc expectations.

Findings

Mean metallicity [Fe/H] = -0.31 with no dispersion

Ca and Ba slightly enhanced, Ni and Sc solar

Europium (Eu) enhanced, [Eu/Ba]=+0.17

Abstract

We investigate the nature of the chemical composition of the outer disc open cluster Tombaugh 2, that a recent study by Frinchaboy et al. (2008) suggested to possess an intrinsic metal abundance dispersion. We aim to investigate such claims by high resolution spectra obtained for a number of stars in the Tombaugh 2 field, together with independent UBVIc photometry. The spectra, together with input atmospheric parameters and model atmospheres, are used to determine detailed chemical abundances for a variety of elements in 13 members having good spectra. We find the mean metallicity to be [Fe/H]=-0.31+-0.02 with no evidence for an intrinsic abundance dispersion, in contrary to the recent results of Frinchaboy et al. (2008). We find Ca and Ba to be slightly enhanced while Ni and Sc are solar. The r-process element Eu was found to be enhanced, giving an average [Eu/Ba]=+0.17. The Li abundance decreases with Teff on the upper giant branch and maintains a low level for red clump stars. The mean metallicity we derive is in good agreement with that expected from the radial abundance gradient in the disc for a cluster at its Galactocentric distance. The surprising result found by Frinchaboy et al. (2008), that is the presence of 2 distinct abundance groups within the cluster, implying either a completely unique open cluster with an intrinsic metallicity spread, or a very unlikely superposition of a cold stellar stream and a very distant open cluster, is not supported by our new result.