The metallicity of the open cluster Tombaugh 2

Abstract

Context. Open clusters are excellent tracers of the structure, kinematics, and chemical evolution of the disk and a wealth of information can be derived from the spectra of their constituent stars. Aims. We investigate the nature of the chemical composition of the outer disk open cluster Tombaugh 2. This has been suggested to be a member of the GASS/Mon substructure, and a recent study by Frinchaboy et al. (2008) suggested that this was a unique open cluster in possessing an intrinsic metal abundance dispersion. We aim to investigate such claims. Methods. High resolution VLT+GIRAFFE spectra in the optical are obtained and analyzed for a number of stars in the Tombaugh 2 field, together with independent UBVI C photometry. Radial velocities and position in the color−magnitude diagram are used to assess cluster membership. 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 with good spectra. Results. 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, MNRAS, 391, 39). 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 T eff on the upper giant branch and maintains a low level for red clump stars. The mean metallicity we derive agrees well with that expected from the radial abundance gradient in the disk for a cluster at its Galactocentric distance. Conclusions. Tombaugh 2 is found to have abundances as expected from its Galactocentric distance and no evidence for any intrinsic metallicity dispersion. The surprising result found by Frinchaboy et al. (2008), which is the presence of two 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.

On the metallicity of open clusters

“…These two authors analysed 14 and 13 radial velocity members, respectively, using the same instrument, but different wavelength regions. Frinchaboy et al (2008) obtained two different mean metallicities (about solar and −0.3 dex) for the two halves of their sample, while Villanova et al (2010) arrived at a consistent metallicity of about −0.3 dex for all of their stars. The possible reasons for the different results are discussed at length in Villanova et al (2010).…”

“…Frinchaboy et al (2008) obtained two different mean metallicities (about solar and −0.3 dex) for the two halves of their sample, while Villanova et al (2010) arrived at a consistent metallicity of about −0.3 dex for all of their stars. The possible reasons for the different results are discussed at length in Villanova et al (2010). We add that of the five stars in common between the two studies, one has the same atmospheric parameters and metallicity, while three have different parameters and metallicities, and one has different parameters but the same metallicity.…”

“…It is part of the Friel et al (2002) sample, and has been studied at medium resolution by Frinchaboy et al (2008) and Villanova et al (2010). These two authors analysed 14 and 13 radial velocity members, respectively, using the same instrument, but different wavelength regions.…”

Old open clusters and the Galactic metallicity gradient: Berkeley 20, Berkeley 66 and Tombaugh 2★

“…Phelps, Janes & Montgomery (1994) included To 2 in their list of old OCs, assigning it a value of δ V = 1.5, that is, an age of about 2.5 Gyr (following the formula in Janes & Phelps 1994). No further photometric catalogues are freely available, but Villanova et al (2010) re‐determined the parameters of To 2 on the basis of spectroscopy (see below) and photometry, deriving an age of 2 Gyr, ( m − M ) V = 15.1, E ( B − V ) = 0.25, with [Fe/H]=−0.32 dex (see below).…”

“…This result has, however, been questioned by part of the same team; Villanova et al (2010) presented results on GIRAFFE VLT spectra for 37 RGB and RC stars (only 15 are actually members and only 13 with data good enough for the analysis). They found [Fe/H]=−0.31 ± 0.02 dex (rms = 0.07 dex) and no sign of a bimodal distribution.…”