Eileen D. Friel

Metallicities of Old Open Clusters

We present radial velocities and metallicities for a sample of 39 open clusters with ages greater than about 700 million years. For 24 clusters new moderate-resolution spectroscopic data obtained with multiobject spectrographs on the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory 4 m telescopes are used to determine radial velocities and mean cluster metallicities. These new results are combined with data published previously by Friel & Janes to provide a sample of 459 giants in 39 old open clusters, which are used to investigate radial abundance gradients in the Galactic disk. Based on an updated abundance calibration of spectroscopic indices measuring Fe and Fe-peak element blends, this larger sample yields an abundance gradient of -0.06 ± 0.01 dex kpc-1 over a range in Galactocentric radius of 7 to 16 kpc. There is a slight suggestion of a steepening of the abundance gradient with increasing cluster age in this sample, but the significance of the result is limited by the restricted distance range for the youngest clusters. The clusters show no correlation of metallicity with age in the solar neighborhood. Consistent with the evidence for a steepening of the gradient with age, the clusters in the outer disk beyond 10 kpc show a suggestion at the 1.5 σ level of a dependence of metallicity on age.

A CHEMICAL ABUNDANCE STUDY OF 10 OPEN CLUSTERS BASED ON WIYN*-HYDRA SPECTROSCOPY

We present a detailed chemical abundance study of evolved stars in 10 open clusters based on Hydra multi-object echelle spectra obtained with the WIYN 3.5 m telescope. From an analysis of both equivalent widths and spectrum synthesis, abundances have been determined for the elements Fe, Na, O, Mg, Si, Ca, Ti, Ni, Zr, and for two of the 10 clusters, Al and Cr. To our knowledge, this is the first detailed abundance analysis for clusters NGC 1245, NGC 2194, NGC 2355, and NGC 2425. These 10 clusters were selected for analysis because they span a Galactocentric distance range R gc ~ 9-13 kpc, the approximate location of the transition between the inner and outer disks. Combined with cluster samples from our previous work and those of other studies in the literature, we explore abundance trends as a function of cluster R gc, age, and [Fe/H]. As found previously by us and other studies, the [Fe/H] distribution appears to decrease with increasing R gc to a distance of ~12 kpc and then flattens to a roughly constant value in the outer disk. Cluster average element [X/Fe] ratios appear to be independent of R gc, although the picture for [O/Fe] is more complicated with a clear trend of [O/Fe] with [Fe/H] and sample incompleteness. Other than oxygen, no other element [X/Fe] exhibits a clear trend with [Fe/H]; likewise, there does not appear to be any strong correlation between abundance and cluster age. We divided clusters into different age bins to explore temporal variations in the radial element distributions. The radial metallicity gradient appears to have flattened slightly as a function of time, as found by other studies. There is also some indication that the transition from the inner disk metallicity gradient to the ~constant [Fe/H] distribution of the outer disk occurs at different Galactocentric radii for different age bins. However, interpretation of the time evolution of radial abundance distributions is complicated by the unequal R gc and [Fe/H] ranges spanned by clusters in different age bins.

Abundances of Red Giants in the Old Open Cluster Collinder 261

We present an analysis of high-dispersion echelle spectra of four giant stars in the 8 Gyr open cluster Cr 261 obtained with the Cerro Tololo Inter-American Observatorys 4 m telescope. Abundances were determined relative to the bright, slightly metal-poor disk giant Arcturus. Cr 261 is found to have a mean [Fe/H] = -0.22 ± 0.05 (mean error). Oxygen abundances, determined from the forbidden [O I] lines, show roughly solar or slightly depleted abundance ratios. The α-elements Mg, Ca, and Ti also show scaled solar abundance ratios, with suggestions that the Si abundance is slightly enhanced. The odd-Z elements Na and Al are significantly enhanced relative to scaled solar abundances. The abundance results for Cr 261 are compared with those from high-dispersion studies of other old open clusters in the literature, which, overall, show similar behavior in all these elements. None of the abundance ratios show significant trends with overall cluster [Fe/H] or with cluster age.

A Chemical Abundance Study of Red Giants in Open Clusters NGC 2204 and NGC 2243

Detailed element abundances have been determined for 10-13 stars each in the open clusters (OCs) NGCxa02204 and NGCxa02243 based on Hydra multi-object echelle spectra obtained with the CTIO 4xa0m telescope. We have found average cluster metallicities of [Fe/H] = –0.23 ± 0.04 and –0.42 ± 0.05 for NGCxa02204 and NGCxa02243, respectively, from an equivalent width analysis. NGCxa02243 is the most metal-poor cluster at its Galactocentric radius and is one of the most metal-poor OCs currently known. These two clusters lie ~1xa0kpc below the Galactic plane; it is therefore worthwhile to compare their abundance patterns to those of clusters both closer to and further from the plane. To that end, we combined the results of the current study with those of clusters from our previous work as well as from the literature. To minimize systematic differences between different studies, element abundances of many outer disk OCs as well as thin and thick disk field stars have been placed on our abundance scale. Plots of [X/Fe] versus [Fe/H] for NGCxa02204, NGCxa02243, other clusters from the literature, and thin and thick disk field stars show NGCxa02204 and NGCxa02243 to have element abundance patterns comparable to those of other clusters regardless of distance from the plane or center of the Galaxy. Similarly, no individual cluster or group of clusters far from the Galactic mid-plane can be identified as belonging to the thick disk based on their abundance patterns.