Catherine A. Pilachowski

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 OLD OPEN CLUSTERS. V. Be 31, Be 32, Be 39, M 67, NGC 188, AND NGC 1193

We present a detailed abundance analysis based on high resolution spectroscopy for 16 stars in the old open clusters Be 31, Be 32, Be 39, M 67, NGC 188, and NGC 1193. Average cluster metallicities of [Fe/H] = –0.30 ± 0.02, –0.21 ± 0.01, +0.03 ± 0.07, +0.12 ± 0.02, and –0.22 ± 0.14 (s.d.) have been found for Be 32, Be 39, M 67, NGC 188, and NGC 1193, respectively. The two stars observed in the field of Be 31 have disparate radial velocities and elemental abundance patterns, and also disagree with the possible Be 31 star studied by Yong et al. We conclude that membership has yet to be established for this important cluster, and therefore no element abundances measured here or in previous studies of Be 31 should be taken as definitive cluster abundances. A careful comparison of our results for the clusters M 67 and Be 32 to those of other studies shows general good agreement and identifies systematic differences resulting from different analyses. After combination of our results with those of other studies for clusters spanning the full R gc range of the thin disk, we explore the abundance distributions as a function of R gc and age for the elements Fe, O, Na, Mg, Al, Si, Ca, Ti, and Ni. As found in previous work, the [Fe/H] gradient appears to be continuous up to R gc ~ 13 kpc before flattening in the outer disk. [X/Fe] ratios show a scatter of 0.2-0.3 dex at all R gc, for all elements considered. The [X/Fe] values of the six clusters analyzed here are consistent with those of other clusters of similar metallicity and Galactocentric location. Our whole cluster sample shows trends of increasing [O/Fe] and [Al/Fe] with age, although these trends vanish with the inclusion of other clusters from the literature. Larger, homogeneous open cluster samples are necessary to verify the existence and magnitude of abundance trends with age.

Chemical Abundances in 12 Red Giants of the Large Magellanic Cloud from High-Resolution Infrared Spectroscopy

High-resolution infrared spectra (� /D� = 50,000) have been obtained for 12 red giant members of the Large Magellanic Cloud (LMC) with the Gemini South 8.3 m telescope and Phoenix spectrometer. Two wavelength regions, at 15540 and 23400 A ˚ , were observed. Quantitative chemical abundances of carbon (both 12 C and 13 C), nitrogen, and oxygen were derived from molecular lines of CO, CN, and OH, while sodium, scandium, titanium, and iron abundances were obtained from neutral atomic lines. The 12 LMC red giants span a metallicity range from [Fe/H] = � 1.1 to [Fe/H] = � 0.3. It is found that values for both [Na/Fe] and [Ti/Fe] in the LMC giants fall below their corresponding Galactic values (at these same [Fe/H] abundances) by about � 0.1–0.5 dex; this effect is similar to abundance patterns found in the few dwarf spheroidal galaxies with published abundances. The program red giants all show evidence of first dredge-up mixing of material exposed to the CN cycle, that is, low 12 C/ 13 C ratios and lower 12 C with higher 14 N abundances. The carbon and nitrogen trends are similar to what is observed in samples of Galactic red giants, although the LMC red giants seem to show smaller 12 C/ 13 C ratios for a given stellar mass. This relatively small difference in the carbon isotope ratios between LMC and Galactic red giants could be due to increased extra mixing in stars of lower metallicity, as suggested previously in the literature. Comparisons of the oxygen-to-iron ratios in the LMC and the Galaxy indicate that the trend of [O/Fe] versus [Fe/H] in the LMC falls about 0.2 dex below the Galactic trend. Such an offset can be modeled as due to an overall lower rate of supernovae per unit mass in the LMC relative to the Galaxy, as well as a slightly lower ratio of supernovae of Type II to supernovae of Type Ia.

Na, Al, AND O ABUNDANCES OF OPEN CLUSTERS NGC 7142, NGC 6939, AND IC 4756

We present an analysis of echelle spectra of stars in three open clusters obtained with the Hydra multiobject spectrograph on the WIYN 3.5 m telescope. Abundances of Fe, O, Si, Ca, Na, Al, and Ni have been determined via equivalent width analysis and spectrum synthesis. Mean abundances for each cluster are compared to those of previous studies and of other clusters in the literature, with emphasis on exploring the enhancements of Na and Al seen in many open clusters. All three clusters show enhanced values of [Na/Fe] and [Al/Fe], while the abundances of Fe, O, Si, and Ca are consistent with their ages and locations in the Galactic disk.

ABUNDANCES OF RED GIANTS IN OLD OPEN CLUSTERS. IV. NGC 1817, NGC 1883, NGC 2141, AND NGC 2158

We present an analysis of high-dispersion echelle spectra of seven giant stars in the ~1-2?Gyr old open clusters NGC 1817, NGC 1883, NGC 2141, and NGC 2158. Abundances were determined relative to the bright, mildly metal-poor disk giant Arcturus. NGC 1817, NGC 1883, NGC 2141, and NGC 2158 are found to have a mean [Fe/H] = ?0.07 ? 0.04, ?0.01 ? 0.01, +0.00 ? 0.16, and ?0.03 ? 0.14 (s.d.), respectively. All four clusters show modest (~0.1 dex) enhancements of Na and Si, and scaled solar abundances of Al and Mg, while [O/Fe] ~ ?0.15 dex. [Ca/Fe] and [Ti/Fe] values are slightly subsolar in all four clusters, but this may be due to a systematic effect. Given that all four of these clusters are located at R gc > 10?kpc in the disk, and barring any systematic effects, our results imply that any abrupt transition in abundance in the Galactic disk occurs beyond R gc ~ 13?kpc.

ABUNDANCES OF RED GIANTS IN OLD OPEN CLUSTERS. III. NGC 7142

We present an analysis of high-dispersion echelle spectra of four giant stars in the 4 Gyr open cluster NGC 7142 obtained with the KPNO 4 m telescope. Abundances were determined relative to the bright, mildly metal-poor giant Arcturus. NGC 7142 is found to have a mean [Fe/H] = +0.14 ± 0.01 (standard deviation) relative to the Sun, with an estimated uncertainty of 0.12 dex per star. Oxygen abundances, determined from the forbidden [O I] λ6300 A feature, are slightly sub-solar, while the α-elements are slightly enhanced to ~0.1 dex above solar. Na and Al are enhanced ~0.2 dex relative to scaled solar abundances. These abundance patterns are similar to those reported for other open clusters in the literature, and suggest that the Galactic disk underwent rapid chemical enrichment in early phases of its evolution. Finally, comparison of these results to those found from lower-dispersion WIYN 3.5 m HYDRA spectra of the cluster stars shows them to be comparable within the errors, verifying that detailed abundance analyses of HYDRA spectra are reliable and can be directly compared to analyses of higher-dispersion data.

Abundances of Red Giants in Old Open Clusters. II. Berkeley 17

We present an analysis of high-dispersion echelle spectra of three giant stars in the 10 Gyr open cluster Berkeley 17 obtained with the KPNO 4 m Telescope. Abundances were determined relative to the bright, somewhat metal-poor disk giant Arcturus. Be 17 is found to have a mean [Fe/H] = -0.10 ± 0.09 (rms). Oxygen abundances, determined from the forbidden [O I] lines, and the α-elements Mg, Ca, and Ti show scaled solar abundance ratios, while the Si abundance is slightly enhanced. The odd-Z elements Na and Al are significantly enhanced relative to scaled solar abundances. These abundance patterns are similar to those seen in other old open clusters and suggest that the Galactic disk was enriched to solar abundance levels at very early times.

AGB Sodium Abundances in the Globular Cluster 47 Tucanae (NGC 104)

A recent analysis comparing the [Na/Fe] distributions of red giant branch (RGB) and asymptotic giant branch (AGB) stars in the Galactic globular cluster NGC 6752 found that the ratio of Na-poor to Na-rich stars changes from 30:70 on the RGB to 100:0 on the AGB. The surprising paucity of Na-rich stars on the AGB in NGC 6752 warrants additional investigations to determine if the failure of a significant fraction of stars to ascend the AGB is an attribute common to all globular clusters. Therefore, we present radial velocities, [Fe/H], and [Na/Fe] abundances for 35 AGB stars in the Galactic globular cluster 47 Tucanae (47 Tuc; NGC 104), and compare the AGB [Na/Fe] distribution with a similar RGB sample published previously. The abundances and velocities were derived from high resolution spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5m telescope. We find the average heliocentric radial velocity and [Fe/H] values to be =-18.56 km s^-1 (sigma=10.21 km s^-1) and =-0.68 (sigma=0.08), respectively, in agreement with previous literature estimates. The average [Na/Fe] abundance is 0.12 dex lower in the 47 Tuc AGB sample compared to the RGB sample, and the ratio of Na-poor to Na-rich stars is 63:37 on the AGB and 45:55 on the RGB. However, in contrast to NGC 6752, the two 47 Tuc populations have nearly identical [Na/Fe] dispersion and interquartile range values. The data presented here suggest that only a small fraction <20% of Na-rich stars in 47 Tuc may fail to ascend the AGB. Regardless of the cause for the lower average [Na/Fe] abundance in AGB stars, we find that Na-poor stars and at least some Na-rich stars in 47 Tuc evolve through the early AGB phase. [abridged]

Study of CNO elements in barium stars

Carbon, nitrogen, and oxygen abundances are presented for seven mild barium stars and two classical barium stars. The mild barium stars do not show the carbon enhancement typical of the classical Ba II stars. The CNO abundances of the mild barium and normal G and K giants are identical. An s-process enhancement is confirmed for some of the mild barium stars.

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.