Heather R. Jacobson

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.

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.

THE 2001-2003 LOW STATE OF NOVA LACERTAE 1950 (DK LAC)

We report on extensive photometry of DK Lac obtained during the interval 1990–2009, which includes a 2 mag low state during 2001–2003. Much of the photometry consists of exposures obtained with a typical spacing of several days, but also includes 26 sequences of continuous photometry each lasting 2–7 hr. We find no evidence for periodicities in our data. We do find that the random variations in the low state are approximately twice those in the high state, when expressed in magnitudes. The lack of orbital-timescale variations is attributed to the nearly face-on presentation of the disk. There is a 0.2 mag decline in the high-state brightness of the system over 19 years, which is consistent with the behavior of other old novae in the decades following outburst. High-state spectra are also presented and discussed. We find that the equivalent width of Hα falls by about double from 1991 to 2008. The photometric properties are discussed in the context of the hibernation scenario for the behavior of novae between outbursts, in which we conclude that low states in old novae are probably unrelated to their possible entrance into hibernation.