7 research outputs found
WIYN Open Cluster Study. LXIII. Abundance in the Super-Metal-Rich Open Cluster NGC 6253 from Hydra Sprectroscopy of the 7774 Ã… Oxygen Triplet Region
We present a spectroscopic abundance analysis of the old, super-metal-rich open cluster NGC 6253, with emphasis on its O abundance. High-dispersion, 7774 Å O i triplet region spectra of 47 stars were obtained using Hydra II on the CTIO Blanco 4 m. Radial velocity analysis confirms 39 stars consistent with single star membership, primarily at the turnoff. Thirty-six of these are included in our abundance analysis. Our differential analysis relative to the Sun yields primarily scaled-solar values, with weighted cluster averages of [O/H] = +0.440 ± 0.020, [Fe/H] = +0.445 ± 0.014, [Al/H] = +0.487 ± 0.020, [Si/H] = +0.504 ± 0.018, and [Ni/H] = +0.702 ± 0.018 (where the errors are ). We discuss possible origins for the three known super-metal-rich clusters based upon their abundance patterns, Galactic locations, and space motions. The abundance patterns of NGC 6253 are very similar to those of NGC 6791 and NGC 6583. With the possible exception of oxygen, the abundances of these clusters are all close to scaled-solar, and they are similar to patterns seen in metal-rich disk dwarfs and giants. However, they also seem to differ from those of metal-rich bulge stars. We demonstrate that NGC 6253 is unusually oxygen rich (in [O/H]) for its 3.3 Gyr age. While we find [O/Fe] to be scaled-solar for NGC 6253, the more recently reported values for NGC 6791 show a large variation, from values close to scaled-solar down to values at least a factor of two below scaled-solar. We discuss the possibility that the scaled-solar [O/Fe] abundances of NGC 6253 and NGC 6791 might reflect a flattening of the Galactic [O/Fe] versus [Fe/H] relationship. This possibility may be consistent with disk star abundance data, which show an apparent "floor" at [O/Fe] for [Fe/H] , and with chemical evolution model results, which may predict such a flattening due to a decrease in supernova Fe yields at super-solar-metallicities. Orbit solutions for NGC 6791 allow that it may have formed in the inner disk and was then kicked out, but the origins of the other two much younger clusters remain mysterious. We re-evaluate the age of NGC 6583 in view of the evidence that the cluster is super-metal-rich, and confirm a probable age less than 1 Gyr (best range: 500–900 Myr). We also argue that it is unlikely the cluster is more than 3 kpc away (best range: 2–3 kpc) if the apparent turnoff, main sequence, and giants are all cluster members
WIYN Open Cluster Study. XXXIX. Abundances in NGC 6253 from HYDRA Spectroscopy of the Li 6708 A Region
High-dispersion spectra of 89 potential members of the old, super-metal-rich
open cluster, NGC 6253, have been obtained with the HYDRA multi-object
spectrograph. Based upon radial-velocity measurements alone, 47 stars at the
turnoff of the cluster color-magnitude diagram (CMD) and 18 giants are
identified as potential members. Five turnoff stars exhibit evidence of
binarity while proper-motion data eliminates two of the dwarfs as members. The
mean cluster radial velocity from probable single-star members is -29.4 +/- 1.3
km/sec (sd). A discussion of the current estimates for the cluster reddening,
derived independently of potential issues with the BV cluster photometry, lead
to an adopted reddening of E(B-V) = 0.22 +/- 0.04. From equivalent width
analyses of 38 probable single-star members near the CMD turnoff, the weighted
average abundances are found to be [Fe/H] = +0.43 +/- 0.01, [Ni/H] = +0.53 +/-
0.02 and [Si/H] = +0.43 (+0.03,-0.04), where the errors refer to the standard
errors of the weighted mean. Weak evidence is found for a possible decline in
metallicity with increasing luminosity among stars at the turnoff. We discuss
the possibility that our turnoff stars have been affected by microscopic
diffusion. For 15 probable single-star members among the giants, spectrum
synthesis leads to abundances of +0.46 (+0.02,-0.03) for [Fe/H]. While less
than half the age of NGC 6791, NGC 6253 is at least as metal-rich and, within
the uncertainties, exhibits the same general abundance pattern as that typified
by super-metal-rich dwarfs of the galactic bulge.Comment: 5 Tables, 9 figures, 45 page