Laura Kellar Fullton

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.

The distant globular cluster NGC 6229 and its relation to other outer halo clusters

CCD-based BV photometry is presented for over 1100 stars in and around the globular cluster NGC 6229. For the first time, the clusters entire giant branch and its horizontal branch are delineated, from which metallicity (Fe/H about -1.4) and distance estimates ensue. NGC 6229 is found to have a relatively blue horizontal branch for its intermediate metallicity, which is unusual for an outer halo cluster. Its low R value hints that the blue color of its horizontal branch is due to a lower than average helium abundance. Three possible new variable stars are identified, including one RR Lyrae and two long-period variables. Among the 13 globular clusters lying more than 24 kpc from the Galactic center, the unusually red horizontal branch phenomenon is common only among the outermost clusters. An average R value for these clusters suggests the effect is not caused by an overabundance of helium. The Oosterhoff dichotomy apparently persists into the outer halo, although the number of clusters with RR Lyrae variables is small. 55 refs.

Near-Infrared Photometry of Globular Clusters NGC 6287 and NGC 6341 (M92): The Formation of the Galactic Halo*

We present Hubble Space Telescope (HST) Near Infrared Camera and Multi-Object Spectrometer (NICMOS) camera 3 photometry of the inner halo globular cluster NGC 6287 and the intermediate halo globular cluster NGC 6341 (M92). Our photometric measurements range from the lower red giant branch to ≈3 mag below the main-sequence turnoff, allowing us to define accurate mean loci of the two clusters. Our relative age estimate between NGC 6287 and M92 using the color difference between the main-sequence turnoff and the base of the red giant branch shows that they essentially have the same ages within ±2 Gyr. The slightly different chemical compositions between the two clusters do not significantly influence our result. Our simulations using synthetic spectra show that the temperature dependence of the interstellar reddening law for the HST NICMOS photometric system could be a more serious problem in the relative age estimate using the color difference between the main-sequence turnoff and the base of the red giant branch. However, this effect adds no more than ±0.5–1 Gyr to the uncertainty in our relative age estimate. Our color-magnitude diagram of NGC 6287 shows a well-defined blue horizontal branch population. This is consistent with its age and metallicity. We also present the relative interstellar reddening toward NGC 6287 and distance modulus with respect to those of M92. We find E(B-V) = 0.62 and (m-M)0 = 14.35 for NGC 6287, which are generally in good agreement with previous estimates. Our photometry suggests that there exists a differential reddening across NGC 6287 with an amount of ΔE(B-V) = 0.07–0.09 mag, consistent with the previous study of Stetson & West. The Galactocentric distance of NGC 6287 becomes RGC = 1.6 kpc if R0 = 8.0 kpc. Based on our new radial velocity measurement for NGC 6287, the clusters (minimum) apogalacticon distance is Ra ≈ 5.5 kpc, suggesting that NGC 6287 is an inner halo cluster. Within this framework, our results are consistent with the idea that the globular cluster formation must have been triggered everywhere at the same time in our Galaxy. In the appendices, we discuss that the interstellar extinction law of the HST NICMOS photometric system, particularly in the F110W passband, depends on the temperature of the stars in the sense that hotter stars suffer more interstellar absorption than cooler stars.

Lithium in the binary halo star HD 89499

We report the finding of an unusually large lithium abundance in the tidally locked binary halo star HD 89499. The effective temperature of this star has been determined to be 4900±150 K from photometric and spectroscopic analyses. Its metallicity has been determined to be [Fe/H]=−2.08±0.07 from 26 Fe I lines and its rotational velocity is estimated to be 22±4 km s −1 . If the star is a dwarf, its lithium abundance log e(Li)=1.35 which is a factor of 10-30 above that for other halo dwarfs of the same temperature. In this case, the lithium depletion may have been arrested when the rotational period of the star tidally synchronized with the binary orbital period. We discuss applicable models

The Planetary Nebulae in NGC 6441 and Pal 6

Jacoby & Fullton (1994, BAAS, 26, 1384) searched 133 of the ~ 150 Galactic globular clusters for Planetary Nebulae (PNe) using the on-band/off-band imaging technique at [O III] λ5007A. We present preliminary abundances and central star properties for the 2 new PN identified in that survey (labeled JaFu 1 and JaFu 2 by Acker, Marcout, & Ochsen-bein 1996).

HST Snapshot Images of Planetary Nebulae

During Cycle 5 of the HST General Observer program we have been carrying out a “snapshot” survey of central stars of planetary nebulae. The snapshots are short exposures in the V (F555W) and / (F814W) filters, taken with the Wide Field Planetary Camera (WFPC2) during brief scheduling opportunities.

The Age of the Galactic Inner Halo

Are the inner regions of the Galaxy younger, older or the same age as the outer halo? We have undertaken a study of globular clusters which lie within 4–5 kpc of the Galactic center. Here we present results for three of these clusters: NGC 6723, NGC 6352 and NGC 5927. We have constructed cluster color-magnitude diagrams from which we have determined their ages using the magnitude difference between the horizontal branch and main-sequence turnoff following the procedure of Carney, Storm & Jones (1992, CSJ). We find that 47 Tue, NGC 6352 and NGC 6723 are coeval to within the errors, while NGC 5927 appears younger. The figure below is a reproduction of Figure 20 of CSJ with our clusters added.