Taft Armandroff

Integrated-light spectroscopy of globular clusters at the infrared Ca II lines

Integrated-light spectroscopy has been obtained for 27 globular clusters at the Ca II IR triplet. Line strengths and radial velocities have been measured from the spectra. For the well-studied clusters in the sample, the strength of the Ca II lines is very well correlated with previous metallicity estimates. Thus, the triplet is useful as a metallicity indicator in globular cluster integrated-light spectra. The greatly reduced effect of interstellar extinction at these wavelengths (compared to the blue region of the spectrum) has permitted observations of some of the most heavily reddened clusters in the Galaxy. For several such clusters, the Ca II triplet metallicities are in poor agreement with metallicity estimates from IR photometry by Malkan (1981). The strength of an interstellar band at 8621A has been used to estimate the amount of extinction towards these clusters. Using the new metallicity and radial-velocity data, the metallicity distribution, kinematics, and spatial distribution of the disk globular cluster system have been analyzed. Results very similar to those of Zinn (1985) have been found. The relation of the disk globulars to the stellar thick disk is discussed.

Metallicities for old stellar systems from Ca II triplet strengths in member giants

Spectra of a large sample of giants in six well-studied Galactic globular clusters are used to investigate the utility of Ca II triplet line strengths as an abundance indicator. It is found that the most appropriate diagram for abundance analysis is that in which the sum of the equivalent widths of the two stronger triplet lines is plotted against V - V HB . This diagram shows good abundance sensitivity, particulary for abundances less than approximately [Fe/H] = -1.2 dex. At higher abundances, however, there is a loss of sensitivity and the interpretation is complicated by possible changes in the [Ca/Fe] ratio. The abundances from spectra of giants in Eridanus, Pal 12 and Carina dwarf spheroidal galaxy are derived.

The properties of the disk system of globular clusters

A large refined data sample is used to study the properties and origin of the disk system of globular clusters. A scale height for the disk cluster system of 800-1500 pc is found which is consistent with scale-height determinations for samples of field stars identified with the Galactic thick disk. A rotational velocity of 193 + or - 29 km/s and a line-of-sight velocity dispersion of 59 + or - 14 km/s have been found for the metal-rich clusters. 70 references.

The mass-to-light ratios of the draco and ursa minor dwarf spheroidal galaxies. 2. The binary population and its effect in the measured velocity dispersions of dwarf spheroidal galaxies

We use a large set of radial velocities in the Ursa Minor and Draco dwarf spheroidal galaxies to search for binary stars and to infer the binary frequency. Of the 118 stars in our sample with multiple observations, six are velocity variables with

The dwarf spheroidal companions to M31: Surface-brightness profiles

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A Survey for Low Surface Brightness Galaxies around M31. I. The Newly Discovered Dwarf Andromeda V

probabilities below 0.001. We use Monte Carlo simulations that mimic our observations to determine the efficiency with which our observations find binary stars. Our best, though significantly uncertain, estimate of the binary frequency for stars near the turnoff in Draco and UMi is 0.2--0.3 per decade of period in the vicinity of periods of one year, which is 3--5

What is better than an 8192x8192 CCD Mosaic imager: two Mosaic wide-field imagers, one for KPNO and one for CTIO

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The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda II

that found for the solar neighborhood. This frequency is high enough that binary stars might significantly affect the measured velocity dispersions of some dwarf spheroidal galaxies according to some previous numerical experiments. However, in the course of performing our own experiments, we discovered that this previous work had inadvertently overestimated binary orbital velocities. Our first set of simulations of the effects of binaries is based on the observed scatter in the individual velocity measurements for the multiply-observed Draco and Ursa Minor stars. This scatter is small compared to measured velocity dispersions and, so, the effect of binaries on the dispersions is slight. This result is supported by our second set of experiments, which are based on a model binary population normalized by the observed binary frequency in Draco and Ursa Minor. We conclude that binary stars have had no significant effect on the measured velocity dispersion and inferred mass-to-light ratio of any dwarf spheroidal galaxy.

IC 10 - A 'poor cousin' rich in Wolf-Rayet stars

CCD images and surface photometry are presented for the three dwarf spheroidal companions to M31. The derived light profiles are the msot accurate to date for galaxies of this type and serve to define the faint end of the surface-brightness-luminosity relation for early type dwarfs. New structural parameters are also compared for a subset of the galactic dwarf spheroidals, which indicate, when compared to the M31 dwarfs, that both sets of dwarf galaxies are similar. A B−V color is measured for And I and found to be in accord with that expected from the metallicity derived by Mould and Kristian (1990, ApJ, 354) from their color-magnitude diagram study. However, to a high degree of accuracy, we rule out their claim of a color gradient in this galaxy

Wolf-Rayet stars in NGC 6822 and IC 1613

We present B-, V-, and I-band images, as well as an Hα image, of Andromeda VI (And VI). This is the second newly identified dwarf spheroidal (dSph) companion to M31 found using a digital filtering technique applied to the second Palomar Sky Survey for which about 1550 deg2 now have been surveyed. And VI was confirmed to be a nearby dSph galaxy when it resolved into stars easily with a short V-band exposure with the 4 m telescope. Subarcsecond images taken at the Kitt Peak WIYN 3.5 m telescope provided (I, V-I) and (V, B-V) color-magnitude diagrams that yield a distance of 775 ± 35 kpc using the tip of the red giant branch method, and a mean metallicity of [Fe/H] = -1.58 ± 0.20 with a dispersion of approximately 0.3 dex. And VI has a galactocentric distance of roughly 271 kpc and MV = -11.3. All observed properties of And VI are consistent with its classification as a dSph companion to M31. Despite the recent identification of the And V, And VI, and Cas dwarfs, the Local Group luminosity function remains highly deficient in faint galaxies relative both to CDM simulations of its formation, and to the luminosity functions for richer clusters of galaxies.