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On the Automatic Determination of Light-Curve Parameters for Cepheid Variables
Stetson, Peter B.
A computerized algorithm for the automatic detection of Cepheid variables and for the estimation of their periods, amplitudes, and mean magnitudes from sparse data sets is presented. It is intended to be suitable for use in such programs as the measurement of Cepheid distances to external galaxies, for example with the Hubble Space Telescope. The …
A Supergiant-Dominated Starburst in the Nucleus of NGC 4569
Keel, William C.
The Virgo spiral NGC 4569 has a very compact optical nucleus whose spectrum is highly composite, which strong Balmer absorption indicating that A stars dominate the optical light. Recent HST imaging shows that the nucleus is extremely compact in the mid-ultraviolet, so that either there must be a central AGN or an extraordinarily luminous and comp…
The Diverse Optical/Ultraviolet Spectra of the Globular Cluster X-ray Sources in NGC 7078 and NGC 6712
Anderson, Scott F.; Margon, Bruce; Downes, Ronald A.
Among the about one dozen luminous X-ray bursters in the cores of globular clusters, only three have optical/UV counterparts unambiguously identified. We have observed two of these counterparts spectroscopically with the low resolution optical and UV gratings of the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope (HST), yielding …
Low Mass Stars in an Outer Field in NGC 6397
Casertano, Stefano; Ballester, Gilda E.; Crisp, David +13 more
We have imagined a field 10' from the center of the globular cluster NGC 6397 in the visible and I bands with WFPC-2 on the HST. AT a level which is severely limited by counting statistics in the small area so far studied, this field is richer in dwarfs with I > 21.5 than the 4.6' radius parallel field discussed by Paresce and colleagues. This …
Formation of the Galaxy: Clues from Globular Cluster Ages and Abundances
Fullton, Laura Kellar
In an effort to bring more data to bear on the problem of the age and formation history of the inner regions of the Milky Way, I undertook a photometric and spectroscopic study of a small sample of globular clusters within 5 kpc of the Galactic center: NGC~6723, NGC~6352 and NGC~5927. The ages and chemical compositions of these clusters provide cl…