Stellar Phoenix Photo Recovery 4 Serial Number
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We present the results of a search for variable stars in the Local Group dwarf galaxy Phoenix. Nineteen Cepheids, six candidate long-period variables, one candidate eclipsing binary, and a large number of candidate RR Lyrae stars have been identified. Periods and light curves have been obtained for all the Cepheid variables. Their distribution in the period-luminosity diagram reveals that both anomalous Cepheids (ACs) and short-period classical Cepheids (s-pCC's) are found in our sample. This is the first time that both types of variable stars are identified in the same system even though they likely coexist, but have gone unnoticed so far, in other low-metallicity galaxies such as Leo A and Sextans A. We argue that the conditions for the existence of both types of variable stars in the same galaxy are a low metallicity at all ages and the presence of both young and intermediate-age (or old, depending on the nature of AC) stars. The RR Lyrae candidates trace, together with the well-developed horizontal branch, the existence of an important old population in Phoenix. The different spatial distributions of s-pCC's, ACs, and RR Lyrae variables in the Phoenix field are consistent with the stellar population gradients found in Phoenix, in the sense that the younger population is concentrated in the central part of the galaxy. The gradients in the distribution of the young population within the central part of Phoenix, which seem to indicate a propagation of the recent star formation, are also reflected in the spatial distribution of the s-pCC's.
We present narrowband photometry of 91 dwarf elliptical galaxies in the Coma and Fornax Clusters taken with the Strömgren (uvby) filter system. Dividing the sample by dwarf morphology into nucleated (dEN) and nonnucleated (dE) dwarfs reveals two distinct populations of early-type systems based on integrated colors. The class of dEN galaxies are redder in their continuum colors as compared to bright cluster ellipticals and dE type dwarfs, and their position in multicolor diagrams can only be explained by an older mean age for their underlying stellar populations. By comparison with the narrowband photometry of the M87 globular cluster system, we find that dEN's are a higher metallicity continuation of the old, metal-poor color sequence of galactic globular clusters and the blue population of M87 globular clusters. Bright ellipticals and dE dwarfs, on the other hand, follow the color sequence of the metal-rich, red population of M87 globular clusters. A comparison to SED models, convolved to a simple metallicity model, finds that dEN's and blue globular clusters are 3 to 4 Gyr older than cluster ellipticals and 5 Gyr older than dE type galaxies. The implication is that globular clusters and dEN galaxies are primordial and have metallicities set by external constraints such as the enrichment of their formation clouds. Bright ellipticals and dE galaxies have metallicities and ages that suggest an extended phase of initial star formation that produces a younger mean age, even if their formation epoch is similar to that of dEN's and blue globular clusters, and an internally driven chemical evolutionary history.
A photometric monitoring program in the field of the Galactic cluster NGC 6939 was conducted using the 0.91 m WIYN and the US Naval Observatory Flagstaff Station 1.3 m and 1 m telescopes, with the primary goal being to search for cataclysmic variables (CVs) in the cluster. Although no CVs were found during the 5 months of monitoring, a number of short-period variables were discovered in the field of the cluster. A large portion of them are of the W Ursae Majoris type, whereas WOCS V7 is an RR Lyrae star. Although the survey was somewhat compromised by the bright stars around the center of the cluster, it is intriguing that NGC 6939 appears to be deficient in faint variables.
The distribution of N*, the number of OB stars per association or cluster, appears to follow a universal power-