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.
Stellar Phoenix Photo Recovery 4 Serial Number
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-law form N in the local universe. We evaluate the distribution of N* in the Small Magellanic Cloud using recent broadband optical and space-ultraviolet data, with special attention to the lowest values of N*. We find that the power-law distribution in N* continues smoothly down to N* = 1. This strongly suggests that the formation of field massive stars is a continuous process with those in associations and that the field stars do not originate from a different star formation mode. Our results are consistent with the model that field massive stars represent the most massive members in groups of smaller stars, as expected if the clustering law applies to much lower masses as is expected from the stellar initial mass function (IMF). These results are consistent with the simultaneous existence of a universal IMF and a universal clustering law. Jointly, these laws imply that the fraction of field OB stars typically ranges from about 35% to 7% for most astrophysical situations, with an inverse logarithmic dependence on the most populous cluster, and hence on galaxy size and/or star formation rate. There are important consequences for global feedback effects in galaxies: field stars should therefore contribute proportionately to the volume of the warm ionized medium, and equal relative contributions by superbubbles of all sizes to the interstellar porosity are expected.
We examine HD 207651 as a possible example of a star exhibiting both γ Doradus and δ Scuti type pulsations. We find photometric periods of 0.06479 and 0.06337 days with peak-to-peak amplitudes in Johnson B of 21 and 13 mmag, respectively, clearly indicating δ Scuti pulsations. Additional light variation with a period of 0.73540 days and an even larger amplitude of 31 mmag is within the range of γ Doradus pulsation periods but results instead from the ellipticity effect. HD 207651 has a composite spectrum with a weak, narrow absorption line superposed near the center of each broad metal line. The broad-lined component is the primary of a short-period, single-lined binary, which has a period of 1.4708 days, twice the period of the ellipsoidal variations seen in the photometry. We determine the primary to be an A8 giant and estimate the unseen secondary of the short-period binary to be a mid-M dwarf. The narrow-lined star, an F7: dwarf, shows velocity variability with a period of months or perhaps years. It is thus a more distant companion to the binary, making HD 207651 a triple system. All light variations come from the A8 giant primary star. Since the 0.73540 day variation results from the ellipticity effect, HD 207651 is not an example of a star that exhibits both δ Scuti and γ Doradus pulsations. The growing number of confirmed γ Doradus stars that also occur within the δ Scuti instability strip but fail to show additional δ Scuti variability makes it increasingly unlikely that the two types of pulsation can coexist in the same star. 2ff7e9595c
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