We have obtained the first B,V CCD color-magnitude diagram (CMD) of the galactic globular cluster Arp 2. About 1600 stars have been measured between the tip of the red giant branch at V ~ 15.5 and V ~ 23.5, about two magnitudes below the main sequence turnoff. A centrally concentrated population of blue stragglers has been detected. The slope of the red giant branch and the overall CMD morphology are consistent with that of a metal-poor cluster, with [Fe/H] = 18.4+/-0.25. A preliminary spectroscopic measurement based on the Ca II triplet yields [Fe/H] = -1.73+/-0.05. The comparison of the CMD of Arp 2 with that of other clusters favors a value more metal-poor than that indicated by the Ca II triplet. In this respect, Arp 2 is similar to Ruprecht 106 [Buonanno et al. =1990AJ....100.1811B and =1993AJ....105..184B]. Differential ages between Arp 2 and a number of reference clusters are obtained from the vertical age parameter Delta V^TO_HB, and the horizontal age parameter, {Delta}(B-V)^TO_RGB. By requiring both age estimators to give consistent results, we find that Arp 2 is ~ 3 Gyr younger than the group of the metal-poor clusters and slightly older than Ruprecht 106. The detection of young metal-poor clusters ([Fe/H] <= -1.8) implies a complex scenario for the origin of the galactic halo, possibly involving interactions with satellite galaxies and their cluster systems.
We conduct a search for radio stars by combining radio and optical data from the FIRST survey (Cat. VIII/71) and the Sloan Digital Sky Survey (SDSS-DR6, Cat. II/282, obsoleted by II/294). The faint limit of SDSS makes possible a homogeneous search for radio emission from stars of low optical luminosity. We select a sample of 112 candidate radio stars in the magnitude range 15<i<~19.1 and with radio flux S_20_>=1.25mJy, from about 7000deg^2^ of sky. The selection criteria are positional coincidence within 1", radio and optical point source morphology, and an SDSS spectrum classified as stellar. The sample contamination is estimated by random matching to be 108+/-13, suggesting that at most a small fraction of the selected candidates are genuine radio stars. Therefore, we rule out a very rare population of extremely radio-loud stars: no more than 1.2 of every million stars in the magnitude range 15<i<19.1 stars has radio flux S_20_>=1.25mJy. We investigate the optical and radio colors of the sample to find candidates that show the largest likelihood of being real radio stars. The significant outliers from the stellar locus, as well as the magnetically active stars, are the best candidates for follow-up radio observations.
The planet-metallicity correlation serves as a potential link between exoplanet systems as we observe them today and the effects of bulk composition on the planet formation process. Many observers have noted a tendency for Jovian planets to form around stars with higher metallicities; however, there is no consensus on a trend for smaller planets. Here, we investigate the planet-metallicity correlation for rocky planets in single and multi-planet systems around Kepler M-dwarf and late-K-dwarf stars. Due to molecular blanketing and the dim nature of these low-mass stars, it is difficult to make direct elemental abundance measurements via spectroscopy. We instead use a combination of accurate and uniformly measured parallaxes and photometry to obtain relative metallicities and validate this method with a subsample of spectroscopically determined metallicities. We use the Kolmogorov-Smirnov (K-S) test, Mann-Whitney U-test, and Anderson-Darling (AD) test to compare the compact multiple planetary systems with single-transiting planet systems and systems with no detected transiting planets. We find that the compact multiple planetary systems are derived from a statistically more metal-poor population, with a p-value of 0.015 in the K-S test, a p-value of 0.005 in the Mann-Whitney U-test, and a value of 2.574 in the AD test statistic, which exceeds the derived threshold for significance by a factor of 25. We conclude that metallicity plays a significant role in determining the architecture of rocky planet systems. Compact multiples either form more readily, or are more likely to survive on gigayear timescales, around metal-poor stars.
In the plate collection of the Harvard College Observatory, we have obtained 528 photographic magnitude estimates for the recently discovered long-period classical Cepheid ASAS 101538-5933.1 (P=51.4-days). Together with the published photoelectric and CCD observations, our data have allowed us to construct an O-C diagram spanning a time interval of 120 years. The O-C diagram has the shape of a parabola, which has made it possible to determine for the first time the quadratic light elements and to calculate the rate of evolutionary increase in the period, dP/dt=51.8(+/-4.8)s/yr or (dP/dt)/P=7.3(+/-0.7)s, in agreement with the results of theoretical calculations for the third crossing of the instability strip. The available data reduced by the method of Eddington and Plakidis reveal small random period fluctuations that do not distort the evolutionary trend in the O-C residuals.
We have started a spectroscopic survey to identify new chromospherically active components and low-mass pre-main sequence (PMS) stars in recently discovered All Sky Automated Survey (ASAS) eclipsing binaries. In this paper, we briefly describe our scientific motivation, the observing tools and the results obtained from the first phase of this survey. Using the available observing facilities in India, the spectroscopic observations of a sample of 180 candidate eclipsing binary stars selected from ASAS-I&II releases were carried out during 2004-2006. The strength of Halpha emission was used to characterize the level of chromospheric activity. Our spectroscopic survey reveals that out of 180 stars about 36 binary systems show excess Halpha emission. One of the objects in our sample, ASAS 081700-4243.8, displays very strong Halpha emission. Follow-up high-resolution spectroscopic observations reveal that this object is indeed very interesting and most likely a classical Be-type system with K0III companion.
ROSAT All Sky Survey (RASS). The combination results in 836 eclipsing binaries that display coronal activity and is the largest sample of active binary stars assembled to date. By using the (V-I) colors of the ASAS eclipsing binary catalog, we are able to determine the distances and thus bolometric luminosities for the majority of eclipsing binaries that display significant stellar activity.
We have analyzed 1455 fundamental mode RR Lyr stars of the Galactic field, using the All Sky Automated Survey (ASAS) data. The sample covers 75% of the sky and contains objects in the close neighborhood of the Sun, within 4kpc distance. Unlike in the previous analysis of the close field RRab stars, we see a clear manifestation of the Oosterhoff groups on the period-amplitude diagram. The relation for Oosterhoff I type variables becomes strongly flattened at large V amplitudes, which was not observed for globular cluster RR Lyr.
We present the results of our detailed spectroscopic and photometric analysis of two previously unknown <1M_{sun}_ detached eclipsing binaries: ASAS J045304-0700.4 and ASAS J082552-1622.8.
Photometric data from the ASAS - South (declination less than 29{deg}) survey have been used for identification of bright stars located near the sources from the ROSAT All Sky Survey Bright Source Catalog (RBSC). In total 6028 stars brighter than 12.5mag in I- or V-bands have been selected and analyzed for periodicity. Altogether 2302 variable stars have been found with periods ranging from 0.137d to 193d. Most of these stars have X-ray emission of coronal origin with a few cataclysmic binaries and early type stars with colliding winds. Whenever it was possible we collected data available in the literature to verify periods and to classify variable objects.
We present a catalog of 307 optical counterparts of the bright ROSAT X-ray sources, identified with the ASAS North survey data and showing periodic brightness variations. They all have declination north of -25{deg}. Other data available from the literature for the listed stars are also included. All the tabulated stars are new variables, except for 13 previously known, for which the revised values of periods are given.
