W UMa-type Binary Stars in Globular Clusters

A sample of 86 contact binary systems in 14 globular clusters with available color index data in (B-V) or in (V-I) has been analyzed. At least one third of all systems are numerous foreground Galactic Disk projections over long lines of sight to the clusters. Since the selection of the cluster members has been based on the MV(logP, color) calibrations, the matter of a metallicity-correction required particular attention with the result that such a correction is apparently not needed at the present level of accuracy. Analysis of the color-magnitude and period-color relations shows that globular cluster members are under-luminous relative to the Galactic Disk contact systems mainly because of the smaller sizes and, consequently, shorter orbital periods; the color-index effect of the diminished blanketing is less important, especially for (V-I). The variability amplitudes for the Blue Straggler systems show a significantly different distribution from that for systems below the Turn Off Point (TOP): The BS systems show only small amplitudes while the distribution for the systems below the TOP is peculiar in containing only large amplitude systems. This difference is linked to an observational selection effect so that efforts at determining the frequency of occurrence of the contact systems below the TOP have been judged to be premature; the frequency among the BS stars could be at about 45+-10 BS stars per one contact BS binary.Comment: submitted for publication in Astronomical Journal; 8 figures, 3 tables (Table 1 in landscape

Search for Variable Stars in the Globular Cluster M3

We describe here results of a photometric time-sequence survey of the globular cluster M3 (NGC 5272), in a search for contact and detached eclipsing binary stars. We have discovered only one likely eclipsing binary and one SX Phe type star in spite of monitoring 4077 stars with $V<20.0$ and observing 25 blue stragglers. The newly identified SX Phe star, V237, shows a light curve with a variable amplitude. Variable V238 shows variability either with a period of 0.49 d or with a period of 0.25 d. On the cluster colour-magnitude diagram, the variable occupies a position a few hundredths of magnitude to the blue of the base of the red giant branch. V238 is a likely descendent of a binary blue straggler. As a side result we obtained high quality data for 42 of the previously known RR Lyrae variables, including 33 of Bailey type ab, 7 type c and 2 double-mode pulsators. We used equations that relate the physical properties of RRc stars to their pulsation periods and Fourier parameters to derive masses, luminosities, temperatures and helium parameters for five of the RRc stars. One of the RRd stars (V79) has switched modes. In previous studies, it was classified as RRab, but our observations show that it is an RRd star with the first overtone mode dominating. This indicates blueward evolution on the horizontal branch.Comment: 21 pages including 14 figures, Latex, requires mn.sty, psfig.sty. Submitted, MNRA

UV Spectroscopy of AB Doradus with the Hubble Space Telescope. Impulsive flares and bimodal profiles of the CIV 1549 line in a young star

We observed AB Doradus, a young and active late type star (K0 - K2 IV-V, P= 0.514 d) with the Goddard High Resolution Spectrograph of the post-COSTAR Hubble Space Telescope with the time and spectral resolutions of 27 s and 15 km, respectively. The wavelength band (1531 - 1565 A) included the strong CIV doublet (1548.202 and 1550.774, formed in the transition region at 100 000 K). The mean quiescent CIV flux state was close to the saturated value and 100 times the solar one. The line profile (after removing the rotational and instrumental profiles) is bimodal consisting of two Gaussians, narrow (FWHM = 70 km/s) and broad (FWHM =330km/s). This bimodality is probably due to two separate broadening mechanisms and velocity fields at the coronal base. It is possible that TR transient events (random multiple velocities), with a large surface coverage, give rise to the broadening of the narrow component,while true microflaring is responsible for the broad one. The transition region was observed to flare frequently on different time scales and magnitudes. The largest impulsive flare seen in the CIV 1549 emission reached in less than one minute the peak differential emission measure (10**51.2 cm-3) and returned exponentially in 5 minutes to the 7 times lower quiescent level.The 3 min average line profile of the flare was blue-shifted (-190 km/s) and broadened (FWHM = 800 km/s). This impulsive flare could have been due to a chromospheric heating and subsequent evaporation by an electron beam, accelerated (by reconnection) at the apex of a coronal loop.Comment: to be published in AJ (April 98), 3 tables and 7 figures as separate PS-files, print Table 2 as a landscap

Eclipsing Binaries in the OGLE Variable Star Catalogs.V. Long-Period Beta Lyrae-type Systems in the Small Magellanic Cloud and the PLC-beta Relation

Thirty eight long-period (P>10 days), apparently contact binary stars discovered by the OGLE-II project in the SMC appear to be Beta Lyrae-type systems with ellipsoidal variations of the cool components dominating over eclipse effects in the systemic light variations and in the total luminosity. A new period-luminosity- color (PLC) relation has been established for these systems; we call it the PLC-beta relation, to distinguish it from the Cepheid relation. Two versions of the PLC-beta relation - based on the (B-V)0 or (V-I)0 color indices - have been calibrated for 33 systems with (V-I)0>0.25 spanning the orbital period range of 11 to 181 days. The relations can provide maximum-light, absolute-magnitude estimates accurate to epsilon-M_V~0.35 mag. within the approximate range -3<M_V<+1. In terms of their number in the SMC, the long-period Beta Lyrae-type binaries are about 50 times less common than the Cepheids. Nevertheless, their large luminosities coupled with continuous light variations make these binaries very easy to spot in nearby galaxies, so that the PLC-beta relation can offer an auxiliary and entirely independent method of distance determination to nearby stellar systems rich in massive stars. The sample of the long-period Beta Lyrae systems in the SMC analyzed in this paper is currently the best defined and uniform known sequence of such binaries.Comment: submitted for publication in Astronomical Journal; 8 PS figures, 2 table

Long-term variability survey of the old open cluster NGC 6791

We present the results of a long-term variability survey of the old open cluster NGC 6791. The BVI observations, collected over a time span of 6 years, were analyzed using the ISIS image subtraction package. The main target of our observations were two cataclysmic variables B7 and B8. We have identified possible cycle lenghts of about 25 and 18 days for B7 and B8, respectively. We tentatively classify B7 as a VY Scl type nova-like variable or a Z Cam type dwarf nova. B8 is most likely an SS Cygni type dwarf nova. We have also extracted the light curves of 42 other previously reported variable stars and discovered seven new ones. The new variables show long-period or non-periodic variability. The long baseline of our observations has also allowed us to derive more precise periods for the variables, especially for the short period eclipsing binaries.Comment: 13 pages LaTeX, including 8 PostScript figures and 4 tables. To appear in June 2003 issue of The Astronomical Journa

Radial Velocity Studies of Close Binary Stars.XIII

Radial-velocity measurements and sine-curve fits to the orbital radial velocity variations are presented for ten close binary systems: EG Cep,V1191 Cyg, V1003 Her, BD+7_3142, V357 Peg, V407 Peg, V1123 Tau, V1128 Tau, HH UMa, and PY Vir. While most of the studied eclipsing systems are contact binaries, EG Cep is a detached or a semi-detached double-lined binary and V1003 Her is a close binary of an uncertain type seen at a very low inclination angle. We discovered two previously unknown triple systems, BD+7_3142 and PY Vir, both with late spectral-type (K2V) binaries. Of interest is the low-mass ratio (q = 0.106) close binary V1191 Cyg showing an extremely fast period increase; the system has a very short period for its spectral type and shows a W-type light curve, a feature rather unexpected for such a low mass-ratio system.Comment: Accepted by AJ. 19 pages including 5 figure

Eclipsing Binaries in the OGLE Variable Star Catalog. IV. The Pre-Contact, Equal-Mass Systems

We used the database of eclipsing binaries detected by the OGLE microlensing project in the pencil-beam search volume toward Baade's Window to define a sample of 74 detached, equal-mass, main-sequence binary stars with short orbital periods in the range 0.19<P<8 days. The logarithmic slope of the period distribution, logN propto (-0.8 pm 0.2) logP, was used to infer the angular-momentum-loss (AML) efficiency for the late, rapidly-rotating members of close binaries. It is very likely that the main cause of the negative slope is a discovery selection bias that progressively increases with the orbital period length. Assuming a power-law dependence for the correction for the bias: bias propto -C logP (with C ge 0), the AML braking-efficiency exponent alpha in dH/dt = P^-alpha can take any value alpha = (-1.1 pm 0.2) + C. Very simple considerations of discovery biases suggest C simeq 4/3, which would give an AML braking law very close to the "saturated" one, with no dependence on the period. However, except for plausibility arguments, we have no firm data to support this estimate of C, so that alpha remains poorly constrained. The results signal the utmost importance of the detection bias evaluation for variable star databases used in analyses similar to the one presented in this study.Comment: accepted by AJ, October 1999. AASTEX-4. 9 PS figures and 3 table

TT Arietis - Observations of a Cataclysmic Variable Star with the MOST Space Telescope

We measured the photometric flux of the cataclysmic variable TT Arietis (BD+14 341) using the MOST space telescope. Periodic oscillations of the flux reveal the orbital period as well as other features of this binary system. We applied a Discrete Fourier Transform (DFT) on a reduced dataset to retrieve the frequencies of TT Arietis. The analysis of the system revealed a photometric period of 3.19 hours. Though the MOST data has a high cadence of 52.8 seconds, a fine structure of the accretion disk is not obvious.Comment: 3 pages, 1 figure, JENAM 2008 proceeding

Variable stars in the field of the old open cluster Melotte 66

We report the results of photometric monitoring of the Melotte 66 field in BVI filters. Ten variables were identified with nine being new discoveries. The sample includes eight eclipsing binaries of which four are W UMa type stars, one star is a candidate blue straggler. All four contact binaries are likely members of the cluster based on their estimated distances. Ten blue stars with U-B<-0.3 were detected inside a 14.8 x 22.8 arcmin^2 field centred on the cluster. Time series photometry for 7 of them showed no evidence for any variability. The brightest object in the sample of blue stars is a promising candidate for a hot subdwarf belonging to the cluster. We show that the anomalously wide main sequence of the cluster, reported in some earlier studies, results from a combination of two effects: variable reddening occuring across the cluster field and the presence of a rich population of binary stars in the cluster itself. The density profile of the cluster field is derived and the total number of member stars with 16<V<21 or 2.8<M_{V}<7.8 is estimated conservatively at about 1100.Comment: 8 pages, 12 figures, accepted to MNRAS - 29 June 200