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

Contact Binaries of the Galactic Disk: Comparison of the Baade's Window and Open Cluster Samples

The paper integrates the available data for contact binaries of the disk population in a deep galactic field and in old open clusters. The two basic data sets consist of 98 systems in the volume-limited 3 kpc sub-sample of contact binaries detected by the OGLE microlensing project toward Baade's Window (BW3) and of 63 members of 11 old open clusters (CL). Supplementary data on the intrinsically bright, but spatially rare, long-period binaries are provided by 238 systems in the BW sample to the distance of 5 kpc (BW5). The basic BW3 sample and the CL sample are remarkably similar in the period, color, luminosity and variability-amplitude distributions, in spite of very different sample selections. The contact systems are found in the color interval 0.3 < (B-V)0 < 1.2 where the turn-off points (TOP) of the considered clusters are located; however, they are not concentrated at the respective TOP locations but, once the TOP happens to fall in the above color interval, they can appear anywhere within it. The luminosity function for the BW sample appears to be very similar in shape to that for the solar neighborhood main-sequence stars when corrections for the galactic disk structure are applied, implying a flat apparent frequency-of-occurrence distribution. In the interval 2.5 < M_V < 7.5, the frequency relative to MS stars equals about 1/130 for the exponential disk length scale h_R = 2.5 kpc and about 1/100 for h_R = 3.5 kpc. (abridged)Comment: submitted to AJ; 27 pages (aastex), 16 figures (.ps), 7 tables (aastex

CCD Photometry of Faint Variable Stars in the Globular Cluster NGC 6752

We present the results of a photometric survey for variable stars in the field of the nearby globular cluster NGC 6752. The cluster was monitored in 1996 and 1997 for a total of 54 hours with 3 different CCD cameras mounted on the 1.0-m Swope telescope. Eleven new variables were identified: 3 SX Phe stars, 7 contact binaries and 1 candidate detached eclipsing binary. All 3 SX Phe variables are likely members of the cluster while only 1 out of the 7 contact binaries is a potential cluster member. As a by-product of our survey we obtained UBV photometry for a large sample of stars in the cluster field. Two stars with U-B \approx -1.0 and V=19.3 and V=20.6 were identified. They lie along the extended horizontal branch of the cluster, and are likely to be faint sdB stars from NGC 6752.Comment: 18 pages, LaTex, 9 figures (Fig. 1 not available), accepted for publication in the Astronomical Journa

The White Dwarf Distance to the Globular Cluster 47 Tucanae and its Age

We present a new determination of the distance (and age) of the Galactic globular cluster 47 Tucanae (NGC 104) based on the fit of its white dwarf (WD) cooling sequence with the empirical fiducial sequence of local WD with known trigonometric parallax, following the method described in Renzini et al. (1996). Both the cluster and the local WDs were imaged with HST+WFPC2 using the same instrument setup. We obtained an apparent distance modulus of $(m-M)_V=13.27\pm0.14$ consistent with previous ground-based determinations and shorter than that found using HIPPARCOS subdwarfs. Coupling our distance determination with a new measure of the apparent magnitude of the main sequence turnoff, based on our HST data, we derive an age of $13\pm2.5$ Gyr.Comment: Accepted for publication on the Astrophysical Journa

Morphology of Galactic Open Clusters

We analyzed the shapes of Galactic open clusters by the star counting technique with the 2MASS star catalog database. Morphological parameters such as the ellipticity and size have been derived via stellar density distribution, weighed by clustering probability. We find that most star clusters are elongated, even for the youngest star clusters of a few million years old, which are located near to the Galactic disk. The shapes of young star clusters must reflect the conditions in the parental molecular clouds and during the cluster formation process. As an open cluster ages, stellar dynamics cause the inner part of the cluster to circularize, but the overall radius gets larger and the stellar density becomes sparser. We discuss how internal relaxation process competes with Galactic external perturbation during cluster evolution.Comment: 13 pages; 10 pages; accepted Astronomical Journa

Radial Velocity Studies of Close Binary Stars.III

Radial velocity measurements and simple sine-curve fits to the orbital velocity variations are presented for the third set of ten contact binary systems: CN And, HV Aqr, AO Cam, YY CrB, FU Dra, RZ Dra, UX Eri, RT LMi, V753 Mon, OU Ser. All systems but two are contact, double-line spectroscopic binaries with four of them (YY CrB, FU Dra, V753 Mon, OU Ser) being the recent discoveries of the Hipparcos satellite project. The most interesting object is V753 Mon with the mass-ratio closest to unity among all contact systems (q = 0.970 pm 0.003) and large total mass ((M1+M2)sin^3i = 2.93 pm 0.06). Several of the studied systems are prime candidates for combined light and radial-velocity synthesis solutions.Comment: submitted for publication in Astronomical Journal; 3 figures, 2 table

The MACHO Project Large Magellanic Cloud Variable Star Inventory. XIII. Fourier Parameters for the First Overtone RR Lyrae Variables and the LMC Distance

Fourier coefficents have been derived for the $V$ and $R$ light curves of 785 overtone RR Lyrae variables in 16 MACHO fields near the bar of the LMC. The $\phi_{31}$ and $R_{21}$ coefficients have been compared with those of the first overtone RR Lyrae variables in the Galactic globular clusters NGC 6441, M107, M5, M3, M2, $\omega$ Centauri and M68. The results indicate that many of the LMC variables have properties similar to the ones in M2, M3, M5 and the Oosterhoff type I variables in $\omega$ Cen, but they are different from the Oosterhoff type II variables in $\omega$ Cen. Equations derived from hydrodynamic pulsation models have been used to calculate the luminosity and temperature for the 330 bona fide first-overtone variables. The results indicate that they have $\log L$ in the range 1.6 to 1.8\lsun and $\log T_{eff}$ between 3.85 and 3.87. Based on these temperatures, a mean color excess $E(V-R) =0.08$ mag, equivalent to $E(B-V)=0.14$ mag, has been estimated for these 330 stars. The 80 M5-like variables (selected according to their location in the $\phi_{31}-\log P$ plot) are used to determine a LMC distance. After correcting for the effects of extinction and crowding, a mean apparent magnitude $=18.99 \pm 0.02$ (statistical) $\pm 0.16$ (systematic) has been estimated for these 80 stars. Combining this with a mean absolute magnitude $M_V=0.56\pm 0.06$ for M5-like stars derived from Baade-Wesselink analyses, main sequence fitting, Fourier parameters and the trigonometric parallax of RR Lyrae, we derive an LMC distance modulus $\mu=18.43\pm 0.06$ (statistical) $\pm 0.16$ (systematic) mag. The large systematic error arises from the difficulties of correcting for interstellar extinction and for crowding.Comment: 51 pages, 17 figures, 12 tables, accepted to A

A Universal Decline Law of Classical Novae

We calculate many different nova light curves for a variety of white dwarf masses and chemical compositions, with the assumption that free-free emission from optically thin ejecta dominates the continuum flux. We show that all these light curves are homologous and a universal law can be derived by introducing a ``time scaling factor.'' The template light curve for the universal law has a slope of the flux, F \propto t^{-1.75}, in the middle part (from ~2 to ~6 mag below the optical maximum), but it declines more steeply, F \propto t^{-3.5}, in the later part (from ~6 to ~10 mag). This break on the light curve is due to a quick decrease in the wind mass-loss rate. The nova evolutions are approximately scaled by the time of break. Once the time of break is observationally determined, we can derive the various timescales of novae such as the period of a UV burst phase, the duration of optically thick wind phase, and the turnoff date of hydrogen shell-burning. We have applied our template light curve model to the three well-observed novae, V1500 Cyg, V1668 Cyg, and V1974 Cyg. Our theoretical light curves show excellent agreement with the optical y and infrared J, H, K light curves. The WD mass is estimated, from the light curve fitting, to be 1.15 M_\sun for V1500 Cyg, 0.95 ~M_\sun for V1668 Cyg, and 0.95-1.05 M_\sun for V1974 Cyg.Comment: To appear in ApJS, vol.167, 23 pages including 24 figure