The Optical Gravitational Lensing Experiment. Variable Stars in Globular Clusters - IV. Fields 104A-E in 47 Tuc

Five fields located close to the center of the globular cluster NGC 104=47 Tuc were surveyed in a search for variable stars. We present V-band light curves for 42 variables. This sample includes 13 RR Lyr stars -- 12 of them belong to the Small Magellanic Cloud (SMC) and 1 is a background object from the galactic halo. Twelve eclipsing binaries were identified -- 9 contact systems and 3 detached/semi-detached systems. Seven eclipsing binaries are located in the blue straggler region on the cluster color-magnitude diagram (CMD) and four binaries can be considered main-sequence systems. One binary is probably a member of the SMC. Eight contact binaries are likely members of the cluster and one is most probably a foreground star. We show that for the surveyed region of 47 Tuc, the relative frequency of contact binaries is very low as compared with other recently surveyed globular clusters. The sample of identified variables also includes 15 red variables with periods ranging from about 2 days to several weeks. A large fraction of these 15 variables probably belong to the SMC but a few stars are likely to be red giants in 47 Tuc. VI photometry for about 50 000 stars from the cluster fields was obtained as a by product of our survey.Comment: Latex file, l-aa style, 10 pages, 7 ps figures included. Submitted to A&A. Figure 8 available from ftp://sirius.astrouw.edu.pl/pub/jka/47Tuc/fig8.ps.

Are the Ogle Microlenses in the Galactic Bar?

The analysis of the first two years of OGLE data revealed 9 microlensing events of the galactic bulge stars, with the characteristic time scales in the range $8.6 < t_0 < 62$ days, where $t_0 = R_E / V$. The optical depth to microlensing is larger than $( 3.3 \pm 1.2 ) \times 10^{-6}$, in excess of current theoretical estimates, indicating a much higher efficiency for microlensing by either bulge or disk lenses. We argue that the lenses are likely to be ordinary stars in the galactic bar, which has its long axis elongated towards us. A relation between $t_0$ and the lens masses remains unknown until a quantitative model of bar microlensing becomes available. At this time we have no evidence that the OGLE events are related to dark matter. The geometry of lens distribution can be determined observationally when the microlensing rate is measured over a larger range of galactic longitudes, like $-10^o < l < +10^o$, and the relative proper motions of the galactic bulge (bar) stars are measured with the HST.Comment: 10 pages, 2 figures, revised version accepted for the publication in ApJL, uses AAS LaTeX aaspp.sty macro, PostScript figures and PostScript version of the paper available through anonymous ftp from astro.princeton.edu, directory stanek/tau, or on reques

The Clusters AgeS Experiment (CASE). I. V209 omega Cen - An Eclipsing Post-Common Envelope Binary in the Globular Cluster omega Cen

We use photometric and spectroscopic observations of the detached eclipsing binary V209 omega Cen to derive the masses, radii, and luminosities of the component stars. The system exhibits total eclipses and, based on the measured systemic velocity and the derived distance, is a member of the globular cluster omega Cen. We obtain 0.945 +/- 0.043 Msun, 0.983 +/- 0.015 Rsun and 6.68 +/- 0.88 Lsun for the cooler, but larger and more luminous primary component. The secondary component has 0.144 +/- 0.008 Msun, 0.425 +/- 0.008 Rsun and 2.26 +/- 0.28 Lsun. The effective temperatures are estimated at 9370 K for the primary and at 10866 K for the secondary. On the color-magnitude diagram of the cluster, the primary component occupies a position between the tip of the blue straggler region and the extended horizontal branch while the secondary component is located close to the red border of the area occupied by hot subdwarfs. However, its radius is too large and its effective temperature is too low for it to be an sdB star. We propose a scenario leading to the formation of a system with such unusual properties with the primary component ``re-born'' from a former white dwarf which accreted a new envelope through mass transfer from its companion. The secondary star has lost most of its envelope while starting its ascent onto the sub-giant branch. It failed to ignite helium in its core and is currently powered by a hydrogen burning shell.Comment: 24 pages, 9 figures, AJ, in pres

The Clusters AgeS Experiment (CASE). II. The Eclipsing Blue Straggler OGLEGC-228 in the Globular Cluster 47 Tuc

We use photometric and spectroscopic observations of the eclipsing binary OGLEGC-228 (V228) to derive the masses, radii, and luminosities of the component stars. Based on measured systemic velocity, proper motion and distance, the system is a blue straggler member of the globular cluster 47 Tuc. Our analysis shows that V228 is a semi-detached Algol. We obtain M=1.512 +/- 0.022 Msun, R=1.357 +/- 0.019 Rsun, L=7.02 +/- 0.050 Lsun for the hotter and more luminous primary component and M=0.200 +/- 0.007 Msun, R=1.238 +/- 0.013 Rsun, L=1.57 +/- 0.09 Lsun for the Roche lobe filling secondary.Comment: 19 pages, 5 figures, AJ, in pres