Eclipsing Binaries in the OGLE Variable Star Catalog.II. Light Curves of the W UMa-type Systems in Baade's Window

Light curves of the contact systems visible in the direction of Baade's Window have been analyzed using the first coefficients of the Fourier representation. The results confirm that the geometric contact between components is usually weak. Systems showing significant differences in the depths of eclipses are very rare in the volume-limited sample to 3 kpc: only 2 among 98 contact systems show the difference larger than 0.065 mag; for most systems the difference is <0.04 mag. If this relative frequency of 1/50 is representative, then one among 12,500 - 15,000 Main Sequence F-K spectral-type stars is either a semi-detached or poor-thermal-contact system. Below the orbital period of 0.37 day, no systems with appreciable differences in the eclipse depths have been discovered. Since large depth differences are expected to be associated with the "broken-contact" phase of the Thermal Relaxation Oscillations, this phase must be very short for orbital periods above 0.37 day and possibly entirely absent for shorter periods. In the full sample, which is dominated by intrinsically bright, distant, long-period systems, larger eclipse-depth differences are more common with about 9% of binaries showing this effect. Sizes of these differences correlate with the sense of light-curve asymmetries (differing heights of maxima) for systems with orbital periods longer than 0.4 day suggesting an admixture of semi-detached systems with accretion hot spots on cooler components. The light-curve amplitudes in the full sample as well as in its volume-limited sub-sample are surprisingly small and strongly suggest a mass-ratio distribution steeply rising toward more dissimilar components. Many low mass-ratio systems remain to be discovered in the sky field.Comment: latex 14 pages of text and 14 figures (aastex40 and psfig), submitted to AJ; the first paper astro-ph/9607009 will appear in AJ, Jan.199

Time sequence spectroscopy of AW UMa. The 518 nm Mg I triplet region analyzed with Broadening Functions

High resolution spectroscopic observations of AW UMa, obtained on three consecutive nights with the median time resolution of 2.1 minutes, have been analyzed using the Broadening Functions method in the spectral window Doppler images of the system reveal the presence of vigorous mass motions within the binary system; their presence puts into question the solid-body rotation assumption of the contact binary model. AW UMa appears to be a very tight, semi-detached binary; the mass transfer takes place from the more massive to the less massive component. The primary, a fast-rotating star with V sin i = 181.4+\-2.5 km s^-1, is covered by inhomogeneities: very slowly drifting spots and a dense network of ripples more closely participating in its rotation. The spectral lines of the primary show an additional broadening component (called the "pedestal") which originates either in the equatorial regions which rotate faster than the rest of the star by about 50 km s^-1 or in an external disk-like structure. The secondary component appears to be smaller than predicted by the contact model. The radial velocity field around the secondary is dominated by accretion of matter transferred from (and possibly partly returned to) the primary component. The parameters of the binary are: A sin i = 2.73 +/- 0.11 R_odot and M_1 sin^3 i = 1.29 +/- 0.15 M_odot, M_2 sin^3 i = 0.128 +/- 0.016 M_odot. The mass ratio q_rm sp = M_2/M_1 = 0.099 +/- 0.003, while still the most uncertain among the spectroscopic elements, is substantially different from the previous numerous and mutually consistent photometric investigations which were based on the contact model. It should be studied why photometry and spectroscopy give so very discrepant results and whether AW UMa is an unusual object or that only very high-quality spectroscopy can reveal the true nature of W UMa-type binaries.Comment: Accepted for publication in AJ. 16 figures, Tabs.2 and 3 in ascii. Revisions rel 1st submission: 1st prgs of Sec.4.3 and 6.1 rewritten. Added Fig.5. Added Tab.3 with RV's of primary. Re-determined K1 and final element

The shortest-period M-dwarf eclipsing system BW3 V38

The photometric data for a short-period (0.1984 day) eclipsing binary V38 discovered by the OGLE micro-lensing team in Baade's W indow field BW3 have been analyzed. The de-reddened color (V-I_C)_0=2.3 and the light-curve synthesis solution of the I-filter light curve suggest a pair of strongly-distorted M-dwarfs, with parameters between those of YY Gem and CM Dra, revolving on a tightest known orbit among binaries consisting of Main Sequence stars. The primary, more massive and hotter, component maybe filling its Roche lobe. The very small amount of angular momentum in the orbital motion makes the system particularly important for studies of angular momentum loss at the faint end of the Main Sequence. Spectroscopic observations of the orbital radial velocity variations as well as of activity indicators are urgently needed for a better understanding of the angular-momentum and internal-structure evolutionary state of the system.Comment: latex aastex4.0, 16 pages, in that 4 figures (.ps inserted by psfig.sty) and one table; submitted to PAS