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Description
High-resolution spectroscopic observations of AW UMa, obtained on three consecutive nights with a median time resolution of 2.1minutes, have been analyzed using the broadening function method in the spectral window of 22.75nm around the 518nm MgI triplet region. 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 Vsini=181.4+/-2.5km/s, is covered with 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") that originates either in the equatorial regions, which rotate faster than the rest of the star by about 50km/s, 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 Asini=2.73+/-0.11R_{sun}_ and M_1_sin^3^i=1.29+/-0.15M_{sun}_, M_2_sin^3^i=0.128+/-0.016M_{sun}_. The mass ratio, q_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 such discrepant results and whether AW UMa is an unusual object or if only very high-quality spectroscopy can reveal the true nature of W UMa-type binaries.
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