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 of 22.75 nm around the 518 nm Mg I 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 V \sin i = 181.4 \pm 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 \pm 0.11 R _ { \odot } and M _ { 1 } \sin ^ { 3 } i = 1.29 \pm 0.15 M _ { \odot } , M _ { 2 } \sin ^ { 3 } i = 0.128 \pm 0.016 M _ { \odot } .
The mass ratio q _ { sp } = M _ { 2 } / M _ { 1 } = 0.099 \pm 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 .