Radial velocity mapping of Paczyński's star AW UMa: not a contact binary

Abstract

Abstract We present 2D (radial velocity, orbital phase) spectroscopic results for the very low mass-ratio close binary AW UMa which strongly indicate that the spectroscopic mass ratio (qsp= 0.10) does not agree with the photometrically derived one and that the widely adopted contact binary model appears to experience serious inconsistencies and limitations for this object. AW UMa is compared with V566 Oph (qsp= 0.26) which we found to behave according to the contact model. Observed broadening functions of AW UMa can be interpreted by a very strong limb darkening and/or non-solid-body rotation of the dominant primary component; the former assumption is unphysical while the differential rotation is not supported by an apparent stability of localized, dark features on the outer side of the primary. There are indications of the existence of an equatorial belt encompassing the whole system. All deficiencies in the interpretation and the discrepancy between the photometric and spectroscopic mass ratio of AW UMa can be solved within a new model of AW UMa where both components are detached and the system is submerged in a stream of hot, optically thick matter which mimics the stellar contact. While the masses and their ratio are correctly given by spectroscopy, the photometric picture is heavily modified by the matter engulfing both stars in the equatorial plane.