Radial velocity mapping of Paczy\'nski's star AW UMa: Not a contact binary

TL;DR

This study uses two-dimensional spectroscopic analysis to challenge the contact binary model of AW UMa, revealing it is likely a detached system with a complex matter stream, inconsistent with previous photometric interpretations.

Contribution

It introduces a new model for AW UMa where both stars are detached and surrounded by a matter stream, explaining the discrepancies between spectroscopic and photometric data.

Findings

Spectroscopic mass ratio (q_sp=0.10) differs from photometric estimates.

AW UMa's broadening functions suggest differential rotation or limb darkening issues.

A new model with detached components and a matter stream better explains observations.

Abstract

We present two-dimensional (radial velocity, orbital phase) spectroscopic results for the very low mass-ratio close binary AW UMa which strongly indicate that the spectroscopic mass ratio (q_sp = 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 (q_sp = 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.