Revisiting the high-mass transfer close binary star system AU Monocerotis

¹ Institut für Astronomie und Astrophysik, Eberhard-Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: antonio.armeni@astro.uni-tuebingen.de
² Dipartimento di Fisica “Enrico Fermi”, Universitè di Pisa, Pisa, Italy
³ INFN – Sezione Pisa, largo B. Pontecorvo 3, 56127 Pisa, Italy
e-mail: steven.neil.shore@unipi.it

Received: 22 March 2022
Accepted: 16 May 2022

Abstract

Context. AU Monocerotis is an eclipsing, double-lined spectroscopic binary with a period of 11 days that is in a state of extreme mass transfer, consisting of a main sequence B-type embedded in a thick accretion disk fed by a Roche lobe overflowing evolved G-type companion. It is also one of the double periodic variable Algol-type binaries.

Aims. Our aim is to study the accretion environment and the origin of the long cycle in the system. We present revised properties of the gainer by including contributions from the accretion disk and its boundary layer, because the absorption lines used in previous works to estimate the parameters were contaminated by the disk absorption.

Methods. We performed a multiwavelength spectroscopic study using archival high-resolution IUE ultraviolet (1200–3200 Å) spectra and optical spectra (from about 3700–9000 Å) from FEROS, HARPS, and SOPHIE.

Results. Using the optical He I lines and the UV Si III, C II, Si IV lines, we derived new parameters for the temperature, gravity, and rotational velocity of the B star. The IUE spectra delineate a stratified environment around the gainer, with spectral lines such as O I, Mg II, Al II, and Si II formed in the outer accretion disk and a pseudo-photospheric boundary layer that alters the spectrum. Phase-limited discrete outflows, detected in the time-dependent absorption, trace the stream impact site and the disturbance it creates downstream in the disk. The long-term variability is due to changes in the accretion disk structure and circumstellar environment. Enhanced systemic mass outflow is observed at long cycle maximum, reaching at least 1000 km s⁻¹.

Conclusions. These results highlight the complex interplay between physical mechanisms that regulate the evolution of strongly interacting mass-exchanging binary stars.