Accretion and activity on the post-common-envelope binary RR Caeli

Abstract

Context. Current scenarios for the evolution of interacting close binaries - such as cataclysmic variables (CVs) - rely mainly on our understanding of low-mass star angular momentum loss (AML) mechanisms. The coupling of stellar wind with its magnetic field, i.e., magnetic braking, is the most promising mechanism believed to drive AML in these stars. There are basically two properties thought to drive magnetic braking: the stellar magnetic field and the stellar wind. Understanding the mechanisms that drive AML therefore requires a comprehensive understanding of these two properties as well. Aims. RR Cae is a well-known nearby (d = 20 pc) eclipsing DA+M binary with an orbital period of P = 7.29 h. The system harbors a metal-rich cool DA white dwarf (WD) and a highly active M-dwarf locked in synchronous rotation. The metallicity of the WD suggests that wind accretion is taking place, which provides a good opportunity to obtain the mass-loss rate of the M-dwarf component. We aim to reach a better understanding of the AML mechanisms in close binaries by characterizing the relevant properties of the M-dwarf component of this system. Methods. We analyzed multi-epoch time-resolved high-resolution spectra of RR Cae in search for traces of magnetic activity and accretion. We selected a number of well-known chromospheric activity indicators and studied their phase-dependence and long-term behavior. Indirect-imaging tomographic techniques were also applied to provide the surface brightness distribution of the magnetically active M-dwarf. The blue part of the spectrum was modeled using a state-of-the-art atmosphere model to constrain the WD properties and its metal enrichment. The latter was used to improve the determination of the mass-accretion rate from the M-dwarf wind. Results. Doppler imaging of the M-dwarf component of RR Cae reveals a polar feature similar to those observed in fast-rotating solar-type stars. Analysis of tomographic reconstruction of the Hα emission line reveals two components, one traces the motion of the M dwarf and is generated by chromospheric activity, while the other clearly follows the motion of the WD. The presence of metals in the WD spectrum suggests that this component arises from accretion of the M-dwarf wind. A model fit to the WD spectrum provides Teff = (7260 ± 250) K and log g = (7.8 ± 0.1) dex with a metallicity of 〈log [X/X⊙] 〉 = (-2.8 ± 0.1) dex. This maps into a mass-accretion rate of Ṁacc = (7 ± 2) × 10-16M⊙ yr-1 onto the surface of the WD. © ESO, 2013.