RW Aur B: a modest UX Ori-type companion of the famous primary

Abstract

ABSTRACT The secondary of the famous young binary RW Aur is much less studied than the primary. To rectify this shortcoming, we present here the results of UBVRIJHK photometric, VRI polarimetric and optical spectral observations of RW Aur B. The star demonstrates chaotic brightness variations in the optical band, with irregular, short (∼1-d) dimmings with an amplitude ΔV up to 1.3 mag. The dimmings are accompanied by an increase in the linear polarization (up to 3 per cent in the I band), presumably as a result of the scattering of stellar radiation by dust in the circumstellar disc, which means that RW Aur B can be classified as a UX Ori-type star. We conclude that the observed excess emission at λ ≲ 0.45 μm and longwards of ~2 μm as well as the variability of fluxes and profiles of the H i, He i and Na i D emission lines are due to the accretion process. At the same time, emission components of Ca ii lines indicate that RW Aur B has a powerful chromosphere. Assuming solar elemental abundances, we find the following parameters for the star: Teff = 4100–4200 K, AV = 0.6 ± 0.1 (out of the dimming events), L* ≈ 0.6 ${\rm L}_\odot ,\, R_* \approx 1.5$ ${\rm R}_\odot ,\, M\approx 0.85$ M$_\odot ,\, \dot{M}_{\rm acc}\lt 5\times 10^{-9}$ M⊙ yr−1. Finally, we discuss possible reasons for the different levels of accretion activity of the RW Aur binary components and present arguments in favour of the fact that the components are gravitationally bound.