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

Abstract

The secondary of the famous young binary RW Aur is much less studied than the primary. To compensate 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 day) dimmings with an amplitude $\Delta V$ up to $1.3^{\mathrm{m}}.$ The dimmings are accompanied with an increase in the linear polarization (up to 3 per cent in the I band), presumably due to the scattering of the stellar radiation by dust in the circumstellar disc that means that RW Aur B can be classified as a UX Ori type star. We concluded that the observed excess emission at $\lambda \lesssim 0.45$ $\mu$m and longward $\approx$ 2 $\mu$m as well as a variability of fluxes and profiles of HI, HeI and NaI 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 the solar elemental abundances, we found the following parameters of the star: $T_{\mathrm{eff}}=4100-4200$ K, $A_{\mathrm{V}}=0.6\pm 0.1$ (out of the dimming events), $L_{\ast }\approx 0.6$ $L_{\odot },$ $R_{\ast }\approx 1.5$ $R_{\odot },$ $M\approx 0.85$ M${}_{\odot },$ ${\dot{M}}_{\mathrm{acc}}<5\times 10^{-9}$ M${}_{\odot }$ yr${}^{-1}.$ Finally, we discuss possible reasons for the different levels of the accretion activity of RW Aur binary components and present arguments in favour of the fact that they are gravitationally bound.

12 pages, accepted by MNRAS

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