Magnetic Field of the Young Star RW Aur

Results of our measurements of the longitudinal magnetic field B-z for the young star RW Aur A are presented. B-z measured from the so-called narrow component of the He I 5876 line varies in the range from -1.47 +/- 0.15 to + 1.10 +/- 0.15 kG. Our data are consistent with a stellar rotation period of similar or equal to 5.6 days and the model of two hot spots with opposite magnetic field polarities spaced about 180 degrees apart in longitude. Relative to the Earth, the spot with B-z < 0 lies in the hemisphere above the midplane of the accretion disk, while the spot with B-z > 0 is below the midplane. The upper limit for B-z (at the 3 sigma level) obtained by averaging all observations is 180 G for the photosphere and 220 and 230 G for the H alpha and [O I] 6300 line formation regions, respectively. We have also failed to detect a field in the formation region of broad emission line components: the upper limit for B-z is 600 G. In two of 11 cases, we have detected a magnetic field in the formation region of the blue absorption wing of the Na I D doublet lines, i.e., in the wind from RW Aur A: B-z = -180 +/- 50 and -810 +/- 80 G. The radial velocity of the photospheric lines in RW Aur A averaged over all our observations is similar or equal to+10.5 km s(-1), i.e., a value lower than that obtained by Petrov et al. (2001) ten years earlier by 5.5 km s(-1). Therefore, we discuss the possibility that RW Aur is not a binary but a triple system.