RW Aur A, a close binary?

G. F. Gahm, P. P. Petrov, R. Duemmler, J. F. Gameiro, M. T. V. T. Lago

Abstract
We have discovered periodic changes in velocity and equivalent width of a number of different spectral features of RW Aur A, a T Tauri star with a jet, broad emission lines and substantial excess continuous emission. In particular, the photospheric lines vary in radial velocity with an amplitude of 5.7 km s^-1 over a period of 2.77 days, which appears to have been stable over at least 18 years. The present Letter focusses solely on one straightforward interpretation, namely that RW Aur A is a single-lined spectroscopic binary. However, other spectral features vary in or out of phase with the same or the double period. Interpretations, other than a binary model, should be considered but must await the more extended report on our large observational material. In the binary case, the small amplitude leads to a mass function of only 5* 10^-5 Msun , by far the smallest observed for young binaries. The jet velocity and extent restrict the range of inclinations (if the orbital axis is aligned with the jet), implying a close binary with a secondary at the brown dwarf limit. Further restrictions follow if the primary rotates synchronously. Narrow emission components of HeI and HeII vary in anti-phase with the photospheric absorption lines. We show that it is unlikely that these lines reflect the motion of the secondary. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.

Astronomy and Astrophysics
Volume 352, Page L95
December 1999