The UV emission spectrum of RW Aur A is strongly disturbed by absorption lines of singly ionised metals originating in the stellar wind. The lines look symmetrical and blueshifted relative to the star by km s-1. The outflowing gas is dense and its extension along the line of sight is less than The wind gas temperature is definitely below 10 000 K, but metals (and probably sulfur) are almost completely singly ionised by strong stellar emission in the hydrogen lines of the Lyman series. Stellar quanta are responsible for a significant population of the upper levels of the hydrogen in the wind. The photoionization of this hydrogen is the main source of free electrons and subsequent gas heating. The strong emission lines of and Fe II observed in the RW Aur A spectrum are also the result of the fluorescent pumping by stellar quanta. Iron fluorescent lines originate in the warm wind, while molecular hydrogen emission forms far outside, possibly near the Herbig-Haro objects.
Emission in the C IV and Si IV lines is strongly suppressed by Fe II and Ni II wind absorption features. Superposition of strong emission lines onto residual profiles additionally disturbs the picture, so the C IV and Si IV line fluxes derived from low resolution IUE spectra are strongly affected. It may explain the nontrivial character of the variability of UV lines observed by Imhoff & Giampapa (1983). This can also be true for other T Tauri stars - see Gomez de Castro & Franqueira (1997). It is clear anyway that accretion and stellar wind are related phenomena in CTTSs and the emission by a accretion shock looks a valuable link between them.
The observed outflow is too cold and compact to produce the strong UV emission lines of singly ionized metals. We can interpret the UV spectra more easily than optical spectra to derive quantitative physical parameters and geometry of the accretion inflow and the wind. Of course this requires much more (and better quality) data than we have.
© European Southern Observatory (ESO) 2000
Online publication: June 5, 2000