9 Conclusions

Our spectroscopic and photometric observations of the unusual T Tauri star RW Aur A have revealed extremely complex patterns of variability in all emission and absorption lines, both with regard to line profiles, equivalent widths/fluxes and radial velocities of the spectral components. A number of these components vary with a period of $2\hbox{$.\!\!^{\rm d}$ }77$. The variations are regular, and indicate that these patterns have been stable over several years.

In Paper I we reported the discovery of periodic, sinusoidal, small amplitude radial velocity changes in the weak spectrum of narrow absorption lines (WALs). Our new observations in Nov. 1999 confirm the period, phase and amplitude of the WAL velocities. Relative to these, the narrow emission lines of He vary in anti-phase. The broad emission lines, dominating in the spectrum of the star, show also the double period of 5-6 days. The veiling shows no corelation with brightness, although both parameters vary in a wide range. This is partly due to additional variable opacity of the layers above the photosphere (the shell), which imitates lower veiling and also makes the spectrum of RW Aur A vary in spectroscopic luminosity. The presence of chromospheric-like line emission in the photospheric lines most likely also plays a role. In one night we observed a spectrum of RW Aur A with exceptionally strong shell and accretion components present in many lines of neutrals and ions, which makes it possible to estimate the physical conditions in different parts of the accretion stream.

In order to account for all the periodic phenomena, we present two sketches of models, both dependent on the presence of non-axisymmetric accretion.

The first model is based on the binary hypothesis, where the secondary is assumed to generate enhanced accretion from one region of the disk, from where the gas stream originates and finally hits the surface of the primary. With new data on the inclination of the jet (Dougados, private communication), which is presumably aligned with the orbital axis, we get a solution where the secondary is a brown dwarf moving at a distance of only 8-10 $R_{\odot}$from the primary in a nearly circular orbit. The presence of this secondary is considered to be responsible for the exceptional properties of RW Aur compared to other CTTSs.

The second model assumes that RW Aur A is a single star with two major accretion streams within a global magnetosphere that is tilted relative to the rotation axis or is intinsically non-axisymmetric.

Both these model concepts can explain quantitatively and qualitativly most of the complex variations, but both also have their short-comings. These ideas are intended to provide a basis for further developments of the theoretical concepts, and also for selecting key observational tests.

Acknowledgements

Thanks go to E. A. Kolotilov who kindly provided the UBV magnitudes of comparison stars used in our CCD photometry of RW Aur, to David Kennedal who made the UBV photometry at the Swedish 60cm telescope, and to P. Artymowicz and P. Carlqvist for enlightening discussions. We also thank Catherine Dougados for providing information on the jet prior to publication. This work was supported by the Academy of Finland, the Finnish Graduate School of Astronomy and Space Physics, the Crafoord Foundation, the Swedish Natural Science Foundation and the Portuguese foundation for Science and Technology.