A multi-wavelength study of pre-main sequence stars in the Taurus-Auriga star-forming region

Although many lowmass pre-main sequence stars are strong X-ray sources, the origin of the X-ray emission is not well known. Since these objects are va riable at all frequencies, simultaneous observations in X-rays and in other wavelengths are able to constrain the properties of the X-ray emitting reg ions. In this paper, we report quasi-simultaneous observations in X-rays. t he optical, and the radio regime for classical and weak-line T Tauri stars from the Taurus-Auriga star-forming region. We find that all detected T Tau ri stars show significant night-to-night variations of the X-ray emission. For three of the stars, FM Tau and CW Tau, both classical T Tauri stars, an d V773 Tau, a weak-line T Tauri star, the variations are especially large. From observations taken simultaneously, we also find that then is some corr espondence between the strength of Ho and the X-ray brightness in V773 Tau. The lack of a strong correlation leads us to conclude that the X-rap emiss ion of V773 Tau is not a superposition of flares. However, we suggest that a weak correlation occurs because chromospherically active regions and regi ons of strong X-ray emission are generally related. V773 Tau was detected a t 8.46 GHz as a weakly circularly polarised but highly variable source. Ne also find that the X-ray emission and the equivalent width of Ha remained u nchanged, while large variations of the flux density in the radio regime we re observed. This clearly indicates that the emitting regions are different . Using optical spectroscopy we detected a hare in Ha and event which showe d a flare-like light-curve of the continuum brightness in FM Tau. However, ROSAT did not observe the field at the times of these flares. Nevertheless, an interesting X-ray event was observed in V773 Tau, during which the flux increased for about 8 hours and then decreased back to the same level in 5 hours. We interpret this as a long-duration event similar to those seen on the sun and other active stars. In the course of the observations, we disc overed a new weak-line T Tauri star, GSC-1839-5674. Results are also presen ted for several other stars in the ROSAT field.