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Astron. Astrophys. 357, 206-218 (2000)


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

E.W. Guenther 1, B. Stelzer 2, R. Neuhäuser 2, T.C. Hillwig 3, R.H. Durisen 3, K.M. Menten 4, R. Greimel 5,7, H. Barwig 6, J. Englhauser 2 and R.M. Robb 7

1 Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, Sternwarte 5, 07778 Tautenburg, Germany
2 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85740 Garching, Germany
3 Department of Astronomy, SW319, Indiana University, Bloomington, Indiana 47405, USA
4 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
5 Isaac Newton Group, Astronomy Division, Apartado de Correos, 321, 38780 Santa Cruz de La Palma, Canary Islands, Spain
6 Universitäts-Sternwarte München, Scheinerstrasse 1, 81679 München, Germany
7 Department of Physics and Astronomy, University of Victoria, P.O. Box 3055, Victoria, BC, V8W 3P6, Canada

Received 9 December 1999 / Accepted 25 February 2000

Abstract

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 variable at all frequencies, simultaneous observations in X-rays and in other wavelengths are able to constrain the properties of the X-ray emitting regions. In this paper, we report quasi-simultaneous observations in X-rays, the 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 Tauri 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, and V773 Tau, a weak-line T Tauri star, the variations are especially large. From observations taken simultaneously, we also find that there is some correspondence between the strength of H[FORMULA] and the X-ray brightness in V773 Tau. The lack of a strong correlation leads us to conclude that the X-ray emission of V773 Tau is not a superposition of flares. However, we suggest that a weak correlation occurs because chromospherically active regions and regions of strong X-ray emission are generally related. V773 Tau was detected at 8.46 GHz as a weakly circularly polarised but highly variable source. We also find that the X-ray emission and the equivalent width of H[FORMULA] remained unchanged, while large variations of the flux density in the radio regime were observed. This clearly indicates that the emitting regions are different. Using optical spectroscopy we detected a flare in H[FORMULA] and event which showed 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 discovered a new weak-line T Tauri star, GSC-1839-5674. Results are also presented for several other stars in the ROSAT field.

Key words: stars: flare – stars: formation – stars: late-type – stars: pre-main sequence – Galaxy: open clusters and associations: general

Send offprint requests to: E.W. Guenther

Correspondence to: Thüringer Landessternwarte, 07778 Tautenburg, Germany

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000

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