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Astron. Astrophys. 330, 1070-1076 (1998)

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1. Introduction

Low-mass pre-main-sequence stars are characterized by magnetic activity phenomena, i.e. temporal highly variable bursty radiative emissions (X-ray, radio, as well as optical and UV), during the stellar T-Tauri phase (Feigelson and DeCampli 1981; Montmerle 1991; White et al. 1992; André 1996). Recently, ASCA and ROSAT observations have proven that stellar objects also exhibit non-thermal flare-like radiation during an earlier phase of the stellar evolution, namely during the protostellar class I phase (Koyama et al. 1996; Grosso et al. 1997). It has often been argued that magnetic activity phenomena in the context of young stellar objects (YSO) should be understood in analogy to solar coronal flares. However, besides the fact that flares in YSO differ from solar flares in the total released energy amount, the harder spectrum as well as the size of the emitting region, the plasma parameter regime in the YSO-context is totally different from the one of the solar corona (cf. Montmerle 1991; Königl 1994; Koyama et al. 1996). In the YSO context we have to deal with a partially ionized dusty plasma filling the magnetospheres around T-Tauri stars and protostellar class I objects, since the relatively large size of the emitting regions indicates that the magnetic activity processes are not restricted to the more or less fully ionized narrow region immediately above the stellar surface.

A well known macroscopic plasma instability widely believed to be an important process which leads to the formation of coherent structure and eruptive energy conversion in laboratory (e.g. Yur et al. 1995) as well as astrophysical (e.g. Priest 1984) plasmas is the resistive tearing instability (Furth et al. 1963). In the context of solar flares it could be shown that the tearing instability indeed plays a dominant role (e.g. van Hoven 1981; Janicke 1982; Steinolfson and van Hoven 1984; Birk and Otto 1991). It is the aim of this contribution to discuss tearing instabilities in the context of YSO magnetic activity for the appropriate partially ionized dust plasma regime. In the next section the scenario of tearing unstable current sheets/ flux tubes in YSO magnetospheres is outlined. The governing equations are formulated in Sect. 3. In Sect. 4 the dispersion relation is presented (some technical details are given in an Appendix) and numerical solutions are shown. Eventually, Sect. 5 is devoted to a discussion of the results.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998