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

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5. Discussion

Magnetic activity in the context of YSO should be understandable on the grounds of plasma physics. It was the aim of this contribution to show that resistive instabilities can be excited in YSO magnetospheres thereby playing an important role in the generation of spontaneous temporal variable non-thermal radiative emission. It was shown that the instability for quite `typical' plasma parameters can evolve on temporal scales comparable to the observed 'flare'-like activity (Montmerle 1991; André 1996). This means that the rate of energy conversion is fast enough to explain the observed time scale of magnetic activity, in principle. Moreover, the wave length of the most unstable mode is comparable to the expected extent of the emitting region for a reasonable thickness of the current sheet formed within a magnetic flux tube. Contrary to the solar corona applications the tearing instability has not to be triggered by some microturbulence giving rise to anomaleous resistivity in order to operate on temporal scales comparable to the observations (e.g. Kuperus 1976, Birk and Otto 1991). However, it should be noted that the physical parameters may vary significantly resulting in different temporal and spatial scales. On the other hand, observations indeed shows variability of magnetic activity processes within the same YSO which in the context of the introduced model can be explained as the consequence of different local plasma parameters (e.g. ionization rate, dust density).

It remains to answer the question if during the tearing dynamics the observed luminosity can be produced. It is very probable that the observed radio and X-ray flares with luminosities of [FORMULA] in magnetospheres of (weak) T-Tauri stars (as well as protostellar class I objects) are of nonthermal origin (Montmerle 1991; André 1996; Koyama 1996). Consequently, the GHz-radio emission, for example, must be emitted by [FORMULA] -electrons which in the proposed scenario can be accelerated during the reconnection process (cf. Schindler et al. 1991; Lesch and Birk 1997) along the magnetic flux tubes. In fact, for the above cited parameters a magnetic field-aligned potential structure with


evolves (the integral has to be calculated along magnetic field lines penetrating the reconnection region), if we assume the length of acceleration region [FORMULA] to be of the order of the wave length of the most unstable mode which should be comparable to the width of the current sheet, in which electrons can gain energies up to [FORMULA], in principle. However, for any effective particle acceleration the acceleration length [FORMULA] must not exceed the loss length due to synchrotron radiation [FORMULA] ([FORMULA] is the Lorentz factor of the accelerated particles) or inverse Compton scattering [FORMULA] (R is the length scale of the emitting region), in the first place. For a magnetic field of [FORMULA] we obtain [FORMULA]. The inverse Compton scattering loss length is [FORMULA] for [FORMULA] and [FORMULA]. Thus, we can conclude that electrons can be accelerated within the reconnection current sheets/magnetic flux tubes up to the observed energies.

Eventually, it should be noted that whereas the quantitative result were obtained for (weak) T-Tauri parameters the tearing scenario is also applicable for magnetic activity in the context of protostellar class I objects. In fact, in this case comparable spatial and temporal scales should be involved but the relevant plasma parameters seem to be not as well fixed down as in the case of T-Tauri stars.

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© European Southern Observatory (ESO) 1998

Online publication: January 27, 1998