It is difficult to study YSO magnetic fields directly and in most cases indirect tracers of magnetic activity such as cool starspots or high energy radiation produced by violent field reconnection must be used. Table 1 provides a bibliographic guide to the observational literature on magnetic activity tracers. It is organized by waveband and star formation regions. We have attempted to be complete in the X-ray and radio listings, but only a representative selection of the large optical literature is included. The latter includes studies of chromospheres, flares, Zeeman effects, photospheric spots, distances, kinematics, binarity, masses, ages and other properties of magnetically active YSOs. We briefly examine here what can be learned about the magnetic fields and their reconnection from these observations.
The traditional method for measuring magnetic fields on the surfaces of late-type stars is the detection of Zeeman splitting in magnetically susceptible absorption lines. Applying this method to YSOs has been difficult due to their faintness and (except WTT/Class III stars) profusion of emission features. Success has recently been achieved in a few cases, indicating fields around 1-3 kG covering a large fraction of the photosphere. Photometric and Doppler imaging show a patchy distribution of large cool starspots, suggesting that the surface fields are complex and multipolar, as in the Sun. However, these results thus tell us little about large-scale fields important for star-disk interactions (Figure 2 right).
Satellite spectral measurements show that YSO X-rays are optically thin thermal bremsstrahlung with associated ionized metal emission lines from multitemperature plasmas with 1 < Tx < 100 MK (Montmerle et al 1991). The spectral parameters of the emitting plasma - temperature distribution, foreground column density NH, metallicity - are similar to those of other magnetically active late-type stars. The importance of flaring is revealed by the X-ray variability. Virtually all YSOs examined at different epochs are variable and, at any given moment, several percent exhibit luminous flare events with a fast rising curve followed by a slower decay over several hours.
For simple flare models, quantitative properties of the magnetic structures can be inferred from these X-ray flares (e.g. Montmerle et al 1983, Walter & Kuhi 1984). Assuming that radiative cooling dominates conductive cooling in the emitting plasma, and assuming a uniform temperature and density, one can calculate that the plasma density 10#10 cm-3, which is similar to solar values. The equipartition magnetic field strength 11#11, which is the minimum possible value for the field strength dynamic flare loops, is 12#12 G.
The morphology of the magnetic structure is a key question, which will be raised several times in this review. X-ray flares give information on the emitting volume, not their geometry. If one considers solar-type cylindrical magnetic tubes with length 13#13 and aspect ratio 14#14, luminous YSO X-ray flares require 15#15 cm 16#16 (note that typically 17#17), assuming radiative cooling. If however reheating occurs during the flare decline, as sometimes seen in solar and stellar flares, smaller values may be invoked (Reale 1997). However, the magnetic field morphology may be far more complicated. For example, Skinner et al 1997 considers cooling loops, two-ribbon flares, interbinary flares and star-disk magnetic reconnection in a discussion of a giant X-ray flare on the Class II/III star V773 Tau. Solar two-ribbon flares (Schmitt 1994, Güdel et al 1999) and rising magnetic arches associated with eruptive solar flares (Svestka 1995), which involve complex evolving magnetic geometries and continuous injection of energy, may be valuable analogies for YSO flares.
Radio continuum flares also provide clues to the magnetic fields in YSOs. The emission, seen in many Class III stars and one Class I object, is highly variable and is sufficiently bright to have detectable circularly polarization in a few cases. The emission mechanism is quite clearly gyrosynchrotron radiation, as seen in the Sun and other late-type magnetically active stars, produced by mildly relativistic electrons with energies around 1 MeV spiraling in 18#18 G fields (Dulk 1985). In one dramatic case, linear polarization is also present, implying electron acceleration up to several MeV, which has not been seen in other stellar flares (Phillips et al 1996). Simple single-loop models of a rapid radio flare on the Class III star DoAr 21 suggests a radio loop size significantly larger than X-ray loops (Montmerle et al 1993, André 1987a).
We proceed with a more detailed presentation of magnetic tracers in YSOs, first treating the X-ray emission, for which CTT and WTT stars have many features in common, and then consider separately CTT and WTT properties at other wavelengths.