Unresolved radio continuum emission is seen in several dozen WTT stars at levels of 1015-1018 erg s-1 Hz-1, or 3-6 orders of magnitude brighter than powerful contemporary solar flares (e.g. Garay et al 1987, Stine et al 1988, White et al 1992a, Chiang et al 1996). Roughly 3/4 of observed stars are undetected due to instrumental sensitivity limits; the NRAO Very Large Array has been the most effective telescope. All sources exhibit high-amplitude variability on long timescales, and a few flares on timescales of hours have been caught in two nearby WTT stars, HD 283447 and DoAr 21 (Feigelson & Montmerle 1985, Phillips et al 1996). Although radio emission from Class I-II objects can be either thermal or nonthermal, the absence of circumstellar disks and ejecta from Class III objects points to a nonthermal mechanism (Montmerle et al 1991). This possibility is confirmed in several of the stronger sources where circular polarization at a level of a few percent is seen (White et al 1992b, Skinner 1993) and where VLBI measurements show the emitting region is too small for thermal processes (Phillips et al 1991). In nearly all respects, WTT radio emission closely resembles that seen in the class of RS CVn magnetically active late-type binary stars.
Optical and ultraviolet studies reveal magnetic effects on the stellar surface in several ways. First, a large body of literature of photometric and Doppler imaging studies show rotational modulations of starspots which cover < 5% to nearly 50% of the surface with temperatures around 500-1000 K below the photospheric temperature (e.g. Rydgren & Vrba 1983, Bouvier et al 1995, Joncour et al 1994, Strassmeier et al 1994). The WTT star V410 Tau, for example, has both a large high-latitude and low-latitude spots; one large active region may have persisted for >1000 rotations (Vrba et al 1988, Rice et al 1996). Second, magnetic flares can be seen photometrically or spectroscopically, despite the overwhelming photospheric emission. The X-ray discovered star V826 Tau exhibited an excursion of 33#33 in 40 minutes (Rydgren & Vrba 1983), and later showed a 1-hour X-ray flare (Carkner et al 1996). Sudden increases in Balmer continuum and line emission have also been seen with total power around 1033-1034 ergs (Gahm et al 1995, Guenther et al 1997). Third, there has been recent success in measurements of Zeeman effects on photospheric absorption lines. Magnetic enhancement of the equivalent width of Fe I lines in the WTT star LkCa 16 is clearly seen, and can be interpreted as B = 2.4 kG fields covering f = 0.6 of the stellar surface (Guenther et al 1998). Spectropolarimetric observations have detected fields in V410 Tau, HD 283472 and HD 155555 (Donati et al 1997).
To illustrate the interwoven phenomenology of magnetic activity at different wavelengths, we briefly describe the most active T Tauri star in the Taurus-Auriga cloud complex, HD 283447 = V773 Tau (Feigelson et al 1994, Phillips et al 1996, Skinner et al 1997, Tsuboi et al 1998). It is a hierarchical triple system comprising a K2-K3 close binary and a distant K3 companion. The binary system has 34#34 L35#35, age 1 Myr, and rapid rotation with vsini = 44 km/s and Prot = 3.4 days. It has a truncated circumstellar disk and very faint broad H29#29 emission, and thus might be considered an intermediate WTT/CTT object. About 17 percent of the surface is covered with a rotationally modulated cool spot, and the MgII chromospheric emission is strongly elevated. The continuous quiescent X-ray emission of the system is unusually strong with 36#36 erg s-1, and the star repeatedly exhibits powerful day-long flare with peak 37#37 erg s-1 with peak plasma temperatures in excess of 100 MK. Both inner companions are radio-loud. The centimeter radio emission is highly variable around several 38#38 erg s-1Hz-1. Unique among late-type stars, it exhibits both circular and linear polarization indicating electron acceleration to energies significantly above 1 MeV. The unusually high levels of magnetic activity in V773 Tau might be attributed to the early loss of its interacting disk, leaving the star with a rapid rotation rate and consequently strong magnetic dynamo. Similar systems include V410 Tau and HDE 283572 in Taurus-Auriga (Rice et al 1996, Walter et al 1987), DoAr 21 in Ophiuchus (Feigelson & Mntmerle 1985), and Par 1724 in Orion (Neuhäuser et al 1998a).
The more typical solar-mass WTT star, however, is 2-20 Myr old with slow rotation ( Prot > 10 days), 39#39 erg s-1, no known X-ray flares and undetectable radio emission or photometric spots. By studying the initial mass function, we know that the most common Class III stars must be pre-main sequence M stars with 40#40 M35#35 and that significant numbers of brown dwarfs exist. Because of the (unexplained) Lx-mass correlation, these are underrepresented in existing X-ray studies. At least two brown dwarfs have been detected in X-rays to date at low levels (Neuhäuser et al 1999).
WTT stars extend empirical relations between magnetic activity tracers seen in older solar-like stars. A sample of 1 M35#35 main sequence stars with a wide range of ages show several consistent patterns: as one moves from the oldest disk population to ZAMS stars, radio luminosity rises by 104, X-ray luminosity rises by 103, and the fraction of plasma at Tx>10 MK rises by 102 in emission measure (Güdel et al 1997). These trends are explained by a scaling of magnetic activity with rotational velocity, where X-ray (but not radio) luminosity is limited by saturation at the stellar surface and average plasma temperatures rise with increased fraction of plasma in microflares rather than the quiescent corona. Although the data are still fragmentary, T Tauri stars appear to follow this pattern: X-ray luminosities are generally below saturation levels, and their Lr/Lxratios and average plasma temperatures are several times higher than ZAMS stars (Skinner et al 1998).