The protostellar Sun was deeply embedded in its molecular cloud envelope. Direct optical observation of protostars is preluded by high extinction; the best access to these objects is through infrared and X-ray observations, the former, however, picking up much of the emission from dust in the disk and the envelope. Recent efforts have succeeded in obtaining photospheric spectra of Class I protostars from light scattered off the bipolar cavities carved by the outflows (White and Hillenbrand, 2004). Somewhat perplexingly, the analysis of the Taurus sample of embedded objects reveals very little statistical difference between Class I and CTTS objects in Taurus, for example with respect to rotation rates, accretion rates, bolometric luminosities, spectral classes, and disk masses, providing evidence that this sample is co-eval with the CTTS sample; the embedded objects seem to be those with the longest envelope dispersal time, while they are all past their main accretion phase (White and Hillenbrand, 2004).
Magnetic field measurements are commensurately challenging. Johns-Krull (2007b) recently succeeded in obtaining near-infrared diagnostics to measure the photospheric magnetic field on a class I protostar. He reports a field strength of 3.6 kG, making this the highest mean surface field so far detected on any young stellar object.
Direct evidence for magnetic activity is seen in X-rays. Strong X-ray activity is found in considerable numbers of “Class I” protostars thanks to Chandra’s and XMM-Newton’s hard-band sensitivity (see, e.g., Imanishi et al., 2001; Preibisch and Zinnecker, 2001, 2002; Preibisch, 2003b; Getman et al., 2002; Güdel et al., 2007a). Their measured characteristic temperatures are very high, of order 20 – 40 MK (Tsujimoto et al., 2002; Imanishi et al., 2001). Some of these values may, however, be biased by strongly absorbed (“missing”) softer components in particular in spectra with limited signal-to-noise ratios. It is correspondingly difficult to characterize the LX values in traditional soft X-ray bands for comparison with more evolved stars.
Strong X-ray flaring is a characteristic of protostellar solar analogs. Many of these events are extremely large, with total soft X-ray energies of up to 1037 erg (Koyama et al., 1996; Kamata et al., 1997; Grosso et al., 1997; Ozawa et al., 1999; Imanishi et al., 2001; Preibisch, 2003a; Imanishi et al., 2003). Such flares realistically require large volumes, in fact to an extent that star-disk magnetic fields become a possibility for the flaring region (Grosso et al. 1997 for YLW 15 in Oph), with important consequences for the irradiation of the stellar environment by high-energy photons and particles (see Section 6.5).
At radio wavelengths, genuine, embedded class I protostars have most often been detected as thermal sources, and this emission is predominantly due to collimated thermal winds or jets. These jets are probably ionized by neutral winds that collide with the ambient medium at distances of around 10 AU and that are aligned with molecular outflows (e.g., Bieging and Cohen 1985; Snell et al. 1985; Brown 1987; Curiel et al. 1989; Rodríguez et al. 1989, 2003; Anglada 1995; Anglada et al. 1998). Ionized circumstellar material and winds easily become optically thick and therefore occult any non-thermal, magnetic emission from close to the star. However, the discovery of polarization in T Tau(S) (Phillips et al., 1993; Smith et al., 2003), in IRS 5 (Feigelson et al., 1998), in protostellar jet sources (Yusef-Zadeh et al., 1990) and the jet outflows themselves (Curiel et al., 1993; Hughes, 1997; Ray et al., 1997), as well as variability and negative spectral indices in T Tau(S) (Skinner and Brown, 1994) provided definitive evidence for magnetic fields and particle acceleration around these class I objects.
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