Modern theory of star formation together with results from comprehensive observing programs have converged to a picture in which a forming low-mass star evolves through various stages with progressive clearing of a contracting circumstellar envelope. In its “class 0” stage (according to the mm/infrared classification scheme), the majority of the future mass of the star still resides in the contracting molecular envelope. “Class I” protostars have essentially accreted their final mass while still being deeply embedded in a dust and gas envelope and surrounded by a thick circumstellar disk. Jets and outflows may be driven by these optically invisible “infrared stars”. Once the envelope is dispersed, the stars enter their “Classical T Tauri” stage (CTTS, usually belonging to infrared class II with an IR excess) with excess H line emission if they are still surrounded by a massive circumstellar disk; the latter results in an infrared excess. “Weak-line T Tauri” stars (also “naked T Tauri stars”, Walter 1986; usually with class III characteristics, i.e., essentially showing a photospheric spectrum) have lost most of their disk and are dominated by photospheric light (Walter et al., 1988).
Moving back in time from the MS into the PMS era of solar evolution, we encounter changes in the Sun’s internal structure and in fundamental stellar parameters such as radius and Teff. The typical “T Tauri” Sun at an age of 0.5 – 3 Myr was bolometrically 1 – 4 times more luminous and 1.7 – 3.6 times larger in radius, while its surface Teff 4260 K, corresponding to a K5 star (after Siess et al., 2000). The interior of T Tauri stars evolving on the Hayashi track is entirely convective. The operation of an dynamo should not be possible, yet very high levels of magnetic activity are clearly observed on T Tauri stars. Alternative dynamos such as convective dynamos may be in operation. “Solar analogy” no longer holds collectively for all stellar parameters, except (roughly) for stellar mass. Also, there is a complex stellar environment, including an accretion disk, outflows and jets, and probably a large-scale stellar magnetosphere that interacts with these structures (Camenzind 1990; Königl 1991; Collier Cameron and Campbell 1993; Shu et al. 1994; Figure 29). Magnetic interactions between the star and its environment are important in the following contexts:
Given the similarity between WTTS and active, ZAMS stars, “activity” in WTTS as seen in spots, chromospheric and transition-region lines, and X-ray emission has conventionally been attributed to solar-like magnetic activity. Non-thermal gyrosynchrotron radio emission in WTTS suggests processes similar to those observed in active MS stars and subgiant binaries, processes that are thought to be related to electron acceleration in coronal flares (White et al., 1992a,b; Güdel, 2002). As for CTTS, strong optical and ultraviolet emission lines were initially assumed to give evidence for extremely active chromospheres and transition regions (Joy 1945; Herbig and Soderblom 1980, see review by Bertout 1989). Flares and “flashes” (e.g., Ambartsumian and Mirzoyan, 1982) also suggest analogy with magnetic flaring in more evolved stars and the Sun. Arguments in favor of solar-like magnetic coronal activity in all TTS include i) dominant, typical electron temperatures of order 107 K that require magnetic confinement, and ii) the presence of flares (Feigelson and Decampli, 1981; Feigelson and Kriss, 1981, 1989; Walter and Kuhi, 1984; Walter et al., 1988).
However, the solar analogy seems to break down in the optical/UV range where strong flux excesses are recorded; these excesses are uncorrelated with X-rays (Bouvier, 1990) but seem to relate to the accretion process (Section 6.3.2). A similar excess recently discovered in soft X-rays may be related to both coronal magnetic fields and accretion (Güdel and Telleschi 2007, see Section 6.3.4). Also, thermal radio emission observed in CTTS, probably due to optically thick winds but also due to bipolar jets, reaches beyond the solar analogy (Cohen et al., 1982; Bieging et al., 1984). Clearly, emission across the electromagnetic spectrum needs to be understood in the context of the new features around accreting stars, and in particular the related magnetic fields, summarized in Section 6.2.2.
A summary of the present view of activity and other properties of PMS stars from the earliest stages to the arrival on the MS is given in Figure 30 (from Feigelson and Montmerle, 1999).
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