New features in the pre-main sequence Sun

6.2 New features in the pre-main sequence Sun

6.2.1 Evolutionary stages: Overview

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 $α$ 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 ).

6.2.2 New features: Accretion, disks, and jets

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 T_eff. 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 T_eff $≈$ 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 $α Ω$ 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:

Star-disk magnetic fields may form large magnetospheres that lead to star-disk locking and disk-controlled spin rates of the star. Generally, disk-surrounded CTTS rotate relatively slowly, probably due to disk locking, with P = 5–10 d, compared to only a few days for diskless, non-accreting WTTS (Bouvier et al., 1993 ); if a rotation-induced magnetic dynamo is at work in CTTS, then it may be dampened compared to dynamos in freely (and rapidly) rotating diskless WTTS.
Non-synchronized rotation may be possible in particular in very young systems in spite of magnetic fields connecting the star with the inner border of the disk. The different rotation rates produce shear in the star-disk magnetic fields, eventually leading to field lines winding up around the star and reconnecting, releasing energy and plasmoids that stream out along large-scale magnetic fields (Hayashi et al., 1996 ; Montmerle et al., 2000 ). This may be a viable model for the production of protostellar jets.
Star-disk magnetic fields are thought to define the basic route of material accreting from the disk to the star (Uchida and Shibata, 1984 ; Camenzind, 1990 ; Königl, 1991 ). This star-disk magnetospheric complex (Figure 29 ) is in the center of present-day studies of PMS magnetic “activity” as it links the accretion process to stellar magnetic fields and probably to outflows and jets. Although the details of the magnetic interface between the star and the circumstellar disk are far from being understood, the physics of mass-loading of the magnetic field lines is being addressed in detail based on theoretical (e.g., Shu et al., 1994 ; Ferreira and Zanni, 2007 ) and numerical investigations (e.g., Romanova et al., 2006 ). Magnetically funneled material forms “hot spots” on the stellar surface that can be observed as an ultraviolet excess (e.g., Calvet and Gullbring, 1998 ).

Figure 29:

Sketch of the environment of a classical T Tauri star or a protostar, showing the star, the circumstellar disk, magnetic field lines, and closed star-disk magnetic field structures that funnel material from the disk to the star (adapted from Camenzind, 1990 , © Wiley-VCH Verlag, reproduced with permission).

6.2.3 New emission properties: Solar-like or not?

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 ).

Figure 30: