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 Hα 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., 1988Jump To The Next Citation Point).

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 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., 2000Jump To The Next Citation Point). 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 1990Jump To The Next Citation PointKönigl 1991Jump To The Next Citation PointCollier Cameron and Campbell 1993Shu et al. 1994Jump To The Next Citation Point; Figure 29View Image). Magnetic interactions between the star and its environment are important in the following contexts:

View Image

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., 1992aJump To The Next Citation Point,bJump To The Next Citation PointGüdel, 2002). As for CTTS, strong optical and ultraviolet emission lines were initially assumed to give evidence for extremely active chromospheres and transition regions (Joy 1945Herbig and Soderblom 1980, see review by Bertout 1989Jump To The Next Citation Point). 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, 1981Feigelson and Kriss, 19811989Walter and Kuhi, 1984Walter 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, 1990Jump To The Next Citation Point) 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 2007Jump To The Next Citation Point, 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., 1982Jump To The Next Citation PointBieging et al., 1984Jump To The Next Citation Point). 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 30View Image (from Feigelson and Montmerle, 1999Jump To The Next Citation Point).

View Image

Figure 30: Summary of properties of PMS objects, in comparison with MS stars (from Feigelson and Montmerle, 1999,  1999 by Annual Reviews, reprinted with permission).

  Go to previous page Go up Go to next page