Where was the cradle of the Sun?
The present-day galactic environment is in no ways related anymore to the place where the Sun was born. The contemporaneous Sun is obviously not a member of a bound cluster. There has consequently been some speculation regarding the environment in which the Sun formed. The large-scale solar environment controls interactions with the outflowing solar wind and is therefore relevant for the influx of cosmic rays into the inner solar system.
During the past 106 years, the Sun has moved from the very hot (≈ 106 K) low-density “local bubble” environment into the cooler and denser (≈ 6900 K, n(H0 + H+) ≈ 0.3 cm−3) “local interstellar cloud” (Frisch, 2000; Frisch and Slavin, 2005, 2006). The pressure characteristics of these two environments are similar, so that the radii of the heliosphere would have been similar (120–130 AU; Frisch 2000). However, the heliospheric structure would shrink to ten percent of its present size if the Sun encountered molecular clouds, and such encounters are likely to have occurred several times in the past (Frisch, 2000).
Moving back in time, the evidence becomes indirect. The best “memory” of the earliest episodes of the Sun’s life is contained in meteorites (Section 6.6). The presence of live 60Fe in the early solar system (see Section 6.6) inferred from meteoritic trace elements (Tachibana and Huss, 2003) cannot be explained by local processes (such as stellar flares, see Section 6.6) but is thought to require supernova explosions (Meyer and Clayton, 2000). The local environment of the forming solar system was therefore likely reminiscent of a high-mass star-forming region like the Orion region; the young Sun and its circumstellar disk may have resided in a H II region for a considerable amount of time (Hester et al., 2004); the intense ultraviolet radiation field from massive stars might have contributed to the evaporation of the molecular environment of the Sun (so-called proplyds in the Orion Nebula, O’Dell 2001, or “evaporating gaseous globules” [EGGs], Hester et al. 1996). These structures contain protostars that can be detected in X-rays (Kastner et al., 2005; Linsky et al., 2007), i.e., deterioration of the environment is due both to the larger-scale “interstellar” environment and the stellar magnetic activity itself.
New features in the pre-main sequence Sun
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., 1988).
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., 2000). The interior of T Tauri stars evolving on the Hayashi track is entirely convective. The operation of an αΩ 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:
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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.
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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.
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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).
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).
The T Tauri Sun
The magnetic field of the T Tauri Sun
Surface magnetic fields have been successfully measured on “solar-analog” T Tauri stars, using Zeeman broadening (Johns-Krull et al., 1999; Valenti and Johns-Krull, 2004; Johns-Krull, 2007a). For example, BP Tau (a 0.65 M⊙ CTTS) maintains a mean magnetic field strength (∑Bf) of 2.6±0.3 kG, i.e., the equivalent of sunspots for f = 1. Such field strengths exceed the equipartition value of T Tauri photospheres (≈ 1 kG, Johns-Krull et al. 1999; Johns-Krull 2007a), which may be a consequence of near-total surface filling (Solanki, 1994; Johns-Krull, 2007a). T Tauri photospheres are therefore dominated by magnetic pressure rather than thermal pressure, in contrast to MS stars and the Sun but similar to stellar coronae in general. The average surface magnetic fields of CTTS also exceed a prediction from X-rays, based on a correlation between X-ray luminosity and photospheric magnetic flux valid for MS stars and the Sun (Pevtsov et al., 2003). Upper limits of the net polarization of photospheric lines suggest that the photospheric magnetic fields form predominantly in small-scale structures, although a dipole may dominate at large distances (Johns-Krull et al., 1999; Valenti and Johns-Krull, 2004; Johns-Krull, 2007a). Large dipole components are also suggested from observations of spots concentrated at the poles of T Tauri stars, similar to active MS solar analogs (e.g., Joncour et al., 1994; Hatzes, 1995; Rice and Strassmeier, 1996). In any case, the observed fields should be strong enough to truncate circumstellar disks indeed (Johns-Krull et al., 1999; Johns-Krull, 2007a), even if rather complex (non-dipolar) surface magnetic field distributions are assumed (Gregory et al., 2006).
Zeeman-Doppler Imaging techniques have successfully been applied to extremely active solar analogs in the PMS phase. A particularly clear case was presented by Donati et al. (2000), finding solar-like differential rotation on a post-T Tauri star (see also Section 4.3.1). More recently, Donati et al. (2007) have reconstructed a rather complex, large-scale magnetic topology on the CTTS V2129 Oph; they found a relatively weak dipole but stronger octupolar fields that are tilted against the rotation axis, with strong near-polar spots. The accretion footpoints are also found to be located at high latitudes. An attempt was made at extrapolating the fields to the inner rim of the disk, showing that some field lines should successfully accrete toward the observed hot spots.
At coronal levels, X-ray rotational modulation provides information on the large-scale distribution of stellar magnetic fields. Rotational modulation is widespread among extremely active T Tauri stars (Flaccomio et al., 2005). Some 10% of the studied stars in the Orion X-ray sample show such evidence, suggesting that: i) the X-ray emitting active regions are not homogeneously distributed on the surface, i.e., despite the X-ray saturation level reached by these stars, the surface cannot be filled with X-ray-bright magnetic loops; ii) the X-ray emitting regions responsible for the rotational modulation are directly associated with the surface and cannot extend much beyond R* (Flaccomio et al., 2005). A comparison of the modulation depth with the Sun’s modulation in fact shows that the longitudinal inhomogeneities are similar (Flaccomio et al., 2005).
The ultraviolet T Tauri Sun
A defining property of (accreting) classical T Tauri stars is their strong line emission of, e.g., Hα or Ca ii H & K. These strong lines were initially thought to be evidence of massive chromospheres similar to those seen on the Sun or in cool stars (see review by Bertout 1989), and the discovery of strong UV lines such as those of Si ii, Si iv, and C iv — equivalent to “transition region” lines in the Sun formed above 104 K — supported this picture.
However, when compared with MS stars, including chromospherically very active examples, UV line and continuum emission is up to 102–104 times stronger in CTTS (see example of TW Hya in Table 4; Canuto et al. 1982, 1983; Bouvier 1990; Valenti et al. 2000), regardless of the photospheric effective temperature or the stellar rotation period but correlated with the mass accretion rates derived from optical continuum data (Bouvier 1990; Johns-Krull et al. 2000; Figure 32a below). Further, UV or Hα line surface fluxes of CTTS show, in contrast to more evolved stars, no correlation with coronal X-rays, the latter being in the range of RS CVn-type active binary systems or very active MS stars but the UV/Hα lines showing a wide range of excess flux (Bouvier, 1990).
Coronal and “chromospheric/transition region” fluxes are thus not correlated in CTTS, contrasting strongly with MS and subgiant stars for which a sharp correlation is taken as evidence for a common physical heating mechanism (operating in related magnetic fields; Section 5.6, Figure 22). An additional mechanism must be responsible for the optical/UV line flux excess. Apart from the line excess fluxes, there is also a strong blue continuum excess that leads to “veiling” in the optical spectrum, i.e., a filling-in of absorption lines by continuum emission; this emission is also not compatible with chromospheric radiation. The most obvious property common only to CTTS among the stars considered above is accretion; downfalling material could provide the energy to generate the optical/UV excess (Bertout et al., 1988; Basri and Bertout, 1989).
Nearly free-falling gas can be heated to maximum temperatures
$${T_s} = 8.6 \times {10^5}\;{\rm{K}}\left({{M \over {0.5\;{M_ \odot}}}} \right){\left({{R \over {2{R_ \odot}}}} \right)^{- 1}}$$
(21)
in shocks forming at the bottom of magnetic accretion funnels (Calvet and Gullbring, 1998). UV and optical line emission could thus provide diagnostics for the accretion velocity, the mass accretion rate, and possibly the surface filling factor of accretion funnels.
The present consensus, based on such concepts as well as line profile properties and correlations with the mass accretion rate, is that the UV excess emission originates from material heated in accretion shocks (e.g., Calvet and Gullbring, 1998; Gullbring et al., 1998). Some of the emission lines (e.g., Hα, Ca ii) may also form in the accretion funnels themselves, or in stellar winds (Ardila et al., 2002).
The X-ray T Tauri Sun in time
Feigelson et al. (2002b), Wolk et al. (2005), and Telleschi et al. (2007b) presented X-ray studies of near-solar-mass stars (stars in the ranges of 0.7 M⊙ ≤ M ≤ 1.4 M⊙ and 0.9 M⊙ ≤ M ≤ 1.2 M⊙, respectively, in the former two studies of the Orion Nebula cluster, and wider in the latter study of the Taurus star-forming region). The sample ages typically comprise the log t = 5.5–7 range and contain both disk-surrounded and disk-less T Tauri stars. The median X-ray luminosity in the Orion sample is found at log LX = 30.25, i.e., three orders of magnitude above the average solar X-ray output, but there is evidence for a slow decay with age, log LX ∝ t−1.1 (Feigelson et al., 2002b; Wolk et al., 2005). A shallower decay was reported by Preibisch and Feigelson (2005) for the same stellar cluster, with an exponent between −0.2 and −0.5, but when considering normalized LX/Lbol or average surface X-ray flux, then both Feigelson et al.’s and Preibisch & Feigelson’s studies indicate that the LX decay law is roughly compatible with full saturation (i.e., LX/Lbol ≈ 10−3) as the star descends the Hayashi track and its bolometric luminosity is decreasing (Feigelson et al., 2002b). Telleschi et al. (2007b) used the Taurus sample over a wider mass range but removed the strong LX versus mass correlation in order to normalize the X-ray evolutionary behavior to a solar-mass star. The slope of the LX vs. age correlation is fully compatible with the Orion results, with a power-law index of −0.36 ± 0.11 although the correlation is dominated by scatter from other sources, and its significance is marginal.
The evolutionary LX decay is thus qualitatively different from that in MS stars: it is due to stellar contraction (and perhaps a change in the internal dynamo while the star transforms from a fully convective to a convective-radiative interior); in contrast, the decay of LX in MS stars is due to stellar spin-down while the stellar structure and size remain nearly constant. Figure 31 shows the long-term evolution of the median X-ray output from PMS stages to the end of the MS evolution, for G-type stars with ages > 10 Myr and K-type stars with ages < 10 Myr (because the predecessors of MS G stars are PMS K stars; data from Güdel 2004). The slight trend toward decreasing LX at ages < 10 Myr follows approximately LX ≈ t−0.3, in agreement with the individual trends for the Orion and the Taurus samples, albeit the scatter is large. No “onset ”of activity can be seen back to ages < 1 Myr.
In summary, the age evolution of the X-ray output is modest in PMS stars, the bulk of the X-ray output being determined by other stellar properties. There are at least four such parameters that have been discussed in the recent literature: bolometric luminosity, mass, rotation, and mass accretion rate. I briefly summarize these parameter dependencies in turn:
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Similar to young MS stars, CTTS and WTTS straddle the empirical X-ray saturation limit, i.e., LX ≈ 10−3.5Lbol (e.g., Preibisch et al., 2005; Telleschi et al., 2007b), pointing to a dynamo process somehow related to the dynamo in MS stars. The long-term evolution of Lbol may then be the principal parameter for the long-term evolution of LX.
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In contrast to MS stars, PMS stars show a correlation between LX and mass (e.g., Feigelson et al., 1993; Preibisch et al., 2005; Telleschi et al., 2007b). However, if one restricts the MS sample to saturated, young stars, then they follow such a relation as well, owing to the well-known mass-Lbol relation on the MS, Lbol ∝ M3. Conversely, the flatter Lbol − M relation for a given PMS isochrone combined with the saturation law yields an LX vs. mass correlation that is compatible with the observed relation (Telleschi et al., 2007b), i.e., the two relations are interdependent.
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Rotation is one of the main drivers of the magnetic dynamo and therefore of magnetic activity in MS stars. The near-saturation state of most PMS stars, however, suggests that rotation cannot be a key parameter. This is borne out by explicit correlation studies of the Orion (Preibisch et al., 2005) and the Taurus (Briggs et al., 2007) samples of T Tauri stars that show little in the way of a correlation as seen in MS stars. On the contrary, some apparent trends in this direction are the result of population bias (Briggs et al., 2007). The absence of a decrease in the LX/Lbol ratio up to rotation periods of at least 10 d (in contrast to MS stars) can be explained by the convective turnover time in PMS stars being much larger, yielding smaller Rossby numbers for a given rotation period and therefore saturation up to longer rotation periods (Preibisch et al., 2005; Briggs et al., 2007).
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Accretion could influence LX either by generating X-rays itself, or by suppressing coronal X-ray production. Empirically, LX is correlated with the mass accretion rate Ṁ but this is not a physical correlation for the following reason. Muzerolle et al. (2003), Muzerolle et al. (2005), and Calvet et al. (2004) described a relation between M and the stellar mass M, approximately reading Ṁ ∝ M2. Because LX correlates with Lbol and therefore, for a typical age isochrone, with mass, LX must correlate with Ṁ. Telleschi et al. (2007b) removed this trend from the Taurus sample. Although the two parameters still do not seem to be fully independent, a clear correlation cannot be established. Accretion does seem to influence magnetic energy output in more subtle ways, however, to be discussed in the Section 6.3.4.
Coronal excesses and deficits induced by activity?
If the photoelectric absorption by the accreting gas is small, then the softest X-ray range may reveal the high-temperature tail of the shock emission measure thought to be responsible for the UV excesses (Section 6.3.2). Telleschi et al. (2007b) and Güdel et al. (2007b) identified an excess in the O VII/O VIII Lyα flux (or luminosity) ratio in CTTS when compared with WTTS or MS stars, the so-called X-ray soft excess of CTTS (Figure 32b). In the most extreme case of the CTTS T Tau, the excess O VII flux is such that this line triplet, formed at only ≈ 2 MK, is the strongest in the soft X-ray spectrum (see Figure 33; Güdel and Telleschi 2007).
Interestingly, however, the X-ray soft excess in CTTS is comparatively moderate, the L(O VII r)/L(O VIII) line flux ratio being enhanced by factors of typically ≈ 3–4 times that of equivalent MS stars or WTTS (Figure 32). Furthermore, the excess X-ray line fluxes do not seem to be correlated with the UV line excesses but are correlated with the overall stellar coronal activity level traced, for example, by the O VIII Lyα line flux (Güdel and Telleschi, 2007). It appears that the X-ray soft excess depends on the level of magnetic (“coronal”) activity although it is, at the same time, related to the presence of accretion. The two dependencies may point to an interaction between accretion and magnetic activity at “coronal” heights.
The shock interpretation of the softest X-rays and the X-ray soft excess is appealing, but remains controversial until a larger sample of CTTS with various accretion properties has been interpreted. In particular, given the appreciable accretion rates, high shock densities of order 1012–1014 cm−3 are expected, as first indeed reported from density-sensitive line diagnostics of O vii and Ne ix in the CTTS TW Hya, forming at only a few MK (Kastner et al., 2002; Stelzer and Schmitt, 2004). However, some accreting PMS stars show much lower densities, such as AB Aur (Telleschi et al., 2007b) and T Tau (Güdel et al., 2007b); the same discrepancy between expected and observed densities has also been reported from ultraviolet density diagnostics (Johns-Krull et al., 2000).
In stark contrast to the X-ray soft excess and the UV excess described above, it is now well established that CTTS show a moderate suppression of 0.1–10 keV soft X-ray emission, typically by a factor of ≈ 2 when compared with WTTS of similar properties (Strom and Strom, 1994; Damiani et al., 1995; Neuhäuser et al., 1995; Stelzer and Neuhäuser, 2001; Flaccomio et al., 2003; Preibisch et al., 2005; Telleschi et al., 2007a). Although selection/detection bias or different photoelectric absorption has been quoted to be responsible for these differences (see Güdel 2004 and references therein), the luminosity deficit in CTTS is now thought to be real; it appears that accretion suppresses coronal heating in a fraction of the coronal volume (Preibisch et al., 2005), or at least leads to larger amounts of cooler plasma, which is perhaps the same plasma inferred from the X-ray soft excess (Telleschi et al., 2007b; Güdel and Telleschi, 2007). Alternatively, the presence of a circumstellar disk could strip the outer parts of the stellar corona, thus reducing LX (Jardine et al., 2006).
X-ray flaring of the T Tauri Sun
A high level of near-continuous flaring is found in PMS solar-mass stars. As much as half of the emitted X-ray energy, if not more, may be due to strong flares (Montmerle et al., 1983), and many TTS are nearly continuously variable probably also owing to flares (Mamajek et al., 2000; Feigelson et al., 2002a; Preibisch and Zinnecker, 2002; Skinner et al., 2003). Examples with extreme luminosities and temperatures up to 100 MK have been reported (e.g., Preibisch et al., 1995; Skinner et al., 1997; Tsuboi et al., 1998, 2000; Imanishi et al., 2002). The most extreme flares are found on CTTS and protostars, a possible hint at star-disk magnetic interactions during flares. Wolk et al. (2005) studied frequency and properties of flares in the Orion Nebula cluster, concluding that the median peak luminosity of their sample was log LX = 30.97, with extremely hard spectra at peak time. The median electron temperature was found at 7 keV. An analogous study has been presented by Stelzer et al. (2007) for T Tauri stars in the Taurus Molecular Cloud. The extreme flaring recorded on these PMS stars may have an important bearing on coronal heating (see Section 5.8) and on the alteration of solids in the young stellar environment (see Section 6.6).
The radio T Tauri Sun in time
Early VLA surveys quickly reported strong radio emission from both CTTS and WTTS. Somewhat unexpectedly, however, radio emission comes in two principal flavors. The early, pioneering studies by Cohen et al. (1982), Bieging et al. (1984), Cohen and Bieging (1986), Schwartz et al. (1984), and Schwartz et al. (1986) recognized thermal wind-type emission with rising spectra and in cases large angular sizes for several CTTS. This radio emission can then be used to estimate mass loss rates; these are found to range up to ≲ 10−7 M⊙ yr−1 (André et al., 1987). The partly enormous kinetic wind energy derived under the assumption of a uniform spherical wind suggests anisotropic outflows while structural changes in the radio sources indicate variable outflows, probably along jet-like features; at shorter radio wavelength, dust emission from the disk becomes apparent as well (Cohen and Bieging, 1986; Rodríguez et al., 1992, 1994; Wilner et al., 1996). The thermal radio emission tells us nothing about the presence or absence of stellar magnetic fields. As described earlier, CTTS do show many signatures of magnetic activity, but whatever the possible accompanying radio emission, it seems to be absorbed by the circumstellar ionized wind.
The situation is different in WTTS in which the presence of huge flares (Feigelson and Montmerle, 1985; Stine et al., 1988; Stine and O’Neal, 1998), longer-term variability, and falling spectra clearly point to non-thermal gyrosynchrotron emission (Bieging et al., 1984; Kutner et al., 1986; Bieging and Cohen, 1989; White et al., 1992a; Felli et al., 1993; Phillips et al., 1996) analogous to radio emission observed in more evolved active stars. Conclusive radio evidence for the presence of solar-like magnetic fields in WTTS came with the detection of weak circular polarization during flares but also in quiescence (White et al., 1992b; André et al., 1992; Skinner, 1993). Extremely energetic particles radiating synchrotron emission may be involved, giving rise to linear polarization in flares on the WTT star HD 283447 (Phillips et al., 1996). VLBI observations showing large (∼ 10 R*) magnetospheric structures with brightness temperatures up to Tb ≈ 109 K fully support the non-thermal picture (Phillips et al., 1991).
As a WTT star ages, its radio emission drops rapidly on time scales of a few million years from luminosities as high as 1018 erg s−1 Hz−1 to values around or below 1015 erg s−1 Hz−1 at ages beyond 10 Myr. Young age of a star is thus favorable to strong radio emission (O’Neal et al., 1990; White et al., 1992a; Chiang et al., 1996), whereas toward the subsequent ZAMS stage it is only the very rapid rotators that keep producing radio emission at the 1015 erg s−1 Hz−1 level (Carkner et al., 1997; Magazzù et al., 1999; Mamajek et al., 1999).
The composition of the T Tauri Sun’s corona
Initial studies of a few accreting T Tau stars, in particular the old (≈ 10 Myr) TW Hya, have shown an abundance pattern in the X-ray source similar to the IFIP effect although the Ne/Fe abundance ratio is unusually high, of order 10 with respect to the solar photospheric ratio, and the N/O and N/Fe ratios are enhanced by a factor of ≈ 3.
These anomalous abundance ratios have been suggested (Stelzer and Schmitt, 2004; Drake and Testa, 2005) to reflect depletion of Fe and O in the accretion disk where almost all elements condense into grains except for N (Savage and Sembach, 1996; Charnley, 1997) and Ne (Frisch and Slavin, 2003) that remain in the gas phase which is accreted onto the star. If accretion occurs predominantly from the gas phase in the higher layers of the disk while the grains grow and/or settle at the disk midplane, then the observed abundance anomaly may be a consequence.
Larger systematics have made this picture less clear, however. Several CTTS and WTTS have revealed large Ne/Fe ratios (≈ 4 or higher), much larger than in MS active solar analogs (Kastner et al., 2004; Argiroffi et al., 2005, 2007; Telleschi et al., 2005, 2007b; Günther et al., 2006) but similar to RS CVn binaries (Audard et al., 2003b). In contrast, the CTTS SU Aur reveals a low Ne/Fe abundance ratio of order unity (Robrade and Schmitt, 2006; Telleschi et al., 2007b), similar to some other massive CTTS (Telleschi et al., 2007b).
Partial clarification of the systematics has been presented by Telleschi et al. (2007b) (see also Güdel et al., 2007b) who found that
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the abundance trends, and in particular the Ne/Fe abundance ratios, do not depend on the accretion status but seem to depend on spectral type or surface Teff, the later-type stars showing a stronger IFIP effect (larger Ne/Fe abundance ratios); see Figure 34.
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the same trend is also seen in disk-less ZAMS stars.
Anomalously high Ne/O abundance ratios remain, however, for TW Hya (Stelzer and Schmitt, 2004) and V4046 Sgr (Günther et al., 2006) when compared to the typical level seen in magnetically active stars, including PMS objects. The initial idea proposed by Drake et al. (2005) was that the selective removal of some elements from the accretion streams should occur only in old accretion disks such as that of TW Hya where cogulation of dust to larger bodies is ongoing, whereas younger T Tauri stars still accrete the entire gas and dust phase of the inner disk. However, the old CTTS MP Mus does not show any anomaly in the Ne/O abundance ratio (Argiroffi et al., 2007). Larger samples are needed for clarification.
The protostellar Sun
Magnetic activity in the protostellar Sun
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 diference 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.
Magnetic flaring of the protostellar Sun
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 p Oph), with important consequences for the irradiation of the stellar environment by high-energy photons and particles (see Section 6.5).
Radio emission from the protostellar Sun
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.
The pre-main sequence Sun’s environment in time
The molecular and dust environment of the very young (PMS) Sun was affected in several ways by ultraviolet radiation, high-energy radiation and high-energy particle streams emitted by the Sun. These often neglected effects have recently attracted considerable attention. Although they apply to the environment of any forming star that is magnetically active, they have been considered in particular for solar-like stars and specifically for the young Sun itself because of various traces that may be observable in the solid bodies of the present solar system. I give a brief overview of the themes in so far as they may relate to the past solar activity.
Circumstellar disk ionization
In terms of physical processes, ionization of the disk is important for the operation of the magneto-centrifugal instability (MRI; Balbus and Hawley 1991) thought to be the main driver of accretion in young stellar objects. Although cosmic rays have long been suspected to be an effective disk ionization source (Gammie, 1996), the high-level, hard coronal emission and frequent stellar flares may be more effective ionizing sources (Glassgold et al., 1997; Feigelson et al., 2002b). This is even more so as young solar analogs in the T Tau stage drive very strong winds that are very likely magnetized; such winds effectively shield the inner disk from cosmic rays, as does the present-day solar wind, at least for cosmic rays with energies < 100 MeV.
The distance to which stellar X-ray ionization dominates over that produced by galactic cosmic rays can be estimated to be (Glassgold et al., 1997; Montmerle, 2001)
$$D \approx 0.02\;{\rm{pc}}{\left({{{\left[ {{E \over {1\;{\rm{keV}}}}} \right]}^{- 2.485}}{{{L_X}} \over {{{10}^{29}}{\rm{erg}}\;{{\rm{s}}^{- 1}}}}{{\left[ {{\zeta \over {{{10}^{- 17}}\;{{\rm{s}}^{- 1}}}}} \right]}^{- 1}}{J_0}} \right)^{1/2}},$$
(22)
where E is the photon energy in the range of 1–20 keV (for other energies, the first term in the parentheses must be generalized to σ(kTX)/σ(1 keV)), ζ is the cosmic ray ionization rate, and J0 is an attenuation factor (J0 ≈ 0. 13 for optical depth of unity for a 1 keV photon). The values given in the parentheses are characteristic for our situation, with the cosmic-ray ionization rate referring to a UV shielded molecular core. Stellar X-ray ionization therefore dominates cosmic ray ionization out to about 1000 AU, i.e., across most of the radius of a typical circumstellar disk. Taking into account frequent, strong flares, significant portions of molecular cores may predominantly be ionized by the central star rather than by cosmic rays.
Glassgold et al. (1997) and Igea and Glassgold (1999) modeled ionization and heating of circum-stellar disks by stellar coronal X-ray sources. The incoming X-ray photons are subject to Compton scattering and photoelectric absorption as they propagate through the disk. X-ray photons may interact with molecules or atoms by ejecting a fast (primary) photoelectron. This photoelectron collisionally produces on average 27 secondary electrons and ions (for a photon energy of 1 keV). Harder photons on average penetrate deeper and thus ionize layers of the disk closer to the equatorial plane, while softer X-rays ionize closer to the disk surface. The disk ionization fraction is then determined when an equilibrium between ionization and recombination has been reached. Electron fractions of 10−15−10−10 are obtained at vertical disk column densities of NH = 1027−1021 cm−2 (as measured from infinity) for distances of 0.1–10 AU from the central star. The precise results depend somewhat on the hardness of the X-ray spectrum (a modest LX = 1029 erg s−1 has been assumed).
The important points here are: 1) that the ionization fraction at the top of the disk is orders of magnitude higher than the ionization fraction that would result from standard cosmic-ray irradiation, and 2) that at vertical column densities of NH = 1024–1025 cm−2 and less, the disk is sufficiently ionized to become unstable against MRI. The disk surface will thus couple to the magnetic field and accrete to the star. In contrast, the deeper layers remain decoupled and therefore “quiescent”, at least within 5 AU (Figure 35, Igea and Glassgold 1999). These are the likely sites of planet formation (Glassgold et al., 1997). Modifications of these calculations by introducing trace heavy metals and diffusion have been discussed by Fromang et al. (2002) and Ilgner and Nelson (2006).
Disk irradiation and photoionization by stellar UV photons is further responsible for photoevaporation of gaseous disks (Hollenbach et al., 1994; Clarke et al., 2001; Alexander et al., 2006a, b), and therefore the long-term accretion history of the star-disk system. Additional X-ray irradiation is, however, of secondary importance only (Alexander et al., 2004).
Circumstellar disk heating
Apart from disk ionization, X-ray irradiation also leads to disk heating (Igea and Glassgold, 1999; Glassgold et al., 2004). While dust disks are heated by the central star’s optical and UV light to a few 100 K at distances up to a few AU, the gas component may thermally decouple in particular in the upper layers where the density is small. A model calculation based on accretion viscosity heating combined with X-ray heating due to the central star shows that the upper layers of the gaseous disk (NH ≲ 1021 cm−2) can be heated up to ≈ 5000 K (Figure 36). This holds even for low viscous heating efficiency where the X-ray heating contribution entirely dominates (Glassgold et al., 2004). At the same time, the strong temperature gradients in the temperature inversion region lead to the production of large amounts of “warm” CO. Similar calculations by Gorti and Hollenbach (2004) support the above picture of X-rays dominating gas heating at the disk surface.
Observational evidence of disk irradiation
The elevated ultraviolet and X-ray activity level of young low-mass stars leads to significant irradiation of circumstellar accretion disks. Interactions between high-energy photons and disk matter is evident from X-ray photoabsorption in star-disk systems seen edge-on, but also from reprocessed starlight: Spatially unresolved FUV fluorescence lines of H2 have been detected from several CTTS (Brown et al., 1981; Valenti et al., 2000; Ardila et al., 2002; Herczeg et al., 2002, 2006), but usually not from WTTS (Valenti et al., 2000); this emission is reprocessed stellar Lyα emission most intensely radiated from the accretion spots. At least in cases where the line radial velocities are coincident with stellar radial velocities, an origin of the fluorescence in a hydrogenic surface layer of the inner accretion disk at temperatures of 2000–3500 K is likely (Herczeg et al., 2002, 2004) although significantly blueshifted H2 emission points to outflow-related fluorescence in some systems (Brown et al., 1981; Ardila et al., 2002; Walter et al., 2003; Saucedo et al., 2003; Herczeg et al., 2006). Fluorescent emission is also generated by reprocessing of X-ray photons; X-ray fluorescence is seen in particular in the 6.4 keV line of cold iron (Imanishi et al., 2001; Tsujimoto et al., 2005; Favata et al., 2005).
Simple energy considerations are revealing: Herczeg et al. (2004) estimate the rate of energy deposited in the environment of the CTTS TW Hya as a result of Lyα photoexcitation and subsequent far-ultraviolet fluorescence of H2 to be 1.4 × 1029 erg s−1. The total soft X-ray luminosity of ≈ 1.4 × 1030 erg s−1 will at least partially heat the disk surface layer further (Igea and Glassgold, 1999).
Warm H2 has also been detected through infrared 2.12 µm ro-vibrational emission from several T Tauri stars (Weintraub et al., 2000; Bary et al., 2003). This emission is thought to be excited by collisions between H2 molecules and X-ray induced non-thermal electrons, or by an UV radiation field. High temperatures, of order 1000–2000 K, are required, but such temperatures are predicted from disk irradiation by X-rays out to several AU (Glassgold et al., 2004), or by UV radiation from the central star out to ≈ 10 AU if the star shows an UV excess (Nomura and Millar, 2005).
Glassgold et al. (2007) proposed forbidden [Ne II and Ne iii] infrared line emission at 12.81 µm and 15.55 µm, respectively, to be indicative of X-ray irradiation. The high first ionization potential of Ne (21.6 eV) indeed requires Ly continuum or X-ray photons for ionization (or cosmic rays, which are unlikely to be abundant in the inner disk region). The transitions are collisionally excited in warm gas, requiring temperatures of a few 1000 K, attained in disk surface layers out to about 20 AU for X-ray irradiated disks (Glassgold et al., 2004). The [Ne II] 12.81 µm transition has indeed been detected in several CTTS (Pascucci et al., 2007; Lahuis et al., 2007; Ratzka et al., 2007).
As a further consequence, specific chemical reactions may be induced. For example, Lyα itself can dissociate molecules like H2 and H2O and can ionize Si and C (Herczeg et al., 2004). Lyα radiation photodissociates HCN (but not CN), which leads to an enhancement of CN relative to HCN (Bergin et al., 2003).
The T Tauri Sun’s activity and meteoritics
The presence of chondrules and isotopic anomalies in chondritic meteorites has posed one of the most outstanding problems in our understanding of the young solar system. Chondrules are millimeter-sized spheres of igneous rock embedded in the meteoritical matrix; their content and structure suggests that they were heated to melting temperatures (≈ 2000 K) of solid iron-magnesium silicates for only an hour or less. They must have cooled in an ambient magnetic field of ≈ 10 G (Shu et al., 2001). Calcium-aluminum-rich inclusions (CAIs) are structures in meteorites that vary in shape; they may derive from melt or partial melts. CAIs contain evidence for short-lived radionuclides in the young solar system, in particular of 26Al, 41Ca, 53Mn, and 60Fe with half-lives of 1.1, 0.15, 5.3, and 2.2 Myr, respectively (Lee et al., 1998; Gounelle et al., 2001).
Conventionally, it has been assumed that these short-lived isotopes were injected by external stellar nucleosynthetic events (ejecta from AGB stars, Wolf-Rayet stars, supernova explosions) that at the same time triggered the collapse of the parent molecular cloud, to form the solar system. The fundamental problem with external seeds is the short time required between the formation and injection of live radionuclides and their incorporation into solid CAI structures; this time span should not exceed 105 yr (Lee et al., 1998), i.e., the trigger for the formation of the solar system must have been extremely fast. Observations of galactic star-formation regions show star-forming molecular cloud cores to be rarely within the immediate environment of Wolf-Rayet wind bubbles or supernova shells (Lee et al., 1998); the association of asymptotic giant branch stars with star-forming regions is also very small, making injection of 60Fe into the solar system from such a star improbable (Kastner and Myers, 1994). Also, the assembly of CAIs and chondrules into planetesimals takes much longer, of order 5 Myr (Lee et al., 1998).
These problems can be removed if the short-lived radionuclides were formed locally, namely by bombardment with “cosmic rays” ejected by stellar flares (Lee et al., 1998). While this alternative for the production of the radionuclides is not unanimously accepted or may not be responsible for all isotopic anomalies in meteorites (e.g., Goswami et al., 2001; Wadhwa et al., 2007), I discuss only this hypothesis here as it is directly related to the (undisputed) high activity level of the young Sun in its T Tauri stage.
There is indeed substantial evidence for an active early Sun not only from inferences from active, young solar analogs (Section 5), but also from large enrichments of spallation-produced 21Ne and 38Ar in “irradiated” meteorite grains (i.e., grains that show radiation damage trails from solar-flare Fe-group nuclei), compared to “non-irradiated” grains (Caffee et al. 1987; a summary of further, earlier, albeit ambiguous evidence can be found in Newkirk Jr 1980). Galactic cosmic-ray irradiation would require exposure times in excess of 108 yr for some of these grains, incompatible with other features of the meteorites (Caffee et al., 1987). Alternatively, energetic solar flare protons could be responsible, but the present-day level would again be insufficient to explain the anomaly. Caffee et al. (1987) concluded that an elevated particle flux, related to the enhanced magnetic activity of the young Sun, naturally explains the meteoritic spallation-produced 21Ne enrichment. A flux several orders of magnitude in excess of present-day values and a harder energy spectrum would be required.
Energetic protons required for the generation of radionuclides could be generated in various places in the extended stellar magnetosphere. In the “x-wind” model proposed by Shu et al. (1997, 2001), magnetic reconnection flares occur at the inner border of the accretion disk where closed stellar magnetic fields and open disk fields converge. Flares would flash-melt protochondrules, and the x-wind would eject them to larger solar distances. Radionuclides would be synthesized by flare proton bombardment.
Alternatively, the elevated activity of the central star itself may be sufficient to produce the required proton flux at planetary distances. Feigelson et al. (2002b) estimated the proton flux at 1 AU of a solar analog in its T Tauri phase, from a statistical X-ray study of T Tauri stars in the Orion Nebula Cluster. They found that frequent flares on T Tauri stars are 101.5 times more luminous than the largest solar flares (or 104 times more than solar flares that occur with a daily frequency). These same flares occur at a rate about 102.5 higher than the rate of the largest solar flares. As solar proton fluxes scale non-linearly with the solar X-ray luminosity, Feigelson et al. (2002b) estimated a proton flux about 105 times higher than at present (i.e., 107 protons cm−2 s−1 at 1 AU). Given the high flare rate, this flux was probably present almost continuously.
Regardless of the location of the proton acceleration (flaring) source, I now summarize the relevant results for various radionuclides. For example, 41 Ca is predominantly produced through
$$^{42}{\rm{Ca}}{({\rm{p}},{\rm{pn}})^{41}}{\rm{Ca}},$$
(23)
$$^{40}{\rm{Ca}}{({\alpha ^3},{\rm{He}})^{41}}{\rm{Ca}},$$
(24)
$$^{40}{\rm{Ca}}{({\alpha ^3},{\rm{He}})^{41}}{\rm{Sc}},$$
(25)
where 41Sc electron-captures to 41Ca (Lee et al., 1998). Note that the abundance ratio for 42Ca/40Ca is 6.7 × 10−3 (Lee et al., 1998, and references therein). Summing all three production channels, an isotopic ratio 41Ca/40Ca as inferred from CAIs requires a proton flux of 5 × 103–104 times the present-day value during an irradiation time scale of 5 × 105–106 yr (Goswami et al., 2001). This approximately matches the observational implications from T Tauri flares by Feigelson et al. (2002b).
Similar considerations for 26Al lead to an underproduction by a factor of 20 under the same conditions (Lee et al., 1998). Goswami et al. (2001) require a proton flux 105 times as strong as the present flux at 1 AU to explain the inferred 26Al abundance in the forming CAIs, and irradiation times of about 1 Myr; this is in excellent agreement with the observational inferences made by Feigelson et al. (2002b). However, 3He bombardment of 24Mg may efficiently produce 26Al as well. 3He is preferentially accelerated in solar impulsive (as opposed to gradual) flares (see discussion in Lee et al. 1998 and references therein). The problem then arises that 41Ca is overproduced by two orders of magnitude through reactions involving 3He. Shu et al. (1997, 2001) therefore proposed that CAIs consisting of refractory, Ca-Al-rich material are surrounded by thick mantles of less refractory, Mg-rich material. 3He nuclei would therefore be stopped in the outer mantle where 26Al is produced from 24Mg, while the 40Ca-rich interior remains less affected, i.e., 41Ca production is suppressed. Canonical isotopic ratios can then indeed be derived for most of the species of interest (Gounelle et al., 2001).
The most promising support for local irradiation by solar (or possibly, trapped cosmic ray) protons has been the discovery of 10Be (McKeegan et al. 2000; half-life of 1.5 Myr) and possibly also the extremely short-lived 7Be (Chaussidon et al. 2006; half-life of 53 d). The 10Be isotope could be entirely produced by solar protons and 4He nuclei at asteroidal distances (Gounelle et al., 2001; Marhas and Goswami, 2004) while it is destroyed in the alternative nucleosynthetic production sources such as massive stars or supernova explosions. If the presence of 7Be in young meteorites can be confirmed, then its short half-life precludes an origin outside the solar system altogether and requires a local irradiation source.
Despite the successful modeling of radionuclide anomalies in early CAIs, at least the case of 60Fe remains unsolved in this context. It is difficult to synthesize by cosmic-ray reactions; the production rate falls short of rates inferred from observations by two orders of magnitude (Lee et al., 1998; Goswami et al., 2001) and requires stellar nucleosynthesis or, most likely, a supernova event (Meyer and Clayton, 2000).
Although the formation of radionuclides in early meteorites is under debate (Goswami et al., 2001) and may require several different production mechanisms (Wadhwa et al. 2007; for example, to explain the simultaneous presence of the 60Fe and the 10Be isotopes), the above models are at least promising in explaining some nuclear processing of solar-system material without external irradiation source but with sources whose presence cannot be disputed, namely high-energy particle populations that are a direct consequence of the magnetic activity of the young Sun. Isotopic anomalies in meteorites have opened a window to the violent environment of the young solar system.
Summary: The violent pre-main sequence Sun
Both the T Tauri and the protostellar Sun were extremely magnetically active, as far as we can tell from observations of contemporaneous objects at these stages. There is no definitive evidence for a turn-on of magnetic activity (for a contrasting view, see Linsky et al., 2007), but the stellar environment (gas and dust disks and envelopes) makes observations challenging. On the other hand, perhaps the most interesting aspect of magnetic activity in this phase is indeed its influence on the environment itself. High X-ray and UV fluxes produced both by magnetic activity and magnetically funneled accretion flows onto the star heat and ionize the circumstellar disk, thus controlling mechanisms as diverse as gas-disk photoevaporation, accretion through the mag-netorotational instability, and chemical networks across the disk. Strong flaring and observations of non-thermal gyrosynchrotron emission in T Tauri and protostars further indicate the presence of strongly elevated particle fluxes in the pre-main sequence Sun’s environment. The impact on solid-state matter forming in the circumstellar disk may be visible to the present day, in the form of daughter products of short-lived radioactive isotopes formed by proton impact. The study of the young Sun’s environment under the aspect of magnetic activity of the central star is still in its early stage and rapidly developing.