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, 41Ca is predominantly produced through
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.
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