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Register a new userAstronomical Oxygen Isotopic Evidence for Supernova Enrichment of the Solar System Birth Environment by Propagating Star Formation
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Edward D. Young
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New infrared absorption measurements of oxygen isotope ratios in CO gas from individual young stellar objects confirm that the solar system is anomalously high in its 18O/17O ratio compared with extra-solar oxygen in the Galaxy. We show that this difference in oxygen isotope ratios is best explained by 1 per cent enrichment of the proto-solar molecular cloud by ejecta from type II supernovae from a cluster having of order a few hundred stars that predated the Sun by at least 10 to 20 Myr. The likely source of exogenous oxygen was the explosion of one or more B stars during a process of propagating star formation.
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The relative abundances of the radionuclides in the solar system at the time of its birth are crucial arbiters for competing hypotheses regarding the birth environment of the Sun. The presence of short-lived radionuclides, as evidenced by their decay products in meteorites, has been used to suggest that particular, sometimes exotic, stellar sources were proximal to the Suns birth environment. The recent confirmation of neutron star - neutron star (NS-NS) mergers and associated kilonovae as potentially dominant sources of r-process nuclides can be tested in the case of the solar birth environment using the relative abundances of the longer-lived nuclides. Critical analysis of the 15 radionuclides and their stable partners for which abundances and production ratios are well known suggests that the Sun formed in a typical massive star-forming region (SFR). The apparent overabundances of short-lived radionuclides (e.g., $^{26} {rm Al}$, $^{41}{rm Ca}$, $^{36}{rm Cl}$) in the early solar system appears to be an artifact of a heretofore under-appreciation for the important influences of enrichment by Wolf-Rayet winds in SFRs. The long-lived nuclides (e.g., $^{238}{rm U}$, $^{244}{rm Pu}$, $^{247}{rm Cr}$, $^{129}{rm I}$) are consistent with an average time interval between production events of $10^8$ years, seemingly too short to be the products of NS-NS mergers alone. The relative abundances of all of these nuclides can be explained by their mean decay lifetimes and an average residence time in the ISM of $sim200$ Myr. This residence time evidenced by the radionuclides is consistent with the average lifetime of dust in the ISM and the timescale for converting molecular cloud mass to stars.
We investigate the enrichment of the pre-solar cloud core with short lived radionuclides (SLRs), especially 26Al. The homogeneity and the surprisingly small spread in the ratio 26Al/27Al observed in the overwhelming majority of calcium-aluminium-rich inclusions (CAIs) in a vast variety of primitive chondritic meteorites places strong constraints on the formation of the the solar system. Freshly synthesized radioactive 26Al has to be included and well mixed within 20kyr. After discussing various scenarios including X-winds, AGB stars and Wolf-Rayet stars, we come to the conclusion that triggering the collapse of a cold cloud core by a nearby supernova is the most promising scenario. We then narrow down the vast parameter space by considering the pre-explosion survivability of such a clump as well as the cross-section necessary for sufficient enrichment. We employ numerical simulations to address the mixing of the radioactively enriched SN gas with the pre-existing gas and the forced collapse within 20kyr. We show that a cold clump of 10Msun at a distance of 5pc can be sufficiently enriched in 26Al and triggered into collapse fast enough - within 18kyr after encountering the supernova shock - for a range of different metallicities and progenitor masses, even if the enriched material is assumed to be distributed homogeneously in the entire supernova bubble. In summary, we envision an environment for the birth place of the Solar System 4.567Gyr ago similar to the situation of the pillars in M16 nowadays, where molecular cloud cores adjacent to an HII region will be hit by a supernova explosion in the future. We show that the triggered collapse and formation of the Solar System as well as the required enrichment with radioactive 26Al are possible in this scenario.
Heating by short-lived radioisotopes (SLRs) such as aluminum-26 and iron-60 fundamentally shaped the thermal history and interior structure of Solar System planetesimals during the early stages of planetary formation. The subsequent thermo-mechanical evolution, such as internal differentiation or rapid volatile degassing, yields important implications for the final structure, composition and evolution of terrestrial planets. SLR-driven heating in the Solar System is sensitive to the absolute abundance and homogeneity of SLRs within the protoplanetary disk present during the condensation of the first solids. In order to explain the diverse compositions found for extrasolar planets, it is important to understand the distribution of SLRs in active planet formation regions (star clusters) during their first few Myr of evolution. By constraining the range of possible effects, we show how the imprint of SLRs can be extrapolated to exoplanetary systems and derive statistical predictions for the distribution of aluminum-26 and iron-60 based on N-body simulations of typical to large clusters (1000-10000 stars) with a range of initial conditions. We quantify the pollution of protoplanetary disks by supernova ejecta and show that the likelihood of enrichment levels similar to or higher than the Solar System can vary considerably, depending on the cluster morphology. Furthermore, many enriched systems show an excess in radiogenic heating compared to Solar System levels, which implies that the formation and evolution of planetesimals could vary significantly depending on the birth environment of their host stars.
Earth and Moon are shown here to be composed of oxygen isotope reservoirs that are indistinguishable, with a difference in {Delta}17O of -1 +/- 5ppm (2se). Based on these data and our new planet formation simulations that include a realistic model for oxygen isotopic reservoirs, our results favor vigorous mixing during the giant impact and therefore a high-energy high- angular-momentum impact. The results indicate that the late veneer impactors had an average {Delta}17O within approximately 1 per mil of the terrestrial value, suggesting that these impactors were water rich.
The Solar system was once rich in the short-lived radionuclide (SLR) $^{26}$Al, but deprived in $^{60}$Fe. Several models have been proposed to explain these anomalous abundances in SLRs, but none has been set within a self-consistent framework of the evolution of the Solar system and its birth environment. The anomalous abundance in $^{26}$Al may have originated from the accreted material in the wind of a massive $apgt 20$,$M_odot$ Wolf-Rayet star, but the star could also have been a member of the parental star-cluster instead of an interloper or an older generation that enriched the proto-solar nebula. The protoplanetary disk at that time was already truncated around the Kuiper-cliff (at $45$ au) by encounters with another cluster members before it was enriched by the wind of the nearby Wolf-Rayet star. The supernova explosion of a nearby star, possibly but not necessarily the exploding Wolf-Rayet star, heated the disk to $apgt 1500$K, melting small dust grains and causing the encapsulation and preservation of $^{26}$Al into vitreous droplets. This supernova, and possibly several others, caused a further abrasion of the disk and led to its observed tilt of $5.6pm1.2^circ$ with respect to the Suns equatorial plane. The abundance of $^{60}$Fe originates from a supernova shell, but its preservation results from a subsequent supernova. At least two supernovae are needed (one to deliver $^{60}$Fe, and one to preserve it in the disk) to explain the observed characteristics of the Solar system. The most probable birth cluster then has $N = 2500pm300$ stars and a radius of $r_{rm vir} = 0.75pm0.25$ pc. We conclude that Solar systems equivalent systems form in the Milky Way Galaxy at a rate of about 30 per Myr, in which case approximately 36,000 Solar system analogues roam the Milky Way.
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