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Register a new userThe Supernova Triggered Formation and Enrichment of Our Solar System
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Added by
Matthias Gritschneder Dr
Publication date
2011
fields
Physics
and research's language is
English
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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.
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About 4.6 billion years ago, some event disturbed a cloud of gas and dust, triggering the gravitational collapse that led to the formation of the solar system. A core-collapse supernova, whose shock wave is capable of compressing such a cloud, is an obvious candidate for the initiating event. This hypothesis can be tested because supernovae also produce telltale patterns of short-lived radionuclides, which would be preserved today as isotopic anomalies. Previous studies of the forensic evidence have been inconclusive, finding a pattern of isotopes differing from that produced in conventional supernova models. Here we argue that these difficulties either do not arise or are mitigated if the initiating supernova was a special type, low in mass and explosion energy. Key to our conclusion is the demonstration that short-lived Be-10 can be readily synthesized in such supernovae by neutrino interactions, while anomalies in stable isotopes are suppressed.
If the Sun was born in a relatively compact open cluster, it is quite likely that a massive (10MSun) star was nearby when it exploded in a supernova. The repercussions of a supernova can be rather profound, and the current Solar System may still bear the memory of this traumatic event. The truncation of the Kuiper belt and the tilt of the ecliptic plane with respect to the Suns rotation axis could be such signatures. We simulated the effect of a nearby supernova on the young Solar System using the Astronomical Multipurpose Software Environment. Our calculations are realized in two subsequent steps in which we study the effect of the supernova irradiation on the circumstellar disk and the effect of the impact of the nuclear blast-wave which arrives a few decades later. We find that the blastwave of our adopted supernova exploding at a distance of $0.15$--$0.40$,pc and at an angle of $35^circ$--$65^circ$ with respect to the angular-momentum axis of the circumsolar disk would induce a misalignment between the Suns equator and its disk to $5^circ.6pm1^circ.2$, consistent with the current value. The blast of a supernova truncates the disk at a radius between $42$ and $55$,au, which is consistent with the current edge of the Kuiper belt. For the most favored parameters, the irradiation by the supernova as well as the blast wave heat the majority of the disk to $sim 1200$,K, which is sufficiently hot to melt chondrules in the circumstellar disk. The majority of planetary system may have been affected by a nearby supernova, some of its repercussions, such as truncation and tilting of the disk, may still be visible in their current planetary systems topology. The amount of material from the supernova blast wave that is accreted by the circumstellar disk is too small by several orders of magnitude to explain the current abundance of the short live radionuclide $^{26}$Al.
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.
Representative abundances of the chemical elements for use as a solar abundance standard in astronomical and planetary studies are summarized. Updated abundance tables for solar system abundances based on meteorites and photospheric measurements are presented.
A critical constraint on solar system formation is the high $^{26}$Al/$^{27}$Al abundance ratio of 5 $times 10^{-5}$ at the time of formation, which was about 17 times higher than the average Galactic ratio, while the $^{60}$Fe/$^{56}$Fe value was about $2 times 10^{-8}$, lower than the Galactic value. This challenges the assumption that a nearby supernova was responsible for the injection of these short-lived radionuclides into the early solar system. We show that this conundrum can be resolved if the Solar System was formed by triggered star formation at the edge of a Wolf-Rayet (W-R) bubble. Aluminium-26 is produced during the evolution of the massive star, released in the wind during the W-R phase, and condenses into dust grains that are seen around W-R stars. The dust grains survive passage through the reverse shock and the low density shocked wind, reach the dense shell swept-up by the bubble, detach from the decelerated wind and are injected into the shell. Some portions of this shell subsequently collapses to form the dense cores that give rise to solar-type systems. The subsequent aspherical supernova does not inject appreciable amounts of $^{60}$Fe into the proto-solar-system, thus accounting for the observed low abundance of $^{60}$Fe. We discuss the details of various processes within the model and conclude that it is a viable model that can explain the initial abundances of $^{26}$Al and $^{60}$Fe. We estimate that 1-16% of all Sun-like stars could have formed in such a setting of triggered star formation in the shell of a WR bubble.
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