* Astronomy

Title: Was The Sun Born In A Massive Cluster?
Authors: Donald Dukes, Mark R. Krumholz

A number of authors have argued that the Sun must have been born in a cluster of no more than about 1000 stars, on the basis that, in a larger cluster, close encounters between the Sun and other stars would have truncated the outer Solar System or excited the outer planets into eccentric orbits. However, this dynamical limit is in tension with meteoritic evidence that the Solar System was exposed to a nearby supernova during or shortly after its formation; a 1000-star cluster is much too small for supernova contamination to be likely. In this paper we revisit the dynamical limit in the light of improved observations of the properties of young clusters. We use a series of scattering simulations to measure the velocity-dependent cross-section for disruption of the outer Solar System by stellar encounters, and use this cross-section to compute the probability of a disruptive encounter as a function of birth cluster properties. We find that, contrary to prior work, the probability of disruption is small regardless of the cluster mass, and that it actually decreases rather than increases with cluster mass. Our results differ from prior work for three main reasons: (1) unlike in most previous work, we compute a velocity-dependent cross section and properly integrate over the cluster mass-dependent velocity distribution of incoming stars; (2) we adopt realistically-short cluster lifetimes of a few crossing times, rather than assuming lifetimes of 10 to 100 Myr; and (3) following recent observations, we adopt a mass-independent surface density for embedded clusters, rather than a mass-independent radius as assumed many earlier papers. Our results remove the tension between the dynamical limit and the meteoritic evidence, and suggest that the Sun was born in a massive cluster.

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Title: The Supernova Triggered Formation and Enrichment of Our Solar System
Authors: M. Gritschneder, D. N. C. Lin, S. D. Murray, Q.-Z. Yin, M.-N. Gong

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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Title: Searching for Possible Siblings of the Sun from a Common Cluster based on Stellar Space Velocities
Authors: V.V. Bobylev, A.T. Bajkova, A. Myllari, M. Valtonen

We propose a kinematic approach to searching for the stars that could be formed with the Sun in a common "parent" open cluster. The approach consists in preselecting suitable candidates by the closeness of their space velocities to the solar velocity and analysing the parameters of their encounters with the solar orbit in the past in a time interval comparable to the lifetime of stars. We consider stars from the Hipparcos catalogue with available radial velocities. The Galactic orbits of stars have been constructed in the Allen--Santillan potential by taking into account the perturbations from the spiral density wave. We show that two stars, HIP 87382 and HIP 47399, are of considerable interest in our problem. Their orbits oscillate near the solar orbit with an amplitude of about 250 pc; there are short-term close encounters to distances <10 pc. Both stars have an evolutionary status and metallicity similar to the solar ones.

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By analysing the isotopic compositions of several elements, scientists are able to glean clues about the origin of stardust grains in the atmospheres of dying red giant stars or in exploding supernovae.
When a doctoral student heated a primitive meteorite in 1972 and it gave off pure neon-22, scientists were stunned. No known solar system process would produce this isotope without its sister isotopes neon-20 and neon-21. Gradually, they came to suspect that some part of the meteorite had come from outside of the solar system.
But which part? To find out, a group at the University of Chicago starting dissolving meteorites in strong acids - a process their leader Edward Anders, PhD, characterised as "burning down the haystack to find the needle."
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