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Project Progress

Project Summary

Nickolas Moeckel who was supported by the CU Astrobiology program completed his PhD in April 2008. He is currently a post-doc at the University of St. Andrews in Scotland. His PhD thesis, “Massive Stars, Disks, and Clustered Star Formation” demonstrated how disks surrounding massive stars make formation of binaries and multiples by dynamical capture a likely process that operated in dense star clusters.

Henry Throop (SWRI; throop@swri.boulder.swri.edu ) and I have continues our collaboration to explore the effect of clustered environments on the process of planet formation. Most solar mass stars are formed in clusters. Thus, the radiation and dynamical environment of star clusters may be the most common environment in which planetary systems form.

Project Progress

Stochastic UV photo-ablation of disk.s

Our numerical clusters models were used to show that the UV radiation exposure of a typical low-mass star hi highly variable and stochastic. The orbital motions of typical stars are followed. For most stars, their orbits bring them into and out of the cluster core, resulting in a several order of magnitude variation in the UV flux. Most irradiation tends to occur in relatively short time-intervals while the star plunges close to the clusters’ massive members. The UV dosage during these brief encounters can be several orders of magnitude higher than average. Thus, most photo-processing of proto-planetary disks occur during a few percent of the lifetime of the disk.

As the massive stars age and evolve into super-giants and explode as supernovae, their soft-UV radiation fields can increase by orders of magnitude. A typical planetary systems can experience a two orders of magnitude increase in UV dosage for a few hundred thousand years while massive stars evolve off the main sequence. The supernova explosion can produce about 10% of the UV and X-ray radiation generated during the entire 3 to 30 Myr main-sequence lifetime of a massive star. all of this radiation emerges in a pulse lasting less than one year and therefore provides a 10⁵ to 10⁷ fold increase in UV flux.

Late-Phase Bondi-Hoyle Accretion: Formation of Gas Giant and accumulation of short-lived radioisotopes.

A second consequence of planet formation in clusters arises from the feature that the cluster formation time-scale is about an order-of-magnitude longer than the time required to form an individual star; 3 to 10 Myr vs. 0.1 to 1 Myr. The orbit time in the cluster potential is usually about 0.1 Myr, comparable to the star-formation time-scale. Thus, first-to-form-stars can re-enter surviging portions of the cluster-forming cloud millions of years after the stars formed. Such planetary disks can accrete additional material from reservoirs that are chemically distinct from their birth clouds. If the cloud was polluted by a massive star wind, such secondary accretion can introduce short-live radio-active species such as ⁶⁰Fe and ²⁶Al and bring-in fresh hydrogen well after the formation of rocky-planet cores that and removal of the initial supply of hydrogen by photo-ablation.