2008 Annual Science Report
University of Hawaii, Manoa Reporting | JUL 2007 – JUN 2008
Assessing the Likelihood of Supernova Impact of Protoplanetary Disks
This project is investigating the origin, abundance, and distribution of short-lived radioisotopes in the early Solar System, and other planetary systems, possibly from massive stars. Specific focus is on Al-26 and Fe-60 as they played an important role in the early thermal evolution of planetesimals.
We evaluated four scenarios for the incorporation of the shortlived radionu-clide 26 Al from massive stars into circumstellar solids (represented in the Solar System by refractory inclusion in primitive meteorites). Injection into a gaseous disk during Class II protostellar evolution is highly unlikely (< 1%) to produce the canonical early Solar System abundance of 26Al. Models of scenarios in which massive stars pollute the remaining gas of the embedded cluster or a host giant molecular cloud with radionuclides, and hence laterforming stars, can generate abundances closer to the Solar System value if the time delay between star formation and solid condensation is a million years or less. Inclusion of BondiHoyle accretion increases the average abundance of Al in disks if star formation is simultaneous, but does not affect the estimated frequency of systems with an 26Al abundance like the early Solar System. Our models predict an initial abundance of exogenous 26Al in planetary systems from 0 to 26Al/ 27 Al ? 1E?4 . The ma jority, of systems receive no 26Al from massive stars because they formed before the most massive stars evolved away from the main sequence. A major uncertainty in all the models is the formation and transport of 26Albearing dust grains in winds or SN ejecta. Even if the primary mechanism of 26 Al production is, instead, irradiation by energetic particles from the central star, this source will also vary between stars by about two orders of magnitude. Planetesimals in disks with different abundances of 26Al will experience different thermal histories. If the position of the boundary between dehydrated and hydrated bodies, located at 2.5 AU in the Main Asteroid Belt, is set by the relative rates of accretion and the decay of 26Al, the location of this “waterline” will depend on the initial abundance of 26Al and affect the amount of water accreted by terrestrial planets in the habitable zone.