NASA Astrobiology Institute (NAI)

1. The delivery of short-lived radionucleides to the solar system

   Project Investigators: Eric Gaidos

   Other Project Members

   Jonathan Williams (Collaborator)

   Summary

   I have studied various astrophysical scenarios for the delivery of short-lived radionucleides to star forming cores and planet forming disks in order to explain the observed abundances of 26-Al and 60-Fe in primitive meteorites. The latter, in particular, implies the birth of our solar system closely followed the death of a massive star. It is hard to reconcile the astrophysical and cosmochemical pictures but the most likely birth environment of our Sun was in a giant molecular cloud that formed several generations of stars.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of formation and evolution of habitable planets
   • Objective 4.3: Effects of extraterrestrial events upon the biosphere

   Project Progress

   The discovery that the short-lived radionucleide, 60-Fe, was present in the oldest meteorites suggests that the formation of the Earth closely followed the death of a massive star. I have been studying different mechanisms for the delivery of 60-Fe and other short-lived radionucleides to the solar system. This work resulted in one paper published in the Astrophysical Journal (Williams & Gaidos 2007) and I was invited to lecture on the subject at a winter school on planet formation in France in early 2008. The lecture is available on the web at http://www-laog.obs.ujf-grenoble.fr/heberges/Houches08/index.htm and has been written up for publication in the workshop proceedings. I conclude that there are few ways to reconcile the astrophysical and cosmochemical pictures but the most likely is that the Sun formed in a large cluster of stars in a massive molecular clump (about 10,000 solar masses and several parsecs in size) on or near the boundary of an HII region. An example of such a situation is in the Rosette molecular cloud where I find more than half the protostars lie close enough to the most massive stars that they could incorporate supernova ejecta were it to explode at the current time (see figure). The delivery of 60-Fe is also likely accompanied by the delivery of 26-Al, a more abundant short-lived radionucleide that has a significant effect on the thermal history of planetesimals and potentially the snowline in a proto-planetary disk. Eric Gaidos is leading the effort on this related subject.

   
   

   Gas and stars in the Rosette. The red color is an image of the ionized gas emission powered by the radiation from high mass stars. These supernova progenitors are shown as white star symbols. Young stars with ages less than 3 Myr are shown as white dots. The blue and green show the distribution of CO and 13CO respectively and reveal a giant molecular cloud. Individual regions that are both close enough to a massive star and dense enough to collapse rapidly to deliver large quantities of 60-Fe to planet forming material is shown by the white circles. This demonstrates that more than 50% of the young stars in the cloud could potentially be contaminated by supernova ejecta.

   Mission Involvement

   Herschel Space Observatory

   I am the NASA PI on a Herschel Open Time Key Program (OTKP) to measure the gas content in protostellar disks via far-infrared spectroscopy. This project will help constrain the timescale for planet formation, a critical parameter in understanding the delivery of short-lived radionucleides to the solar system.

Publications

Williams, J.P. & Gaidos, E.  (2007).  On the likelihood of supernova enrichment of protoplanetary disks.  Astrophysical Journal, 663(1):L33-L36  [Online].