The abundance of ²⁶Al-rich planetary systems in the Galaxy

¹ IMPMC, Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, UPMC & IRD, 57 rue Cuvier, 75005 Paris, France
e-mail: gounelle@mnhn.fr
² Institut Universitaire de France, 103 boulevard Saint-Michel, 75005 Paris, France

Received: 24 March 2015
Accepted: 17 June 2015

Abstract

One of the most puzzling properties of the solar system is the high abundance at its birth of ²⁶Al, a short-lived radionuclide with a mean life of 1 Myr. Now decayed, it has left its imprint in primitive meteoritic solids. The origin of ²⁶Al in the early solar system has been debated for decades and strongly constrains the astrophysical context of the Sun and planets formation. We show that, according to the present understanding of star-formation mechanisms, it is very unlikely that a nearby supernova has delivered ²⁶Al into the nascent solar system. A more promising model is the one whereby the Sun formed in a wind-enriched, ²⁶Al-rich dense shell surrounding a massive star (M > 32 M_⊙). We calculate that the probability of any given star in the Galaxy being born in such a setting, corresponding to a well-known mode of star formation, is of the order of 1%. It means that our solar system, though not the rule, is relatively common and that many exo-planetary systems in the Galaxy might exhibit comparable enrichments in ²⁶Al. Such enrichments played an important role in the early evolution of planets because ²⁶Al is the main heat source for planetary embryos.