Theoretical Distributions of Short-Lived Radionuclides for Star Formation in Molecular Clouds

Abstract

Short-lived radioactive nulcei (half-life τ 1/2 ∼ 1 Myr) influence the formation of stars and planetary systems by providing sources of heating and ionization. Whereas many previous studies have focused on the possible nuclear enrichment of our own Solar System, the goal of this paper is to estimate the distributions of short-lived radionuclides (SLRs) for the entire population of stars forming within a molecular cloud. Here we focus on the nuclear species 60 Fe and 26 Al, which have the largest impact due to their relatively high abundances. We construct molecular cloud models and include nuclear contributions from both supernovae and stellar winds. The resulting distributions of SLRs are time dependent with widths of ∼ 3 orders of magnitude and mass fractions ρ SLR /ρ * ∼ 10 −11 − 10 −8 . Over the range of scenarios explored herein, the SLR distributions show only modest variations with the choice of cloud structure (fractal dimension), star formation history, and cluster distribution. The most important variation arises from the diffusion length scale for the transport of SLRs within the cloud. The expected SLR distributions are wide enough to include values inferred for the abundances in our Solar System, although most of the stars are predicted to have smaller enrichment levels. In addition, the ratio of 60 Fe/ 26 Al is predicted to be greater than unity, on average, in contrast to Solar System results. One explanation for this finding is the presence of an additional source for the 26 Al isotope.

Rapid Formation of Exoplanetesimals Revealed by White Dwarfs

“…33] featuring Solar System-like abundances. Most works, however, suggest that few systems have significantly higher abundances of 26 Al than the Solar System [34][35][36][37], which is supported by observational evidence from individual star-forming regions [38,39]. Depending on when planetesimal formation occurs, this means that for some exoplanetary systems, with high initial budget of short-lived radioactive nuclides, a large fraction of planetesimals will form an iron core.…”

“…A size distribution of n(D)dD ∝ D −3.5 dD is assumed, and shown are a range of 26 Al budgets, above and below Solar System levels ( 26 Al SS = 5.25 × 10 −5 27 Al SS ) . Few planetary systems have abundances significantly above solar [31,[34][35][36][37][38][39].…”