The formation of solar-system analogs in young star clusters

Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: spz@strw.leidenuniv.nl

Received: 27 July 2018
Accepted: 23 November 2018

Abstract

The solar system was once rich in the short-lived radionuclide (SLR) ²⁶Al but poor in ⁶⁰Fe. Several models have been proposed to explain these anomalous abundances in SLRs, but none has been set within a self-consistent framework of the evolution of the solar system and its birth environment. The anomalous abundance in ²⁶Al may have originated from the accreted material in the wind of a massive ≳20 M_⊙ Wolf-Rayet star, but the star could also have been a member of the parental star-cluster instead of an interloper or an older generation that enriched the proto-solar nebula. The protoplanetary disk at that time was already truncated around the Kuiper-cliff (at 45 au) by encounters with other cluster members before it was enriched by the wind of the nearby Wolf-Rayet star. The supernova explosion of a nearby star, possibly but not necessarily the exploding Wolf-Rayet star, heated the disk to ≳1500 K, melting small dust grains and causing the encapsulation and preservation of ²⁶Al in vitreous droplets. This supernova, and possibly several others, caused a further abrasion of the disk and led to its observed tilt of 5.6 ± 1.2° with respect to the equatorial plane of the Sun. The abundance of ⁶⁰Fe originates from a supernova shell, but its preservation results from a subsequent supernova. At least two supernovae are needed (one to deliver ⁶⁰Fe and one to preserve it in the disk) to explain the observed characteristics of the solar system. The most probable birth cluster therefore has N = 2500 ± 300 stars and a radius of r_vir = 0.75 ± 0.25 pc. We conclude that systems equivalent to our solar system form in the Milky Way Galaxy at a rate of about 30 Myr⁻¹, in which case approximately 36 000 solar-system analogs roam the Milky Way.