Accounting for Short-Lived Radionuclides in the Early Solar System in the Context of a Triggered Star Formation Origin of the Solar System

Abstract:

The early Solar System (ESS) was characterized by a value of ²⁶Al that exceeded the Galactic average, while ⁶⁰Fe in the ESS was about an order of magnitude less than the Galactic value. These results dispute the existence of a supernova near the solar system at the time of formation. An alternative source of ²⁶Al is Wolf-Rayet (W-R) stars. Aluminium-26 is produced in massive stars in the main-sequence phase, and released in the wind in the later W-R phase. No ⁶⁰Fe is ejected in the wind. Dwarkadas et al. (2017, ApJ, 851, 147) showed that a single W-R star above about 50 solar mass could be sufficient to provide the measured amount of ²⁶Al in the ESS. W-R stars have strong winds that sweep up the surrounding medium to form low density wind bubbles bordered by a dense shell. The ²⁶Al is carried out by dust grains in the wind from the star to the dense shell, where it is released. The solar system is subsequently formed by triggered star formation in the dense shell, due to the combined actions of the wind, shock fronts and the ionization front due to ionizing photons from the hot star. Besides ²⁶Al and ⁶⁰Fe, many other short-lived radionuclides (SLRs) were present in the ESS, including ¹⁰Be, ³⁶Cl, ⁴¹Ca, ⁵³Mn, ¹⁰⁷Pd, ¹²⁹I, and ¹⁸²Hf. Herein we investigate whether this triggered star formation model can account for the abundance of these other SLRs. These SLRs could (1) be produced in the star, carried out in the wind (via dust grains) and injected into the dense shell, (2) be already prevalent in the swept-up dense shell which was formed by sweeping up of the surrounding medium by the supersonic winds, or (3) be due to irradiation, from the supernova following the collapse of the W-R star (if a SN explosion occurs), from the early Sun, or via cosmic-rays. We explore the abundance of each SLR, and show that it can be accounted for using one of the 3 modes described above. ⁴¹Ca and ¹⁰⁷Pd, being refractory elements, would be transported via dust grains, similar to ²⁶Al in our model. Others would either be already present in the dense shell or can be formed via irradiation. We use abundance information from stellar evolution models calculated by various groups that are available in the literature. We also include abundances obtained using our own models of rotating and non-rotating massive stars, calculated with the publicly available MESA code.