[P-CG19] Origin and evolution of materials in space

[PCG19-03] The birth environment and current location of the Solar system in the Galactic disc, using Milky-Way simulations and radioactive isotopes

*Yusuke Fujimoto¹ (1.Earth and Planets Laboratory, Carnegie Institution for Science)

Keywords:Galactic disc, meteorites, gamma-rays, hydrodynamics, massive stars, short-lived radioactive isotope

Formation mechanisms of the Solar system and exoplanetary systems that harbor life have been one of the hottest topics. The birth environment of the early Solar system 4.6 Gyr ago has been investigated using meteorites, which contain the oldest solids formed in the Solar protoplanetary disc. Materials from near-Earth asteroids by recent sample-return projects such as JAXA's Hayabusa-2 and NASA's OSIRIS-REx will give us more information about the Solar birth environment. At the same time, many nearby exoplanets have been discovered so far, and after the first observation of a protoplanetary disc of the exoplanetary system HL Tau, many observational and theoretical works have attempted to understand planet formation in the Solar neighborhood. However, we need to be aware of the fundamental issue of a 4.6 Gyr gap between the Solar system formation and the present-day planet formation. It is that the Solar birth environment could be quite different from the current local environment. For example, if the Solar system formed closer to the center of the Galaxy than it is today, the primordial gas of the Solar system was easily abundant in metals because it was a place where star formation and subsequent supernova (SN) explosions, which enriched the interstellar medium (ISM), were active (then the metals played an important role in such as a radiative cooling and chemical reaction of the ISM). Moreover, the morphology of the Galaxy should have changed for 4.6 Gyr. Therefore, it should be noted that the formation process of the Solar system discussed using meteorites and asteroids can be different in space and time from the planet formation in the vicinity of the present-day Solar system. To bridge the gap, I am investigating the evolution of the Galaxy for 4.6 Gyr and trajectories of the Solar system and other planetary systems from their birthplace to the current positions within the evolving Galactic disc.
In this presentation, I will show chemo-hydrodynamical simulations of the entire Milky Way galaxy, including ejections of short-lived radioisotopes (SLRs) such as 26Al and 60Fe from stellar winds and SNe. To understand the birthplace of the early Solar system and its environment in the Galactic disc, we measure the distribution of SLR ratios over star particles in the simulated galaxy and compare them with the initial abundance ratios of SLRs in the early Solar system observed in meteorites. We find that the Solar initial abundance ratios are well in the normal range and that the SLRs are abundant in newborn stars because star formation is correlated on galactic scales so that ejecta preferentially enrich atomic gas that will subsequently be accreted onto existing giant molecular clouds or will form new ones. We conclude that in terms of 26Al and 60Fe the birth environment of the Solar system is not atypical in the Galaxy (Fujimoto, Krumholz & Tachibana 2018, MNRAS, 480, 4025).
In addition, to understand the Solar local interstellar environment in the present-day Galaxy, we perform another galaxy simulation that represents the current state of the Milky Way, and compare it with astronomical and geological observations: galactic-scale distributions of 26Al seen in all-sky gamma-ray maps and detection of live 60Fe in deep-sea archives and Antarctic snow. We find that the current location of the Solar system in the Galactic disc is near the edges of spiral arms and lie inside kpc-scale bubbles that are created by multiple generations of star formation in the arm (Fujimoto, Krumholz, Inutsuka, Boss & Nittler 2020, MNRAS, 498, 5532).