Constraints on stellar rotation from the evolution of Sr and Ba in the Galactic halo

TL;DR

This study investigates how the rotation of massive stars influences the production of Sr and Ba in the Galactic halo, using a stochastic model to match observations and constrain stellar rotation behaviors across different metallicities.

Contribution

It introduces new methods for model-data comparison in chemical evolution, constrains the metallicity dependence of stellar rotation, and demonstrates the importance of rotation in s-process nucleosynthesis.

Findings

Rotation velocity decreases with increasing metallicity.

The model successfully reproduces observed Sr and Ba abundances.

Metallicity-dependent rotation is essential for explaining s-process yields.

Abstract

Recent studies show that the chemical evolution of Sr and Ba in the Galaxy can be explained if different production sites, hosting r- and s-processes, are taken into account. However, the question of unambiguously identifying these sites is still unsolved. Massive stars are shown to play an important role in the production of s-material if rotation is considered. In this work, we study in detail the contribution of rotating massive stars to the production of Sr and Ba, in order to explain their chemical evolution, but also to constrain the rotational behaviour of massive stars. A stochastic chemical evolution model was employed to reproduce the enrichment of the Galactic halo. We developed new methods for model-data comparison which help to objectively compare the stochastic results to the observations. We employed these methods to estimate the value of free parameters which describe the rotation of massive stars, assumed to be dependent on the stellar metallicity. We constrain the parameters using the observations for Sr and Ba. Employing these parameters for rotating massive stars in our stochastic model, we are able to correctly reproduce the chemical evolution of Sr and Ba, but also Y, Zr and La. The data supports a decrease of both the mean rotational velocities and their dispersion with increasing metallicity. Our results show that a metallicity-dependent rotation is a necessary assumption to explain the s-process in massive stars. Our novel methods of model-data comparison represent a promising tool for future galactic chemical evolution studies.