Chemical Evolution of Fluorine in the Milky Way

TL;DR

This study uses chemical evolution models to identify the main sources of fluorine in the Milky Way, emphasizing the role of rapidly rotating massive stars and the increasing importance of AGB stars at higher metallicities.

Contribution

It demonstrates that rapidly rotating massive stars are essential to reproduce observed fluorine abundances, and rules out Wolf-Rayet winds as dominant fluorine sources.

Findings

Rapidly rotating massive stars explain low-metallicity fluorine trends.

AGB stars contribute significantly at higher metallicities.

Wolf-Rayet winds are unlikely the main fluorine source.

Abstract

Fluorine has many different potential sites and channels of production, making narrowing down a dominant site of fluorine production particularly challenging. In this work, we investigate which sources are the dominant contributors to the galactic fluorine by comparing chemical evolution models to observations of fluorine abundances in Milky Way stars covering a metallicity range -2$<$[Fe/H]$<$0.4 and upper limits in the range -3.4$<$[Fe/H]$<$-2.3. In our models, we use a variety of stellar yield sets in order to explore the impact of varying both AGB and massive star yields on the chemical evolution of fluorine. In particular, we investigate different prescriptions for initial rotational velocity in massive stars as well as a metallicity dependent mix of rotational velocities. We find that the observed [F/O] and [F/Fe] abundance ratios at low metallicity and the increasing trend of [F/Ba] at [Fe/H]$\gtrsim$-1 can only be reproduced by chemical evolution models assuming, at all metallicities, a contribution from rapidly rotating massive stars with initial rotational velocities as high as 300km s$^{-1}$. A mix of rotational velocities may provide a more physical solution than the sole use of massive stars with $v_{\text{rot}}$=300$\text{km s}^{-1}$, which are predicted to overestimate the fluorine and average s-process elemental abundances at [Fe/H]$\gtrsim$-1. The contribution from AGB stars is predicted to start at [Fe/H]$\approx$-1 and becomes increasingly important at high metallicity, being strictly coupled to the evolution of the nitrogen abundance. Finally, by using modern yield sets, we investigate the fluorine abundances of Wolf-Rayet winds, ruling them out as dominant contributors to the galactic fluorine.