Fluorine production in He-burning regions of massive stars during cosmic history

¹ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Department of Community Service, Koeki University, 3-5-1 Iimoriyama, Sakata, Yamagata 998-8580, Japan
³ Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles, CP 226, 1050 Brussels, Belgium
⁴ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
⁵ Observatório Nacional, R. Gen. José Cristino 77, 20921-400 Rio de Janeiro, Brazil
⁶ NOIRLab, Tucson, AZ 85719, USA
⁷ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA

^⋆ Corresponding author; sofia.tsiatsiou@unige.ch

Received: 28 November 2024
Accepted: 10 February 2025

Abstract

Context. The origin of fluorine is still a debated question. Asymptotic giant branch stars synthesise this element and likely contribute significantly to its synthesis in the present-day Universe. However, it is not clear whether other sources contribute, especially in the early Universe.

Aims. We discuss variations of the surface abundances of fluorine coming from our massive star models and compare them with available present-day observations. We compute the contribution of single massive stars in producing ¹⁹F over metallicities covering the whole cosmic history (i.e. from zero up to super-solar metallicities).

Methods. We used massive star models in the mass range of 9 M_⊙ ≤M_ini ≤ 300 M_⊙ at metallicities from Population III (Z = 0) up to super-solar (Z = 0.020) while accounting for the required nuclear network to follow the evolution of ¹⁹F during the core H- and He-burning phases. Results from models with and without rotational mixing are presented.

Results. We find that rotating models predict a slight depletion of fluorine at their surface at the end of the main sequence phase. In more advanced evolutionary phases, only models with an initial mass larger than 25 M_⊙ at metallicities Z ≥ 0.014 show phases where the abundance of fluorine is enhanced. This occurs when the star is a Wolf-Rayet star of the WC type. WC stars can show surface abundances of fluorine ten times larger than their initial abundance. However, we obtained that the winds of massive stars at metallicities larger than Z = 0.006 do not significantly contribute to fluorine production, confirming previous findings. In contrast, very metal-poor rapidly rotating massive star models may be important sources of fluorine through the mass expelled at the time of their supernova explosion.

Conclusions. Observations of WC stars at solar or super-solar metallicities may provide very interesting indications on the nuclear pathways that lead to fluorine production in massive stars. The possibility of observing fluorine-rich carbon-enhanced metal-poor stars is also a way to put constrains in present models at very low metallicities.