Probing the first generations of massive stars through fluorine in CEMP-no stars

Abstract

We investigate whether the first discovered fluorine-rich CEMP-no star, CS 29498$-$043, can be explained by a very metal-poor rotating massive star. We consider single rotating stellar models of 20 $M_{\odot }$ at a metallicity of $Z=10^{-5}$, exploring initial rotation rates from ${\upsilon }_{\mathrm{ini}}/{\upsilon }_{\mathrm{crit}}=0$ to $0.7$ in increments of $0.1$ ($0<{\upsilon }_{\mathrm{ini}}<644$ km s${}^{-1}$). Rotational mixing enhances the production of light elements in the H--He layers, including fluorine. The ejected material can be nitrogen-rich without being fluorine-rich, whereas fluorine-rich ejecta are always predicted to be nitrogen-rich. The model providing the best fit to the abundances of CS 29498$-$043 is the ${\upsilon }_{\mathrm{ini}}/{\upsilon }_{\mathrm{crit}}=0.6$ model (${\upsilon }_{\mathrm{ini}}=547$ km s${}^{-1}$), which reproduces C, N, O, Na, Mg, and Al within the observational uncertainties. However, the predicted [F/Fe] $=2.8$ exceeds the observed value of [F/Fe] $=2.0\pm 0.4$. By simultaneously varying the ${}^{15}$N($\alpha ,\gamma$)${}^{19}$F and ${}^{19}$F($\alpha ,p$)${}^{22}$Ne reaction rates within their acceptable ranges, the [F/Fe] ratio in the ${\upsilon }_{\mathrm{ini}}/{\upsilon }_{\mathrm{crit}}=0.6$ model can be reduced to 2.2, providing a plausible solution to the abundance pattern of CS 29498$-$043. Our results support the hypothesis that fluorine-rich CEMP-no stars may originate from material enriched by a single, metal-poor, rotating massive star. A potential observational test of this scenario may be to check whether the nitrogen and fluorine abundances observed at the surface of CEMP-no stars are correlated.

Accepted in EPJA. Special issue titled "About Stars and Nuclei: Honoring the Legacy of Roberto Gallino"