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 $\upsilon_{\rm ini}/\upsilon_{\rm crit} = 0$ to $0.7$ in increments of $0.1$ ($0<\upsilon_{\rm ini}<644$ 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 $\upsilon_{\rm ini}/\upsilon_{\rm crit} = 0.6$ model ($\upsilon_{\rm ini} = 547$ 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($α,γ$)$^{19}$F and $^{19}$F($α,p$)$^{22}$Ne reaction rates within their acceptable ranges, the [F/Fe] ratio in the $\upsilon_{\rm ini}/\upsilon_{\rm crit} = 0.6$ 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.

Figures (6)

• Figure 1: Abundance profiles of $^{14}$N (top panel) and $^{19}$F (bottom panel) in the vv6 model during core helium burning. Different colors indicate various central helium mass fractions (ranging from 0.731 to $4.34 \times 10^{-5}$), as specified in the legend. The dashed lines in the top panel represent the $^{1}$H abundance profiles.
• Figure 2: Abundance profiles of $^{19}$F in the vv6 model within the helium-burning shell. Different colors represent successive times. The numbers indicate the temperature (in $10^6$ K) at the peak of $^{19}$F, located near the base of the helium-burning shell. Dashed and dotted lines show the corresponding $^{14}$N and $^{4}$He profiles, respectively.
• Figure 3: Pre-supernova abundance profiles of $^{14}$N (left panel) and $^{19}$F (right panel) predicted by our models.
• Figure 4: Yields (in $M_{\odot}$) of $^{13}$C, $^{14}$N and $^{19}$F in the H- and He-rich regions, with the mass cut set at the top of the CO core (defined as the location where the $^{4}$He mass fraction falls below $10^{-2}$), shown as a function of initial velocity. The thin dotted horizontal lines represent the initial mass of these isotopes contained within the stars. The dashed patterns show the yields of the 15 (diamonds) and 25 $M_{\odot}$ models at [Fe/H] $=-4$ (set E) of Roberti et al. (2024) roberti24.
• Figure 5: Predicted [N/Fe] versus [F/Fe] ratios as the mass cut is varied in our models, from the final mass (i.e., winds only, shown as colored circles) to the top of the CO core (shown as colored stars). A solid line connects all possible [N/Fe]–[F/Fe] combinations in the corresponding model ejecta. The left panel shows the case where no dilution with ISM is considered (i.e. pure massive star ejecta). The right panel shows the case where the dilution factor (Eq. \ref{['eq:dil']}) is fixed to $f_{\rm dil} = 0.995$. Black symbols mark the two CEMP-no stars CS 29498$-$043 and CS 29502$-$092.