Effects of nuclear cross sections on¹⁹F nucleosynthesis at low metallicities

Abstract

Context. The origin of fluorine is a longstanding problem in nuclear astrophysics. It is widely recognized that asymptotic giant branch (AGB) stars are among the most important contributors to the Galactic fluorine production. Aims. In general, extant nucleosynthesis models overestimate the fluorine production by AGB stars with respect to observations. Although those differences are rather small at solar metallicity, low metallicity AGB stellar models predict fluorine surface abundances up to one order of magnitude larger than the observed ones. Methods. As part of a project devoted to reducing the uncertainties in the nuclear physics that affect the nucleosynthesis in AGB stellar models, we review the relevant nuclear reaction rates involved in the fluorine production or destruction. We perform this analysis on a model with initial mass M = 2 M and Z = 0.001. Results. We found that the major uncertainties are due to the 13 C(α,n) 16 O, the 19 F(α,p) 22 Ne, and the 14 N(p,γ) 15 O reactions. A change in the corresponding reaction rates within the present experimental uncertainties implies surface 19 F variations at the AGB tip lower than 10%, thus much smaller than observational uncertainties. For some α capture reactions, however, cross sections at astrophysically relevant energies are determined on the basis of nuclear models, in which some low-energy resonance parameters are very poorly known. Thus, larger variations in the rates of those processes cannot be excluded. That being so, we explore the effects of the variation in some α capture rates well beyond the current published uncertainties. The largest 19 F variations are obtained by varying the 15 N(α,γ) 19 F and the 19 F(α,p) 22 Ne reactions. Conclusions. The currently estimated uncertainties of the nuclear reaction rates involved in the production and destruction of fluorine produce minor 19 F variations in the ejecta of AGB stars. Analysis of some α capture processes that assume a wider uncertainty range determines 19 F abundances in better agreement with recent spectroscopic fluorine measurements at low metallicity. In the framework of this scenario, the 15 N(α,γ) 19 F and the 19 F(α,p) 22 Ne reactions show the strongest effects on fluorine nucleosynthesis. The presence of poorly known low-energy resonances make such a scenario possible, even if it is unlikely. We plan to measure these resonances directly.

The origin of fluorine: abundances in AGB carbon stars revisited

“…We test whether the inclusion of rotation might alleviate this problem by modifying the 19 F surface distributions in our models (see Piersanti et al 2013). The main fluorine production channel starts with the neutrons released by the 13 C(α, n) 16 O reaction and proceeds through the nuclear chain 14 N(n, p) Cristallo et al 2014, and references therein). The main effect of mixing induced by rotation is to dilute 14 N within the 13 Cpocket.…”

EVOLUTION, NUCLEOSYNTHESIS, AND YIELDS OF AGB STARS AT DIFFERENT METALLICITIES. III. INTERMEDIATE-MASS MODELS, REVISED LOW-MASS MODELS, AND THE pH-FRUITY INTERFACE

“…19 F is very sensitive to a variation of the initial stellar mass (see lower panel of Figure 12). Its production basically depends on the amount of 15 N in the He-intershell, which in turn is correlated to the amount of 13 C in the ashes of the H-burning shell, as well as in the 13 C pocket (see the discussion in Cristallo et al 2014). In IMS-AGBs, fluorine production is strongly suppressed due to the reduced contribution from the radiative 13 C burning and from the increased efficiency of 19 F destruction channels (the 19 F(p,α) 16 O reaction and, above all, the 19 F(α,p) 22 Ne reaction).…”

Fluorine Abundances in the Galactic Disk

“…The abundances of F and Fe illustrated in Figure 10 can be used to evaluate contributions from possible sources for the nucleosynthesis of 19 F based on chemical evolution model predictions by Timmes et al (1995), Alibés et al (2001), Renda et al (2004), Kobayashi et al (2011a;2011b), Spitoni et al (2018), and Prantzos et al (2018); shown in the bottom panel of Figure 10. Likely sources to consider are both neutrino nucleosynthesis (or the "ν-process"; Woosley et al 1990;Timmes et al 1995;Alibés et al 2001;Kobayashi et al 2011a;2011b) and AGB stars Forestini et al 1992;Abia et al 2009;Cristallo et al 2014;Spitoni et al2018). Massive stars have also been proposed as a significant source of fluorine (e.g., Kobayahsi et al 2011a), including mass loss in Wolf-Rayet stars (Meynet & Arnould 2000;Spitoni et al 2018), as well as rapidly rotating metal-poor massive stars (Prantzos et al 2018).…”