Fluorine in carbon-enhanced metal-poor stars: a binary scenario

Abstract

Aims. A super-solar fluorine abundance was observed in the carbon-enhanced metal-poor (CEMP) star HE 1305+0132 ([F/Fe] = +2.90, [Fe/H] = −2.5). We propose that this observation can be explained using a binary model that involve mass transfer from an asymptotic giant branch (AGB) star companion and, based on this model, we predict F abundances in CEMP stars in general. We discuss wether F can be used to discriminate between the formation histories of most CEMP stars: via binary mass transfer or from the ejecta of fast-rotating massive stars. Methods. We compute AGB yields using different stellar evolution and nucleosynthesis codes to evaluate stellar model uncertainties. We use a simple dilution model to determine the factor by which the AGB yields should be diluted to match the abundances observed in HE 1305+0132. We further employ a binary population synthesis tool to estimate the probability of F-rich CEMP stars. Results. The abundances observed in HE 1305+0132 can be explained if this star accreted 3−11% of the mass lost by its former AGB companion. The primary AGB star should have dredged-up at least 0.2 M of material from its He-rich region into the convective envelope via third dredge-up, which corresponds to AGB models of Z 0.0001 and mass 2 M . Many AGB model uncertainties, such as the treatment of convective borders and mass loss, require further investigation. We find that in the binary scenario most CEMP stars should also be FEMP stars, that is, have [F/Fe] > +1, while fast-rotating massive stars do not appear to produce fluorine. We conclude that fluorine is a signature of low-mass AGB pollution in CEMP stars, together with elements associated with the slow neutron-capture process.

Population synthesis of binary carbon-enhanced metal-poor stars

“…Lugaro et al (2008) although this star may represent the tail of the fluorine distribution. Future observations of fluorine in CEMP stars should reveal this population.…”

Accelerator mass spectrometry measurements of theC13(n,γ)C14andWallner1, Bichler2, Buczak3 et al. 2016Phys. Rev. CSelf Cite281170View full textAdd to dashboardCite

“…This difference would have stringent consequences for the role of 14 N as a neutron poison as well as for the production of 19 F, which depends on the amount of protons emitted in the 14 N(n,p) reactions. While the 19 F abundance is not much affected by the 10% uncertainty of most experimental cross sections, this becomes a critical issue if one includes the data of Brehm et al [17,18]. There are also large discrepancies between the evaluated 14 N(n,p) cross sections in the JEFF-3.1, ENDF/B-VII, JENDL, and BROND libraries, especially above about 50 keV.…”

The s-process in AGB stars

“…In [17] we compared nucleosynthesis predictions from models of intermediate-mass AGB stars to the results of [41], and in [42] we presented results for lower mass and lower metallicity AGB stars. Recently, [44] obtained s-process abundances for the metal-poor BoBn1, showing that the PN is highly enriched in Kr, Xe, and Ba, as well as the light-element F. Fluorine is predicted to be highly enriched in carbon enhanced metal-poor (CEMP) stars that show s-process element enhancements as a consequence of binary mass transfer from a former AGB companion [45]. Fluorine enrichment has been found in only a handful of CEMP stars [46] including HE 1305+0132 at a level of [F/Fe] ≈ 2.90 [47] (although see discussion in [46] regarding the uncertainty of the Fe and F abundance of this object).…”