Fluorine in the Solar Neighborhood: The Need for Several Cosmic Sources

Abstract

The cosmic origin of fluorine is still not well constrained. Several nucleosynthetic channels at different phases of stellar evolution have been suggested, but these must be constrained by observations. For this, the fluorine abundance trend with metallicity spanning a wide range is required. Our aim is to determine stellar abundances of fluorine for −1.1 < [Fe/H] < +0.4. We determine the abundances from HF lines in infrared K-band spectra (∼ 2.3 µm) of cool giants, observed with the IGRINS and Phoenix high-resolution spectrographs. We derive accurate stellar parameters for all our observed K giants, which is important since the HF lines are very temperature sensitive. We find that [F/Fe] is flat as a function of metallicity at [F/Fe]∼ 0, but increases as the metallicity increases. The fluorine slope shows a clear secondary behavior in this metallicity range. We also find that the [F/Ce] ratio is relatively flat for −0.6 < [Fe/H] < 0, and that for two metal-poor ([Fe/H] < −0.8), s-process element enhanced giants, we do not detect an elevated fluorine abundance. We interpret all these observational constraints to indicate that several major processes are at play for the cosmic budget of fluorine over time; from those in massive stars at low metallicities, through the asymptotic giant branch-star contribution at −0.6 < [Fe/H] < 0, to processes with increasing yields with metallicity at super-solar metallicities. The origins of the latter, and whether or not Wolf-Rayet stars and/or novae could contribute at super-solar metallicities, is currently not known. To quantify these observational results, theoretical modelling is required. More observations in the metal-poor region are required to clarify the processes there.

The CARMENES search for exoplanets around M dwarfs

“…Mishenina et al (2015) instead reintroduced the intermediate neutron-capture i-process discussed earlier by Cowan & Rose (1977) to explain this fact, in which the neutron density operating the process is approximately 10 15 cm −3 . Also, recently Ryde et al (2020) found a secondary behaviour of [F/Fe] for supersolar metallicities in K giants. We note that F has a significant cosmic contribution from AGB stars (Jorissen et al 1992;Karakas & Lattanzio 2014;Abia et al 2019).…”

ATOMIUM: halide molecules around the S-type AGB star W Aquilae

“…In a detailed study of the uncertainties in the nuclear reaction rates for 19 F, Lugaro et al (2004) concluded that these cause uncertainties in theoretical models of fluorine production of a factor of 2-7, depending on the initial stellar mass. A recent study by Ryde et al (2020) argues for multiple sites of fluorine production, in particular at different metallicities. Nevertheless, it has been clear for some time that AGB stars play a significant role in the production of fluorine.…”

A Hint of a Low-Energy Excess in Cosmic-Ray Fluorine

“…A diagnostic is via observations of molecular HF emission lines in the K and N bands (2.1-2.4 µm and 8-13 µm, respectively), which are observable only in cool giants (T eff < 4500 K). Analysis of the spectra of K giants show that the fluorine-to-oxygen abundance ratio [F/O] increases as a function of oxygen abundance [O/H] with a slope 1±0.2, and can reach [F/O]≈0.4 dex for [O/H]≈0.2 dex (Ryde et al 2020). Behavior like this is typical for the so-called "secondary" elements, whose synthesis in stars involves nuclides pre-existing in the ISM gas of which stars form.…”

“…Novae are also proposed as a possible source of fluorine through proton captures by 17 O. A useful review of fluorine nucleosynthesis is given in Ryde et al (2020).…”