[Paper Review] The Chemical Evolution of Fluorine in the Bulge - High-resolution K-band spectra of giants in three fields

Henrik Jönsson, N. Ryde+7 more|University of Hertfordshire Research Archive (University of Hertfordshire)|Mar 11, 2014

Stellar, planetary, and galactic studies59 references17 citations

TL;DR

This study analyzes high-resolution K-band spectra of red giants in the Galactic bulge to investigate fluorine production mechanisms, finding that AGB stars and Type II supernovae alone cannot explain observed fluorine abundances. The data strongly support a significant contribution from Wolf-Rayet stars, and the authors present a consistent HF molecular line list to resolve prior inconsistencies in fluorine abundance measurements that may have overestimated fluorine by ~0.3 dex in earlier works.

ABSTRACT

Possible main formation sites of F in the Universe include AGB stars, the ν-process in Type II SNe, and/or W-R stars. The importance of the W-R stars has theoretically been questioned and they are probably not needed in the modelling of the chemical evolution of F in the solar neighborhood. It has, however, been suggested that W-R stars are indeed needed to explain the chemical evolution of F in the Bulge. The molecular spectral data of the often used HF-molecule has not been presented in a complete and consistent way and has recently been debated in the literature. In this article we determine the [F/O] vs. [O/H] trend in the Bulge to investigate the possible contribution from W-R stars. Additionally, we present here a HF line list for the K- and L-bands (including the often used 23358.33 Å line) and an accompanying partition function. The F abundances were determined using spectral fitting from hi-res NIR spectra of eight K giants recorded by the spectrograph CRIRES. We have also re-analyzed five previously published Bulge giants using our new HF molecular data. We find that the F-O abundance in the Bulge probably cannot be explained with chemical evolution models including only AGB-stars and the ν-process in SNe Type II, i.e. a significant amount of F production in W-R stars is likely needed to explain the F abundance in the Bulge. Concerning the HF line list, we find that a possible reason for the inconsistencies in the literature, with two different excitation energies being used, is two different definitions of the zero-point energy for the HF molecule and therefore also two accompanying different dissociation energies. Both line lists are correct, as long as the corresponding consistent partition function is used in the spectral synthesis. However, we suspect this has not been the case in several earlier works leading to F abundances 0.3 dex too high.

Motivation & Objective

• To determine the fluorine-oxygen abundance trend in the Galactic bulge as a function of metallicity to assess stellar production sites.
• To evaluate whether Wolf-Rayet stars are necessary to explain fluorine evolution in the bulge, given conflicting theoretical models.
• To resolve inconsistencies in past fluorine abundance measurements caused by mismatched molecular data for the HF molecule.
• To provide a consistent, high-accuracy HF line list for the K- and L-bands with a corresponding partition function for spectral synthesis.

Proposed method

• High-resolution K-band spectra were obtained using the CRIRES spectrograph on the Very Large Telescope (VLT) for eight K-type giants in three bulge fields.
• Fluorine abundances were derived via spectral fitting of the 23358.33 Å HF line, using a newly constructed, consistent HF molecular line list.
• The authors re-analyzed five previously published bulge giants observed with the Gemini/GPS-Phoenix spectrograph using their new molecular data.
• A consistent partition function was derived for the HF molecule, ensuring compatibility with the new line list and correcting for prior inconsistencies in excitation energy definitions.
• Chemical evolution models incorporating AGB stars, Type II supernovae (via ν-process), and Wolf-Rayet stars were compared to observed F/O trends.
• The study used metallicity indicators such as [Fe/H] and [O/H] to trace the evolution of fluorine relative to oxygen across the bulge.

Experimental results

Research questions

• RQ1 Is the observed fluorine abundance in the Galactic bulge consistent with models that include only AGB stars and ν-process from Type II supernovae?
• RQ2 What is the role of Wolf-Rayet stars in producing fluorine in the bulge, and is their contribution necessary to explain the observed F/O trend?
• RQ3 Why do previous studies report inconsistent fluorine abundances, and can this be traced to mismatches in molecular data for the HF molecule?
• RQ4 How can a consistent HF line list be constructed for the K- and L-bands to improve accuracy in future abundance determinations?
• RQ5 Does the observed decline in [Zr/F] with increasing [F/H] support the need for a metallicity-dependent fluorine source like Wolf-Rayet stars?

Key findings

• The observed fluorine-oxygen abundance trend in the bulge is steeper than predicted by models including only AGB stars and the ν-process from Type II supernovae, indicating an additional production mechanism is required.
• The data strongly suggest that Wolf-Rayet stars are a necessary source of fluorine in the bulge, as they can explain the observed trend and metallicity dependence.
• A significant fraction of earlier fluorine abundance measurements may be overestimated by approximately 0.3 dex due to inconsistent use of HF molecular data, particularly mismatched partition functions and dissociation energies.
• The authors present a new, consistent HF line list for the K- and L-bands, including the 23358.33 Å line, with a corresponding partition function that resolves prior discrepancies.
• The observed [Zr/F] trend with increasing [F/H] is inconsistent with AGB star production alone, further supporting the need for an additional source like Wolf-Rayet stars.
• The study confirms that the ν-process in Type II supernovae cannot account for the observed fluorine trend due to its lack of metallicity dependence over the observed range.