Fluorine in the Carbon‐Enhanced Metal‐Poor Star HE 1305+0132

Abstract

The fluorine abundance of the Carbon‐Enhanced Metal‐Poor (CEMP) star HE 1305+0132 has been derived by analysis of the molecular HF (1‐0) R9 line at 2.3357 μm in a high‐resolution (R = 50,000) spectrum obtained with the Phoenix spectrometer and Gemini‐South telescope. Our abundance analysis makes use of a CNO‐enhanced ATLAS12 model atmosphere characterized by a metallicity and CNO enhancements determined utilizing medium‐resolution (R = 3,000) optical and near‐IR spectra. The effective iron abundance is found to be [Fe/H] = −2.5, making HE 1305+0132 the most Fe‐deficient star, by more than an order of magnitude, for which the abundance of fluorine has been measured. Using spectral synthesis, we derive a super‐solar fluorine abundance of A(19F) = 4.96±0.21, corresponding to a relative abundance of [F/Fe] = +2.90. A single line of the Phillips C2 system is identified in our Phoenix spectrum, and along with multiple lines of the first‐overtone vibration‐rotation CO (3‐1) band head, C and O abundances of A(12C) = 8.57±0.11 and A(16O) = 7.04±0.14 are derived. We consider the striking fluorine overabundance in the framework of the nucleosynthetic processes thought to be responsible for the C‐enhancement of CEMP stars and conclude that the atmosphere of HE 1305+0132 was polluted via mass transfer by a primary companion during its asymptotic giant branch phase. This is the first study of fluorine in a CEMP star, and it demonstrates that this rare nuclide can be a key diagnostic of nucleosynthetic processes in the early Galaxy.