Fluorine Abundances in the Orion Nebula Cluster

Abstract

This is a pilot study using cool dwarfs as sources with which to probe fluorine abundances via HF. This molecule is detected for the first time in young K-M dwarf members of an OB association. The targets are three low-mass stars (JW 22, JW 163, and JW 433) belonging to the Orion Nebula Cluster. The target stellar parameters were derived to be Teff = 3650, 4250, and 4400 K and log g = 3.5, 3.4, and 3.6, with corresponding stellar masses of 0.4, 0.6, and 0.7 M☉, for JW 22, JW 163, and JW 433, respectively. Fluorine, oxygen, and carbon abundances were derived from the HF(1-0) R9 line along with samples of OH and CO vibrational-rotational lines present in high-resolution infrared spectra observed with the Phoenix spectrograph on the Gemini South Telescope. The fluorine and oxygen results obtained for these targets, still in the pre-main-sequence stage of evolution, agree well with the general trend defined for the Milky Way disk, the latter being deduced from observations of more evolved giant stars. In addition, the carbon and oxygen abundances obtained for the studied stars overlap results from previous studies of the more massive OB stars and FG dwarf members of the Orion Nebula Cluster. We conclude from this agreement that the fluorine abundances derived for the Orion K-M dwarfs (when there is no conspicuous evidence of disks) can be considered a good representation of the current fluorine abundance value for the Milky Way disk.