Interstellar Fluorine toward Stars in Cep OB2

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Massive stars, through their terminal supernova explosions, seed interstellar clouds with newly synthesized elements. In order to understand the details of chemical evolution in a site of star formation, the interstellar material toward stars in Cep OB2 was studied via absorption lines seen in FUSE spectra. A search for the F I resonance lines at 951 and 954 Å toward HD 208440 and HD 209339A was conducted to help resolve the origin of this element: is F synthesized primarily in Type II supernovae, whose effects should be revealed in a region of ongoing massive star formation, or in asymptotic giant branch stars? Syntheses of H2 Lyman bands near 1060 Å were used to remove H2 absorption in the vicinity of λ 954, the stronger of the two F I lines. In the process, we determined the maximium H2 column density before H2 absorption becomes black in this wavelength region. Our upper limits on λ 951 are consistent with the measures from the 954 Å line. The inferred F abundances are similar to the abundance derived for the gas toward δ Sco from measurements with the Copernicus satellite.
Further comparisons among elemental abundance ratios were performed to discern the presence of enhanced F in an environment shaped by past SNe II. First, we obtained the elemental abundance of Cl (from FUSE spectra of Cl I and II) to determine the expected amount of F depletion onto grains. Then, we derived the elemental abundances of N I and O I from FUSE and HST spectra, respectively, because these two elements show modest amounts of depletion and are synthesized through other processes. Our results indicate that the F abundance and the F/O and F/N elemental abundance ratios are not enhanced for the two sight lines studied to date.
This research was supported by NASA grant NAG5-10305 and by the STScI grant associated with the program GO-08693.

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