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Session 44 - The Local Diffuse ISM.
Display session, Tuesday, June 11
We present high S/N HST GHRS echelle observations of the weak interstellar O I] \lambda1356 absorption toward the stars \gamma Cas, \epsilon Per, \delta Ori, \epsilon Ori, 15 Mon, \tau CMa, and \gamma Ara. In combination with previous GHRS measurements in six other sightlines (\zeta Per, \xi Per, \lambda Ori, \iota Ori, \kappa Ori, and \zeta Oph), these new observations yield a mean interstellar gas-phase oxygen abundance (per 10^6 H atoms) of 10^6 O/H = 316 \pm 9. There are no statistically significant variations in the measured O abundances from sightline to sightline and no evidence of density-dependent O depletion from the gas phase. In particular, using the fractional abundance of molecular hydrogen (f(H_2) = 2N(H_2)/[2N(H_2) + N(H I)]) as a barometer of gas/dust interactions, the mean O abundance in the seven sightlines with log f(H_2) < -2.0 (10^6 O/H = 318 \pm 13) is essentially identical to that in the six sightlines with log f(H_2) > -2.0 (10^6 O/H = 314 \pm 12). Assuming various dust mixtures of oxides and silicates such as olivine, the abundance of interstellar O tied up in dust grains is unlikely to surpass 10^6 O/H \approx 180 (Cardelli et al.\ 1996). It is difficult to further increase this O fraction in grain cores simply because the requisite metals are far less abundant than oxygen. Consequently, the GHRS observations imply that the total abundance of interstellar oxygen (gas plus grains) in the vicinity of the Sun is about 2/3 the solar value of 10^6 O/H = 741 \pm 130 (Grevesse amp; Noels 1993).
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