X‐Ray Absorption Spectroscopy of the Multiphase Interstellar Medium: Oxygen and Neon Abundances

X-ray absorption spectroscopy provides a potentially powerful tool indetermining the metal abundances in various phases of the interstellar medium(ISM). We present a case study of the sight line toward 4U 1820-303 (Galacticcoordinates l, b=2.79, -7.91 and distance = 7.6 kpc), based on Chandra Gratingobservations. The detection of OI, OII, OIII, OVII, OVIII, and NeIX Kalphaabsorption lines allows us to measure the atomic column densities of theneutral, warm ionized, and hot phases of the ISM through much of the Galacticdisk. By comparing these measurements with the 21 cm hydrogen emission and withthe pulsar dispersion measure along the same sight line, we estimate the meanoxygen abundances in the neutral and total ionized phases as 0.3(0.2, 0.6) and2.2(1.1, 3.5) in units of Anders & Grevesse (1989) solar value. Thissignificant oxygen abundance difference is apparently a result of molecule/dustgrain destruction and recent metal enrichment in the warm ionized and hotphases. We also measure the column density of neon from its absorption edge andobtain the Ne/O ratio of the neutral plus warm ionized gas as 2.1(1.3, 3.5)solar. Accounting for the expected oxygen contained in molecules and dustgrains would reduce the Ne/O ratio by a factor of ~1.5. From a joint-analysisof the OVII, OVIII, and NeIX lines, we obtain the Ne/O abundance ratio of thehot phase as 1.4(0.9, 2.1) solar, which is not sensitive to the exacttemperature distribution assumed in the absorption line modeling. Thesecomparable ISM Ne/O ratios for the hot and cooler gas are thus considerablyless than the value (2.85+-0.07; 1sigma) recently inferred from corona emissionof solar-like stars (Drake & Testa 2005). (abridged)Comment: 9 pages, 4 figures and 3 tables, accepted for publication in ApJ 200