CHANDRA DETECTION OF Fe XVII IN ABSORPTION: IRON ABUNDANCE IN THE HOT GASEOUS INTERSTELLAR MEDIUM

The iron depletion level and the gas-phase iron abundance in the hot (∼ 106 K) interstellar medium (ISM) are critical to our understanding of its energy balance as well as the thermal sputtering, cooling, and heating processes of dust grains. Here we report on the first detection of the Fe XVII absorption line at 15.02 Å from the hot ISM in the spectrum of the low mass X-ray binary 4U 1820–303 observed with the Chandra X-Ray Observatory. By jointly analyzing this absorption line with those from O VII, O VIII, and Ne IX ions in the same spectrum, we obtain an abundance ratio as Fe/Ne=0.8(0.4, 2.1) in units of the Anders & Grevesse solar value (90% confidence intervals). We find that the result is robust with respect to different assumed gas temperature distributions. The obtained Fe/Ne abundance ratio, albeit with large uncertainties, is consistent with the solar value, indicating that there is very little or no iron depleted into dust grains, i.e., most of or all of the dust grains have been destroyed in the hot ISM. Subject headings: ISM: abundances — ISM: dust — X-rays: ISM — X-rays: individual (4U 1820–303)