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 ~10^6 Kinterstellar medium (ISM) is critical to the understanding of its energybalance as well as the thermal sputtering, cooling, and heating processes ofdust grains. Here we report on the first detection of the Fe XVII absorptionline at 15.02 A from the hot ISM in the spectrum of the low mass X-ray binary4U~1820-303 observed with Chandra X-Ray Observatory. By jointly analyzing thisabsorption line with those from OVII, OVIII, and NeIX ions in the samespectrum, we obtain an abundance ratio as Fe/Ne=0.8(0.4, 2.1) in units of theAnders & Grevesse solar value (90% confidence intervals). We find that theresult is robust with respect to different assumed gas temperaturedistributions. The obtained Fe/Ne abundance ratio, albeit with largeuncertainties, is consistent with the solar value, indicating that there isvery little or no iron depleted into dust grains, i.e., most of or all of thedust grains have been destroyed in the hot ISM.Comment: Minor changes, to match the published version in ApJ Letter, 2006, 65