The iron depletion level and the gas-phase iron abundance in the hot ( \sim 10 ^ { 6 } 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 .