Si and Fe Depletion in Galactic Star-forming Regions Observed by the Spitzer Space Telescope

We report the results of the mid-infrared spectroscopy of 14 Galactic star-forming regions with the high-resolution modules of the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. We detected [Si II] 35 mm, [Fe II] 26 mm, and [Fe III] 23 mm, as well as [S III] 33 mm and H2 S(0) 28 mm emission lines. Using the intensity of [N II] 122 mm or 205 mm and [O I] 146 mm or 63 mm reported by previous observations in four regions, we derived the ionic abundances of Si+/N+ and Fe+/N+ in the ionized gas and Si+/O0 and Fe+/O0 in the photodissociation gas. For all the targets, we derived the ionic abundances of Si+/S2+ and Fe2+/S2+ for the ionized gas. Based on photodissociation and H II region models the gas-phase Si and Fe abundance are suggested to be 3%-100% and <8% of the solar abundance, respectively, for the ionized gas and 16%-100% and 2%-22% of the solar abundance, respectively, for the photodissociation region gas. Since the [Fe II] 26 mm and [Fe III] 23 mm emissions are weak, the high sensitivity of the IRS enables us to derive the gas-phase Fe abundance widely in star-forming regions. The derived gas-phase Si abundance is much larger than that in cool interstellar clouds and that of Fe. The present study indicates that 3%-100% of Si atoms and <22% of Fe atoms are included in dust grains which are destroyed easily in H II regions, probably by the UV radiation. We discuss possible mechanisms to account for the observed trend: mantles which are photodesorbed by UV photons, organometallic complexes, or small grains.