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

Abstract

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 μm, [Fe II] 26 μm, and [Fe III] 23 μm, as well as [S III] 33 μm and H2S(0) 28 μm emission lines. Using the intensity of [NII] 122 μm or 205 μm and [O I] 146 μm or 63 μm reported by previous observations in four regions, we derived the ionic abundances of Si+/N+and Fe+/N+in the ionized gas and Si+/O0and Fe+/O0in 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 μm and [Fe III] 23 μm 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. © 2008. The American Astronomical Society. All rights reserved.