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Title:
Si and Fe Depletion in Galactic Star-forming Regions Observed by the Spitzer Space Telescope
Authors:
Okada, Yoko; Onaka, Takashi; Miyata, Takashi; Okamoto, Yoshiko K.; Sakon, Itsuki; Shibai, Hiroshi; Takahashi, Hidenori
Affiliation:
AA(Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Kanagawa 229-8520, Japan; .), AB(Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.), AC(Institute of Astronomy, Graduate School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan.), AD(Institute of Astrophysics and Planetary Science, Faculty of Science, Ibaraki University, 2-1-1, Bunkyo, Mito, Ibaraki 310-8512, Japan.), AE(Department of Astronomy, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.), AF(Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.; Current address: Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan.), AG(Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama, Agatsuma, Gunma 377-0702, Japan.)
Publication:
The Astrophysical Journal, Volume 682, Issue 1, pp. 416-433. (ApJ Homepage)
Publication Date:
07/2008
Origin:
UCP
ApJ Keywords:
ISM: Dust, Extinction, ISM: H II Regions, Infrared: ISM, ISM: Abundances
DOI:
10.1086/589229
Bibliographic Code:
2008ApJ...682..416O

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 H2 S(0) 28 μm emission lines. Using the intensity of [N II] 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+/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 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.
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