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Title:
Si and Fe depletion in Galactic star-forming regions observed by the Spitzer Space Telescope
Authors:
Okada, Y.; Onaka, T.; Miyata, T.; Okamoto, Y. K.; Sakon, I.; Shibai, H.; Takahashi, H.
Publication:
The Evolving ISM in the Milky Way and Nearby Galaxies, The Fourth Spitzer Science Center Conference, Proceedings of the conference held December 2-5, 2007 at the Hilton Hotel, Pasadena, CA, Eds.: K. Sheth, A. Noriega-Crespo, J. Ingalls, and R. Paladini, Published online at http://ssc.spitzer.caltech.edu/mtgs/ismevol/
Publication Date:
01/2009
Origin:
AUTHOR
Keywords:
Infrared:ISM , ISM:abundances , dust , H II regions
Bibliographic Code:
2009eimw.confE..48O

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 micron, Fe II 26 micron, and Fe III 23 micron as well as S III 33 micron and H_2 S(0) 28 um emission lines. Based on photodissociation and H II region models the gas-phase Si and Fe abundance are suggested to be 3-100% and <22% of the solar abundance, respectively. Since the Fe II 26 micron, and Fe III 23 micron emissions are weak, the high sensitivity of the IRS enables 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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