Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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 [SiII]
35um, [FeII] 26um, and [FeIII] 23um as well as [SIII] 33um and H2 S(0) 28um
emission lines. Using the intensity of [NII] 122um or 205um and [OI] 146um or
63um reported by previous observations in four regions, we derived the ionic
abundance 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 abundance of
Si+/S2+ and Fe2+/S2+ for the ionized gas. Based on photodissociation and HII
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 [FeII] 26um and [FeIII] 23um 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 HII 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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The categories of this paper are: math
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: mtrl-th
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The categories of this paper are: solv-int
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The categories of this paper are: atom-ph
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The categories of this paper are: nlin
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The categories of this paper are: adap-org
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The categories of this paper are: hep-th
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The categories of this paper are: cond-mat
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The categories of this paper are: plasm-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: quant-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: alg-geom
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The categories of this paper are: econ