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Title:
Large Silicon Abundance in Photodissociation Regions
Authors:
Okada, Yoko; Onaka, Takashi; Nakagawa, Takao; Shibai, Hiroshi; Tomono, Daigo; Yui, Yukari Y.
Affiliation:
AA(Department of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan; ), AB(Department of Astronomy, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan; ), AC(Department of Infrared Astrophysics, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Kanagawa 229-8510, Japan), AD(Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan), AE(Subaru Telescope, National Astronomical Observatory of Japan, Hilo, HI 96720), AF(Earth Observation Research and Application Center, Office of Space Applications, Japan Aerospace Exploration Agency (JAXA), Tokyo 104-6023, Japan)
Publication:
The Astrophysical Journal, Volume 640, Issue 1, pp. 383-390. (ApJ Homepage)
Publication Date:
03/2006
Origin:
UCP
ApJ Keywords:
Infrared: ISM, ISM: Individual: Name: ρ Ophiuchi Cloud, ISM: Lines and Bands
DOI:
10.1086/499801
Bibliographic Code:
2006ApJ...640..383O

Abstract

We have made one-dimensional raster scan observations of the ρ Oph and σ Sco star-forming regions with two spectrometers (SWS and LWS) on board the ISO. In the ρ Oph region, [Si II] 35 μm, [O I] 63 μm, 146 μm, [C II] 158 μm, and the H2 pure rotational transition lines S(0) to S(3) are detected, and the photodissociation region (PDR) properties are derived as the radiation field scaled by the solar neighborhood value G0~30-500, the gas density n~250-2500 cm-3, and the surface temperature T~100-400 K. The ratio of [Si II] 35 μm to [O I] 146 μm indicates that silicon of 10%-20% of the solar abundance must be in the gaseous form in the PDR, suggesting that efficient dust destruction is ongoing even in the PDR and that a fraction of the silicon atoms may be contained in volatile forms in dust grains. The [O I] 63 μm and [C II] 158 μm emissions are too weak relative to [O I] 146 μm to be accounted for by standard PDR models. We propose a simple model, in which overlapping PDR clouds along the line of sight absorb the [O I] 63 μm and [C II] 158 μm emissions, and show that the proposed model reproduces the observed line intensities fairly well. In the σ Sco region, we have detected three fine-structure lines, [O I] 63 μm, [N II] 122 μm, and [C II] 158 μm, and derived that 30%-80% of the [C II] emission comes from the ionized gas. The upper limit of the [Si II] 35 μm is compatible with the solar abundance relative to nitrogen, and no useful constraint on the gaseous Si is obtained for the σ Sco region.

Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the UK) and with the participation of ISAS and NASA.

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