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Centimetre-wave continuum radiation from the ρ Ophiuchi molecular cloud

Casassus, Simon; Dickinson, Clive; Cleary, Kieran; Paladini, Roberta; Etxaluze, Mireya; Lim, Tanya; White, Glenn J.; Burton, Michael; Indermuehle, Balt; Stahl, Otmar and Roche, Patrick (2008). Centimetre-wave continuum radiation from the ρ Ophiuchi molecular cloud. Monthly Notices of the Royal Astronomical Society, 391(3) pp. 1075–1090.

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The ρ Oph molecular cloud is undergoing intermediate-mass star formation. Ultraviolet radiation from its hottest young stars heats and dissociates exposed layers, but does not ionize hydrogen. Only faint radiation from the Rayleigh-Jeans tail of ∼10–100 K dust is expected at wavelengths longwards of ∼3 mm. Yet cosmic background imager (CBI) observations reveal that the ρ Oph W photodissociation region is surprisingly bright at centimetre wavelengths. We searched for interpretations consistent with the Wilkinson Microwave Anisotropy Probe radio spectrum, new Infrared Space Observatory-Long Wavelength Spectrograph (LWS) parallel mode images and archival Spitzer data. Dust-related emission mechanisms at 1 cm, as proposed by Draine & Lazarian, are a possibility. But a magnetic enhancement of the grain opacity at 1 cm is inconsistent with the morphology of the dust column maps Nd and the lack of detected polarization. Spinning dust, or electric-dipole radiation from spinning very small grains (VSGs), comfortably explains the radio spectrum, although not the conspicuous absence from the CBI data of the infrared circumstellar nebulae around the B-type stars S1 and SR3. Allowing for VSG depletion can marginally reconcile spinning dust with the data. As an alternative interpretation, we consider the continuum from residual charges in ρ Oph W, where most of carbon should be photoionized by the close binary HD 147889 (B2IV, B3IV). Electron densities of ∼102 cm−3, or H-nucleus densities nH > 106 cm−3, are required to interpret ρ Oph W as the C ii Strömgren sphere of HD 147889. However, the observed steep and positive low-frequency spectral index would then imply optically thick emission from an hitherto unobserved ensemble of dense clumps or sheets with a filling factor of ∼10−4 and nH∼ 107 cm−3.

Item Type: Journal Item
ISSN: 1365-2966
Keywords: radiation mechanisms; ISM; clouds; radio continuum; submillimetre
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Physical Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 17664
Depositing User: Colin Smith
Date Deposited: 09 Jul 2009 10:50
Last Modified: 07 Dec 2018 09:26
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