Mon. Not. R. Astron. Soc., 391, 1075-1090 (2008/December-2)
Centimetre-wave continuum radiation from the ρ Ophiuchi molecular cloud.
CASASSUS S., DICKINSON C., CLEARY K., PALADINI R., ETXALUZE M., LIM T., WHITE G.J., BURTON M., INDERMUEHLE B., STAHL O. and ROCHE P.
Abstract (from CDS):
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 Ndand 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/cm3, or H-nucleus densities nH> 106/cm3, are required to interpret ρ Oph W as the Cii 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/cm3.
© 2008 Universidad de Chile. Journal compilation © 2008 RAS
radiation mechanisms: general - ISM: clouds - radio continuum: general - radio continuum: ISM - submillimetre
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