Centimetre-wave continuum radiation from the ρ Ophiuchi molecular cloud

December 2008 • 2008MNRAS.391.1075C

Authors • Casassus, Simon • Dickinson, Clive • Cleary, Kieran • Paladini, Roberta • Etxaluze, Mireya • Lim, Tanya • White, Glenn J. • Burton, Michael • Indermuehle, Balt • Stahl, Otmar • Roche, Patrick

Abstract • 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 N_d 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 ~10²cm^-3, or H-nucleus densities n_H > 10⁶cm^-3, 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 n_H ~ 10⁷cm^-3.

Links

• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2008MNRAS.391.1075C
• PDF https://academic.oup.com/mnras/pdf-lookup/doi/10.1111/j.1365-2966.2008.13954.x
• PREPRINT http://arxiv.org/abs/0809.3965
• DATA http://archive.eso.org/bin/ads2eso?2008MNRAS.391.1075C
• DATA https://irsa.ipac.caltech.edu/bibdata/2008/C/2008MNRAS.391.1075C.html
• ELECTR http://dx.doi.org/10.1111/j.1365-2966.2008.13954.x