Centimetre-wave continuum radiation from the ρ Ophiuchi molecular cloud

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 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.

Planckearly results. XX. New light on anomalous microwave emission from spinning dust grains

“…Yet CBI observations revealed that the ρ Oph W PDR is surprisingly bright at centimetre wavelengths (Casassus et al 2008). The WMAP 33 GHz image confirms that the general location of the signal seen by CBI is offset by 10 from the centroid of the WMAP 94 GHz data, located on the Oph A core.…”

“…The situation is even worse for the warm ionised medium (WIM) component (n H < 1 cm −3 ). The spinning dust emission of these regions must therefore include a significant contribution from gas which is denser by at least a factor of 10, as suggested by Casassus et al (2008) and Castellanos et al (2011). In the preliminary models of the Perseus and ρ Ophiuchi regions presented below, we show that, assuming plausible physical conditions and PAH size distributions, spinning dust emission from dense gas can explain most of the AME.…”

“…Another clear example is that of the photodissociation region (PDR) associated with the ρ Ophiuchi molecular dust cloud. Casassus et al (2008) observed the region centred at (l, b) = (353.…”

Planck2015 results

“…The brightest AME regions in the ring are mostly located adjacent to bright thermal dust features, although the brightest AME region does not have a bright thermal dust counterpart. AME has been seen in PDRs around other H ii regions at higher resolution, e.g., in ρ Ophiuchus (Casassus et al 2008) and Perseus (Tibbs et al 2010). λ Orionis has a particularly high emissivity against 545 GHz and τ 353 , but the emissivity against 100 µm is comparable to the average; this indicates that the AME is connected to the colder dust in this region.…”

“…ρ Ophiuchus. This molecular cloud is another well-known source of AME (Casassus et al 2008;Planck Collaboration XX 2011). It is included in the PCCS2 catalogue, so we unmask the point sources when considering this region.…”