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Keywords:

  • radiation mechanisms: general;
  • ISM: individual objects: LDN 1780;
  • ISM: individual objects: ζ Ophiuchi;
  • radio continuum: ISM

ABSTRACT

The diffuse cm wave IR-correlated signal, the ‘anomalous’ CMB foreground, is thought to arise in the dust in cirrus clouds. We present Cosmic Background Imager (CBI) cm wave data of two translucent clouds, ζ Oph and LDN 1780 with the aim of characterizing the anomalous emission in the translucent cloud environment.

In ζ Oph, the measured brightness at 31 GHz is 2.4σ higher than an extrapolation from 5-GHz measurements assuming a free–free spectrum on 8 arcmin scales. The SED of this cloud on angular scales of 1° is dominated by free–free emission in the cm range. In LDN 1780 we detected a 3σ excess in the SED on angular scales of 1° that can be fitted using a spinning dust model. In this cloud, there is a spatial correlation between the CBI data and IR images, which trace dust. The correlation is better with near-IR templates (IRAS 12 and 25 μm) than with IRAS 100 μm, which suggests a very small grain origin for the emission at 31 GHz.

We calculated the 31-GHz emissivities in both clouds. They are similar and have intermediate values between that of cirrus clouds and dark clouds. Nevertheless, we found an indication of an inverse relationship between emissivity and column density, which further supports the VSGs origin for the cm emission since the proportion of big relative to small grains is smaller in diffuse clouds.