Modelling the spinning dust emission from LDN 1780

Abstract

ABSTRACT We study the anomalous microwave emission (AME) in the Lynds Dark Nebula (LDN) 1780 on two angular scales. With publicly available data at an angular resolution of 1°, we studied the spectral energy distribution of the cloud in the 0.408–2997 GHz frequency range. The cloud presents a significant (>20σ) amount of AME, making it one of the clearest examples of AME on 1 ° scales, and its spectrum can be well fitted with a spinning dust (SD) model. We also find at these angular scales that the location of the peak of the emission at lower frequencies (23–70 GHz) differs from the location at the higher frequencies (90–3000 GHz) maps. In addition to the analysis on 1° angular scales, we present data from the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at 31 GHz with an angular resolution of 2 arcmin, in order to study the origin of the AME in LDN 1780. We studied morphological correlations between the CARMA map and different infrared tracers of dust emission. We found that the best correlation is with the 70- μm template, which traces warm dust (T ∼ 50 K). Finally, we study the difference in radio emissivity between two locations within the cloud. We measured a factor of ≈6 difference in 31-GHz emissivity. We show that this variation can be explained, using the SD model, by a variation on the dust grain size distribution across the cloud, particularly changing the fraction of polycyclic aromatic hydrocarbon for a fixed total amount of carbon.