📝 SummarXiv

Abstract

We extend previous theoretical works to gain a better understanding of the origin of observed polarisation degree spectra of molecular clouds, which show a so-called V-shape, i.e. a pronounced minimum around 350~$\mu$m. For this purpose, we present results of two-phase dust models investigated with POLARIS. We also provide a guideline to calculate individual dust temperatures for different grain types in POLARIS. We show that V-shaped polarisation spectra can only be obtained if two dust phases, one dense and cold as well as one warm and dilute phase, are present along the line of sight. We find that the V-shape is the stronger pronounced the larger the density and temperature contrast between both phases is. In contrast to previous results, no correlation between the alignment efficiency of silicate grains and the dust temperature is required; carbonaceous grains are in general assumed to be not aligned with the magnetic field. By matching our model results with actual observations of V-shaped polarisation spectra, we show that in UV-illuminated regions (here the warm and dilute phase) carbon grain destruction might take place. This leads to a more pronounced V-shape with a minimum around 300~$\mu$m. In addition, we show that the dust spectral index and temperature of silicate grains affects the steepness of the polarisation spectrum at long wavelengths. Finally, we present a first polarisation spectrum obtained from a 3D, magneto-hydrodynamical molecular cloud simulation. It shows a flattening or even weakly pronounced minimum around 350~$\mu$m demonstrating the potential of such complex 3D simulations to study polarisation spectra.