Polarization of Dust Emission in Clumpy Molecular Clouds and Cores

Abstract

We observed three molecular clouds and four isolated cores at wavelengths from 3.6− 24 µm. The clouds we observed were Ophiuchus, Perseus, and Serpens and the cores were L204C-2, L1152, L1155C-2, and L1228. Our goal was to use these deep infrared data to map changes in the extinction law and the dust properties throughout the observed regions. In our clouds, we found the lowest density regions have an IRAC extinction law similar to the one observed in the diffuse ISM. At higher extinctions, there is evidence for grain growth because the extinction law flattens compared to the diffuse ISM law and becomes more consistent with a model utilizing larger dust grains. In the densest regions of Serpens and Perseus, A K s ≥ 2, it appears icy mantles are forming on the dust grains. We detected one low extinction region in Perseus with an anomalous extinction law that is not explained by current ideas about grain growth or the formation of ices onto dust grains. The extinction law in the cores shows only a slight flattening of the extinction law with increased extinction. Even at the lowest extinctions, the extinction law is more consistent with a dust model containing grain growth, rather than with the diffuse ISM. Two of the four cores have evidence for ices forming the densest regions. Molecular outflows appear to have an impact on the dust grains in two of our cores: L1152 and L1228. In both our clouds and cores, the extinction law at 24 µm is almost always higher than the value predicted by current dust models, but is consistent with other observations. We find some evidence for the 24 µm extinction law decreasing as the extinction increases. Overall, there are relatively few stars with detections ≥ 3σ at 24 µm. More observations are needed to understand the nature of the extinction law at this wavelength.

Grain Alignment and Polarized Emission from Magnetized T Tauri Disks

“…However, being driven by anisotropic radiation, grains get aligned and it was argued in Lazarian (2007) that the parameterization of the radiative torques in terms of the maximal achievable angular velocity is a valid one. Incidentally, this was also the parameterization that we used in CL05 and in the subsequent paper by Bethell et al (2007).…”

Modelling grain alignment by radiative torques and hydrogen formation torques in reflection nebula

“…For the latter, we adopt the radiation field from Mathis, Mezger & Panagia (1983) (hereafter MMP), where the mean radiation intensity J MMP λ inside a giant molecular cloud located at 5 kpc from the Galactic center is calculated for different visual extinctions A MMP V from the surface (see also Cho & Lazarian 2005). A degree of anisotropy γ rad = 0.35 is assumed, as numerically calculated in Bethell et al (2007). The stellar radiation from γ Cas is completely anisotropic with γ rad = 1.…”

“…Grain alignment by RATs in starless, dense clouds has been studied by a number of authors (Cho & Lazarian 2005;Bethell et al 2007;Whittet et al 2008). These studies dealt with the alignment of grains by RATs induced by the attenuated ISRF.…”

“…Lastly, with our aim to model the fundamental properties of RAT alignment theory, our present study does not consider explicitly the effects of the decrease of polarization due to magnetic turbulence (see Cho & Lazarian 2005;Bethell et al 2007). We expect our shallow slope for moderate AV will become steeper in the presence of magnetic turbulence.…”

“…Grain alignment by RATs in molecular clouds has been studied extensively (Cho & Lazarian 2005;Bethell et al 2007;Whittet et al 2008;Pelkonen et al 2009). These studies dealt with the alignment of grains by the attenuated diffuse interstellar radiation field (ISRF).…”