Looking into the spectral response of the polarized dust emission can tell us a great deal about the population of dust grains including size distribution, shape, and composition. In order to discern between the possibilities there have been extensive efforts to model the spectral response while varying any number of the aforementioned parameters. However, most models are only loosely constrained in the FIR and submillimeter part of the spectrum due to the sparseness of available data which has resulted in no currently available comprehensive model. BLASTPol data provides a robust contribution to the field of study with the polarization spectra we are able to produce across our three bands which will help refine models. Additionally, maps at 850µm from Planck were used to increase our spectral coverage. However, doing so required smoothing all BLASTPol maps to the Planck resolution of 50.
In order to create a spectrum we first calculated ratios of prelative to the 350µm band to allow easy comparison of the shape of the spectra to other studies. This also removes the variability between studies of the absolute magnitude of the polarization fraction. Once maps of p250/p350,p500/p350, and p850/p350 had been made, a rigorous
set of cuts were performed to ensure only the highest signal to noise and most reliable data was used. To reduce the possibility that different detector bands are sampling different point along the LOS, points were removed where the polarization angle φ
differs by more than 10 degrees between any two of the maps. Additionally, a cut was done to remove low signal to noise points where p < 3σp in any of the four bands observed. The analysis is also restricted to specified regions, for example in Gandilo et al. (2015) only points within the Hill et al. (2010) regions were used which effectively provided a cutoff of AV <7. Additionally, the region around RCW36 was masked in the analysis discussed here.
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Median p-ratio Slope of p vs p350 Median power-law Median ± MAD power lawMedian polynomial 68% error polynomial
Figure 5.13: Red data points are derived from BLASTPol maps with the 850µm point from the Planck polarization map. The distribution over the whole cloud is used to derive the points and their errors as detailed in Gandilo et al. (2015). The gray points are from previous studies.
The p ratio points and error bars were determined in two ways. In the first the median of all points in the ratio map was taken and then a median of the distribution around the median was taken to determine error bars, referred to as Median Absolute Deviation (MAD). As an additional check to ensure the robustness of the polarization spectra shape, a linear fit was performed to p250 vs. p350, p500 vs. p350, and p850 vs. p350 to determine the ratios with error bars derived from the uncertainty in the fit.
The result of the spectral analysis is very interesting as it predicts a flat polariza- tion spectra in the Vela GMC around 350µm whereas previous studies had found a steeply falling or rising spectrum as seen in the gray data points in Figure 5.13. In
Hildebrand et al. (1999) it was postulated that the steep spectrum was caused by dust grains with at least two temperature components along the line of sight. In this case the rise to shorter wavelengths is due to the warmer dust grain component experience better alignment. This possibility fits with RAT as the warmer component would likely be exposed to a more intense radiative environment which would increase the efficiency of RAT for that environment.
In Vaillancourt et al. (2008) it was proposed that the rise at longer wavelengths could also be consistent with a two temperature component model with the additional effect of different grain emissivities explaining the rising spectrum. For both these arguments it is important to note that the gray data points in Figure 5.13 were generally looking at clouds that showed stronger evidence of internal heating than the regions we examine in Vela C. Therefore, in the other studies there may be two temperature components along the line of sight, cooler areas of the cloud that were ISRF heated along with a hot component heated internally by active star forming regions. Additional research could focus on discerning the temperature components along the LOS in these clouds in an attempt to better constrain the environments that were sampled by the polarization measurements.
In the case of Vela C, after excluding RCW36, we expect to be sampling a single ISRF heated dust grain population with most the intensity coming from the high column density portion that is internal to the cloud and radiatively shielded. In such a case of a single population of dust grains we would not expect to see any change in the efficiency of grain alignment with wavelength in our spectral range which serves well to explain the relatively flat spectrum observed. Our result is significant as it is one of the first submillimeter measurements that both demonstrates a flat spectrum and has complementary data sets that allow us to understand the environment being measured.