SIMPLIFI-Study of Interstellar Magnetic Polarization: A Legacy Investigation of Filaments. II. Enhancement of grain alignment near embedded protostars in the DR21 Ridge

Abstract

Thermal dust continuum polarimetry is a powerful indirect probe of magnetic field geometry in dense molecular clouds while at the same time providing information on the alignment of dust grains with the magnetic field. The leading theory of grain alignment, Radiative Torque Alignment (RAT), has been successful in explaining a variety of observations, including the loss of polarization fraction toward high column densities. One prediction of RAT is that an increase in grain alignment efficiency should be observed in the environments surrounding protostars, due to radiation from the embedded source. However, observational confirmation of this prediction remains scarce. In this study, we sought to test the theoretical prediction of enhanced grain alignment near protostars in the high-mass star forming region DR21 using 214 $μm$ SOFIA/HAWC+ observations. We investigated the correlation of the polarization fraction of dust emission, $p$, and the polarization angular dispersion, $S$, with respect to total intensity. We also probed intrinsic dust polarization properties using the product $S\times p$ as a proxy. We detected significant polarization fractions even at the highest intensities, where strong depolarization is typically expected. The polarization fraction-intensity trend flattens at $I > 1.6^{+0.3}_{-0.3} \times 10^4$ MJy/sr ($N_{H_2}$ $\sim 2\times 10^{23}$ ${cm}^{-2}$). We compared the observed trends with predictions from an analytical model of a centrally heated envelope surrounding an embedded luminous protostar. The predictions from the simple model agree well with the observed trends. Our results provide strong support for enhancement of grain alignment by local radiation from embedded sources.

Figures (13)

• Figure 1: Comparison of observed SOFIA 214 µ m brightness $I_{\textit{SOFIA}{}}$ versus 250 µ m Herschel brightness $I_{Herschel}$ (left A panel) and $N_{\rm H_2}$ (right panel B). In panel A, blue, orange and green dotted lines represent the reference line for 100 % ($I_{\textit{SOFIA}{}}$ = $I_{ Herschel}$), 50 % ($I_{\textit{SOFIA}{}}$ = $0.5I_{ Herschel}$) and 25 % ($I_{\textit{SOFIA}{}}$ = $0.25 I_{Herschel}$) recovery, respectively. The dark blue dashed line is the best‐fit $I_{\textit{SOFIA}{}}\propto I_{ Herschel}^{1.421\pm 0.004}$. Similarly in panel B, the dark-blue dashed line is the best‐fit $I_{\textit{SOFIA}{}}\propto I_{N_{\rm H_2}}^{1.24\pm 0.01 }$.
• Figure 2: Main data products derived from SOFIA/HAWC+ observations used in the study, after application of quality cuts. The SOFIA/HAWC+ map footprint is shown as a light shadow in panel A. Panel A: map of Stokes $I$, panel B: map of polarization fraction $p$, panel C: map of debiased angular dispersion $S$, with $\delta = 20\overset{\prime\prime}{.}{}3$ and panel D: map of $S\times p$. The two massive proto-stellar regions, DR21(OH) and DR21(M) are labeled in panel A. The DR21 Ridge is inside the broken ellipse outlined in red, the DR21 sub-filaments are located outside of the red ellipse. The mask for the outflow is shown as a gray, filled ellipse.
• Figure 3: Trends of $p$ versus $I$ (top) and $p$ versus $S$ (bottom). The data points are plotted in gray with the Ridge in panels A and C and the sub-filament pixel values plotted in B and D. The single power-law fit is shown as a dashed dark blue line in all four panels. In the case of $p$ vs $I$, for the Ridge (panel A), a broken power-law fit (solid light blue) is also included. The break in intensity is marked by a vertical red dot-dashed line and the error in the break point location is shown as a red shading ($\pm1\sigma$) around the vertical line. In the case of $p$ vs $S$, for both the plots (bottom), the saturation limit of angular dispersion is marked by a magenta dot-dashed line at $29.2^{\mathrm{o}}$.
• Figure 4: Similar to Figure \ref{['fig:p_vs_S_and_I']} but showing the relationship between $S$ versus $I$ (panels A and B) and $S\times p$ versus $I$ (panels C and D). The data points are plotted in gray with the Ridge in panels A and C and the sub-filaments in B and D. The saturation limit of the angular dispersion is marked by a magenta dot-dashed line at $29.2^\circ$. In the $S\times p$ plot for the Ridge (panel C), in addition to a single power law, a two break broken power law is fitted shown in solid light blue. The breaks are marked by vertical red dot-dashed lines, with error ranges ($\pm1\sigma$) shown with red shading
• Figure 5: Panel A (left) displays the Herschel/PACS 70 µ m intensity map of DR21 with the Ridge outlined in red. Contours are plotted at levels of [2.61, 3.05, 3.40, 4.21] (log MJy/sr), corresponding to the 50th, 90th, 95th, and 99th pixel value histogram percentiles, respectively. The light-green outline indicates the spatial extent where the increase in grain alignment efficiency is observed. Panel B (right) shows $S \times p$ versus $I$ for the Ridge, with data points in gray. Data points exhibiting increased grain alignment efficiency are shown in light green. The transition is marked by a red dot-dashed line at $1.5^{+0.2}_{-0.1} \times 10^4$ MJy/sr, with the $\pm1\sigma$ uncertainty in the break location shown as red shading.