Probing the magnetic fields and dust properties in the young embedded star-forming region AFGL 6366S using Near Infrared and Optical linear polarimetry

Abstract

We present Near-Infrared (NIR) and Optical linear polarimetry towards the partially embedded cluster AFGL 6366S. The polarization ranges from 0.44-10.3 per cent in NIR and 0.16-11.22 per cent in Optical bands. The position angle spans $1^\circ - 179^\circ$ in both NIR and Optical bands. About 22 stars exhibit intrinsic polarization signatures. A polarization hole is evident towards the densest ($\sim 3.4 \times 10^{23} \mathrm{cm}^{-2}$) and warmest ($\sim 28.8 \mathrm{K}$) central cluster region. It is attributable to depolarization induced by Radiative Torque Disruption (RAT-D) of large grains and a modest contribution from magnetic-field tangling. The local magnetic field towards the cluster's central region is significantly misaligned with both the large-scale Galactic field and the long axis of the filament present in the region. The field morphology wraps around two dense molecular clumps of radii 0.34 pc and 0.22 pc and $\mathrm N(\mathrm H_2)$ = $(7.9 \pm 1.1) \times 10^{22}$ cm$^{-2}$ and $(4.3 \pm 0.5) \times 10^{22}$ cm$^{-2}$, respectively. The clumps are embedded in the filamentary structure and represent locally accelerated stages of mass accumulation. Gravitationally driven mass flows, largely perpendicular to the local magnetic field, produce a U-shaped field curvature across the filament axis. The plane-of-sky magnetic field strengths towards the two clumps are $ 447.91 \pm 83.81 μ\mathrm{G}$ and $396.66 \pm 73.64 μ\mathrm{G}$. The corresponding mass-to-flux ratios ($λ\sim 1.34$ and $0.82$) indicate that one clump is magnetically supercritical and the other is subcritical. The Alfven Mach numbers ($\mathcal{M}_A$) $\sim$ 0.395 and 0.393 indicate that both the clumps are in sub-Alfvénic state.

Figures (11)

• Figure 1: Polarization map of AFGL 6366S showing the NIR (red square) and Optical (large blue circle) polarimetric observation fields superimposed on a 15$'$ × 15$'$ WISE 3.4 $\mu m$ image. The sources observed in NIR and Optical are marked by small red and blue circles, respectively. The observed star IDs according to Table \ref{['Tab:NIR_pol']} and/or Table \ref{['Tab:Opt_pol']}, are annotated. The green star marks represents the cluster members identified from https://academic.oup.com/mnras/article/541/2/1557/8173845. The red and/or blue polarization vectors on each star represent their $H$ band and/or $R_c$ band polarization, respectively. The length of the polarization vectors is proportional to degree of polarization (in per cent). The inclination of the polarization vectors with respect to the north-south direction indicates the position angle of polarization. A line of magnitude of 1 per cent polarization value is drawn for reference. The dashed black line indicates the orientation of the projection of the Galactic plane at b = $0.78^{\circ}$. The yellow cross indicates the centre of the cluster AFGL 6366S at R.A. (J2000) = $06^h08^m41^s$ and Dec. (J2000) = $+21^{\circ}31'01"$.
• Figure 2: (a) Histograms of polarization degree (per cent) for all stars (in upper panel) and member stars (in lower panel) in $JHK_sBVR_cI_c$ bands. (b) Histograms of polarization angle (degrees) for all stars (in upper panel) and member stars (in lower panel) in $JHK_sBVR_cI_c$ bands.
• Figure 3: The plot of $P/P_{\text{max}}$ against $\lambda_{\text{max}}/\lambda$ showing the normalized polarization-wavelength dependence for 19 stars towards AFGL 6366S in the $BV(RI)_c$ wavelength bands. The black points denote stars satisfying the $P/e_p > 3$ criterion in all four Optical bands. The colored points indicate stars meeting this criterion in three bands only: red ($VR_cI_c$), green ($BR_cI_c$), and purple ($BVI_c$). These are also indicated in the legend. The uncertainties for the measurements of $P/P_{\text{max}}$ are represented by the error bars and the solid curve reflects the Serkowski polarization relation for general diffuse ISM. The co-efficient of determination of fit ($R^2$) between the observed data and serkowski relation is shown in the top right corner.
• Figure 4: Plots of (a) $P_H$ versus $P_J$ and (b) $P_H$ versus $P_{K_s}$ for 36 sources (black filled circles) satisfying $P/e_P > 3$ in all three $JHK_s$ bands and exhibiting a monotonic decrease in polarization from 1.25 to 2.14 $\mu$m. Error bars represent the corresponding polarization uncertainties. The red dashed lines and their corresponding shaded regions denote the best-fit relations and their 1$\sigma$ uncertainties, while the black dashed line and gray shaded region represent the whittet_1992 model. These relations are indicated by the corresponding colors in the legends of both panels.
• Figure 5: $P_H$ versus $(H-K_s)$ diagram for 53 stars (black filled circles) from Table \ref{['Tab:NIR_pol']}, satisfying the $P/e_p > 3$ criterion and having high-quality 2MASS photometry. The horizontal and vertical error bars represent uncertainties in color and polarization, respectively. The black solid line represents the best-fit linear relation. The best-fit slope is shown in legend at the top left corner. The dashed red and blue lines correspond to slopes twice and half of the best-fit slope, respectively. Probable sources of intrinsic polarization and stars exhibiting depolarization are marked using red and blue crosses, respectively.