Magnetic field structures in star-forming regions: mid-infrared imaging polarimetry of K3-50

Abstract

We report new imaging polarimetry observations of the Galactic compact HII region K3-50 using CanariCam at the Gran Telescopio Canarias. We use a standard polarimetric analysis technique, first outlined by Aitken, to decompose the observed polarisation images centred at 8.7, 10.3, and 12.5 µm into the emissive and absorptive components from silicate grains that are aligned with the local magnetic field. These components reveal the spatially-resolved magnetic field structures across the mid-infrared emission area of K3-50. We examine these structures and show that they are consistent with previously observed features and physical models of K3-50, such as the molecular torus and the ionised outflow. We propose a 3D geometry for all the structures seen at different wavelengths. We also compute relevant physical quantities in order to estimate the associated magnetic field strengths that would be implied under various physical assumptions. We compare these results with MHD simulations of protostar formation that predict the magnetic field strength and configuration. We find that the magnetic field may be dynamically important in the innermost 0.2 pc of the molecular torus, but that the torus is more likely to be rotationally-supported against gravity outside this radius. Similarly, magnetic fields are unlikely to dominate the global physics of the ionised outflow, but they may be important in helping confine the flow near the cavity wall in some locations. Ours is the first application of the Aitken technique to spatially-resolved magnetic field structures in multiple layers along the line of sight, effectively a method of "polarisation tomography."

Mid-infrared polarization of Herbig Ae/Be discs

“…It also assumes that the absorptive polarization profile found for the BN Object in the Orion Molecular Cloud applies to other objects (see Aitken et al 2004 for more discussion). Despite these limitations, the effectiveness of this method to interpret the mid-IR polarization from YSOs has been demonstrated in a number of studies to date (e.g., Smith et al 2000, Barnes et al 2015, L16, and Zhang et al 2017b). We applied this method to our sample, with results presented in Table 5, in which the columns 2-6 show the fitted components when both emissive and absorptive polarization are included in the fit.…”

The magnetic field and dust filaments in the Polaris Flare

“…We can obtain an alternative estimate of Bpos by using the observed angular dispersion δθ obs found in section 2.4.1 from the Gaussian fit to the distribution of polarization angles (see for example section 4.2.3 of Barnes et al 2015). Ostriker, Stone, & Gammie (2001) applied the DCF method to numerical simulations of MHD turbulence using this type of angle dispersion calculation and found that the method provides a good estimate of Bpos when δθ 25 • .…”

The mid-infrared polarization of the Herbig Ae star WL 16: an interstellar origin?

“…In practice, the absorptive profile, p abs (λ ), is taken as that of the Becklin-Neugebauer (BN) object, which contains absorptive polarization alone, and the extinction curve, τ(λ ), is derived from the observations of the Trapezium region of Orion. Aitken's method has been successfully used by a number of studies to separate emissive and absorptive polarization components from the observed total polarization when spectral polarimetry, or imaging polarimetry at multiple wavelengths, is available (e.g., Smith et al 2000;Barnes et al 2015).…”

“…Polarimetry is a potentially incisive observational probe of B-field morphology (Barnes et al 2015;Crutcher 2012;Matthews et al 2009;Hull et al 2014). Dust grains can polarize light by scattering, dichroic absorption, or dichroic emission, the latter two processes attributed to nonspherical dust grains with their long axes aligned perpendicular to the B-field lines, perhaps as a result of radiative torque (Lazarian 2007;Hoang & Lazarian 2014).…”

“…In the midinfrared (mid-IR, 8-30 µm), the situation becomes more complicated, since the observed polarization can be a combination of dichroic absorption, emission, and/or scattering (Aitken 2005). Nevertheless, mid-IR polarimetry has some distinct advantages compared to other wavelength regions: the predicted emissive polarization is much larger than that at longer wavelengths (Cho & Lazarian 2007), and we can potentially disentangle both the absorptive and emissive polarization components simultaneously along the line of sight and thereby infer the three-dimensional structure of the Bfields (Barnes et al 2015;Smith et al 2000).…”