Polarization Structure of Magnetically Supported Molecular Filaments

Observations of thermal dust emissions with Herschel satellite have revealed that molecular clouds consist of many filaments (Menshchikov et al. 2010). That is, the molecular filaments are the building blocks of interstellar clouds. On the other hand, near IR interstellar polarization indicates the filaments are extending in the perpendicular direction to the interstellar magnetic field (Sugitani et al. 2011).

Magnetohydrostatic equilibrium solutions of isothermal clouds, in which the gravity is balanced with the Lorentz force, thermal pressure and the external pressure, are obtained with a self-consistent field method. We obtained an empirical relation between the maximum supported mass against the self-gravity, λ_max(Tomisaka 2014). Here, we studied polarization structures of the thermal dust emissions expected for these filaments.

The polarization of the thermal dust emissions comes from dusts which are aligned to the interstellar magnetic field. In the figure, color and bars represent, respectively, the polarization degree and the direction of polarization (B-vector of the electromagnetic wave). We chose a line-of-sight of θ=80deg and φ=90 deg in the left panel. That is, a filament extending in the z-direction is assumed to be observed from the lateral direction nearly parallel to the large-scale magnetic field. The middle panel corresponds to a low-density filament (center-to-surface density ratio F = 10) and the right one is a high-density filament with F=300.

When we observe the filament from direction of the interstellar magnetic field, we expect extremely low polarization degree since dusts have no special alignment in the plane of sky (middle panel). On the contrary, high-density filaments indicate relatively large polarization degree whose directions are perpendicular to the filament (right panel). This may explain the reason why the perpendicular configuration is often found between the interstellar magnetic field and the filaments.
(2015/5/19)
Polarization Structure of Filamentary Clouds, by Tomisaka, Kohji, 2015, ApJ, 807, 47(10pp)
[ADS] [arXiv]
Kohji Tomisaka (private website)