Signposts of shock-induced magnetic field compression in star-forming environments

In star-forming environments, shock-compressed magnetic fields occur in cloud-cloud collisions, in molecular clouds exposed to supernova remnants (SNRs), and in photo-dissociation regions (PDRs). Besides their dynamical role, they increase the cosmic ray flux above the Galactic average, and the trapped particles contribute to the heating of the shocked gas. The associated dust emission is polarized perpendicularly to the sky plane projection of the field, B_sky. In edge-on viewed shock planes, highly ordered polarization patterns are expected. In search of such a signature, the dust emission from the Orion bar (a prototypical PDR) and from a molecular cloud/SNR interface (IC443-G) was studied with a λ870μm polarimeter at the APEX (Wiesemeyer etal 2014 and references therein). While our polarization map of OMC1 confirms the hourglass shape of B_sky (e.g., Schleuning 1998, Houde etal 2004), a deep integration towards the Orion bar reveals an alignment of B_sky with the shock forming in response to the wind and to the ionizing radiation from the Trapezium cluster (Fig. 1). This structure suggests a compressed magnetic field accelerating cosmic-ray particles, a scenario proposed by [Pellegrini et al. (2009)] to explain the high excitation temperature of rotationally warm H₂ and CO (Shaw et al. 2009, Peng et al. 2012, respectively).