Optical/Near-IR Polarization Survey of Sh 2-29: Magnetic Fields, Dense Cloud Fragmentations and Anomalous Dust Grain Sizes

Abstract

Sh 2-29 is a conspicuous star-forming region marked by the presence of massive embedded stars as well as several notable interstellar structures. In this research, our goals were to determine the role of magnetic fields and to study the size distribution of interstellar dust particles within this turbulent environment. We have used a set of optical and near-infrared polarimetric data obtained at OPD/LNA (Brazil) and CTIO (Chile), correlated with extinction maps, 2MASS data and images from DSS and Spitzer. The region's most striking feature is a swept out interstellar cavity whose polarimetric maps indicate that magnetic field lines were dragged outwards, pilling up along its borders. This led to a higher magnetic strength value ($\approx 400\mu$G) and an abrupt increase in polarization degree, probably due to an enhancement in alignment efficiency. Furthermore, dense cloud fragmentations with peak $A_V$ between 20 and 37 mag were probably triggered by its expansion. The presence of $24\mu$m point-like sources indicates possible newborn stars inside this dense environment. A statistical analysis of the angular dispersion function revealed areas where field lines are aligned in a well-ordered pattern, seemingly due to compression effects from the HII region expansion. Finally, Serkowski function fits were used to study the ratio of the total-to-selective extinction, reveling a dual population of anomalous grain particles sizes. This trend suggests that both effects of coagulation and fragmentation of interstellar grains are present in the region.

20 pages, 12 figures, 2 tables; Accepted for The Astrophysical Journal

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