Magnetic fields and Star Formation around HII regions: The S235 complex

Abstract

Magnetic fields are ubiquitous and essential in star formation. In particular, their role in regulating formation of stars across diverse environments like HII regions needs to be well understood. In this study, we present magnetic field properties towards the S235 complex using near-infrared (NIR) $H$-band polarimetric observations, obtained with the Mimir and POLICAN instruments. We selected 375 background stars in the field through combination of Gaia distances and extinctions from NIR colors. The plane-of-sky (POS) magnetic field orientations inferred from starlight polarization angles reveal a curved morphology tracing the spherical shell of the HII region. The large-scale magnetic field traced by Planck is parallel to the Galactic plane. We identified 11 dense clumps using $1.1\,\mathrm{mm}$ dust emission, with masses between $33-525\,\rm M_\odot$. The clump averaged POS magnetic field strengths were estimated to be between $36-121\,\mathrm{\mu G}$, with a mean of ${\sim}65\,\mathrm{\mu G}$. The mass-to-flux ratios for the clumps are found to be sub-critical with turbulent Alfv\'{e}n Mach numbers less than 1, indicating a strongly magnetized region. The clumps show scaling of magnetic field strength vs density with a power-law index of $0.52\pm0.07$, similar to ambipolar diffusion models. Our results indicate the S235 complex is a region where stellar feedback triggers new stars and the magnetic fields regulate the rate of new star formation.