Pinched Magnetic Fields in the High-mass Protocluster W3 IRS5

Abstract We present polarization maps of dust emission at 340 GHz in the luminous high-mass protocluster W3 IRS5, observed with the Submillimeter Array. The projected magnetic fields appear fairly organized, with a pinched morphology in the northern part and a concave shape in the southern part. We fit the polarization maps with a two-component magnetic field model: an hourglass model centered at the continuum peak, SMM2, and an empirical sphere centered at the O-type star, IRS7. Using the Davis–Chandrasekhar–Fermi method, we calculate a projected field strength of B pos = 1.4 mG. Along with the Zeeman measurement, a total magnetic field strength of B tot = 1.6 mG is obtained. We find that gravitational energy is the most dominant, followed by magnetic energy and then turbulent energy. Small values of the virial parameter, α vir = 0.8, and the ratio of timescales, t ff / t cross = 0.6, suggest an ongoing collapse. We also show collimated molecular outflows in the CO (3−2) and SiO (8−7) transitions. The morphology of the magnetic fields and the surrounding H ii regions put forward a scenario for W3 IRS5. A gravitationally unstable dense core formed within a neutral gas ridge plowed by the expansions of W3 A and W3 B. The core began to contract, causing gravity to pull the magnetic field lines inward, which resulted in a pinched field morphology. The subsequent expansion of W3 F, ionized by IRS7, perturbed the magnetic field, creating the concave patterns. The dynamical interactions among protostars led to misalignment of their outflows.