Very Large Array H i Zeeman Observations toward the W49 Complex

Abstract

We report VLA HI Zeeman observations toward the W49A star-forming region and the SNR W49B. Line of sight magnetic fields (Blos) of 60 to 300 muG at 25" resolution were detected toward W49A at velocities of ~4 and ~7 km/s. The Blos values measured toward W49A show a significant increase in field strength with higher resolution especially for the ~4 km/s HI gas. The HI gas in the velocity range -5 to 25 km/s toward W49A shows good agreement both kinematically and spatially with molecular emission intrinsically associated with W49A. Based on comparisons with molecular data toward W49A, we suggest that the 4 km/s HI gas is directly associated with the northern part of the HII region ring, while the 7 km/s HI gas seems to originate in a lower density halo surrounding W49A. We estimate that the W49A North core is significantly subvirial, and that the total kinetic +magnetic energies amount to less than 1/3 of the total W49A North gravitational energy. These magnetic field results suggest that W49A North is unstable to overall gravitational collapse in agreement with evidence that the halo is collapsing onto the W49A North ring of HII regions. The majority of the HI column density toward W49B comes from Sagittarius Arm clouds along the line of sight at ~40 km/s and ~60 km/s. No significant magnetic fields were detected toward W49B. Comparison of the spectral distribution of HI gas toward W49A and W49B suggests that evidence placing W49B 3 kpc closer to the sun (i.e. at 8 kpc) than W49A is quite weak.