VLBA Observations of the Zeeman Effect in H 2 O Masers in OH 43.8‐0.1

Abstract

We present VLBA observations of the Zeeman effect in H 2 O masers in the high-mass star-forming region OH 43.8-0.1, where we observed 116 maser features. These masers may be arranged in several groups: the most prominent are an arc-shaped structure to the north, a central cluster, two groups located symmetrically around the central cluster to its northeast and southwest, and a group in the extreme south. The highest velocity (redshifted) masers are in the center of the northern arc. The observed morphology of masers in OH 43.8-0.1 suggests a stellar object (or objects) located within the central cluster of masers, driving outflows to the north and south; the redshifted and blueshifted group in the northern arc may represent the leading edge of two or more such outflows. The two groups located symmetrically around the central cluster may suggest a circumstellar disk of diameter 3000 AU. Seven masers in OH 43.8-0.1 are above our Zeeman detection limit. We detected magnetic fields in the range 10-20 mG in four of these masers and imposed sensitive upper limits on the other three. Three detections are for masers in the northern arc; the fourth is in the central cluster. We find no significant difference between the magnetic field strengths in these two groups. In the northern arc we detect a magnetic field reversal over a scale as small as 170 AU. We use our Zeeman-effect results to examine connections between the pre-and postshock magnetic fields and densities. The predicted preshock magnetic field strength and density are consistent with the fields and densities observed in typical preshock regions. The predicted postshock density also appears to be in the regime for optimal H 2 O maser pumping. Finally, we find that the magnetic and kinetic energy densities are likely in equilibrium in both pre-and postshock regions, meaning that the magnetic field must affect significantly the outflow dynamics.