A strong magnetic field in the disk of MWC 349

Using Zeeman observations of the H30α recombi-nation line maser transition at 1.3 mm we have detected a magnetic field which is associated with the corona of the circum-stellar disk of MWC 349. At a radial distance of 40 a.u., where the H30α maser is located, the line–of–sight component of the field is approximately parallel to the plane of this edge–on disk, and its average strength is 22 mG. The corresponding magnetic energy density is ∼ 70% of the thermal energy density of the plasma where the maser emission originates, very likely making the detected field dynamically important. Spectral fine structure of the detected Zeeman pattern suggests that the field may have a strong radial component, although other models for the field configurations are possible. The strength of the field at such a large distance from the star makes it unlikely that the field is of stellar origin. We suggest that it is generated by a local disk dynamo.