Detection of a High-velocity Jet from MWC 349A Traced by Hydrogen Recombination Line Maser Emission

Abstract

MWC 349A is one of the rare stars known to have hydrogen radio recombination line (RRL) masers. The bright maser emission makes it possible to study the dynamics of the system at milliarcsecond (mas) precision. We present Atacama Large Millimeter/submillimeter Array observations of the 1.4 and 0.8 mm continuum emission of MWC 349A, as well as the H30α and H26α RRLs. Using the most extended array configuration of C43-10 with a maximum baseline of 16.2 km, we spatially resolved the H30α line and 1.4 mm continuum emission for the first time. In addition to the known H30α and H26α maser emission from a Keplerian disk at LSR velocities from −12 to 28 km s−1 and from an ionized wind for velocities between −12 to −40 km s−1 and 28 to 60 km s−1, we found evidence of a jet along the polar axis at V LSR from −85 to −40 km s−1 and +60 to +100 km s−1. These masers are found in a linear structure nearly aligned with the polar axis of the disk. If these masers lie close to the polar axis, their velocities could be as high as 575 km s−1, which cannot be explained solely by a single expanding wind as proposed in Báez-Rubio et al. We suggest that they originate from a high-velocity jet, likely launched by a magnetohydrodynamic wind. The jet appears to rotate in the same direction as the rotation of the disk. A detailed radiative transfer modeling of these emissions will further elucidate the origin of these masers in the wind.