Extremely broad radio recombination maser lines toward the high-velocity
  ionized jet in Cepheus A HW2
  We present the first detection of the H40a, H34a and H31a radio recombination
lines (RRLs) at millimeter wavelengths toward the high-velocity, ionized jet in
the Cepheus A HW2 star forming region. From our single-dish and interferometric
observations, we find that the measured RRLs show extremely broad asymmetric
line profiles with zero-intensity linewidths of ~1100 kms-1. From the
linewidths, we estimate a terminal velocity for the ionized gas in the jet of
>500 kms-1, consistent with that obtained from the proper motions of the HW2
radio jet. The total integrated line-to-continuum flux ratios of the H40a, H34a
and H31a lines are 43, 229 and 280 kms-1, clearly deviating from LTE
predictions. These ratios are very similar to those observed for the RRL maser
toward MWC349A, suggesting that the intensities of the RRLs toward HW2 are
affected by maser emission. Our radiative transfer modeling of the RRLs shows
that their asymmetric profiles could be explained by maser emission arising
from a bi-conical radio jet with a semi-aperture angle of 18 deg, electron
density distribution varying as r^(-2.11) and turbulent and expanding wind
velocities of 60 and 500 kms-1.
