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We present the first detection of the H40a, H34a and H31a radio recombinationlines (RRLs) at millimeter wavelengths toward the high-velocity, ionized jet inthe Cepheus A HW2 star forming region. From our single-dish and interferometricobservations, we find that the measured RRLs show extremely broad asymmetricline profiles with zero-intensity linewidths of ~1100 kms-1. From thelinewidths, 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 HW2radio jet. The total integrated line-to-continuum flux ratios of the H40a, H34aand H31a lines are 43, 229 and 280 kms-1, clearly deviating from LTEpredictions. These ratios are very similar to those observed for the RRL masertoward MWC349A, suggesting that the intensities of the RRLs toward HW2 areaffected by maser emission. Our radiative transfer modeling of the RRLs showsthat their asymmetric profiles could be explained by maser emission arisingfrom a bi-conical radio jet with a semi-aperture angle of 18 deg, electrondensity distribution varying as r^(-2.11) and turbulent and expanding windvelocities of 60 and 500 kms-1.

Extremely broad radio recombination maser lines toward the high-velocity ionized jet in Cepheus A HW2