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We report the detection of a new radio recombination line (RRL) maser objecttoward the IRS2 source in the MonR2 ultracompact HII region. The continuumemission at 1.3mm and 0.85mm and the H30a and H26a lines were observed with theSubmillimeter Array (SMA) at angular resolutions of about 0.5"-3". The SMAobservations show that the MonR2-IRS2 source is very compact and remainsunresolved at spatial scales <=400AU. Its continuum power spectrum atmillimeter wavelengths is almost flat (alpha=-0.16, with S_nu proportional tonu^alpha), indicating that this source is dominated by optically thin free-freeemission. The H30a and H26a RRL emission is also compact and peaks toward theposition of the MonR2-IRS2 source. The measured RRL profiles are double-peakedwith the H26a line showing a clear asymmetry in its spectrum. Since the derivedline-to-continuum flux ratios (80 and 180kms-1 for H30a and H26a, respectively)exceed the LTE predictions, the RRLs toward MonR2-IRS2 are affected by maseramplification. The amplification factors are however smaller than those foundtoward the emission line star MWC349A, indicating that MonR2-IRS2 is a weaklyamplified maser. Radiative transfer modelling of the RRL emission toward thissource shows that the RRL masers arise from a dense and collimated jet embeddedin a cylindrical ionized wind, oriented nearly along the direction of theline-of-sight. High-angular resolution observations at sub-millimeterwavelengths are needed to unveil weakly amplified RRL masers in very youngmassive stars.

A new radio recombination line maser object toward the MonR2 HII region