  We report the detection of a new radio recombination line (RRL) maser object
toward the IRS2 source in the MonR2 ultracompact HII region. The continuum
emission at 1.3mm and 0.85mm and the H30a and H26a lines were observed with the
Submillimeter Array (SMA) at angular resolutions of about 0.5"-3". The SMA
observations show that the MonR2-IRS2 source is very compact and remains
unresolved at spatial scales <=400AU. Its continuum power spectrum at
millimeter wavelengths is almost flat (alpha=-0.16, with S_nu proportional to
nu^alpha), indicating that this source is dominated by optically thin free-free
emission. The H30a and H26a RRL emission is also compact and peaks toward the
position of the MonR2-IRS2 source. The measured RRL profiles are double-peaked
with the H26a line showing a clear asymmetry in its spectrum. Since the derived
line-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 maser
amplification. The amplification factors are however smaller than those found
toward the emission line star MWC349A, indicating that MonR2-IRS2 is a weakly
amplified maser. Radiative transfer modelling of the RRL emission toward this
source shows that the RRL masers arise from a dense and collimated jet embedded
in a cylindrical ionized wind, oriented nearly along the direction of the
line-of-sight. High-angular resolution observations at sub-millimeter
wavelengths are needed to unveil weakly amplified RRL masers in very young
massive stars.
