2013ApJ...764L...4J

We report the detection of a new radio recombination line (RRL) maser object toward the IRS2 source in the MonR2 ultracompact H II region. The continuum emission at 1.3 mm and 0.85 mm and the H30α and H26α lines were observed with the Submillimeter Array (SMA) at angular resolutions of ∼0.''5-3''. The SMA observations show that the MonR2-IRS2 source is very compact and remains unresolved at spatial scales ≤ 400 AU. Its continuum power spectrum at millimeter wavelengths is almost flat (α = -0.16, with S_ν∝ν^α), indicating that this source is dominated by optically thin free-free emission. The H30α and H26α RRL emission is also compact and peaks toward the position of the MonR2-IRS2 source. The measured RRL profiles are double peaked with the H26α line showing a clear asymmetry in its spectrum. Since the derived line-to-continuum flux ratios (∼80 and 180 km/s for H30α and H26α, 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 modeling 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 submillimeter wavelengths are needed to unveil weakly amplified RRL masers in very young massive stars.