A NEW RADIO RECOMBINATION LINE MASER OBJECT TOWARD THE MonR2 H II REGION

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 H30 alpha and H26 alpha lines were observed with the Submillimeter Array (SMA) at angular resolutions of similar to 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 (alpha = - 0.16, with S-nu proportional to nu(alpha)), indicating that this source is dominated by optically thin free-free emission. The H30 alpha and H26 alpha RRL emission is also compact and peaks toward the position of the MonR2-IRS2 source. The measured RRL profiles are double peaked with the H26 alpha line showing a clear asymmetry in its spectrum. Since the derived line-to-continuum flux ratios (similar to 80 and 180 km s(-1) for H30 alpha and H26 alpha, 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.