Abstract Radio recombination line (RRL) maser is a useful tool to study massive star formation regions with ionized gas close to new born massive stars. Masers often show sharp line profiles and/or extreme narrow widths, and high brightness temperatures. However, RRL masers were rarely detected only in several sources. Here we report the detection of sharp line profiles of the RRL H29 α , which can be interpreted as maser candidates, in two sources within W49A, a mini-starburst region in our Galaxy. These observations, conducted with high resolution (∼ 0.03″) using the Atacama Large Millimeter/sub-millimeter Array (ALMA), reveal high brightness temperatures up to ∼ 9000K for H29 α emission in another two sources, which might also be regarded as maser candidates. Additionally, suggestions for efficiently identifying RRL maser candidates are also provided.
Lu et al. (Tue,) studied this question.