Sharp line profile of radio recombination line H29 $\alpha$ in W49A

Abstract Radio recombination line (RRL) maser is a useful tool to study massive star formation regions with ionised 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 $\alpha$ , 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 ( $\sim0.03''$ ) using the Atacama Large Millimeter/sub-millimeter Array (ALMA), reveal high brightness temperatures up to $\sim$ 9 000 K for H29 $\alpha$ emission in another two sources, which might also be regarded as maser candidates. Additionally, suggestions for efficiently identifying RRL maser candidates are also provided.