First Detection of CS Masers around a High-mass Young Stellar Object, W51 e2e

We report the discovery of maser emission in the two lowest rotational transitions of CS toward the high-mass protostar W51 e2e with the Atacama Large Millimeter/submillimeter Array and the Jansky Very Large Array. The masers from CS J = 1-0 and J = 2-1 are neither spatially nor spectrally coincident (they are separated by ∼150 au and ∼30 {km} {{s}}{}^-1), but both appear to come from the base of the blueshifted outflow from this source. These CS masers join a growing list of rarely detected maser transitions that may trace a unique phase in the formation of high-mass protostars.