Discovery of an H₂CO 6 Centimeter Maser in IRAS 18566+0408

We report VLA observations toward IRAS 18566+0408 conducted to determine the nature of the H 2 CO 6 cm emission line recently detected by Araya and coworkers. Our observations clearly show that the H 2 CO source is due to maser emission, making IRAS 18566+0408 only the fourth Galactic H 2 CO 6 cm maser source, out of five known H 2 CO emitters. We also report detection of a weak 2 cm continuum source that is coincident with the H 2 CO maser. Given the current observational constraints, the maser could be due to the radiative pumping mechanism proposed by Boland & de Jong; however, the coincidence of the new H 2 CO maser with 22 GHz H 2 O masers suggests that shocked molecular gas could also play a role in its excitation.