New H2O maser near NGC7538

Abstract

A NEW, strong, water vapour maser has appeared at the position of a very compact radio and infrared source near the H II region NGC7538. The radio source is NGC7538-B (infrared source IRS-1), the most intense of a group of three compact H II regions1–4 at a distance of 3 kpc from the Earth. The continuum source is optically thick below 20 GHz (ref. 5), and thus has one of the highest emission measures yet observed in a compact H II region. At 10 µm, it has one of the strongest silicate absorption features known6, and this source and the KL nebula in Orion are the only sources in the Galaxy which show formaldehyde in emission at 4.8 GHz (ref. 7). The area has been searched many times during the past 5 yr without the detection of this particular H2O maser. Its sudden appearance may be related to the dynamics of the high-density shell surrounding the compact ionised region.

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