¹⁵NH₃ millimasers toward NGC 7538—IRS 1

Abstract

Ammonia towards the UCHII—region NGC 7538—IRS 1 shows an extremely hot component observed in absorption in several transitions and a so far unexplained (9,6) maser (see Fig. 1). This source is the only known source in the Galaxy where the 141 GHz line (1450 K above ground) of vibrationally excited Ammonia is seen in absorption (Schilke et al. 1990). An additional unique feature is the ¹⁵NH₃(3,3) maser observed by Mauersberger et al. (1986) in Effelsberg and by Johnston et al. (1989) with the VLA. There are theoretical models to explain such a maser (see e.g. Flower, Offer and Schilke 1990) in low density regions where ortho-H₂ is underabundant. In this study, our aim was to investigate ¹⁵NH₃ in more detail. We succeeded in observing the (1,1) and (2,2) lines of ¹⁵NH₃ in absorption and, surprisingly, the (4,3) and (4,4) lines in emission (see Fig. 1).