PECULIAR (NH3)-N-15 TOWARD NGC-7538-IRS-1

We present recent observations of several (NH3)-N-15 transitions toward the ultracompact HII (UCHII) region NGC 7538-IRS 1. While the (J,K) = (1,1), (2,2), (5,5) and (6,6) lines are observed in absorption against the ultracompact HII region, as are the highly excited (NH3)-N-14 lines, the (NH3)-N-15 (4,3) and (4,4) lines are seen in weak emission. It is argued that the (4,3) and (4,4) lines must be weak masers amplifying the continuum of the UCHII region. The well studied (NH3)-N-15(3,3) maser was remeasured and does not appear to vary on a timescale of years. We report sensitive upper limits to the (7,7) and several non-metastable lines of (NH3)-N-15. The existence of the (4,3) and (4,4) masers is a challenge to the usual NH3 maser pumping models since such models do not allow simultaneous masing of the (4,3) and the (3,3) line and do not predict (4,4) masing. We investigate the question if it is possible to explain all masers in the frame of the current model or if it has to be extended. One possible explanation of the observations would be the pumping of the maser by 10-mu-m continuum radiation via the v2 = 1 level. This is not totally ad hoc because vibrationally excited ammonia has been detected in NGC 7538 by Schilke et al. (1990a).