A Survey Of High Mass Star Forming Regions In The Line Of Singly Deuterated Ammonia NH2D

Abstract

The present survey represents a continuation of our study of high mass star forming regions in the lines of deuterated molecules, the first results of which were published in Trofimova et al. (2020). This paper presents the results of observations of 50 objects in the line of ortho modification of singly deuterated ammonia NH$_2$D $1_{11}^s - 1_{01}^a$ at frequency 85.9 GHz, carried out with the 20-m radio telescope of the Onsala Space Observatory (Sweden). This line is detected in 29 sources. The analysis of obtained data, as well as the fact that gas density in the investigated sources, according to independent estimates, is significantly lower than the critical density for this NH$_2$D transition, indicate non-LTE excitation of NH$_2$D. Based on non-LTE modeling, estimates of the relative content of the NH$_2$D molecule and the degree of deuterium enrichment were obtained, and the dependencies of these parameters on temperature and velocity dispersion were analyzed with and without taking into account detection limits assuming the same gas density in all sources. An anti-correlation between the NH$_2$D relative abundances and the kinetic temperature is revealed in the temperature range 15-50K. At the same time, significant decrease in the ratio of the NH$_2$D/NH$_3$ abundances with increasing temperature, predicted by the available chemical models, is not observed under the adopted assumptions. An anti-correlation was also revealed between the relative content of the main isotopologue of ammonia NH$_3$ and the velocity dispersion, while no statistically significant correlation with the kinetic temperature of sources in the same temperature range was found.