A compact NH$_2$D source without NH$_3$ counterpart in the S187 high-mass star-forming region

Abstract

Our goal is to investigate a so far only tentatively detected compact NH$_2$D source devoid of a notable NH$_3$ counterpart, located rather close to the luminous IRAS 01202+6133 source (the projected distance is $\lesssim$0.3 pc) in the S187 high-mass star-forming region. A high ammonia deuteration in such an environment is unusual. We performed observations of this area with the 20-m OSO radio telescope in several frequency ranges from $\sim$72 to $\sim$112 GHz. The area was mapped at the lower frequencies while at the higher frequencies only the position of the NH$_2$D emission peak was observed. The data were analyzed using non-LTE modeling. The data confirm a compact NH$_2$D source with a size of $\lesssim$0.2 pc. In addition, emission from about 20 molecules was detected, but only the DCO$^+$(1-0) peak coincides with the NH$_2$D clump. The clump is not associated with any known astronomical source or an NH$_3$ emission peak but the dust continuum map shows an enhancement at approximately the same position. The gas kinetic temperature derived from various indicators is $\sim$20-30 K, which possibly characterizes the surrounding material. The intrinsic NH$_2$D line width ($\sim$0.4 km/s) implies a clump with subsonic or transonic turbulence. The ortho/para NH$_2$D line intensity ratio is consistent with an abundance ratio of 3, expected from the spin statistics. The NH$_2$D/NH$_3$ abundance ratio may be $\sim$1. In general, the data hint at a compact cold gravitationally stable dense core within the warm environment, which may represent pristine material. However this picture still needs a verification with observations at a higher angular resolution.