Knowledge Bank

An emission line has been observed in the giant molecular cloud NGC 2264 at the frequency expected for the $110+111$ transition of $H2D+$, while a similar search in the dark cloud TMC 1 produced a negative result. These observations were made using the Kuiper Airborne Observatory to avoid the severe attenuation at 372 GHz by atmospheric water vapor. The strength of the observed line is in approximate agreement with the $H2D+$ abundance anticipated from models of interstellar chemistry. The failure to observe emission in TMC 1, where the $H2D+$ abundance is expected to be large, is inferred to result from the low degree of excitation of the $110$ level in this very cold object and can be related to the extent of equilibration between ortho and para $H2$ in the interstellar medium.