AN EXPERIMENTAL APPROACH TO THE PREDICTION OF COMPLETE MILLIMETER AND SUBMILLIMETER SPECTRA AT ASTROPHYSICAL TEMPERATURES: APPLICATIONS TO CONFUSION-LIMITED ASTROPHYSICAL OBSERVATIONS

摘要

Unidentified features in interstellar spectra (U lines) have persisted almost from the beginning of the field. In recent years, the number of such lines has rapidly increased in parallel with the sensitivity and frequency range of new observational facilities. Initially, the U lines often were from species considered exotic at the time, such as HCO~+, but now the origins of these unidentified "weeds" are overwhelmingly from previously observed large molecules with dense spectra. The origin of the weeds problem lies in the nature of the spectroscopic approach that has typically been used in the millimeter and submillimeter spectral region: the bootstrap narrowband-observation, assignment, and theoretical prediction cycle. Unfortunately, the weeds arise from complex spectra involving many low-lying and often interacting vibrational states that are not typically part of the bootstrap process. This paper describes a purely experimental approach to this problem that does not require spectra assignment, but rather relies on the observation of complete spectra over a range of temperatures. It also discusses the potential for the use of these complete spectra in conjunction with the multiplex capabilities of modern radio telescopes for the detection of large species whose spectra consist of many relatively weak and ordinarily unobservable lines. This latter application will be made particularly challenging by the inhomogeneities and nonequilibrium characteristics of the interstellar medium. This approach is enabled by the FASSST (fast scan submillimeter spectroscopy technique) spectroscopic system and the use of collisional cooling cells to provide reference spectra at low temperature. Experimental and theoretical results are presented.