Infrared Heterodyne Spectroscopy of Ammonia and Ethylene in Stars

Abstract

The study of molecules in stars has greatly expanded in scope during the past decade, principally because of the development of high resolution heterodyne spectrometers for radio astronomy and the subsequent discovery of maser emission from OH, H₂0, and SiO. Although infrared frequencies have a fundamental advantage for the detection of new and more complicated molecules, the development of infrared spectrometers with the required spectral resolution and sensitivity has heretofore lagged behind equivalent advances in microwave instrumentation. In stellar sources, the linewidths of molecular transitions are generally dominated by large scale mass motions rather than simple thermal or pressure-broadening effects. Nevertheless, Doppler-broadened linewidths as narrow as 1 km/s (100 MHz at 30 THz) can still be expected. Generally, the line profiles are asymmetrical, and the details of the shapes made visible by high resolution are of critical importance for unraveling the dynamics of the stellar environment. Fortunately, developments in laser and photodiode technology now bring the advantages of coherent heterodyne detection to the infrared, with the result that the vibrational transitions of molecules can now be observed with the same Doppler-limited resolution commonly used for observations of the rotational, hyperfine, and inversion transitions which occur at radio frequencies. The importance of laser heterodyne spectroscopy for astronomical observations has been demonstrated by the first detection of ammonia [1,2] and ethylene [3] in stars. In heterodyne spectroscopy, the signal radiation collected through a telescope is mixed with a local oscillator (LO) beam from a stabilized laser in a high speed photodetector. The resulting difference frequencies are then amplified over a broad radio-frequency band and analyzed in a contiguous set of radio-frequency filters. The actual resolution is determined by the chosen widths of the filters, but the ultimate achievable resolution is limited only by the spectral linewidth of the laser.