Infrared Spectra of Stars

Abstract

This review of infr·ared spectroscopy will be restricted to stars, and to wavelengths longer than 9000 A (0.9 p.). Infrared studies of the Sun will not be considered for their own sake but will frequently be called upon to help interpret the spectra of other stars. Infrared stellar photometry will be discussed only when it can be used to study spectral features. Finally, our concern wiII be the stellar spectra, rather than the problems and triumphs of instrumental technique. According to the terminology now in fairly general use, the infrared is divided into the following four regions: the near infrared, roughly 0.7-0.9 po, invisible to the eye but accessible to the N-type photographic emulsion; the one-micron region, roughly 0.9-1 . 1 p., accessible to the Z emulsion and to S1 photocathodes; the two-micron region, roughly 1.2-2.5 J.L, in which PbS photoconductive cells are often used (hence sometimes called the lead sulfide region) i and the far infrared, extending from about 3 p. to millimeter wavelengths and the domain of radio astronomy. These regions are quite distinct to an infrared observer; the completeness of our knowledge of stel­ lar spectra drops sharply from region to region toward longer wavelengths but is fairly uniform within each region. By omitting the vast quantity of material accumulated with N plates in the near infrared-which, by virtue of its relative completeness, belongs perhaps more properly in a general review of stellar spectroscopy-we can give a fairly comprehensive account of the work done at longer wavelengths. In the 1 J.I region, the identifications of spectral-features are now in a satisfactory state for G, K, and M stars, and several results of astrophysical interest have been obtained; in the 2 J.L region, important studies of molecular bands have. been made but the work on atomic-line identification is just beginning ; in the far infrared, very few spectral features have yet been detected. Infrared spectroscopy is a young field which seems now to be entering a stage of rapid development, and we have depended rather heavily upon unpublished material. There are many reasons for observing stellar spectra in the infrared, despite the difficulties imposed by low detector sensitivity and atmospheric absorption. The abundant molecules Hz, CO, and H20 can be observed only in the infrared, while some other molecules (e.g. VO and CN) and atoms