The Far Violet Region in the Spectra of the Cool Carbon Stars.

A survey of the spectra of a number of the brighter N-type stars in the far violet has been made. Spectrographs of iow dispersion were necessarily used because of the great decrease in intensity of light from the blue-green toward the violet. The spectrograms obtained are described. Perhaps the most interesting features found are a number of strong absorptions in the XX 4100-3950 region of the spectrum of Y Canum Venaticorum. These are tentatively identified with the "X 4050 group" of CH2 bands. If correct, this identification provides direct observational evidence of polyatomic molecules in a stellar atmosphere and opens new possibilities of accounting for the very strong absorption of violet light in the atmospheres of the N-type stars. INTRODUCTION The extreme decrease in intensity from the blue-green to the violet region in the spectra of the cool carbon stars (spectral type N) has been recognized for many years. C. D. Shane' commented upon the phenomenon in his early comprehensive study of the carbon stars and was well aware that the effect was far more than would be expected on the basis of temperature alone. He evaluated the falling-off of intensity in arbitrary units and used it as one criterion in the spectral classification of the R- and N-type stars. Recently, G. Shajn and 0. Struve2 have studied the phenomenon from a spectro- photometric point of view. They photographed the spectrum3 of UU Aurigae (N3: CS3) and compared it with that of an M-type star. A marked weakening of absorption lines in the N-type spectrum in the XX 3900-4100 region, compared with the corresponding lines in the M-type spectrum, was found-an effect judged inappreciable at suitable comparison regions in the ~XX 4400-4700 interval. It was therefore concluded, on the basis of the theory of blended lines, that the continuous absorption arose, in part at least, in the layers of the atmosphere where the absorption lines are formed. From a visual comparison of the spectra of UU Aurigae and the M-type star in the far violet, with special attention being paid to the parts that appeared to be nearest to the continuous background, certain differences were noted. These led Shajn and Struve to suggest that the great weakening of the continuum in the violet more likely arose from as yet unidentified overlapping molecular absorption bands than from an increase in continuous absorption. Spectrograms covering the far violet region have been obtained during the past year at Victoria for a number of cool carbon stars. The purpose of the present article is to de- scribe the spectra. It is hoped that some of the features found may contribute to the solution of the interesting problem of the origin of the very strong absorption in the violet. THE SPECTROGRAPHS The observations were made with the two-prism ultraviolet-transmitting glass spec- trograph, as adapted for use at the Cassegrain focus of the 72-inch reflector. With its * Contributions from the Dominion Astrophysical Observatory, No. 15. 1 Lick Obs. Bull., 13, 123, 1928. 2 Ap. J., 106, 86, 1947