3.1. The Spectrum of the Nuclear Region
Spectrograms were obtained by G. H. Herbig and Morgan in the autumn of 1955 with the 36-inch Crossley reflector of the Lick Observatory; the two-prism nebular spectrograph was used, together with Kodak plates of I-N emulsion, which were hypersensitized in ammonia. The slit was centered on the nuclear region and the nucleus was trailed during the exposure to give a uniformly widened spectrum. A number of exposures of varying duration were made in order to obtain a record of the whole spectral interval 3700-8000 with optimum density; the dispersion is 430 Å/mm at Hy. A series of plates of standard stars in the MK system also was obtained, under nearly identical spectroscopic conditions. This material made possible a comparison of the nuclear region of M 31 with MK standard stars over the full range of wavelength from the ultraviolet to the infrared.
Although the dispersion is very low, certain unique conclusions may be drawn; these are as follows: (a) The principal contributors to the luminosity of the inner, brightest region of M 31 in the spectral interval 3880-4300 are "cyanogen giants" - that is, stars in the general category gG8 to gK3. (b) The principal contributors in the neighborhood of 4600 are probably K4 to K5 giants. (c) Bands of TiO are present in the red and infrared regions. The luminosities of the M-type stars responsible for these bands have not yet been observationally determined, but it seems highly probable that they are normal giants. Certain luminosity criteria described by Öhman 4 should settle this point when observations with higher dispersion in the red region have been obtained. (d) In spectral regions below 3900 there is evidence for the presence of an appreciable number of stars of types G0 to G5. Their presence is suggested by a softening in the contrast of the K-type absorptions in the ultraviolet. It is very improbable, however, that a considerable proportion of stars of spectral types earlier than F8 is present; in particular, no indication of hydrogen absorption lines in the ultraviolet has been observed. The spectrum of the inner region of M 31 is illustrated in Plate I.
Plate I. Spectra of the central region of M 31 and of comparison stars in the blue (left) and red (right). The MK spectral type of Tauri is K0 III and of Draconis K0 V. The conspicuous CN absorption near 4200 shows both in the giant star and in M 31; the same spectral region in the dwarf is quite different. The MK spectral type of Andromedae is M0 III, and the TiO absorptions near 5900 and 6200 show both in the M giant and in M 31. Crossley reflector spectrograms with Kodak 33 emulsion were used for Draconis, and Kodak I-N hypersensitized for the others.