THE PRE-OUTBURST SPECTRUM OF NOVA DELPHINI 1967

Abstract

The spectrum of Nova Delphini 1967 * ( Alcock 1967 ) was recorded on two objective-prism plates taken for one of my observing programs seven years before the current outburst. This is the third case of the observation of the pre-outburst spectrum of a nova not known to be recurrent ( cf. Cannon 1920; Stephenson and Herr 1963; Gotz 1965 ), and the first instance in which the pre-outburst spectrum was not badly underexposed. The identification of the pre-outburst spectrum with the nova was accomplished as follows. In the first place, the prenova spectrum shows no discrete spectral features, so that direct measurement of a pre-outburst plate would yield a coordinate of very low precision in the direction parallel to the spectral dispersion. However, the candidate spectrum is that of a star no fainter than twelfth magnitude, and only one star of those shown on the prints of the National Geographic Society-Palomar Observatory Sky Survey could be responsible for the prenova spectral image. This star appears to have about the same photographic magnitude on the prints of the Lick Sky Atlas and FranklinAdams charts that show it; moreover, in agreement with the pre-outburst spectrum, a comparison of the red and blue Palomar prints shows that the star in question was extremely blue. To verify that this star was indeed the prenova, its image and those of four nearby A-type reference stars were measured on a glass contact copy of the blue Palomar print ( print O-831 ) and on a post-outburst objective-prism plate of the nova taken July 13, 1967. On the