Spectra of the late N-type stars in the ultra-violet, violet and blue-green regions

摘要

Spectrograms of several late N-type stars have been obtained covering the far violet spectral region from λ 4100 to λ 3900. One plate of Y CVn extends the observations into the ultra-violet, radiation being detectable below λ 3500. A list of the wave-lengths of the absorption features and the emission-like spaces between them is given for the interval λ 3400 to λ 4100.The ultra-violet spectrum of Y CVn is dominated by a series of newly discovered broad absorption bands centred about λλ 3790, 3700, 3595 and 3480, for which no identification is found.The λ 4100-λ 3900 region of the several late N-type stars exhibits the series of absorption bands previously found for Y CVn, these bands being separated by comparatively sharp regions of background radiation. The λ 4050 group of bands as found in the laboratory and in cometary spectra, currently provisionally ascribed to C3, is discussed. Comparison of the stellar absorptions with the laboratory and cometary bands is made. While the stellar bands cannot be identified with the others for certain, the previous tentative identification is believed strengthened.The unidentified blue-green bands characteristic of late N-type spectra, as photographed with moderately high resolution, are found to reveal no rotational structure. Hence the suggestion that they might arise from a diatomic hydride must be abandoned. It is noted that the spectrum of the irregular variable, U Hya, sometimes shows the blue-green bands and, when it does, the λ 4050 group and high opacity in the far violet, associated with late N-type spectra, are also present. When the blue-green bands are absent, the violet bands are also absent and the spectrum is of early N-type. Various considerations are believed to favour a polyatomic molecular origin for the blue-green bands ; as for the high opacity in the far violet, it is related to the formation of the molecules responsible for the λ 4050 absorption.