Temperature and spectrum of VX Aquilae

The spectrum of the red variable VX Aql contains strong CN bands and falls in the intermediate SC category between types S and C. A careful attempt to detect bandheads of ZrO and C2, involving the comparison of computed CN spectra with tracings of image-tube spectrograms, gave negative results. We interpret this to mean that the abundances of oxygen and carbon are more exactly balanced in VX Aql than in most other SC stars, which show either weak ZrO or weak C2 bands. Thus VX Aql provides a link between these two subgroups of SC stars, supporting the view that they are all part of a continuous sequence of stars ranging from the S stars to the carbon stars, in which the variable is the oxygen/carbon ratio. Other interesting spectral characteristics include the great strength of the Li I, CaH, and CaCl features and the presence of the Merrill-Sanford bands, which have previously been found only in carbon stars; all of these features are strongly favored by high pressure as well as by low temperature