VX Aquilae: the missing link in the evolutionary sequence of S and carbon stars

Spectroscopic observations (4500 to 9000 A) of the intermediate CS Mira variable, VX Aql have been obtained which cover nearly two cycles of the star's light variation. At certain phases it is found that bands of the CaCl A2PI - X2Σ+ and B2Σ+ - X2Σ+ systems are the strongest molecular absorption features in the 4500 to 9000 A region. At other phases the CaCl bands are absent, and the optical spectrum is characterized only by strong resonance lines of Na I, Li I and K I, as well as moderately strong bands of the CN A2PI - X2Σ system. An unidentified band at 8610 A observed in the spectrum of VX Aql is tentatively identified with CrH. No evidence is found for the presence of either C2 or ZrO bands at any phase in the spectra of VX Aql. However, very weak bands of SeC2 are observed. The observations appear to support the suggestion of Greene and Wing that the C:O abundance ratio in the atmosphere of VX Aql is closer to 1 than for any known cool Sc or CS star. Finally, the possibly that the collective dissociation continua of abundant molecular hydrides with thresholds near 2.8 eV might contribute significantly to the violet depression in SC, CS and S stars is discussed. (author)