Observations of the first-overtone silicon monoxide bands in late-type stars

Abstract

Scans of the spectral region 3.98--4.07 ..mu..m, which includes the (2,0) and (3,1) rotation-vibration bands of silicon monoxide, have been made at a resolution of 5.5 cm/sup -1/ (88 A) in 74 late-type stars with a grating spectrometer and InSb detector. All the M giants and bright giants observed have detectable SiO absorption, and the bands strengthen rapidly with advancing type until at least M6. A similar trend is found in synthetic SiO strengths computed with model atmospheres. The SiO band strength varies considerably among M supergiants of the same spectral type and is strongly time-dependent in Mira variables. Since the observed spectra of even very late M stars and cool stars of type S exhibit the characteristic shape of SiO absorption, it appears that SiO is the dominant absorber in this region in all cool oxygen-rich stars. The highly luminous supergiants S Per and RW Cep mayhave the SiO bands in emission, while VY CMa, VX Sgr, and NML Cyg have featureless spectra at the resolution employed. Absorption from /sup 29/Si/sup 16/O was probably detected in the Mira variable R Cas near minimum light.