HIGH-RESOLUTION INFRARED-SPECTRA OF SILICON MONOXIDE AND SILICON ISOTOPIC ABUNDANCES IN COOL LUMINOUS STARS

We report on high spectral resolution observations of the 4 mu m SiO f irst overtone bands in six late-type M giants (M5-8III) and two M supe rgiants. For M supergiants as well as for M5-6 giants, line intensitie s and profiles of most (SiO)-Si-28 lines can be understood by a unique set of silicon abundance, micro-and macro-turbulence, once a model at mosphere is specified by a T-eff which is confirmed by angular diamete r measurement. In the latest M giants (M7-8III), strong lines of (SiO) -Si-28 show excess emission over the predictions based on classical mo del atmospheres. We suggest that emission of SiO from the outer atmosp here fills in the photospheric absorption. Many resolved lines and ban d heads due to (29)Si0 and (SiO)-Si-30 can be clearly identified in al l the late M giant spectra surveyed. Silicon isotopic ratios are deter mined by the analysis of the well isolated weak absorption features, i ncluding doublet features due to the returning R-branch. A reasonable number of weak spectral lines for use as abundance indicators were fou nd for each isotopic species. Si-28/Si-29 and Si-28/Si-30 have approxi mately the terrestrial values but more neutron-rich nuclei tend to be more abundant. Si-29/Si-30 has slightly less than the terrestrial valu e of 1.51 in all the M giants surveyed. Similar analysis turned out to be more difficult in M supergiants, but Si-28/Si-29 shows the same te ndency as in M giants. Information on silicon isotopic ratios outside the solar system is still meager. Our results suggest that the silicon isotopic ratios may not be necessarily homogeneous throughout the Gal axy, possibly reflecting different contributions of types I and II sup ernovae.