1994A&A...289..469T

We report on high spectral resolution observations of the 4 µm SiO first overtone bands in six late-type M giants (M5-8III) and two M supergiants. For M supergiants as well as for M5-6 giants, line intensities and profiles of most ²⁸SiO lines can be understood by a unique set of silicon abundance, micro- and macro-turbulence, once a model atmosphere is specified by a T_eff which is confirmed by angular diameter measurement. In the latest M giants (M7-8III), strong lines of ²⁸SiO show excess emission over the predictions based on classical model atmospheres. We suggest that emission of SiO from the outer atmosphere fills in the photospheric absorption. Many resolved lines and band heads due to ²⁹SiO and ³⁰SiO can be clearly identified in all the late M giant spectra surveyed. Silicon isotopic ratios are determined by the analysis of the well isolated weak absorption features, including doublet features due to the returning R-branch. A reasonable number of weak spectral lines for use as abundance indicators were found for each isotopic species. ²⁸Si/²⁹Si and ²⁸Si/³⁰Si have approximately the terrestrial values but more neutron-rich nuclei tend to be more abundant. ²⁹Si/³⁰Si has slightly less than the terrestrial value of 1.51 in all the M giants surveyed. Similar analysis turned out to be more difficult in M supergiants, but ²⁸Si/²⁹Si shows the same tendency 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 Galaxy, possibly reflecting different contributions of types I and II supernovae.