Warm molecular envelope of M giants and Miras: a new molecule forming region unmasked by the ISO SWS.

Abstract

Our observations with the ISO SWS revealed that H_2_O band at 2.7μm appears as absorption already in an M2 giant β Peg and strengthens towards later M giants. Further, CO_2_ 4.2μm band appears as absorption in an M7 giant SW Vir and especially strong in a Mira variable S Vir. The excitation temperatures of these molecular bands estimated from their band shapes turn out to be 750-1,250K in M giants as well as in a Mira variable. Absorption features of CO and SiO at 4μm region appear to be weaker in cooler M giants and almost disappear in S Vir (Mira), suggesting filling in by the emission of CO and SiO themselves. The absorption and/or emission may be produced in a rather warm molecule forming region above the photosphere. This molecular envelope should be related to the quasi-static molecular zone previously suggested from the high resolution FTS spectra of CO in the near IR.