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

Our observations with the ISO SWS revealed that H₂O band at 2.7 μm appears as absorption already in an M2 giant β Peg and strengthens towards later M giants. Further, CO₂ 4.2 μm 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 - 1,250 K in M giants as well as in a Mira variable. Absorption features of CO and SiO at 4 μm 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.