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Astron. Astrophys. 320, L1-L4 (1997)

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Letter to the Editor

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

T. Tsuji 1, K. Ohnaka 1, W. Aoki 1 and I. Yamamura 1, 2

1 Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo, 181 Japan
2 Space Research Organization of the Netherland, P.O.Box 800, 9700 AV Groningen, The Netherlands

Received 31 January 1997 / Accepted 11 February 1997


Our observations with the ISO SWS revealed that H2 O band at 2.7 µm appears as absorption already in an M2 giant [FORMULA] 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.

Key words: Infrared: stars - molecular processes - stars: atmospheres - stars: circumstellar matter - stars: late-type

* Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998