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Astron. Astrophys. 348, L17-L20 (1999)

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1. Introduction

Recent ISO observations in the "thermal infrared" spectral region (about [FORMULA]m) have revealed new constraints on the outer atmospheres of oxygen-rich AGB stars. The physical conditions present in these layers close to the photosphere are of special interest concerning various long-standing astrophysical problems, as for example the occurrence of chromospheres and shock waves, the formation of molecules and dust, the acceleration of stellar winds, and how these phenomena are related to each other.

Free from the disturbances of the Earth's atmosphere, new molecular absorption and/or emission features have been detected in several O-rich AGB stars including Miras and semi-regular variables which appeared quite different from what could be expected from standard model atmospheres. The magnitude and the spectral shapes of the bands attributed to CO2, SO2 and H2O can be used to probe the physical and chemical conditions in these outer atmospheres (Table 1). The discovery of the excess absorption/emission features led several authors to the conclusion that rather dense, quasi-static molecular envelopes at temperatures of [FORMULA] K exist around these stars - termed as "warm molecular layers". These layers are clearly distinct from the much cooler circumstellar envelopes already known from radio observations.


Table 1. Observed molecular bands and derived quantities for various O-rich AGB stars including Miras and semi-regular variables. The excitation temperatures [FORMULA] are deduced from the shape (i.e. the rotational fine-structure) of the bands. Consequently, [FORMULA] reflects the rotational temperature of the lower vibrational state in case of an absorption feature, and of the upper vibrational state in case of an emission feature.
abs.: absorption feature, em. : emission feature, -: not stated, *: total number of molecules in the excited vibrational state. (1): Tsuji et al. (1997), (2): Justtanont et al. (1998), (3): Ryde et al. (1999), (4): Yamamura et al. (1999), (5): de Jong (1999), (6): Cami et al. (1997).

In this letter, we report on clues to the origin of such molecular layers based on hydrodynamical model calculations. This is in contrast to previous works, which have assumed the temperature and density of these layers in order to obtain agreement with the observations and allowed for the first quantitative interpretations. In a simple, but quantitative model, where dust formation is not taken into account, we demonstrate the contribution of the pulsation to the formation of such layers.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999