The infrared excess of cool giant stars - A chromospheric contribution

The idea that the infrared excesses of evolved M stars may contain a contribution from a chromosphere is explored using alpha Ori and W Hya as test cases. The spectrum of alpha Ori between 8 and 30 millimicrons can be interpreted satisfactorily in terms of three components: a photosphere, a silicate dust cloud, and a cool chromosphere (temperature about 5000 K), which is optically thick at 14 millimicrons. A similar modelling for W Hya suggests a hotter chromosphere (temperature about 8000 K), with unit optical depth at 30 millimicrons. Some consequences of these chromospheres are briefly discussed.