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Astron. Astrophys. 348, L55-L58 (1999)

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Letter to the Editor

Infrared emission of hot water in the atmosphere of Mira *

I. Yamamura 1, T. de Jong 2,1 and J. Cami 1,3

1 University of Amsterdam, Astronomical Institute `Anton Pannekoek', Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
2 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
3 SRON-Groningen, P.O.Box 800, 9700 AV Groningen, The Netherlands

Received 21 June 1999 / Accepted 19 July 1999

Abstract

The ISO/SWS spectrum of o Cet taken at its maximum exhibits an absorption-like feature between 3.5 and 4.0 µm. We present evidence that the feature is due to emission of H2O and SiO molecules, in a layer extended to about two stellar radii with an excitation temperature of 2000 K. These hot molecules are also observed in a spectrum of Z Cas near minimum, but this time in absorption. A simple plane-parallel model is used to fit the spectra of these two stars. The H2O column densities and excitation temperatures in the layers are found to be similar in both stars. The difference of the H2O band is thus primarily due to the layer size. The H2O layers seem to be more extended at visual maximum, probably related to the stellar pulsation. The estimated lower limit to the local gas density in the layers of [FORMULA] cm-3 is in good agreement with theoretical predictions from dynamical model atmospheres.

Key words: stars: AGB and post-AGB – stars: atmospheres – stars: late-type – infrared: stars

* 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) with the participation of ISAS and NASA. The SWS is a joint project of SRON and MPE.

Send offprint requests to: I.Y. (yamamura@astro.uva.nl)

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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