Proposal type : Open Time

Scientific category : Stellar_Physics

Title :
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A STUDY OF WATER EMISSION IN O-RICH EVOLVED STARS THE WATER ABUNDANCE AND THE 17O/18O ISOTOPIC ABUNDANCE RATIO IN CSE

P.I. : J. CERNICHARO
Instituto de Estructura de la Materia
Dpto. Física Molecular
Serrano 123
28006 Madrid

e-mail : cerni@astro.iem.csic.es
Tel : 91-5901611
Fax : 91-5855184

Keywords :
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abundances
AGB stars
circumstellar envelopes
line formation

Abstract :
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Water vapor plays an important role in the chemistry and in the gas cooling of the envelopes of O-rich stars. Although all chemical models predict a high water abundance, there is no way to observe the rotational transitions of this conspicuous molecule from ground based telescopes. Only its line at 22 GHz, frequency for which the atmosphere is transparent, has been observed in many star forming regions, circumstellar envelopes and external galaxies. However, water emission at this frequency is maser in nature and the physical conditions necessary to pump the involved rotational levels are so extreme that the emission is arising from very small clumps.
ISO is the only instrument allowing to observe thermal emission of water vapor with a high sensitivity and high spectral resolution in O-rich CSEs with mass loss rates larger than 1E-06 Ms/y. Many observations of water lines are proposed in the central programme in the dusty O-rich CSEs. In these objects the water lines will be strong but certainly optically thick. However, no sensitive observations of CSEs with moderate mass loss rates, which are the main suppliers to the ISM of processed material and in which the water abundance could be more accurately derived, are proposed.
In order to derive the water abundance in O-rich CSEs we would like to observe 8 backbone transitions towards a sample of objects that we have already observed at millimeter and submillimeter wavelengths in the emission of para water at 183.3 and 325 GHz with the 30 m IRAM radiotelescope. In addition, we would like to observe the dusty envelopes having high mass loss rates in two lines of HH17O and HH18O and four non-backbone transitions of water. From these observations we expect to derive the water abundance and the 16O/17O and 16O/18O isotopic abundance ratios. These observations will constitute a major contribution to the study of O-rich circumstellar envelopes.

Publications :

The rich far-infrared water vapour spectrum of W Hya. Barlow et al. A.A. 315, L241

Updated: March 17, 1997.