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Issue A&A
Volume 450, Number 1, April IV 2006
Page(s) 167 - 179
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20053759

A&A 450, 167-179 (2006)
DOI: 10.1051/0004-6361:20053759

CO line emission from circumstellar envelopes

D. Teyssier1, 2, R. Hernandez3, V. Bujarrabal3, H. Yoshida4 and T. G. Phillips5

1  Departamento de Astrofísica Molecular e Infrarroja, IEM, CSIC, 28006 Madrid, Spain
2  Space Research Organization Netherlands, P.O. Box 800, 9700 AV Groningen, The Netherlands
    e-mail: teyssier@sron.rug.nl
3  Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
    e-mail: v.bujarrabal@oan.es
4  Caltech Submillimeter Observatory, 111 Nowelo Street, Hilo, HI 96720, USA
    e-mail: hiro@submm.caltech.edu
5  California Institute of Technology, Pasadena, CA 91125, USA
    e-mail: phillips@submm.caltech.edu

(Received 4 July 2005 / Accepted 30 December 2005 )

Abstract
Aims.We present the results of a multi-transition CO observational program conducted on a sample of AGB and post-AGB stars envelopes. We have collected maps and single pointing observations of these envelopes in 5 rotational transitions ranging from J = 1-0 to J = 6-5, including in particular new observations of the CO line at 691 GHz at the CSO. The use of such a set of mm and submm CO line on stellar envelopes is rare and limited to the work of some authors on IRC+10216.
Methods.Using a model for the CO emission of an AGB circumstellar envelope, in combination with a standard LVG approach, we have conducted a systematic modelling analysis using the whole set of CO data collected for a sample of 12 sources. We simultaneously fit all five transitions, taking into account the spatial information provided by the maps.
Results.We find mass-loss rates in the range 1 $\times$ 10-7 to 4 $\times$ 10 $^{-4}~M_\odot$/yr, and envelope temperatures ranging from 20 K to 1000 K at a radius of 1016 cm. There seem to be a general anti-correlation between mass loss rates and temperature, the high mass loss rate AGBs having low temperatures, and vice versa. We show that most AGB data can be fitted using a constant mass loss rate, at least within the calibration uncertainties associated with the data collected at different frequencies. For some cases though (e.g. CIT 6, R Hya, $\chi$ Cyg), a change in the mass loss rate history needs to be invoked to reconcile data at low- and high-J, a scenario already mentioned by several authors to explain observations of WX Psc.


Key words: stars: AGB and post-AGB -- stars: circumstellar matter, mass-loss -- infrared: stars

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© ESO 2006


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