Properties of detached shells around carbon stars

Free access article

Issue		A&A Volume 436, Number 2, June III 2005


Page(s)		633 - 646
Section		Interstellar and circumstellar matter
DOI		http://dx.doi.org/10.1051/0004-6361:20042510

A&A 436, 633-646 (2005)
DOI: 10.1051/0004-6361:20042510

Evidence of interacting winds

F. L. Schöier¹, M. Lindqvist² and H. Olofsson¹

¹ Stockholm Observatory, AlbaNova, 106 91 Stockholm, Sweden
e-mail: fredrik@astro.su.se
² Onsala Space Observatory, 439 92 Onsala, Sweden

(Received 9 December 2004 / Accepted 2 March 2005 )

Abstract
The nature of the mechanism responsible for producing the spectacular, geometrically thin, spherical shells found around some carbon stars has been an enigma for some time. Based on extensive radiative transfer modelling of both CO line emission and dust continuum radiation for all objects with known detached molecular shells, we present compelling evidence that these shells show clear signs of interaction with a surrounding medium. The derived masses of the shells increase with radial distance from the central star while their velocities decrease. A simple model for interacting winds indicates that the mass-loss rate producing the faster moving wind has to be almost two orders of magnitudes higher (~10 $^{-5}~M_{\odot}$ yr^-1) than the slower AGB wind (a few 10 $^{-7}~M_{\odot}$ yr^-1) preceding this violent event. At the same time, the present-day mass-loss rates are very low indicating that the epoch of high mass-loss rate was relatively short, on the order of a few hundred years. This, together with the number of sources exhibiting this phenomenon, suggests a connection with He-shell flashes (thermal pulses). We report the detection of a detached molecular shell around the carbon star DR Ser , as revealed from new single-dish CO (sub-)millimetre line observations. The properties of the shell are similar to those characterising the young shell around U Cam .

Key words: stars: AGB and post-AGB -- stars: carbon -- stars: late-type -- stars: mass-loss

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