IRC +10 216 (CW Leo) is the nearest and best-studied carbon star. It is a long-period variable star with a period of about 650 d and a spectral type of C9,5 (see e.g. Olofsson et al. 1982 ). Estimates of its distance range from 100 pc (Zuckerman et al. 1986 ) to 290 pc (Herbig & Zappala 1970 ). IRC +10 216 is surrounded by a dust shell which is expanding at kms , thereby carrying a mass loss rate of yr-1 (e.g. Loup et al. 1993 ). The first high-resolution IR observations of the dust shell of IRC +10 216 were reported by Toombs et al. (1972 ), McCarthy et al. (1980 , 1990 ), Mariotti et al. (1983 ), Dyck et al. (1984, 1987, 1991 ), Ridgway & Keady (1988 ), Christou et al. (1990 ), Le Bertre et al. (1988 ), Danchi et al. (1994 ), Osterbart et al. (1997 ), and Weigelt et al. (1997 ). Detailed radiation transfer calculations for IRC +10 216 have been carried out, for instance, by Groenewegen (1997 ) using a large amount of spectroscopic and visibility data. Consistent time-dependent models describing the circumstellar shells of dust forming long-period variables have been presented by Fleischer et al. (1992 ). A general result of these models is the formation of discrete dust layers, causing pronounced time-varying, step-like surface intensity distributions (Winters et al. 1995 ).
In this Letter we present diffraction-limited 76 mas speckle masking observations of the clumpy dust shell of IRC +10 216. We speculate about the origin of these structures in the light of consistent time-dependent model calculations and discuss the red giants' large-scale surface convection (Schwarzschild 1975 ) as a possible mechanism for inhomogeneous mass loss.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998