1998A&A...339..489G

A CO(1-0) on-source spectrum of the well-known carbon star IRC +10 216 taken with the IRAM 30m telescope in June 1996 shows excess emission between -18.3 and -14.3km/s at the red wing of the underlying profile. The excess emission is confirmed in January 1997 but is absent in April 1997 and June 1998 IRAM spectra. Such a transient feature has not been seen before in this star or any other AGB star. In April 1997 we mapped the circumstellar shell out to 110''. Both the J=1-0 and J=2-1 spectra show ``spikes'' or components which vary in strength with position in the envelope. One of these components corresponds to the velocity interval mentioned above. An immediate conclusion is that the circumstellar shell is not spherically symmetric, contrary to what was believed based on lower spectral resolution data. We are probably seeing emission from a complex geometrical structure. Neither a disk structure nor a double-wind structure seem to be able to explain the observations. The on-source transient behaviour of the red excess emission can reasonably well be explained by a single large (∼5x10¹³cm) blob, that expands due to internal motion.