  Y CVn is a carbon star surrounded by a detached dust shell that has been
imaged by the Infrared Space Observatory at 90 microns. With the Nancay Radio
Telescope we have studied the gaseous counterpart in the 21-cm HI emission
line. New data have been acquired and allow to improve the signal to noise
ratio on this line. The high spectral resolution line profiles obtained at the
position of the star and at several offset positions set strong constraints on
the gas temperature and kinematics within the detached shell; the bulk of the
material should be at ~ 100-200 K and in expansion at ~ 1-2 km/s. In addition,
the line profile at the central position shows a quasi-rectangular pedestal
that traces an 8 km/s outflow of ~ 1.0 10^-7 Msol/yr, stable for about 2 10^4
years, which corresponds to the central outflow already studied with CO
rotational lines. We present a model in which the detached shell results from
the slowing-down of the stellar wind by surrounding matter. The inner radius
corresponds to the location where the stellar outflow is abruptly slowed down
from ~ 8 km/s to 2 km/s (termination shock). The outer radius corresponds to
the location where external matter is compressed by the expanding shell (bow
shock). In this model the mass loss rate of Y CVn has been set constant, at the
same level of 1.0 10^-7 Msol/yr, for ~ 4.5 10^5 years. The gas temperature
varies from ~ 1800 K at the inner limit to 165 K at the interface between
circumstellar matter and external matter. Our modelling shows that the presence
of a detached shell around an AGB star may not mean that a drastic reduction of
the mass loss rate has occurred in the past. The inner radius of such a shell
might only be the effect of a termination shock rather than of an interruption
of the mass loss process.
