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.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical Societ

We report the detection of an H i counterpart to the extended, far-ultravioletemitting tail associated with the asymptotic giant branch star Mira (o Ceti). Using the Nançay Radio Telescope (NRT), we have detected emission as far as 88 ′ north of the star, confirming that the tail contains a significant atomic component (M HI ∼ 4 × 10 −3 M ⊙ ). The NRT spectra reveal a deceleration of the tail gas caused by interaction with the local interstellar medium. We estimate an age for the tail of ∼ 1.2 × 10 5 years, suggesting that the mass-loss history of Mira has been more prolonged than previous observational estimates. Using the Very Large Array (VLA) we have also imaged the H i tail out to ∼ 12 ′ (0.4 pc) from the star. The detected emission shows a "head-tail" morphology, but with complex substructure. Regions with detected H i emission correlate with farultraviolet-luminous regions on large scales, but the two tracers are not closely correlated on smaller scales ( < ∼ 1 ′ ). We propose that detectable tails of H i are likely to be a common feature of red giants undergoing mass-loss. 4 All physical quantities quoted in this paper assume a distance of 107 pc (Knapp et al. 2003).

We report the detection of the H i line at 21 cm in the direction of α Orionis (α Ori) with the Nançay Radiotelescope and with the Very Large Array. The observations confirm the previous detection of H i emission centred on α Ori, but additionally reveal for the first time a quasi‐stationary detached shell of neutral atomic hydrogen ∼4 arcmin in diameter (0.24 pc at a distance of 200 pc). The detached shell appears elongated in a direction opposite to the star’s space motion. A simple model shows that this detached atomic gas shell can result from the collision of the stellar wind from α Ori with the local interstellar medium (ISM). It implies that α Ori has been losing matter at a rate of ∼1.2 × 10−6 M⊙ yr−1 for the past 8 × 104 yr. In addition, we report the detection of atomic hydrogen associated with the far‐infrared arc located 6 arcmin north‐east of α Ori, which has been suggested to trace the bow shock resulting from the motion of the star through the ISM. We also report the detection by the Galaxy Evolution Explorer of a far‐ultraviolet counterpart to this arc.