Circumstellar atomic hydrogen in evolved stars

We present new results of a spectroscopic survey of circumstellar H I in the direction of evolved stars made with the Nancay Radiotelescope. The H I line at 21 cm has been detected in the circumstellar shells of a variety of evolved stars: asymptotic giant branch stars, oxygen-rich and carbon-rich stars, semiregular and Mira variables, and planetary nebulae. The emissions are generally spatially resolved, i.e., larger than 4 ', indicating shell sizes on the order of 1 pc, which opens the possibility of tracing the history of mass loss over the past similar to 10(4)-10(5) yr. The line profiles are sometimes composite. The individual components generally have a quasi-Gaussian shape; in particular, they seldom show the double-horn profile that would be expected from the spatially resolved optically thin emission of a uniformly expanding shell. This probably implies that the expansion velocity decreases outward in the external shells (0.1-1 pc) of these evolved stars. The H I line profiles do not necessarily match those of the CO rotational lines. Furthermore, the centroid velocities do not always agree with those measured in the CO lines and/or the stellar radial velocities. The H I emissions may also be shifted in position with respect to the central stars. Without excluding the possibility of asymmetric mass ejection, we suggest that these two effects could also be related to a nonisotropic interaction with the local interstellar medium. H I was detected in emission toward several sources (rho Per, alpha Her, delta(2) Lyr, U CMi) that otherwise have not been detected in any radio lines. Conversely, it was not detected in the two oxygen-rich stars with substantial mass-loss rate, NML Tau and WX Psc, possibly because these sources are young, with hydrogen in molecular form, and/or because the temperature of the circumstellar H I gas is very low (< 5 K).