21-CM line and radio continuum emission from circumstellar envelopes around late-type giants

Searches for atomic hydrogen in the circumstellar envelopes produced by mass loss from late-type giants and from planetary nebulae using the 21-cm line have so far proven negative, with the possible exception of the planetary nebula NGC 6302. These observations show that the mass shed by red giants is overwhelmingly in molecular form. Comparison between the upper limits and the amount of H I expected to be produced by photodissociation by interstellar UV sets an upper limit of ≡20,000 years on the age of the envelope around IRC+10216. The available continuum observations suggest that the mass-loss mechanism may be different for cool stars than for hotter stars. They also allow a preliminary estimate of the lifetime of the copious mass loss phase of extreme AGB stars of 20,000 - 30,000 years.