The HI emission profile of RS Cnc

The H I line at 21 cm has been detected in the direction of the S-type AGB star RS Cnc with the Nançay Radio Telescope (NRT). The profile is composite and similar to the CO rotational line profiles. It is composed of a narrow rectangular component overimposed on a wider Gaussian one, both centred on the stellar radial velocity. The narrow component has a width of 4 km s^-1 and is unresolved (with the NRT beam size of 4ʹ in (alpha)). The Gaussian component has a width (FWHM) of 12 km s^-1 and seems resolved by the NRT. These components trace at least 2 different episodes of mass loss by RS Cnc. We argue that the atomic hydrogen responsible for the narrow rectangular profile was likely formed in the stellar atmosphere which could thus be devoid of molecular hydrogen, although we cannot strictly exclude that up to 25% may have been produced by photodissociation of H2. As predicted 20 years ago by Glassgold & Huggins, such a situation should be common to giants with stellar effective temperature larger than 2500 K. This property is relevant to the atmospheric structure of these sources and probably also to the characteristics of their winds.