IRAS flux densities of S stars

This dichotomy states that only Tc-rich ("intrinsic") S stars are genuine thermally-pulsing asymptotic giant branch stars, possibly involved in the M-S-C evolutionary sequence.

Tc-poor S stars are referred to as "extrinsic" S stars, because they are the cooler analogs of barium stars, and like them, owe their chemical peculiarities to mass transfer across their binary system. Accordingly, an extensive data set probing the circumstellar environment of S stars (IRAS flux densities, maser emission, CO rotational lines) has been collected and critically evaluated. This data set combines new observations (9 stars have been observed in the CO J=2-1 line and 3 in the CO J=3-2 line, with four new detections) with existing material (all CO and maser observations of S stars published in the literature).