CARBON STARS WITH EPISODIC MASS-LOSS - OBSERVATIONS AND MODELS OF MOLECULAR-EMISSION FROM DETACHED CIRCUMSTELLAR SHELLS

We have obtained detailed CO radio line maps of the circumstellar medi um around the bright carbon stars R Scl, U Ant, S Sct, and TT Cyg. The y provide direct evidence for the existence of large [radii between 10 - 70'', or (1 - 5) x 10(17) cm], geometrically thin (we estimate that the shell widths are less than or similar to 10'', over-all spherical ly symmetric shells of CO line-emitting gas around these stars. The sh ells expand with velocities in the range 13 - 20 km s(-1), i.e., their ages lie in the range (1 - 10) x 10(3) years. Less extensive CO obser vations of the carbon star V644 Sco suggest that also it is surrounded by a detached shell. The expansion velocities of the present mass los s winds, as evidenced by weak CO emission from regions close to the st ars, are considerably lower, of the order 5km s(-1). We conclude that the mass loss characteristics of these, otherwise apparently normal, c arbon stars have changed significantly over the last 10(4) years. For such a shell structure, the most reasonable cause is a short period of very intense mass loss (i.e., a mass loss eruption), although an inte racting-wind scenario cannot be excluded. The CO brightness distributi ons are very patchy, suggesting an inhomogeneous circumstellar medium. Using a model where the shell consists of a large number of small, ho mogeneous clumps, we estimate that the Hz-masses of the four, spatiall y resolved shells are all around 0.01M. (for an adopted CO abundance w ith respect to H-2 of approximate to 10(-3)), and that in the ''mass l oss eruption''-scenario the H-2-mass loss rates of the stars were appr oximate to 10(-5) x (10''/Delta R)M(.)yr(-1) during the formation of t he shells (Delta R being the unresolved shell width in are seconds). T he present mass loss rates are very low, less than or similar to 10(-7 )M(.)yr(-1) These results suggest that the four stars have all gone th rough a type of event that led to a dramatic change in the mass loss c haracteristics. The adopted model is an initial, relatively crude, att empt to provide a more realistic base for the interpretation of line e mission from a circumstellar medium in which, in general, the physical conditions are very likely quite inhomogeneous. It is the accidental overlap along the line-of-sight and in velocity space of the many smal l clumps that in the model produces a clumpy appearance of the brightn ess distribution, at the larger scale set by the observational resolut ion, that resembles the observed ones. In the ''mass loss eruption''-s cenario the estimated life time of a CO line-emitting shell of the typ e discussed in this paper is approximate to 10(4) years, and it is det ermined by the photodissociation of the CO molecules. Only shells youn ger than approximate to 10(3) years are expected to be observable in m olecular radio lines other than those of CO. There is a period after f ormation when such shells should be characterized by very anomalous li ne intensity ratios. For instance, in our model the line intensity rat io between the photodissociation product CN and the parent molecule HC N increases drastically on a time scale of hundreds of years as the sh ell recedes from the star. We suggest that the shell around R Scl is i n this phase, since this is the only object, among the five observed, in which we have clearly detected also lines of HCN and CN, albeit wit h anomalous line intensity ratios.