Carbon stars with episodic mass loss: observations and models of molecular emission from detached circumstellar shells.

Details Carbon stars with episodic mass loss: observations and models of molecular emission from detached circumstellar shells. Carbon stars with episodic mass loss: observations and models of molecular emission from detached circumstellar shells.

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...311..587o&link_type=abstract

Astronomy and Astrophysics, v.311, p.587-615

Astronomy and Astrophysics

Astrophysics

103

Stars: Carbon, Circumstellar Matter, Stars: Mass-Loss, Radio Lines: Stars

Scientific paper

We have obtained detailed CO radio line maps of the circumstellar medium around the bright carbon stars R Scl, U Ant, S Sct, and TT Cyg. They provide direct evidence for the existence of large [radii between 10-70", or (1-5)x10^17^cm], geometrically thin (we estimate that the shell widths are <~10"), over-all spherically symmetric shells of CO line-emitting gas around these stars. The shells expand with velocities in the range 13-20km/s, i.e., their ages lie in the range (1-10)x10^3^years. Less extensive CO observations of the carbon star V644 Sco suggest that also it is surrounded by a detached shell. The expansion velocities of the present mass loss winds, as evidenced by weak CO emission from regions close to the stars, are considerably lower, of the order 5km/s. We conclude that the mass loss characteristics of these, otherwise apparently normal, carbon 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 interacting-wind scenario cannot be excluded. The CO brightness distributions are very patchy, suggesting an inhomogeneous circumstellar medium. Using a model where the shell consists of a large number of small, homogeneous clumps, we estimate that the H_2_-masses of the four, spatially resolved shells are all around 0.01Msun_ (for an adopted CO abundance with respect to H_2_ of =~10^-3^), and that in the mass loss eruption -scenario the H_2_-mass loss rates of the stars were =~10^-5^x(10"/{DELTA}R)Msun_/yr during the formation of the shells ({DELTA}R being the unresolved shell width in arc seconds). The present mass loss rates are very low, <~10^-7^Msun_/yr. These results suggest that the four stars have all gone through a type of event that led to a dramatic change in the mass loss characteristics. The adopted model is an initial, relatively crude, attempt to provide a more realistic base for the interpretation of line emission 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 small clumps that in the model produces a clumpy appearance of the brightness distribution, at the larger scale set by the observational resolution, that resembles the observed ones. In the mass loss eruption -scenario the estimated life time of a CO line-emitting shell of the type discussed in this paper is =~10^4^years, and it is determined by the photodissociation of the CO molecules. Only shells younger than =~10^3^years are expected to be observable in molecular radio lines other than those of CO. There is a period after formation when such shells should be characterized by very anomalous line intensity ratios. For instance, in our model the line intensity ratio between the photodissociation product CN and the parent molecule HCN increases drastically on a time scale of hundreds of years as the shell recedes from the star. We suggest that the shell around R Scl is in 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 with anomalous line intensity ratios.

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