The carbon-bearing material in the outflows from luminous carbon-rich stars

Summary

Within the neighbourhood of the Sun, a number of highly evolved stars are carbonrich in the sense that they have more carbon than oxygen so their outer atmospheres contain molecules such as CN, CH and C₂H₂. These stars are cool with atmospheric temperatures near 3000 K and they are also luminous, typically 10⁴ times more powerful than the Sun. The outer envelopes of these stars are tenuously bound, and they all are losing mass at a very high rate, in some cases more than 10⁻⁵M_Oa a⁻¹ (where M_O denotes the mass of the Sun). These high luminosity carbon stars remain in this phase for a time, very approximately, near 10⁵ years. They exhibit a large amount of carbon in their atmospheres because the products of the nuclear burning that occurs in the very centre of the star, including the synthesis of carbon, appear on the surface.

In the extended envelopes around these stars, there is a very active chemistry, and the gas is sufficiently cool that nucleation of solid dust grains occurs. These solid particles may grow to sizes as large as 1 μm although a more typical size is near 0.05 μm. We therefore can identify both relatively small carbon-bearing molecules (for example HC₇N) and much larger carbon-containing dust grains in the outflows. The amount of intermediate size particles or molecules, such as C₆₀, and their possible role in the circumstellar chemistry is not yet well understood. At least in the envelope of the well studied carbon star IRC+10216, there appears to be more carbon in CO and solid grains than in poly cyclic aromatic hydrocarbons.