Carbon Stars

In this review we rst describe basic information for carbon stars starting with the most recent spectral classi cation system of P.C. Keenan and luminosities derived from data in the Hipparcos Catalogue. For the SRb and Lb variables MK = 6:8 1:2 (21 stars). Only two R stars (non-variables) have measured K magnitudes and showMK = 5:2. The four N stars show MV = +0:76 1:06. Using the Magellanic Clouds, in which the stars are selected by apparent magnitude, carbon stars as bright as MK = 9:2 are found. The distribution of carbon stars perpendicular to the Galactic plane is best understood by two exponentials of scale height about 65 and 300 parsecs. The CH stars, however, are a true halo population of low metalicity. E ective temperatures of C-N stars derived from lunar occultations and near-IR interferometry range from 2000 to 3300 K with little correlation with spectral subtype. Stellar radii derived from angular diameter measurements of C-N stars with Hipparcos parallaxes fall within the limits of 2.4 to 4.7 A.U. Separate short sections describe special carbon stars such as the carbon dwarfs, R stars, and CH stars. The chemical composition of the C-N stars derived by Lambert and collaborators shows that their C/H and N/H ratios are actually very near to being solar. In addition, their 12C=13C ratios (excluding the so-called J stars with 12C=13C below about 10) are similar to the present interstellar ratios. This indicates that much of the C and N in our Galaxy came from mass-losing carbon stars. Carbon stars are surrounded by cool dusty molecular envelopes produced by mass loss, whose composition follows that of the photosphere. Mass loss rates up to several 10 5 M yr 1 are observed. Carbon stars appear to contribute about half of the total mass return to the local interstellar medium, at a rate which can replace it in about a Hubble time. Infrared colors and CO observations show that all N stars lose mass but the R stars do not; although the data are far from conclusive, these may be carbon-rich red giant branch stars. The mass loss rate distributions for semiregular and irregular variables are very similar, as are the luminosities, suggesting that these stars belong to the same population. The Mira variables, however, have mass loss rates higher by about a factor of 10. The peak in the mass loss rate distribution for all carbon stars is 10 7 M yr 1, interestingly close to the rate of growth of the core mass, and demonstrating the close relationship between mass loss and the evolution of the star. Observations of the silicate and SiC dust features in the infrared spectra of carbon stars suggest that 2 dust mixtures can occur in the envelope: this could be due to temperature inhomogeneities across the stellar atmosphere. Finally we discuss recent developments in observations of detached shells, which appear to form on the timescale of helium shell ashes. The statistics of the occurrence of these shells, and the properties of the stars with detached shells, suggest that they are a normal occurrence in the evolution of carbon stars. A discussion of the complex circumstellar chemistry of carbon stars is followed by a description of recent observations which show marked deviations from spherical symmetry in many carbon stars, and the presence of very fast molecular winds which appear to accompany the nal burst of mass ejection and the beginning of evolution to the planetary nebula stage. 3