Carbon Stars

Abstract

The defining characteristics of carbon stars are presented and interpreted in an evolutionary context. Some occurences are listed with their known physical properties.

Keywords: carbon stars

Introduction

Carbon stars, also known as type C stars, overlap with late G, K and M stars in most respects but are distinguished by compositional differences, typically an unusually high concentration of carbon.

Characteristics

Spectra

C stars, known as carbon stars, overlap with late G, K and M stars in most respects but are distinguished by compositional differences, typically an unusually high concentration of carbon.

Type R is the approximate carbon analogue of the K stars, and is very rare; N is the approximate carbon analogue of the M stars (see Table 1).

Mass and Luminosity

xxx.

Variability/Mass Loss

Carbon stars are typically long period variables.

Interpretation

During late phases of stellar evolution, ‘ordinary’ stars (i.e. excluding the super-massive stars which take the LBV–WR route) become long period variables on the AGB. "Because of the sudden increase in energy flux from the helium-burning shell during a flash episode, a convective zone is established between it and the hydrogen-burning shell. At the same time, the depth of the envelope convection zone increases with pulse strength. For stars that are massive enough (M > 2M_¤), the convection zones will merge and eventually extend down into regions where carbon has been synthesized, carrying freshly processed material to the surface. This third dredge-up phase appears to explain the difference between oxygen-rich giants (Xo > Xc) and carbon-rich giants (Xc > Xo), called carbon stars..." (Carroll & Ostlie 1996, p. 504).

Occurrence/Examples

The following table provides a little additional information about some bright examples of these rare stars. Examples are provided for both southern and northern observers.

References

Burnham, Robert 1989a: Burnham’s Celestial Handbook. Volume 1 – Andromeda to Cetus. Dover.

— 1989b: Burnham’s Celestial Handbook. Volume 2 – Chamaeleon to Orion. Dover.

— 1992: Burnham’s Celestial Handbook. Volume 3 – Pavo to Vulpecula. Dover.

Carroll, Bradley W.; Ostlie, Dale A. 1996: Introduction to Modern Astrophysics. Addison Wesley.

Hoffleit et al. 1991: The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version).

Kaler, James B. 1997: Stars and Their Spectra. Cambridge. (Corrected paperback ed.) 300 pp.

Tirion, Wil 1991: Cambridge Star Atlas 2000.0. Cambridge.