Annu. Rev. Astron. Astrophys. 1996. 34: 511-550
Copyright © 1996 by . All rights reserved

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2.8. The CH Stars

Another stellar type often associated with old populations are CH stars. They comprise a rather ill-defined class of carbon stars that are seen in dSph galaxies, the Galactic globular cluster omega Cen, and in the Galactic halo. Although CH stars do have a number of spectral peculiarities (strong CH bands, s-process enhancements, low metallicities, and bluer colors than N-type carbon stars), there is presently no spectroscopic way to identify a carbon star as a CH star unambiguosly. The existence of "CH-like" stars (Yamshita 1975), which have disk-like motions in the Galaxy but otherwise appear the same as CH stars, compounds the problem. The classification of a star as a CH star is therefore associated with its kinematics. While the kinematics coupled with spectral characteristics may allow the identification of a star as a CH star in our Galaxy, the unambigous identification of a star as a CH star in another galaxy cannot be made without other information, such as velocities or bolometric luminosities.

Hartwick & Cowley (1988), Cowley & Hartwick (1991) have identified blue carbon stars with enhanced s-process features in the LMC. Infrared photometry of these stars by Suntzeff et al (1993), Feast & Whitelock (1992) have shown these stars to be very luminous [Mbol = - 5.3 in the mean]. They are probably carbon stars that are younger (t < 1 Gyr) than the dominant C-star population (Section 3.2 below) discovered with infrared plates (Blanco et al 1980). A deeper survey in the peripheries of the Clouds should reveal a number of true CH stars near the tip of the old RGB tip at MV = -3, but no convincing population of these old carbon stars have been made to date.

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