CH stars at high Galactic latitudes

Abstract

Carbon-rich stars of Population II, such as CH stars, can provide direct information on the role of low- to intermediate-mass stars of the halo in early Galactic evolution. Thus accurate knowledge of the CH stellar population is a critical requirement for building up scenarios for early Galactic chemical evolution. In the present work, we report on several CH stars identified in a sample of faint high-latitude carbon stars from the Hamburg survey and discuss their medium-resolution spectra covering the wavelength range 4000–6800 Å. Estimation of the depths of the (1,0)12C12C λ4737 and (1,0)12C13C λ4744 bands in these stars indicates an isotopic ratio 12C/13C ~ 3, apart from a few exceptions; these ratios are consistent with existing theories of CH stellar evolution. The stars of the Hamburg survey, a total of 403 objects, were reported to be carbon star candidates with strong C2 and CN molecular bands. In the first phase of observation, we acquired spectra of 91 objects. Inspection of the spectra of those objects shows 51 objects with C2 molecular bands in their spectra, of which 13 stars have low flux below about 4300 Å. There are 25 objects that show weak or moderate CH and CN bands, 12 objects that show weak but detectable CH bands, and three objects that do not show any molecular bands due to C2, CN or CH in their spectra. Objects with C2 molecular bands and with good signals bluewards of 4300 Å that show prominent CH bands in their spectra are potential candidate CH stars. There were 35 such candidates found in the present sample of 91 objects observed so far. The set of CH stars identified could be the targets of subsequent observation at high resolution for a detailed and comprehensive analysis to understand their role in early Galactic chemical evolution.