# CH star
A CH star is a metal-poor, carbon-enhanced giant star whose spectrum shows an unusually strong G band of the CH molecule near 4308 Å together with overabundances of heavy elements made by the slow neutron-capture (s-) process. Philip C. Keenan separated them from ordinary carbon stars by this strong CH band in 1942, and Bidelman and Keenan added the heavy-element criterion in 1951.[1](https://doi.org/10.1051/0004-6361/202555701) They are now understood as members of binary systems whose surfaces were polluted by a companion that has since become a white dwarf.
| Key fact | Detail |
|---|---|
| Defining abundances | Iron deficiency with carbon enhancement: −2 ≤ [Fe/H] ≤ −1, [C/Fe] ≥ 0.7, [Ba/Fe] ≥ 1.0 (Keenan's 1942 description identified CH stars spectroscopically, by the great strength of their CH molecular bands, not by abundance criteria)[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) • [8](https://articles.adsabs.harvard.edu/pdf/1942ApJ....96..101K) |
| Spectral signature | Strong CH molecular G band near 4308 Å, distinguishing them from other carbon stars[1](https://doi.org/10.1051/0004-6361/202555701) |
| Origin | Binary mass transfer from a companion that evolved through the asymptotic giant branch (AGB) and is now an invisible white dwarf[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) |
| Binarity | All, or almost all, CH stars are binaries, tighter than the average solar-neighborhood system; most likely maximum period ~10,000–20,000 days[3](https://academic.oup.com/mnras/article/441/2/1217/1067494) |
| Population | High-velocity Population II halo carbon stars, the most populous type of halo carbon star known[4](https://lweb.cfa.harvard.edu/~pberlind/atlas/htmls/cstars.html) • [5](https://academic.oup.com/mnras/article/359/2/531/987142) |
| Metal-poor counterparts | CEMP-s stars, defined as [C/Fe] > 1 and [Fe/H] < −2 (Beers & Christlieb 2005), share the same binary mass-transfer origin[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) |
| s-process enrichment | [s/Fe] = +1.0 to +2.0 in CH giants, with the heavy s-process peak more enhanced than the light one[6](https://doi.org/10.1086/116375) |
## Definition and spectral appearance
CH stars are G- and K-type giants whose spectra show weak metallic (iron-group) lines but enhanced lines of heavy elements and a conspicuous CH G band.[7](https://doi.org/10.1093/mnras/stv2011) The class rests on two abundance criteria: the stars are iron-deficient and carbon-enhanced, with metallicities between −2 and −1, [C/Fe] at least 0.7, and [Ba/Fe] at least 1.0.[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) Keenan's 1942 paper noted that the known CH stars were high-velocity objects; he suggested that all carbon stars with high velocities might be CH stars, since the remaining high-velocity carbon stars were too faint or too red for their CH bands to be observed.[8](https://articles.adsabs.harvard.edu/pdf/1942ApJ....96..101K)
A related class, the subgiant CH stars, shares the same carbon and s-process surface enhancements at lower luminosity; the two groups were originally separated by luminosity class.[3](https://academic.oup.com/mnras/article/441/2/1217/1067494) • [9](https://pure.psu.edu/en/publications/subgiant-ch-stars-ii-chemical-compositions-and-the-evolutionary-c)
## The binary mass-transfer picture
Radial-velocity surveys by McClure and colleagues in the 1980s and 1990s established that CH stars are binaries with now-invisible companions.[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) The accepted scenario is that the observed star accreted carbon- and s-process-rich material when its companion was on the asymptotic giant branch; that companion is today a white dwarf.[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) A Bayesian analysis of long-term radial-velocity monitoring found that all, or almost all, CH stars and CEMP-s stars are binaries, and that these binaries are tighter than the average solar-neighborhood system, with a most likely maximum period of about 10,000 to 20,000 days.[3](https://academic.oup.com/mnras/article/441/2/1217/1067494)
**Orbital range.** Direct monitoring of 13 low-metallicity carbon stars with the HERMES spectrograph found binarity for all but one, with periods from 3.4 days for the dwarf carbon star HE 0024-2523 to about 54 years for the CH star HD 26, the longest known among barium, CH, and extrinsic S stars.[10](https://arxiv.org/abs/1510.05840) The period-eccentricity diagram of these systems shows two groups: short-period systems with mostly circular orbits and longer-period, eccentric systems with e > 0.1.[10](https://arxiv.org/abs/1510.05840) The mass-function distributions correspond to companions of 0.5 to 0.7 solar masses, consistent with white dwarfs if the primaries have 0.8 to 0.9 solar masses.[10](https://arxiv.org/abs/1510.05840)
Because pure Roche-lobe overflow from an AGB star is usually unstable and leads to negligible accretion, much of the modeling effort has focused on wind accretion, particularly wind Roche-lobe overflow, in which the AGB wind is directed toward the companion.[3](https://academic.oup.com/mnras/article/441/2/1217/1067494)
## Relation to barium stars and CEMP stars
CH stars occupy a metallicity bridge between two related classes of chemically peculiar binaries. Barium stars, generally the metal-rich [Population I](https://www.edgepedia.org/population-i) analogs of CH stars, mostly lie at −0.6 ≤ [Fe/H] ≤ 0.2; CEMP-s stars lie at [Fe/H] ≤ −2.0; and CH stars have intermediate metallicities of −1.5 ≤ [Fe/H] ≤ −0.5, a range in which barium stars are rare.[11](https://academic.oup.com/mnras/article/459/4/4299/2623989) Barium stars are less carbon-enhanced than their metal-poor counterparts because their pre-transfer composition already contained more carbon and oxygen, so the accreted material does not push the C/O ratio above 1 and the CH and C2 lines stay weaker.[12](https://www.researchgate.net/publication/23954982_A_holistic_approach_to_carbon-enhanced_metal-poor_stars)
The CEMP-s stars, defined by Beers and Christlieb (2005) as carbon-enhanced metal-poor stars with [C/Fe] > 1 and [Fe/H] < −2, are considered the more metal-poor counterparts of CH stars with the same origin.[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) A radial-velocity campaign of 22 CEMP-s and CEMP-r/s stars found 18 of them (82 percent ± 26 percent) to be bona fide binaries, the rest being single stars or systems with extremely long periods.[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) The similarity in binary properties between CH and CEMP-s stars places the hypothesis that CEMP-s stars are the lower-metallicity equivalents of CH stars on firmer footing.[3](https://academic.oup.com/mnras/article/441/2/1217/1067494) Boundaries between the classes are not sharp: some stars appear in both CH and CEMP-s samples, which reflects how loosely the classes are defined.[3](https://academic.oup.com/mnras/article/441/2/1217/1067494)
## s-process enrichment and nucleosynthesis diagnostics
Because the pollution came from an AGB companion, the surface composition of a CH star is a fossil record of AGB nucleosynthesis, and abundance studies use CH stars to constrain models of low-mass AGB stars.[2](https://academic.oup.com/mnras/article/486/3/3266/5382052) • [13](https://doi.org/10.3847/1538-4357/833/2/181) In seven CH giants, s-process abundances run from [s/Fe] = +1.0 to +2.0, the heavy s-process peak (Ba through Sm) is more enhanced than the light peak (Sr, Y, Zr) by about +0.9 dex on average, and the patterns are best fit by a single neutron irradiation of seed nuclei, pointing to the 13C(α, n)16O reaction as the neutron source.[6](https://doi.org/10.1086/116375)
Lead is often strongly enhanced. In the Subaru analysis of HD 26, HD 198269, and HD 224959, [Pb/Fe] reached 2.11, 2.4, and 3.7 respectively, and [Pb/hs] decreases with increasing metallicity as models predict.[7](https://doi.org/10.1093/mnras/stv2011) The same study found that the neutron-capture elements in HD 26 ([Fe/H] = −1.13) come mainly from the s-process, while in the more metal-poor HD 198269 and HD 224959 the r-process dominates, possibly from material pre-enriched by r-process products.[7](https://doi.org/10.1093/mnras/stv2011) Models still fall short in places: theoretical AGB models reproduce only part of the observed CH-star abundance spread at fixed metallicity, and cannot reach the large [hs/ls] ratios of some objects, prompting speculation about an intermediate neutron-capture (i-) process.[13](https://doi.org/10.3847/1538-4357/833/2/181)
## Galactic population and surveys
CH stars are high-velocity [Population II](https://www.edgepedia.org/population-ii) halo objects and the most populous type of halo carbon star known, which makes them useful tracers of the [Galactic halo](https://www.edgepedia.org/galactic-halo) and of nucleosynthesis in metal-poor low-mass stars.[4](https://lweb.cfa.harvard.edu/~pberlind/atlas/htmls/cstars.html) • [5](https://academic.oup.com/mnras/article/359/2/531/987142) Kinematic studies with Gaia astrometry show that CH-type stars in the LAMOST survey sit far from the [Galactic plane](https://www.edgepedia.org/galactic-plane), at an average height of 2.7 ± 2.5 kpc, and share luminosities and kinematics with R-hot carbon stars, although CH stars are binaries with s-process enhancement while R-hot stars show neither property.[14](https://arxiv.org/abs/2206.00405)
Large spectroscopic surveys keep expanding the census. The GALAH survey built a catalog of carbon-enhanced stars and CEMP candidates and identified 32 lithium-rich CH stars among its detections.[15](https://academic.oup.com/mnras/article/483/3/3196/5199221) Using the Gaia-2MASS diagram, most carbon stars identified in LAMOST as AGB stars are probably R-hot or CH-type stars rather than genuine AGB carbon stars.[14](https://arxiv.org/abs/2206.00405)
## References
1. [Modeling the progenitors of low-mass post-accretion binaries, Astronomy & Astrophysics (2025)](https://doi.org/10.1051/0004-6361/202555701)
2. [Chemical analysis of CH stars – III. Atmospheric parameters and elemental abundances, MNRAS 486, 3266 (2019)](https://academic.oup.com/mnras/article/486/3/3266/5382052)
3. [Binarity in carbon-enhanced metal-poor stars, MNRAS 441, 1217 (2014)](https://academic.oup.com/mnras/article/441/2/1217/1067494)
4. [C Stars, CfA spectral atlas](https://lweb.cfa.harvard.edu/~pberlind/atlas/htmls/cstars.html)
5. [CH stars at high Galactic latitudes, MNRAS 359, 531 (2005)](https://academic.oup.com/mnras/article/359/2/531/987142)
6. [The CH Stars. III. Heavy Element Abundances, Vanture, Astronomical Journal 104, 1997 (1992)](https://doi.org/10.1086/116375)
7. [Subaru/HDS study of CH stars, MNRAS (2015)](https://doi.org/10.1093/mnras/stv2011)
8. [Keenan, P. C. (1942). The Spectra of CH Stars, ApJ 96, 101](https://articles.adsabs.harvard.edu/pdf/1942ApJ....96..101K)
9. [Subgiant CH stars. II. Chemical compositions and the evolutionary connection with barium stars (1991)](https://pure.psu.edu/en/publications/subgiant-ch-stars-ii-chemical-compositions-and-the-evolutionary-c)
10. [Binary properties of CH and Carbon-Enhanced Metal-Poor stars, Jorissen et al. (2015), arXiv:1510.05840](https://arxiv.org/abs/1510.05840)
11. [Chemical abundances and kinematics of barium stars, MNRAS 459, 4299 (2016)](https://academic.oup.com/mnras/article/459/4/4299/2623989)
12. [A holistic approach to carbon-enhanced metal-poor stars](https://www.researchgate.net/publication/23954982_A_holistic_approach_to_carbon-enhanced_metal-poor_stars)
13. [Constraints of the physics of low-mass AGB stars from CH and CEMP stars, ApJ 833, 181 (2016)](https://doi.org/10.3847/1538-4357/833/2/181)
14. [Characterisation of Galactic carbon stars and related stars from Gaia-EDR3, Abia et al. (2022), arXiv:2206.00405](https://arxiv.org/abs/2206.00405)
15. [GALAH survey: a catalogue of carbon-enhanced stars and CEMP candidates, MNRAS 483, 3196 (2019)](https://academic.oup.com/mnras/article/483/3/3196/5199221)
---
*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Stars and galaxies › Stellar astrophysics, structure, evolution, and variables › Rotational and chemically peculiar variables › Barium and mass-transfer chemically peculiar stars*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 11, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license