Annu. Rev. Astron. Astrophys. 1997. 35: 503-556
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7.1. Carbon

Carbon is one of those elements that can be greatly affected by late stages of stellar evolution. In the red giant stage, a star will dredge up material processed by the CNO cycle, which results in C depletions, increased 13C, and increased N abundances, and sometimes mild O depletions. In this review, I am mostly concerned with evolution of abundances in the Galaxy as a whole and not with the self-pollution of individual stars, unless this has a significant effect on the Galactic picture. For an excellent discussion of mixing in red giant branch stars, see Kraft (1994), Kraft et al (1997), Shetrone (1996a, b).

Wheeler et al (1989) reviewed carbon abundances from Peterson & Sneden (1978), Clegg et al (1981), Laird (1985), Tomkin et al (1986), Carbon et al (1987) and concluded that [C/Fe] ~ 0.0 independent of [Fe/H], but with a possible increase in [C/Fe] below [Fe/H] ~ -1.5.

Recent abundance studies of carbon indicate that [C/Fe] is enhanced with declining [Fe/H] in the Galactic disk (e.g. Friel & Boesgard 1992, Andersson & Edvardsson 1994, Tomkin et al 1995), such that at [Fe/H] = -0.8, [C/Fe] ~ +0.2. Thus, in the disk [C/Fe] and [alpha / Fe] show morphologically similar trends with [Fe/H].

Tomkin et al (1992) found that [C/O] ~ -0.6 for halo dwarfs in the interval -1 geq [Fe/H] geq -2.6, based on high excitation C I and O I lines. Although the C I and O I results showed evidence of unaccounted non-LTE effects, Tomkin et al suggested that the errors cancel out for the [C/O] ratio. Tomkin et al also measured [C/Fe] from CH lines and found a trend of increasing [C/Fe] with decreasing [Fe/H]: Near [Fe/H] = -1, [C/Fe] ~ -0.3, with the trend suggesting that at [Fe/H] = -2, [C/Fe] ~ 0.0. Balachandran & Carney (1996) measured C and O from infrared CO and OH lines for one halo dwarf at [Fe/H] = -1.2, and they found [C/Fe] = -0.32.

Two puzzles arise from the Tomkin et al (1992) halo results: If [C/O] is constant, but [C/Fe] increases with declining [Fe/H], then [O/Fe] must also increase with declining [Fe/H]; yet Figure 3a rules out the required 0.3 dex change in [O/Fe] between [Fe/H] = -1 and -2. It is likely that the constant [C/O] ratio implied from the high excitation C I and O I lines may be suspect. The second difficulty, pointed out by Balachandran & Carney (1996), results from the large change in [C/Fe] between the halo and disk at similar metallicity; in the halo [C/Fe] ~ -0.3 near [Fe/H] = -1.2, while in the disk [C/Fe] = +0.2 at [Fe/H] = -0.8. According to Balachandran & Carney (1996), this would require a large contribution to Galactic carbon from intermediate-mass stars.

McWilliam et al (1995b) measured [C/Fe] values for 33 halo giants with -4 leq [Fe/H] leq -2 and combined the results with the sample of Kraft et al (1982); no compelling evidence was found for a deviation of the mean [C/Fe] from the solar ratio. However, when compared with the results of Carbon et al (1987), a slight trend of increasing [C/Fe] could not be ruled out at the level of about 0.07 dex/dex in [Fe/H]. In either case, [C/Fe] is roughly constant over a range of 3.5 decades in [Fe/H]. McWilliam et al (1995b) found a large scatter in [C/Fe] for their giant sample, with a range of 1.6 dex, which is much larger than the measurement uncertainties. It seems possible that the scatter in [C/Fe] is due to an intrinsic dispersion in composition of the gas that formed the stars. If this is the case, then some of the halo carbon stars may not be the products of nucleosynthesis on the AGB, or mass transfer from an AGB star, but occurred because of stochastic enhancements in the carbon abundance of Galactic gas. This idea was supported by Kipper et al (1996), who claimed to have found at least three objects that formed as intrinsic carbon stars.

Thus, although the disk carbon abundances may resemble the alpha element pattern, in the halo the abundance trends are quite different and quite uncertain. It is clear that more work is required to properly understand [C/Fe] as a function of [Fe/H]; this might best be executed by taking advantage of new infrared spectrometers to measure C and O abundances from lines of CO and OH (as pointed out by Balachandran 1996).

For the bulge, the only carbon abundance measurement is that of A McWilliam, A Tomaney & RM Rich (in progress), who used the published values of the narrowband CO index for several bulge giants to estimate the average bulge [C/Fe] ratio, which was found to be ~ -0.2 dex. This value is consistent with typical [C/Fe] ratios seen in solar neighborhood red giants (Lambert & Ries 1981); the slight deficiency from the solar value is due to the normal red giant dredge-up of material processed though the CN cycle.

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