Duane Francis Carbon

Carbon and nitrogen abundances in giant stars of the metal-poor globular cluster M92

Zinn in 1973 and 1977 and Norris and Zinn in 1977 showed that in M92 and several other metal-poor globular clusters the G bands (mostly due to CH) in the spectra of asymptotic giant branch (AGB) stars are systematically weaker than those found in the less highly evolved subgiant branch (SGB) stars. If carbon is depleted in the atmospheres of evolved stars because material at the base of the envelope, processed through the CN cycle, has been mixed with the material above, then the atmospheric nitrogen abundance should be correspondingly increased. In this paper we test the hypothesis that C and N abundances in M92 giants are negatively correlated as the evolutionary state becomes more advanced. We find that this simple hypothesis is not adequate to describe the complex behavior of C and N in the cluster giants.

Carbon and nitrogen abundances in metal-poor dwarfs of the solar neighborhood

The authors have analyzed spectra of 83 subdwarfs for C, N, and Fe abundances. They deduced [C/Fe] from the CH bands (λ ≡ 4300 A), [N/Fe] from the NH bands (λ ≡ 3360 A), and [Fe/H] from strong mostly Fe I features (3400 A ≤ λ ≤3550 A). They find excellent agreement between the [Fe/H] values and the results obtained by other investigators based on high-resolution spectra. They find [C/Fe] to be essentially constant, with a scatter of σ ≡ 0.18 dex, over the range -0.75 ≥ [Fe/H] > -2.5, confirming that C is a primary nucleosynthetic species in this metallicity domain. It is found that the behavior of as [Fe/H] declines is compatible with the assumption of a primary, as opposed to a secondary, origin of N, but the limiting value of is found to be several tenths dex smaller than in previous investigations.

On the carbon abundance of subgiant stars in the globular cluster M 92

The authors find that >[C/Fe]< ≈ 0.0 in stars at the base of the subgiant branch and approaches +0.1 in stars near the main-sequence turnoff in the metal-poor globular cluster M92. The carbon abundance falls on the average by about 0.5 dex by the time stars reach MV0 ≡ +1.5. These carbon depletions occur early enough and are approximately the right size to account for the large average nitrogen enhancement ([N/Fe] ≡ +0.65) observed in M92 subgiants near MV0 ≡ +1.5. Although the present observations do not absolutely prove that C→N conversion and mixing alone control the C- and N-abundances seen in M92 giants, neither is there any longer a compelling reason to believe there were large primordial nitrogen abundance variations in M92. The mechanism responsible for the large (Δ[C/Fe] ≈ 1.0) carbon depletions observed as stars ascend the M92 giant branch to the red-giant tip remains unknown.

Carbon and nitrogen abundances in giant stars of the metal-poor globular cluster M15

We derive carbon and nitrogen abundances for 33 giant-star members of the globular cluster M15, thus extending the earlier work of Carbon et al. in M92 to another cluster of comparably low metallicity ((M/H)< or approx. =-2). We describe a technique for obtaining C and N abundances which is based on spectrophotometric indices that measure the strengths of NH, CN, and CH bands in low-resolution scanner spectra. The spectrophotometric indices are calibrated using scanner spectra of M92 giants stars in which estimates of C and N abundances had previously been obtained by the method of spectrum synthesis. The C abundances derived here for M15 giants are comparable in quality to those given for M92 giants previously; observational errors associated with the N abundances for M15 stars are typically about twice those derived for M92 stars.

CARBON AND NITROGEN ABUNDANCES IN EXTREMELY METAL-DEFICIENT RED GIANTS.

The relative carbon and nitrogen abundances of a sample of metal-poor halo giants are analyzed with respect to their similarity with the abundances of similar stars in globular clusters. Spectra of 64 giants selected from the objective prism survey of Bond (1980) were obtained at a resolution of 8 A in the spectral region 3100-5300 A. C/Fe and N/Fe abundance ratios were estimated from the spectra by the method of spectrum synthesis using metallicities estimated from H and K line strengths. Halo giants with metallicities greater than or equal to -2.0, which are characteristic of giants in the M3 and M13 clusters, are found to have C/Fe and N/Fe abundance distributions similar to those of M3 but not M13, for which a greater carbon depletion and nitrogen enhancement is observed. Halo giants with metallicities less than -2.0 showed less carbon depletion and nitrogen enhancement than the corresponding M29 stars. Globular cluster and field halo stars with the same metallicities thus do not appear to be derived from the same parent population.

Carbon and Nitrogen Abundances in Giant Stars of the Metal Poor Globular Cluster M92 — A Preliminary Report

Observational data on the C12/C13 and O/H-abundance ratios in red giants with near solar metal abundance have recently been put forward by Lambert, his associates and students. In summarizing these results, Dearborn, Eggleton, and Schramm (1976) concluded that in the atmospheres of pre-He-core flash red giants there is evidence for substantial depletion of C12 and enhancement of C13 presumably as a result of deep mixing of envelope material into layers that have undergone CNO-cycle processing. Moreover, values of the ratio C12/C13 in the range 10–20 seem to be achieved by stars with luminosities too low, and evolutionary states too early, for compatibility with convectively mixed standard stellar models. It is true, however, as recognized by Dearborn, et al., that red giants of the general star field have checkered backgrounds: giants in the same part of the HR diagram may have quite different masses and ages, even for a fixed initial metal-abundance. Moreover, comparison of a given star with theoretical models may be rendered somewhat uncertain by errors in parrallax or other criteria of absolute magnitude.