Carbon, abundance

A weak but useful carbon line [Cl] 8727.13 A disappears in halo dwarfs with metallicities below —1. To measure carbon abundance in halo stars one can use four Cl high excitation lines near 9100 A and the CH band at 4300 A. The Cl lines at 9100 A together with the OI triplet at 7771 A have been used by Tomkin et al. (1992) and Akerman et al. (2004) to study the behaviour of C/O versus metallicity. However, Cl and OI lines employed in these papers are sensitive to a non-LTE effects and one has to bare in mind that this sensitivity is different for C and O. The CH band at 3145 A used by Israelian et al. (1999) is almost saturated in disk stars and several blends makes the abundance analysis less accurate. To ensure a homogeneous analysis of the C/O and N/O ratio from NH,CH and OH lines in the near-UV, we used the same model atmospheres and tools as in our previous studies. The oxygen abundances were compiled from Israelian et al. (1998, 2001) and Boesgaard et al. (1999). 

Carbonaceous material (Fig. 12.8b) is intimately mixed with silicates and is very abundant (carbon abundance averages 13% and varies up to 50%) in CP IDPs. Some carbon is elemental (graphite), but C-H stretching resonances in infrared spectra show that aliphatic hydrocarbons are also present. Polycyclic aromatic hydrocarbons (PAHs) also occur. Nanodiamonds have been identified in cluster IDPs, but not in smaller CP IDPs. Enormous D/H and 15N/14N anomalies have been measured in bulk IDPs, and the hydrogen isotopic anomalies are correlated with organic-rich domains. Ratios of D/H as high as 25 times the solar ratio suggest the presence of molecular cloud materials. 

Carbon stars earlier than C3 cannot be well detected due to the weakness of C2 Swan bands. A majority of the stars belong to C4 and C5 stars with high carbon abundance, while there are a few C8 and C9 stars. 

The evolution of the carbon abundance at the surface of both components of a mass-exchanging (Algol-type) binary is examined (fig. 1). Distinction is made between case B and case AB (fig. 2) of mass transfer, in view of the different timescales involved. In the mass accreting component thermohaline mixing is adopted when matter with decreasing hydrogen abundance is deposited on the surface. 

Figure 1. Carbon abundance as a function of mass for both components of a close binary system at the onset of mass transfer. The region from Mx=0 to Mr=Mgi=8.1 Mo corresponds to the originally less massive component (gainer), whereas the carbon distribution of the loser is plotted from 8.1 Mo (surface) to 17.1 M (center). The first occurrence of hydrogen depleted layers (Xat<0.7) and the end of the Roche Lobe Overflow are indicated.

The interstellar light extiction curve at 2,175 A can be modeled using fulleranes with different degree of hydrogenation (Webster 1995 Cataldo 2003 Cataldo et al. 2009). The fraction of interstellar carbon abundance needed to produce fulleranes is high, but not prohibitively and carbon stars and planetary nebulae could provide these molecules in sufficient quantity. The experimental determination of optical and infrared spectra for laboratory-isolated fulleranes would be very valuable in assessing their role as carriers of the DIBs and of the unidentified infrared emission features. 

The cross section obtained for single fullerenes and buckyonions reproduce the behaviour of the interstellar medium UV extinction curve. A power-law size distribution n(R) R m with in = 3.5 1.0 for these molecules can explain the position and widths observed for the 2,175 A bump and, partly, the rise in the extinction curve at higher energies. We infer ISM densities of 0.2 and 0.1 ppm for small fullerenes and buckyonions (very similar to the densities measured in meteorites). If as expected the cosmic carbon abundance is close to the solar atmosphere value, individual fullerenes may lock up 20-25% of the total carbon in the diffuse interstellar space. 

Carbon abundances are measured in planetary nebulae by observing from space the ultraviolet lines of doubly ionized C++. The observed C/O abundance ratio is large and variable, indicating C production in the intermediate-mass stars thatare the immediate precursors to the planetary nebulae. Evidence of different nuclei synthesized in different types of stars is established by such data. They also reveal the nucleosynthesis of carbon to be more complex than that of oxygen. 

Fig. 2. Total carbon abundances (by combustion) in basaltic rocks, breccias and bulk fines from Apollo 11, 12, 14 and 15 missions. Horizontal bars indicate different samples...

Was not obtainable in natural carbon abundance samples-e Olah etal., 1964, 1970, 1971. 

It is important here to call attention to the revised determinations of the oxygen and carbon abundances in the Sun. Allende Prieto et al. (2001) derived an accurate oxygen abundance for the Sun of log s(0) = 8.69 0.05 dex, a value approximately a factor of 2 below that quoted by Anders and Grevesse (1989). Subsequently, Allende Prieto et al. (2002) determined the solar carbon abundance to be log s(C) = 8.39 0.04 dex, and the ratio C/O = 0.5 0.07. The bottom line here is a reduction in the abundances of the two most abundant heavy elements in the Sun, relative to hydrogen and helium, by a factor 2. The implications of these results for stellar evolution, nucleosynthesis, the formation of carbon stars, and galactic chemical evolution remain to be explored. 

Bulk nitrogen and carbon abundances and isotopic compositions... 

Although TES and THEMIS are sensitive to carbonates and sulfates, these minerals have not yet been detected unambiguously from orbit (Bandfield, 2002). The low carbon abundance in APXS-analyzed soils rules out much carbonate, although appreciable sulfur and chlorine are present in all soils. Thermodynamic stability considerations suggest that sulfates and iron carbonates should be present under martian conditions (Clark and Van Hart, 1981 Catling, 1999). It is unclear whether sulfate formed by reactions with acidic vapor from volcanic exhalations (Banin et al., 1997) or evaporation of surface brines (Warren, 1998 McSween and Harvey, 1998). 

The C/Si ratio in chondritic IDPs is systematically higher than all classes of chondritic meteorites. The mean carbon abundance is —10 wt.% versus 3.22 wt.% for Cl (see Chapter 1.03). Nitrogen has been detected in chondritic IDPs but as yet not quantified, although Keller et al. (1995) report that the C/N ratio is approximately chondritic. Electron energy-loss spectra show that nitrogen is carried in amorphous carbonaceous material and that it is heterogeneously distributed as hot spots. There is indirect evidence that the nitrogen is associated with polyaromatic hydrocarbons (Section 1.26.3.1). 

Mathez E. A., Blade J. D., Beery J., Maggiore C., and Hollander M. (1984) Carbon abundances in mantle minerals determined by nuclear reaction analysis. Geophys. Res. Lett. 11, 947-950. 

---