Carbon excess

Recent surveys of metal-deficient stars have discovered a large number of carbon-rich objects, with a marked increase in their frequency at [Fe/H] < —2.5. In order to constrain the origin(s) of their carbon excesses, we have performed elemental abundance analyses for 40 objects selected from candidate metal-poor stars with strong CH G bands identified in the HK and Hamburg/ESO surveys. High-resolution spectroscopy has been obtained with AAT/UCLES and Subaru/HDS a portion of these studies have already been published [1—3]. 

Fig. la shows the abundance ratio [Ba/Fe] for this sample as a function of [C/Fe]. Thirty stars (77% of the sample) have [Ba/Fe] > +0.7, while the others have [Ba/Fe] < 0.0. There is a clear gap in the Ba abundances between the two groups, suggesting at least two different origins of the carbon excesses. Ba-enhanced stars The Ba-enhanced stars exhibit a correlation between the Ba and C abundance ratios (Fig. la). This fact suggests that carbon was enriched in the same site as Ba. The Ba excesses in these objects presumably originated from the s-process, rather than the r-process, because (1) nine stars in this group for which detailed abundance analysis is available clearly show abundance patterns associated with the s-process [2], and (2) there is no evidence of an r-process excess in the other 21 objects. Hence, the carbon enrichment in these objects most likely arises from Asymptotic Giant Branch (AGB) stars, which are also the source of the s-process elements. 

It should be noted that at least three of the Ba-rich stars in our sample exhibit no clear variation in their radial velocities over the last 8-10 years. Either their periods are quite long, or the mass-accretion scenario may not apply to these objects. Further investigation of the binarity for these objects is clearly required. Ba-normal stars The other nine objects in our sample have relatively low Ba abundances ( — 1.0 < [Ba/Fe] < —0.5). These values are typical in metal-deficient stars that show no carbon excess ([C/Fe]< +0.5), hence the scenario of carbon enrichment by AGB stars cannot be simply applied to these stars. 

Figure 7.17 Same as Figure 7.16 for pCOl, seawater alkalinity A, runoff [Ca2+] and the fraction F of precipitated calcite which is preserved on the ocean floor. It takes a little less than 10000 years for runoff calcium to neutralize the excess dissolved COz, but calcite precipitation takes much longer to eliminate Ca and carbonate excess. 

Furan was distilled over anhydrous potassium carbonate. Excess furan is necessary to trap the aryne intermediates. 

Cohall is present in vitamin Bi to Ihe extent of about 4ci-. Lack of cobalt in tlie soil and feedstuffs prevents tuniinants from synthesizing all of the vilamin B j for their needs. Thus, cobalt can be added to feedstuffs as the chloride, sulfate, oxide, nr carbonate. Excessive cobalt intakes are toxic, causing a reduction in feed intake and body weight, accompanied by emaciation, anemia, debility, and elevated levels of cobull in the liver. It is of interest to note that clinical coball tnxiciiy closely resembles clinical cobalt deficiency. 

Over 50 ADP-Glc PPases (mainly bacterial but also plant) have been studied with respect to their regulatory properties and, in almost all cases, glycolytic intermediates activate ADP-Glc synthesis, while AMP, ADP, and/or P are inhibitors. Glycolytic intermediates in the cell can be considered as signals of carbon excess and therefore, under conditions of limited growth with excess carbon in the media, accumulation of glycolytic intermediates would be indicative for the activation of ADP-Glc synthesis. Thus, the enzyme seems to be affected by the availability of ATP in the cell and the presence of glycolytic intermediates. 

The excess of DIC taken up by C. crispus in comparison with its CO2 fixation indicated by the linear net O2 evolution (Fig. 7), suggests the build up of an internal DIC pool. Since the concept of an active HCO3 transport has been deduced from the usual observation of a photosynthetic depletion of HCO3 in comparison to CO2 (11), this carbon excess is concluded to represent an accumulation driven by HCO3 transport. 

It should be noted that during the implantation of low-energy ions (e.g., 100-keV B ) in polyethylene, polyamide, and some other polymers, the carbon enrichment turns out to be maximal at some depth below the surface of the irradiated target, which is evidenced by the depth profile of carbon excess in the implanted layer reconstructed from RBS spectra. In this case the oxygen depth profile is, generally, saddle shaped, because the additional maximum of oxygen concentration... 

The main drawback of this material is a low intrinsic electronic conductivity, which may lead to poor electrochemical performances. The synthesis procedure is aiso a sensitive point, as the oxidation of iron has to be avoided. A relatively simple method to deal with both the reducing synthesis conditions and the low conductivity of the product is to use carbon additives, also known as carbothermal process. Carbon additives have a two-fold effect. On the one hand, iron is protected against oxidation during the heating process. On the other hand, the use of carbon excess may provide an intimate mixture of LiFeP04 and carbon that has a much higher electronic conductivity than the phosphate alone. 

Calculated Raman spectra for (I) ideal SiC cubic-like fragment (II) hexagonal-like (a) and cubic-like (b) SiC fragment with carbon excess (20%) and (III) with excess of silicon (2%) (Reproduced with permission from Kassiba et al. (2002a))... 

The Raman signatures of SiC nanoparticles inform on the vibrational properties of the involved structures and on the degrees of organization and their stoichiometry. Any departure from perfect crystalline order, silicon or carbon excess leads to additional features, such as broad... 

Furthermore, for the SiC238 and SiC229 samples, annealing under argon at 1,400°C contributes to the emergence of well-defined SiC Raman bands and reduction of the broad PL features (Kassiba et al. 2002b). Improvement of the crystalline structures and modification of the particle surfaces by a release of carbon excess are the main effects of the annealing treatment (Charpentier et al. 1999). 

However, it is well known that real nanoparticles exhibit a non-negligible amount of dangling bonds at the outermost surfaces or vacancies in the nanoparticle cores. Furthermore, a stoichiometry departure at the surface due to carbon excess or stacking faults in the particle cores are present in real nanoparticles. Also, as a large molecular complex, SiC duster should exhibit intermediate energy levels and electronic states allowed inside the bandgap. 

Efficient ionization of the vapor species was achieved in the DC discharge method during the ion plating process however, careful optimization of the deposition conditions was needed in the RF discharge method during the process. Stoichiometric TiC deposits could easily be obtained by the former method, but the formation of carbon-excess TiC films was characteristic of the latter method. 

Carbon-excess Ti,Ci-, deposits showed a strongly preferred orientation and a much higher hardness than nearly stoichiometric deposits. 

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