Metal deficient

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]. 

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. 

A quick impression of a star s metallicity can often be derived from inspection -either qualitative or quantitative - of strong spectral features such as the CN band A, 4215 in giants or comparison of hydrogen and Ca+ K-line (A 3933) intensities, which has been particularly useful in the discovery of extremely metal-deficient stars (Beers, Preston Shectman 1992). Numerical comparisons of digital spectra with low signalinoise ratio can also be carried out for this purpose (Carney et al. 1987). 

Fig. 3.21. Two-colour plot of (U-B) against (B—V). The curve shows the main sequence for stars with the metallicity of the Hyades, loci of black bodies (with temperatures marked in units of 1000 K) and a deblanketing vector illustrating schematically the effect of metal deficiency in F and G stars. Adapted from Unsold (1977).

The most metal-deficient stars comprise field stars in the solar neighbourhood (where in some cases distances and luminosities can be found from parallaxes) and stars in globular clusters where the morphology of the HR diagram can be studied (Fig. 4.8). Such stars are of particular interest because their content of heavy elements (synthesized in still earlier generations of stars) is so low that they can... 

Thus the 13 C neutron source (with a little assistance from 22Ne) in thermally pulsing low- and intermediate-mass stars is well established as the chief source of the main component of s-process nuclides in the Solar System. It is not quite clear, however, whether the r0 parameter is something unique, or just the average over a more-or-less broad distribution of values nor is it clear why a similar s-process pattern is seen in stars that are metal-deficient by factors of up to 100 (see Pagel Tautvaisiene 1997). 

The site of the r-process is also not clear, but it seems that the conditions needed to reproduce Solar-System r-process abundances may hold in the hot bubble caused by neutrino winds in the immediate surroundings of a nascent neutron star in the early stages of a supernova explosion (see Fig. 6.10). Circumstantial evidence from Galactic chemical evolution supports an origin in low-mass Type II supernovae, maybe around 10 M (Mathews, Bazan Cowan 1992 Pagel Tautvaisiene 1995). Another possibility is the neutrino-driven wind from a neutron star formed by the accretion-induced collapse of a white dwarf in a binary system (Woosley Baron 1992) leading to a silent supernova (Nomoto 1986). In stars with extreme metal-deficiency, the heavy elements sometimes display an abundance pattern characteristic of the r-process with little or no contribution from the s-process, and the... 

Fig. 8.9. Even-numbered element iron ratios measured in ultra-metal-deficient Galactic halo stars, after Cayrel et al. (2004). 

Oxygen and moisture has to be excluded carefully during the preparation procedures to avoid oxygen-centered cages. Then unusual metal deficient phosphanediides of lithium of the type [(Li2PR) (PR)m] with Li2 P( +m) cages are isolated. Investigations of phosphanides of the heavier alkali metals are far less common [37]. 

Fuerstenau (1980) found that sulphide minerals are naturally floatable in the absence of oxygen. Yoon (1981) ever attributed the natural floatability of some sulphide minerals to their very low solubility. Finkelstein et al. (1975) considered that the natural floatability of sulphide minerals are due to the formation of elemental sulphur and related to the thickness of formation of elemental sulphur at the surface. Some authors reported that the hydrophobic entity in collectorless flotation of sulphide minerals were the metal-deficient poly sulphide (Buckley et al., 1985). No matter whichever mechanism, investigators increasingly concluded that most sulphide minerals are not naturally floatable and floated only under some suitable redox environment. Some authors considered that the natural floatability of sulphide minerals was restricted to some special sulphide minerals such as molybdenite, stibnite, orpiment etc. owing to the effects of crystal structure and the collectorless floatability of most sulphide minerals could be classified into self-induced and sulphur-induced floatability (Trahar, 1984 Heyes and Trahar, 1984 Hayes et al., 1987 Wang et al., 1991b, c Hu et al, 2000). 

Above 1127°C, a single oxygen-rich non-stoichiometric phase of UO2 is found with formula U02+, ranging from UO2 to U02.25- Unlike FeO, where a metal-deficient oxide was achieved through cation vacancies, in this example the metal-deficiency arises from interstitial anions. 

Type B interstitial cations formula M1+/O Metal deficiency (oxidised metal)... 

We looked in some detail earlier at the structure of FeO this falls into the type D category, with metal deficiency and cation vacancies resulting in oxidized metal. (Other compounds falling into this category are MnO, CoO, and NiO.)... 

FIGURE 5.41 Structural possibilities for binary oxides, (a) Type A oxides metal excess/anion vacancies, (i) This shows the two electrons that maintain charge neutrality, localized at the vacancy, (ii) The electrons are associated with the normal cations making them into M. (b) Type B oxides metal excess/interstitials. (i) This shows an interstitial atom, whereas in (ii) the atom has ionized to and the two liberated electrons are now associated with two normal cations, reducing them to M. (c) Type C oxides metal deficiency/interstitial anions. The charge compensation for an interstitial anion is by way of two ions, (d) Type D oxides metal deficiency/cation vacancies. The cation vacancy is compensated by two cations. 

Partial pressure of oxygen controls the nature of defects and nonstoichiometry in metal oxides. The defects responsible for nonstoichiometry and the corresponding oxidation or reduction of cations can be described in terms of quasichemical defect reactions. Let us consider the example of transition metal monoxides, M, 0 (M = Mn, Fe, Co, Ni), which exhibit metal-deficient nonstoichiometry. For the formation of metal vacancies in M, 0, the following equations can be written ... 

The studies on Cu2 aO mentioned above concluded that CujO is a metal-deficient p-type semiconductor with cation vacancies. It was not established, however, which kinds of defects (Vcu, Vcu) were dominant and what the effect of Q (interstitial oxygen) was on non-stoichiometry. To clarify these points, Peterson and Wiley measured the diffusion coefficient, D, of Cu in Cu2 O, by use of "Cu as a tracer over the temperature range 700-1153 °C and for oxygen partial pressures, greater than 10 atm. It has been widely accepted that lattice defects play an important role in the diffusion of atoms or ions. Accordingly it can be expected that the measurement of D gives important information on the lattice defects. 

The CH stars and the CH-like stars are included in Table 1 for completeness, but we don t know much about them. The CH stars are metal-deficient, Population II giants enriched in carbon and s-process elements. They may be enriched in (Lambert 1985). They appear to be binaries (McClure 1985). The CH stars are probably Pop II barium stars, and to accept their designation as Pop II barium stars. Not much is known about the CH-like stars of Yamashita (1972, 1975), which are Pop I, carbon-rich giants. At low spectral resolution the 14554 line of Ba II is enhanced. These stars may be related to the barium or barium-carbon stars, but we need more information about their compositions, and especially about their binary status. 

Crystal Self-Diffusion in Nonstoichiometric Materials. Nonstoichiometry of semiconductor oxides can be induced by the material s environment. For example, materials such as FeO (illustrated in Fig. 8.14), NiO, and CoO can be made metal-deficient (or O-rich) in oxidizing environments and Ti02 and Zr02 can be made O-deficient under reducing conditions. These induced stoichiometric variations cause large changes in point-defect concentrations and therefore affect diffusivities and electrical conductivities. 

Figure 8.14 Addition of a neutral O atom to FeO to produce O-rich (metal-deficient)...

Plant roots are known to exude a fluid containing a number of amino and carboxylic acids, the amount of the exudate increasing under conditions of metal deficiency. Chlorotic plants, i.e. those suffering from iron defidency, have been found to contain more of the citric and malic acids than their normal green counterparts.21 Differences in the susceptibilities of plant species to trace metal deficiencies have indeed frequently been attributed to variations in organic acid production. 

Also the composition of the Sr-HA complexes apparently differs somewhat from those of Co-HA and Eu-HA, as the latter two types exhibit pure 1 1 character, whereas a tendency to a metal-deficient complex is found in the strontium case ... 

---