Iron peak

The derived supernovae ratios of the metal-rich systems in Table 1 indicate that both the low-a and low iron-peak abundances are consistent with significant contributions from Type la supernovae. In general, when compared with the abundances of stars born within the MWG, the metal-rich stars associated with the Sgr dSph possess low iron-peak abundances relative to iron (e.g., [3], [4]). 

By comparing the observed chemical abundance ratios to supernova model yields, one can calculate , the ratio of the number of SNe la to SNe II events that fit the observations and the synthesized mass of the elements from the model yields. In a study adopting the same analysis techniques as those performed here, [5] found large values of for a trio of low-a stars of [Fe/H] -2. Employing the abundances derived in this study of stars with comparable metallicities, I find that the metal-poor systems presented here possess a- and iron-peak abundances (and based on Na, Mg, Si and Fe) consistent with those observed in metal-poor stars of the MWG (e.g., [6]). 

These setups have been chosen to measure abundances of iron peak, a-elements and neutron-capture elements. In parallel, a subset of 14 stars has been observed with the high resolution spectrograph UVES (R=48000) to serve as calibrators for the GIRAFFE sample. 

Fig. 1. Abundances distributions for iron-peak elements (upper 3 panels) and for a-elements (lower 3 panels). References for the figures are filled circles - our sample open squares - [2] open triangles [6] crosses - [1].

For the Sgr dSph we present the UVES DIC1 spectra for 12 giants. Complete analysis of two of them has already been published [2], while for the other ten only iron and a-elements abundances have been published so far (see [3]). Details on the reduction and analysis procedures, and physical parameters for the stars are provided in [3], but they can be briefly resumed here the spectra have been analyzed by means of LTE, one dimensional atmosphere models, using ATLAS, WIDTH and SYNTHE codes (see [7] and [10]). Te// for the stars are in the range 4800 - 5050 K, log g between 2.3 and 2.7. We analyzed abundances of proton capture (Na, Al, Sc, V), a (Mg, Si, Ca, Ti), Iron-peak (Cr, Fe, Co, Ni, Zn) and heavy neutron-capture (Y, La, Ce, Nd) elements. 

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

Figure 6.2 X-ray diffraction identifies phases in a manganese-promoted iron Fischer-Tropsch catalyst after reduction (middle) and after CO hydrogenation or Fischer-Tropsch synthesis bottom). The spectra show that Mn is present as slightly distorted MnO (see the MnO reference measurement at the top), and that bcc iron (peak at 2 6 = 57.0°) converts to iron carbides (peaks around 55°) during the Fischer-Tropsch reaction (from van Dijk et al. [7]).

The most tightly bound nuclei, i.e. the most stable and robust, in the iron peak are not symmetric arrangements bringing together equal numbers of protons and neutrons (N = Z). Rather, they possess a neutron excess (N — Z) between 2 and 4. Close to iron, the most stable nucleus Fe has a number of neutrons which exceeds the number of protons by 4 units N — Z = 4). 

A small growth in the n/p ratio has a considerable effect on the composition of the iron peak. For values very close to unity, the most abundantly produced isotope in the absence... 

We know today that nuclear statistical equilibrium in a neutron-poor environment (p/n = 1.01), dominated by nickel-56 rather than iron-56, gives a good overall explanation of the abundance table in the neighbourhood of the iron peak. This is a natural consequence of high-temperature combustion. The corresponding combustion times are... 

The critical factor which thus determines the nature of the emerging iron peak is the combustion time. Only a hydrodynamic model can provide us with a full appreciation of fusion times, describing the sudden and brutal temperature increase as the shock wave passes through, and the equally sudden cooling that follows it. 

Finally, consider a temperature of 4 billion k rather than 3 billion, and a density of 20 000 000 g cm , but leave the weak interaction switched on. We find that nickel-56 once again predominates over other iron peak nuclei, and this very rapidly indeed. 

The proportions with respect to iron of vanadium-51, manganese-55 and cobalt-59, formed as manganese-51, cobalt-55 and copper-59, respectively, also agree with abundances observed in the Solar System. The overall coherence of this behaviour suggests that appropriate conditions for synthesis of iron peak elements should be sought in... 

The main mechanism by which nuclides beyond the iron peak are produced is by neutron capture. The basic processes involved in neutron capture were laid out by Burbidge, Burbidge, Hoyle, and Fowler (1957) (this classic paper is commonly known as B2FH). The common ingredient in these processes is the capture of a neutron by a nucleus, increasing the atomic mass by one unit. If the resulting nucleus is stable, it remains an isotope of the original element. If not, the atom P-decays (a neutron emits an electron and becomes a proton) and becomes an isotope of the next heavier element. Any isotope, whether stable or unstable, can capture another neutron. The rate of capture compared to the rate of decay leads to two basic end-member processes, the -process and the r-proccss. The s-process is capture of neutrons on a time scale that is slow compared to the rate of P-decay. The r-process is neutron capture on such a rapid time scale that many neutrons can be captured before P-decay occurs. 

Distribution of iron on the lunar surface, as measured by the Clementine spacecraft, compared with the average iron contents (arrows) of Apollo highlands crustal rocks and lunar highlands meteorites. The meteorites, which presumably come from the nearside and farside, more closely match the global iron peak. The compositional ranges for various lunar rock types are shown as horizontal bars. 

Two dominant themes run throughout the evolution of late type star compositions the abundances of the isotopes of carbon, nitrogen, and oxygen, and the abundances of the metals heavier than the iron peak - the neutron capture elements usually associated with the s-process. In addition to these elements, the abundance of lithium can also be a distinguishing characteristic of some groups, and can be used to interpret possible origins for some of these peculiar stars. 

The present calculations show that the Sc, Sr, and Ba abundances are higher with respect to Fe than found in the sun. Since these elements are secondary nucleosynthesis products, synthesized from CNO and the iron-peak nuclei by the... 

A new R-matrix approach for calculating cross-sections and rate coefficients for electron-impact excitation of complex atoms and ions is reviewed in [307]. It is found that accurate electron scattering calculations involving complex targets, such as the astrophysically important low ionization stages of iron-peak elements, are possible within this method. 

An opportunity also existed with these materials to provide techniques for determining small amounts of certain elements that appear within the spectral envelopes of other elements present in high concentrations. For example, lead has been found to be present in such high concentrations in currency that it introduces uncertainties as to the presence of observed elements such as arsenic, mercury, bismuth, and rubidium, which appear at or near the ubiquitous lead peaks. There are also extremely strong iron peaks appearing at or near important trace elements such as manganese, nickel, cobalt, and copper, which require further study to avoid serious error. 

In the area of the iron chelate peak This observation immediately raised the issue of possible poor reproducibility throughout the extraction of all the metals Additional work indicated that the variability was associated with the pH of the chelation reaction. In order to find the optimum pH we chelated and extracted a series of metals from the same stock solution at different pH values The iron was extracted well at pH < 4 while the zinc and copper showed a maximum at pH 6 5 In our present work we use a pH of 6 0 because of our Interest in zinc and copper and at this pH one can still see an iron peak However, if our interest were to change to iron we would certainly move the pH lower We have concluded that some of the variability that we and others have observed in our analysis could have been accounted for by a lack of careful control of the pH of our solutions ... 

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