Nitrogen abundance predictions

Theoretical models for nucleosynthesis in asymptotic giant branch stars predict a large contribution to the cosmic nitrogen abundance from intermediate-mass stars [1], In particular, hot-bottom-burning in stars above a certain mass produces [C/N] —1 [2]. However, observations of C and N abundances in C-rich, metal-poor stars, usually using the CH and CN bands, show [C/N] values that vary between —0.5 and 1.5. (Fig. 1). If any of these stars have been polluted by intermediate mass AGB stars, then they should have lower [C/N] ratios. However, most of the CH stars with detailed abundances have [C/Fe] > 1.0, and it is more likely than stars mildly enhanced in C have been polluted by N-rich stars. 

Venn et al. (2002) re-investigated this problem. Fig. 11 shows that the prediction from the rotating stellar models and the observed boron versus nitrogen abundances agree well. As noted above, this is not trivial since the thermonuclear processes which affect the boron and the nitrogen abundances occur in completely different stellar layers. In particular, the existence of boron depleted stars with a normal nitrogen abundance (i.e. the stars HD 886, HD 29248, and HD 216919 cf. Venn et al. 1996) appears to specifically support the idea of rotational mixing, since such abundance ratios are not considered a possible result of close binary evolution. 

A large number of studies have been devoted to microstructures in PNe, and their nature is still debated. Fast Low Ionization Emission Regions (FLIERs) have first been considered to show an enhancement of N and were interpreted as being recently expelled from the star (Balick et al. 1994). However, Alexander Balick (1997) realized that the use of traditional ionization correction factors may lead to specious abundances. Do-pita (1997) made the point that enhancement of [N II] A6584/Ha can be produced by shock compression and does not necessarily involve an increase of the nitrogen abundance. Gongalves et al. (2001) have summarized data on the 50 PNe known to have low ionization structures (which they call LIS) and presented a detailed comparison of model predictions with the observational properties. They conclude that not all cases can be satisfactorily explained by existing models. 

Fig. 1. Abundance gradient of N/O predicted by models adopting stellar yields where rotation is not taken into account (as model 7 of [3] - thin solid line) and the same models computed with MM02 yields ([2] - thick solid line). A model where we increased only the amount of primary N in massive stars for metallicities below Z=10-B overlaps with the thick solid line shown here [1], This shows that the N/O gradient along the MW disk is affected mainly by the amount of nitrogen production in low and intermediate mass stars and not the primary N in massive stars. For the abundance data see [3] and references therein - asterisks are B stars (see Cunha, this conference).

The thermodynamic predictions presented in Figure 7.8b indicate that at equilibrium, nitrate should be the dominant form of nitrogen in oxic seawater. This is not observed. Rather, nitrate is the second most abundant nitrogen species, with N2 being 100 times more abundant in surfece seawater and 25 times more abundant in the deep sea. 

Figure 8 shows corresponding curves for ozone buildups predicted in the computer experiment. Again the combination seems to exceed the pure propylene in its rapidity to produce ozone. This occurs because of the enhanced conversion rate of nitric oxide to nitrogen dioxide stimulated by the early abundance of RO2 generated by the high-reactivity fraction. 

The noble gas, carbon, nitrogen, and sulfur abundances in Jupiter can be compared to the predicted compositions of icy planetesimals to provide details on when and how material was accreted during the formation of Jupiter. Unfortunately the oxygen abundance in Jupiter is unknown, and since water as the primary oxygen carrier was the dominant ice in planetesimals as well (based on observations of comets), one requires this abundance to decide among models. In its absence, the current heavy element inventory can be explained by a model in... 

A few informative properties of life come from easy category distinctions, such as the fact that all known life makes essential use of carbon and carbon-oxygen-nitrogen molecules in liquid water solution. The seemingly trivial observation that such carbaquist chemistry is ruled out if astrophysical carbon abundance lies below a certain threshold enabled Hoyle [1] to predict the 7.6 MeV carbon-12 ( C) nuclear resonance with remarkable precision because the discovery of the triple-alpha reaction synthesis of in stars happens to be a bottleneck for stellar nucleosynthesis of all the heavy elements. The pragmatic information in this prediction is easy to measure because it guided experimental characterization of nuclear structure where the existing computational capabilities could not. Similar sensitive dependence of the physical state of water has been used to define a habitable zone in planetary physics [10], which is not predictive in the same sense as carbon abundance (we already knew where the earth s orbit lies), but which creates a useful filter in the search for extraterrestrial life. 

The most abundant species on Pluto s surface appears to be nitrogen, followed by carbon monoxide, methane, and water ice, all of which occur in the solid state. Based on the planet s density, scientists predict that its interior consists of some type of hydrated silicate mixed with up to 30 percent water ice. 

These predictions, based on the composition of early rocks, are supported by a second line of evidence, which provides a clue to the origin of this early atmosphere. This is the rarity of inert unreactive gases, particularly neon, in the Earth s atmosphere today. Neon is the seventh most abundant element in the Universe. It was abundant in the clouds of dust and gas from which the Earth and the other planets of the Solar System condensed. As an inert gas, neon is even more unreactive than nitrogen. If any of the Earth s original atmosphere had survived the meteorite bombardment, it should have contained about the same amount of neon as nitrogen. In fact, the ratio of neon to nitrogen is 1 to 60000. If there ever had been a Jupiter-like atmosphere on Earth, then it must have been swept away during that first ferocious period of meteorite bombardment. 

The isotopic distributions of several elements commonly found in organic compounds are shown in Table 13.2. From the isotopic distributions, we see why the M -b 1 peak can be used to determine the number of carbon atoms in a compound It is because the contributions to the M + 1 peak by isotopes of H, O, and the halogens are very small or nonexistent. This formula does not work as well in predicting the number of carbon atoms in a nitrogen-containing compound because the natural abundance of is relatively high. 

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