Nebula burning

Planetary nebulae are often even more rich in carbon than cool carbon stars, and those classified by M. Peimbert as Type I are rich in nitrogen, indicating effects of hot-bottom burning in intermediate-mass progenitor stars. The s-process elements are not normally detectable in PN or their central stars, but a remarkable case is that of FG Sagittae, the central star of a fossil planetary nebula, which has cooled in the course of the twentieth century from around 25 000 K to around 5000 K at constant bolometric luminosity. This star suddenly showed an enhancement of s-process elements in its atmosphere between 1965 and 1972 (see Jeffrey Schoenberner 2006, and references therein). 

As for Allende s inclusions, variable contributions of a component produced in neutron-rich nuclear statistical equilibrium best explains the Ti- Ca data. Some parts of the solar nebula were depleted in these isotopes as deficits are also seen. There are several possibilities for explaining the variations in Ti. 1) The neutron-rich component itself may be heterogeneous and incorporate locally less neutron-rich statistical equilibrium products (Hartmann et al. 1985). 2) Ti may result from another process like explosive Si or He burning (Clayton 1988). This component would be associated with the neutron-rich component but not completely homogenized. In all cases, carriers are solid grains which may have behaved differently than the gaseous nebula during the formation of the solar system. A minimum number of components may be calculated to account for the Ca and Ti isotopic data, which number up to 3 (Ireland 1990) but to be conservative at the 5ct level, clearly resolved effects are present only on 3 isotopes ( Ca, Ti, Ti). 

Observations of isotopic abundances provides information on the nucleosynthesis operating in the compact core of stars and supernova explosions and on the chemical evolution of the Galaxy. The CNO nuclides in late-type stars are affected by freshly synthesized core material brought up by dredge-up events. On the other hand, the Si isotopes are involved in later phases of nuclear burning, a narrow span of the red giant lifetime before planetary nebulae or supernovae. Therefore relative abundances of Si isotopes we observe remain unchanged from those of interstellar matter from which a star was formed. 

Hydrogen was the most abundant element in the solar nebula. The D/H ratio of the solar nebula has not been preserved in the Sun due to deuterium burning to He, but has been deduced from the solar He/ He and He/H ratios to be 5D = -880%o (Geiss and Gloeckler, 1998). A large range of values has been measured in meteorites due to fractionation between chemical... 

Fig. 4. Isotopic variations of hydrogen in the Solar System (adapted from Robert et al. 2000). The deuterium/ hydrogen ratio of different components is normalized to the D/H ratio of the Sun (as it was before deuterium burning), which is thought to represent H2 in the protosolar nebula. Numbers along the y-axis represent the numbers of cases. Terrestrial hydrogen is enriched in deuterium by a factor of about six relative to solar. Among Solar System objects analysed so far, carbonaceous chondrites, Antarctic micrometeorites (Engrand Maurette 1998) and chondruies from LL3 chondrites present a distribution of D/H values that centre around the terrestrial D/H ratio. Notably, comets analysed so far (Halley, Hale Bopp and Hyakutake, references given by Dauphas et al. (2000)) present D/H values about two times higher than the terrestrial value.

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