Showing papers on "Stellar nucleosynthesis published in 1981"

Anomalous isotopic composition of cosmic rays

Abstract: Recent measurements of nonsolar isotopic patterns for the elements neon and (perhaps) magnesium in cosmic rays are interpreted within current models of stellar nucleosynthesis. One possible explanation is that the stars currently responsible for cosmic-ray synthesis in the Galaxy are typically super--metal-rich by a factor of 2--3. Other possibilities include the selective acceleration of certain zones or masses of supernovae or the enhancement of /sup 22/Ne in the interstellar medium by mass loss from red giant stars and planetary nebulae. Measurements of critical isotopic ratios are suggested to aid in distinguishing among the various possibilities. Some of these explanations place signficant constraints on the fraction of cosmic-ray nuclei that must be fresh supernova debris and the masses of the supernovae involved.

Neutron-capture nucleosynthesis of nature's rarest stable isotope

Abstract: Many attempts1–6 have been made to explain the solar abundances of the stable, heavy odd–odd nuclei: 50V, 138La, 176Lu and 180Ta. Among these, 180Ta (actually an isomeric state, 180Tam) has the distinction of being the rarest, stable naturally occurring isotope (0.012% of all Ta) with a solar abundance7 of only 2.46×10−6 (per 106 Si atoms) and a half life of >1013 yr against β decay. Although the variety of mechanisms previously proposed can in some special cases account adequately for the small abundance of 180Tam, it is difficult to test quantitatively those special types of synthesis invented only to explain such a small number of nuclei. In contrast, we show here from our experimental work that 180Tam can also be accounted for by relatively small branchings in the much more common slow (s)-and rapid (r)-processes of stellar nucleosynthesis by neutron capture.

Radio observations of molecules in the interstellar gas

Abstract: Since 1968, radio astronomy has made it possible to identify nearly 50 molecules in the dense concentrations of the interstellar gas now generally termed molecular clouds. Most interstellar molecules are familiar stable compounds. However, one-fifth of the discovered species are ions, radicals, and acetylenic carbon chains so reactive in the laboratory that before being detected in space they had rarely been observed or were entirely unknown. The heavy atom backbone of the known interstellar molecules is a linear chain of C, N, O, or S. Si is found in two diatomic molecules. Rings and branched chains are missing. The most readily observed spectral lines of most interstellar molecules correspond to rotational transitions at millimeter wavelengths. These are generally excited by H2 collisions. A number of rare isotopic species are observed in interstellar molecules. Isotopic ratios differing from those on Earth exist, and can in almost all cases be attributed to stellar nucleosynthesis since the formation of the solar system.

Current rate of nucleosynthesis and its implications

Abstract: The current rate of nucleosynthesis in the solar neighbourhood is re-evaluated on the basis of Arnett’s (1978) stellar yields, the mass loss models of Chiosi, Nasi and Sreenivasan (1978) and the initial mass function determined by Lequeux (1978). If massive stars are held responsible for most of the metals we observe, a higher birthrate of these stars in the past is indicated in view of the low current rate of nucleosynthesis. The intermediate mass stars may not supply the bulk of the metals unless total disruption of their carbon core takes place. While a declining birthrate is in conflict with the result obtained from the age-metallicity relation of stars, it is supported by some galactic evolution models which interpret successfully the white dwarf mass distribution data. If the constraint of a nearly time-invariant birthrate were strictly accepted, then models of the prompt initial enrichment type are required to explain the observed abundances in terms of nucleosynthesis in massive stars.