Stellar Spectroscopy and the s-Process

Neutron capture reactions can occur at low temperatures but stars can activate neutron sources only at high temperatures. It was apparent very early in the search for the origins of the chemical elements that two different neutron capture processes are at work in the Universe - see the classical papers by Cameron (1957) and Burbidge, Burbidge, Fowler Hoyle (1957). The processes are distinguished by the neutron density achieved at the 

One may dissect the origins of the heavy nuclides in the valley of stability - see Fig. 4. Some may be synthesized only by the s-process such a nucleus (Z, N) is shielded 

This latter qualification applies to all but a few stars. Exceptions include the S-type stars in which s-processing occurs internally. An extreme S star has an atmosphere enriched 10 to 100 times in s-process products - these products mask the s-process contribution of the original star. But for stars the information on heavy element abundances is very much less complete than for the solar system. Not only are many heavy elements not measureable in the stellar spectrum but information on the isotopic abundances is almost completely missing. The isotopic abundances are crucial to full resolution of the s- from the r-process in the solar system composition. All is not lost. Ratios of elemental abundances may serve as measures of the relative contributions of the s- and r-process. For example, the Ba/Eu ratio is high for the s-process and lower for the r-process. 

In this section, I discuss some theoretical aspects of the s-process and a few observational investigations of the operation of the s-process in individual stars like the the S stars. 

The cross-section for neutron capture depends in part on the transit time of the neutron across the target nucleus a oc 1/v where a is the cross-section and v is the relative velocity of the neutron and the nucleus. Neutrons of velocity v are captured at a rate proportional to av which is independent of v. It follows immediately that the neutron capture rate in gas at temperature T is independent of T as long as the transit time is the dominant consideration. 

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