A study of U Aquarii and the nucleosynthesis of neutrons and S-process elements in evolved stars of low mass

[摘要] The origin of the extreme hydrogen deficiency observed in the RCoronae Borealis (RCrB) stars remains poorly understood. A likelymechanism for producing this deficiency is one whereby the originalhydrogen envelope of the star is engulfed and destroyed in itsinterior. One of the few RCrB stars so far analysed, U Aquarii, in factshows emphatic evidence of such an event having indeed occurred. Thisevidence takes the form of the large s-process enhancements observed inU Aquarii. Such enhancements are caused by the production ofsignificant neutron fluxes which are directly produced by envelopeengulfmentgiving rise to the ¹²C(p, γ) ¹³N(β⁺γ) ¹³C(α,n) ¹⁶O sequence ofnuclear reactions. In order to obtain vital information regarding thenature of the envelope mixing event from observed s-processenhancements, detailed nucleosynthesis calculations investigatingneutron production and s-process synthesis are carried out using aseries of nuclear reaction networks and covering a wide range ofparameter space. These calculations are mainly based on low-mass AGBand post-AGB stellar models since it is widely believed that the RCrB'sare in some way related to this group of stars. It is shown how theingestion rate of envelope material, the initial abundances and the typeof mixing model used have a large influence on the neutron and s-processproduction.New spectral observations of U Aquarii are presented, and it isshown how these new observations allow a new interpretation of themixing event which occurred in this star and how further improvedabundance data of the star will lead to an unambiguous determination ofthe nature of the mixing event. The RCrB stars are also thought to berelated to the extreme helium stars. Discovery of variability in two ofthese latter stars is presented.

[发布机构] University:University of St Andrews;Department:Physics & Astronomy (School of)