Neutron capture reaction rates for silicon and their impact on the origin of presolar mainstream SiC grains

Abstract

We have made new, improved measurements of the Si28-30 (n, gamma) cross sections and have done a resonance analysis of these data including previous total cross sections. Together with the calculated contributions due to direct capture, we calculated the astrophysical (n, gamma) reaction rates and investigated the s-process abundances of the Si isotopes. Measured isotopic anomalies of intermediate and heavy elements in SiC grains from meteorites appear to be attributable to the s-process in asymptotic giant branch (AGB) stars. But the Si isotopic ratios in these grains are substantially different than s-process models predict. Therefore, recent papers have invoked galactic chemical evolution or other effects to explain the Si isotope ratios in these grains. Our new reaction rates are significantly different than previous rates, and s-process calculations using these rates lead to much larger isotopic shifts in Si-30. However, these exploratory calculations demonstrate that even with these substantially different rates the large observed variation in SiC grain from AGB stars cannot be explained by standard s-process models

STELLAR ORIGINS OF EXTREMELY 13C- AND 15N-ENRICHED PRESOLAR SIC GRAINS: NOVAE OR SUPERNOVAE?

“…For neutron capture on 30 Si, W&H07 adopted the experimental MACS (Maxwellian Averaged Cross Section) at 30 keV from Bao et al (2000), 6.5±0.6 mb. On the other hand, P15 adopted the more recent 30 Si MACS values reported by Guber et al (2003), which is more than a factor of three times lower (1.82±0.33 mb) than that used by W&H07 at 30 keV. Because of the lowered probability for neutron capture on 30 Si by adopting the Guber et al (2003) Si/ 28 Si is about 20 orders of magnitude lower, because of the lowered neutron density and the consequent negligible production of 32 Si ( Figure 5).…”

Decoding the Message From Meteoritic Stardust Silicon Carbide Grains

“…3. We derived the age of the parent star age from its mass as obtained from Δ 30 Si using the set of FRANEC C-rich AGB models with the neutron-capture cross sections of the Si isotopes by Guber et al (2003) presented in Zinner et al (2006), to which we added the age of the Sun. In these models the mass-loss rate was included using the parameterization given by Reimers (1975).…”

Evaluation of Silicon Neutron Resonance Parameters in the Energy Range Thermal to 1800 keV

“…The impact of the new Maxwellian average capture cross sections on the origin of presolar mainstream SiC was investigated, and the results are given in Ref. 23. …”