Neutron-Capture Elements in the Double-Enhanced Star HE 1305-0007： a New s- and r-Process Paradigm

The star HE 1305-0007 is a metal-poor double-enhanced star with metallicity [Fe/H] = -2.0, which is just at the upper limit of the metallicity for the observed double-enhanced stars. Using a parametric model, we find that almost all s-elements were made in a single neutron exposure. This star should be a member of a post-commonenvelope binary. After the s-process material has experienced only one neutron exposure in the nucleosynthesis region and is dredged-up to its envelope, the AGB evolution is terminated by the onset of common-envelope evolution. Based on the high radial-velocity of HE 1305-0007, we speculate that the star could be a runaway star from a binary system, in which the AIC event has occurred and produced the r-process elements.

Are Fulleranes Responsible for the 21 Micron Feature?

Recent detections of C60,C70,and C60^+ in space induced extensive studies of fullerene derivatives in circumstellar environments.As the promising fullerene sources,protoplanetary nebulae(PPNe)shows a number of unidenti ed bands in their infrared spectra,among which a small sample exhibits an enigmatic feature at ~21μm.Hydrogenation converts fullerenes into fulleranes,which breaks the symmetry of fullerene molecules and produces new infrared bands.In this work,we investigate the possibility of fulleranes(C60Hm)as the carrier of the 21μm feature in terms of theoretical vibrational spectra of fulleranes.The evidences favoring and disfavoring the fullerane hypothesis are presented.We made an initial guess for the hydrogen coverage of C60Hm that may contribute to the 21μm feature.

Thermonuclear ^19F(p,α0)^16O reaction rate

The thermonuclear^19F（p,α0）16O reaction rate in the temperature region 0.007–10 GK has been derived by re-evaluating the available experimental data, together with the low-energy theoretical R-matrix extrapolations.Our new rate deviates by up to about 30% compared to the previous results, although all rates are consistent within the uncertainties. At very low temperature（e.g. 0.01 GK） our reaction rate is about 20% lower than the most recently published rate, because of a difference in the low energy extrapolated S-factor and a more accurate estimate of the reduced mass used in the calculation of the reaction rate. At temperatures above ^1 GK, our rate is lower, for instance, by about 20% around 1.75 GK, because we have re-evaluated the previous data（Isoya et al., Nucl. Phys.7, 116（1958）） in a meticulous way. The present interpretation is supported by the direct experimental data. The uncertainties of the present evaluated rate are estimated to be about 20% in the temperature region below 0.2 GK,and are mainly caused by the lack of low-energy experimental data and the large uncertainties in the existing data.Asymptotic giant branch（AGB） stars evolve at temperatures below 0.2 GK, where the^19F（p,α）16O reaction may play a very important role. However, the current accuracy of the reaction rate is insufficient to help to describe, in a careful way, the fluorine over-abundances observed in AGB stars. Precise cross section（or S factor） data in the low energy region are therefore needed for astrophysical nucleosynthesis studies.