INVESTIGATION FOR THE ENRICHMENT PATTERN OF THE ELEMENT ABUNDANCES IN r+s STAR HE 0338-3945: A SPECIAL r-II STAR?

The very metal-poor star HE 0338-3945 shows a double-enhanced pattern of the neutron-capture elements. The study of this sample could help people gain a better understanding of s- and r-process nucleosynthesis at low metallicity. Using a parametric model, we find that the abundance pattern of the neutron-capture elements could be best explained by a binary system formed in a molecular cloud that had been polluted by r-process material. The observed abundance pattern of C and N can be explained by an asymptotic giant branch (AGB) model. Combined with the parameters obtained from Cui & Zhang, we suggest that the initial mass of the AGB companion is most likely to be about 2.5M(circle dot), which excludes the possibility of forming a Type 1.5 supernova. By comparing with the observational abundance pattern of CS 22892-052, we find that the dominant production of O should accompany the production of the heavy r-process elements of r+s stars. Similar to r-II stars, the heavy r-process elements are not produced in conjunction with all the light elements from the Na to Fe group. The abundance pattern of the light and r-process elements for HE 0338-3945 is very close to the pattern of the r-II star CS 22892-052. Therefore, we suggest that HE 0338-3945 should be a special r-II star.