ASYMPTOTIC GIANT BRANCH STARS AS A SOURCE OF SHORT-LIVED RADIOACTIVE NUCLEI IN THE SOLAR NEBULA

被引:136
作者
WASSERBURG, GJ
BUSSO, M
GALLINO, R
RAITERI, CM
机构
[1] OSSERV ASTRON TORINO, I-10025 PINO TORINESE, ITALY
[2] UNIV TURIN, IST FIS GEN, I-10125 TURIN, ITALY
关键词
ISM; ABUNDANCES; NUCLEAR REACTIONS; NUCLEOSYNTHESIS; SOLAR SYSTEM; FORMATION; STARS; AGB AND POST-AGB;
D O I
10.1086/173899
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
We carried out a theoretical evaluation of the contribution of Asymptotic Giant Branch (AGB) stars to some short-lived (10(6) less-than-or-equal-to tau less-than-or-equal-to 2 x 10(7) yr) isotopes in the ISM and in the early solar system using stellar model calculations for thermally pulsing evolutionary phases of low-mass stars. The yields of s-process nuclei in the convective He-shell for different neutron exposures tau0 were obtained, and AGB stars were shown to produce several radioactive nuclei (especially 107Pd, Pb-205, Fe-60, Zr-91, Tc-99, Cs-135, and Hf-182) in different amounts. Assuming either contamination of the solar nebula from a single AGB star or models for continuous injection and mixing from many stars into the ISM, we calculate the ratios of radioactive to stable nuclei at the epoch of the Sun's formation. The dilution factor between the AGB ejecta and the early solar system matter is obtained by matching the observed 107Pd/108Pd and depends on the value of tau0. It is found that small masses M(He) of He-shell material (10(-4)-10(-7) M.) enriched in s-process nuclei are sufficient to contaminate 1 M. of the ISM to produce the 107Pd found in the early solar system. Predictions are made for all of the other radioactive isotopes. The optimal model to explain several observed radioactive species at different states of the proto-solar nebula involves a single AGB star with a low neutron exposure (tau0 = 0.03 mbarn-1) which contaminated the cloud with a dilution factor of M(He)/M. approximately 1.5 x 10(-4). This will also contribute newly synthesized stable s-process nuclei in the amount of approximately 10(-4) of their abundances already present in the proto-solar cloud. Variations in the degree of homogenization (approximately 30%) of the injected material may account for some of the small general isotopic anomalies found in meteorites. It is also found that Fe-60 is produced in small but significant quantities that may be sufficient to explain the observations if the time elapsed (DELTA) from the contamination of the ISM to the formation of protoplanetary bodies is not higher than DELTA = 5 x 10(6) yr. If DELTA is longer, up to 10 x 10(6) yr, this would require the single AGB star to experience enhanced neutron densities (n(n) approximately 3 x 10(9)n CM-3 ) in the s-processing zone in order to compensate for the branching at Fe-59. The alternative model of long-term continuous ejection of matter from many AGB stars does not appear to match the observations. We also estimate the Al-26 production from the H-shell and find that the Al-26 abundance in the early solar system may be readily explained in a self-consistent manner. Moreover, Al-26 from AGB stars may contribute substantially to the galactic Al-26 gamma-source, while no significant gamma-flux from Co-60 (deriving from Fe-60 decay) is to be expected.
引用
收藏
页码:412 / 428
页数:17
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