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Titolo:
EVOLUTION AND NUCLEOSYNTHESIS IN LOW-MASS ASYMPTOTIC GIANT BRANCH STARS - II - NEUTRON-CAPTURE AND THE S-PROCESS
Autore:
GALLINO R; ARLANDINI C; BUSSO M; LUGARO M; TRAVAGLIO C; STRANIERO O; CHIEFFI A; LIMONGI M;
Indirizzi:
UNIV TURIN,DIPARTIMENTO FIS GEN TURIN ITALY OSSERV ASTRON TORINO TURIN ITALY OSSERVATORIO ASTRON COLLURANIA TERAMO ITALY CNR,IST ASTROFIS SPAZIALE I-00044 FRASCATI ITALY OSSERV ASTRON ROMA I-00136 ROME ITALY
Titolo Testata:
The Astrophysical journal
fascicolo: 1, volume: 497, anno: 1998,
parte:, 1
pagine: 388 - 403
SICI:
0004-637X(1998)497:1<388:EANILA>2.0.ZU;2-J
Fonte:
ISI
Lingua:
ENG
Soggetto:
REACTION CROSS-SECTION; METEORITIC SIC GRAINS; ASTROPHYSICAL IMPLICATIONS; CARBON STARS; SOLAR-SYSTEM; RED GIANTS; CHEMICAL-COMPOSITION; INTERSTELLAR GRAINS; STELLAR TEMPERATURE; SILICON-CARBIDE;
Keywords:
NUCLEAR REACTIONS, NUCLEOSYNTHESIS, ABUNDANCES; STARS, AGB AND POST-AGB; STARS, EVOLUTION; STARS, LOW-MASS, BROWN DWARFS;
Tipo documento:
Review
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
118
Recensione:
Indirizzi per estratti:
Citazione:
R. Gallino et al., "EVOLUTION AND NUCLEOSYNTHESIS IN LOW-MASS ASYMPTOTIC GIANT BRANCH STARS - II - NEUTRON-CAPTURE AND THE S-PROCESS", The Astrophysical journal, 497(1), 1998, pp. 388-403

Abstract

We present a new analysis of neutron capture occurring in low-mass asymptotic giant branch (AGB) stars suffering recurrent thermal pulses. We use dedicated evolutionary models for stars of initial mass in the range 1 to 3 M. and metallicity from solar to half solar. Mass loss istaken into account with the Reimers parameterization. The third dredge-up mechanism is self-consistently found to occur after a limited number of pulses, mixing with the envelope freshly synthesized C-12 and s-processed material from the He intershell. During thermal pulses, thetemperature at the base of the convective region barely reaches T-8 similar to 3 (T-8 being the temperature in units of 10(8) K), leading to a marginal activation of the Ne-22(alpha, n)Mg-25 neutron source. The alternative and much faster reaction C-13(alpha, n)O-16 must then play the major role. However, the C-13 abundance left behind by the H shell is far too low to drive the synthesis of the s-elements. We assumeinstead that at any third dredge-up episode, hydrogen downflows from the envelope penetrate into a tiny region placed at the top of the C-12-rich intershell, of the order of a few 10(-4) M.. At H reignition, aC-13-rich (and N-14-rich) zone is formed. Neutrons by the major C-13 source are then released in radiative conditions at T-8 similar to 0.9during the interpulse period, giving rise to an efficient s-processing that depends on the C-13 profile in the pocket. A second small neutron burst from the Ne-22 source operates during convective pulses over previously s-processed material diluted with fresh Fe seeds and H-burning ashes. The main features of the final s-process abundance distribution in the material cumulatively mixed with the envelope through the various third dredge-up episodes are discussed. Contrary to current expectations, the distribution cannot be approximated by a simple exponential law of neutron irradiations The s-process nucleosynthesis mostlyoccurs inside the C-13 pocket; the form of the distribution is built through the interplay of the s-processing occurring in the intershell zones and the geometrical overlap of different pulses. The C-13 pocketis of primary origin, resulting from proton captures on newly synthesized C-12. Consequently, the a-process nucleosynthesis also depends onFe seeds, a lower metallicity favoring the production of the heaviestelements. This allows a wide range of s-element abundance distributions to be produced in AGE stars of different metallicities, in agreement with spectroscopic evidence and with the Galactic enrichment of the heavy s-elements at the time of formation of the solar system. AGE stars of metallicity Zr similar or equal to 1/2 Z. are the best candidates for the buildup of the main component? i.e., for the s-distribution of the heavy elements from the Sr-Y-Zr peak up to the Pb peak, as deduced by meteoritic and solar spectroscopic analyses. A number of AGE stars may actually show in their envelopes an s-process abundance distribution almost identical to that of the main component. Eventually, theastrophysical origin of mainstream circumstellar SiC grains recoveredfrom pristine meteorites, showing a nonsolar s-signatures in a numberof trace heavy elements, is likely identified in the circumstellar envelopes of AGE stars of about solar metallicity, locally polluting theinterstellar medium from which the solar system condensed.

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Documento generato il 04/12/20 alle ore 12:54:19