16 research outputs found
Effect of Reaction & Convective Mixing on the Progenitor Mass of ONe White Dwarfs
Stars in the mass range ~8 - 12 are the most numerous massive
stars. This mass range is critical because it may lead to supernova (SN)
explosion, so it is important for the production of heavy elements and the
chemical evolution of the galaxy. We investigate the critical transition mass
(), which is the minimum initial stellar mass that attains the
conditions for hydrostatic carbon burning. Stars of masses < evolve to
the Asymptotic Giant Branch and then develop CO White Dwarfs, while stars of
masses ignite carbon in a partially degenerate CO core
and form electron degenerate ONe cores. These stars evolve to the Super AGB
(SAGB) phase and either become progenitors of ONe White Dwarfs or eventually
explode as electron-capture SN (EC-SN). We study the sensitivity of to
the C-burning reaction rate and to the treatment of convective mixing. In
particular, we show the effect of a recent determination of the
fusion rate, as well as the extension of the convective core during
hydrogen and helium burning on in solar metallicity stars. We choose
the 9 model to show the detailed characteristics of the evolution
with the new C-burning rate.Comment: Submitted to AIP Conference proceedings of Carpathian Summer School
of Physics-201
Synthesis of C-rich dust in CO nova ourbursts
Context. Classical novae are thermonuclear explosions that take place in the
envelopes of accreting white dwarfs in stellar binary systems. The material
transferred onto the white dwarf piles up under degenerate conditions, driving
a thermonuclear runaway. In those outbursts, about 10-7 - 10-3 Msun, enriched
in CNO and, sometimes, other intermediate-mass elements (e.g., Ne, Na, Mg, or
Al, for ONe novae) are ejected into the interstellar medium. The large
concentrations of metals spectroscopically inferred in the nova ejecta reveal
that the (solar-like) material transferred from the secondary mixes with the
outermost layers of the underlying white dwarf.
Aims. Most theoretical models of nova outbursts reported to date yield, on
average, outflows characterized by O > C, from which only oxidized condensates
(e.g, O-rich grains) would be expected, in principle.
Methods. To specifically address whether CO novae can actually produce C-rich
dust, six different hydrodynamic nova models have been evolved, from accretion
to the expansion and ejection stages, with different choices for the
composition of the substrate with which the solar-like accreted material mixes.
Updated chemical profiles inside the H-exhausted core have been used, based on
stellar evolution calculations for a progenitor of 8 Msun through H and
He-burning phases.
Results. We show that these profiles lead to C-rich ejecta after the nova
outburst. This extends the possible contribution of novae to the inventory of
presolar grains identified in meteorites, particularly in a number of
carbonaceous phases (i.e., nanodiamonds, silicon carbides and graphites).Comment: 5 pages, accepted for publication in Astronomy & Astrophysic
Iron Implantation in Presolar Supernova Grains
We consider the potential of measured iron isotopic ratios within presolar grains from supernovae (as discovered in meteorites) for identifying the gas from which the grains condensed. We show that although iron isotopic ratios vary dramatically with radial coordinate in the initial supernova, it seems likely that the concentration of iron that thermally condenses in SiC grains within the supernova interior may be smaller than the concentration that will later be implanted by high-speed grain-gas collisions following the penetration of the reverse shock into the supernova flow. In that case, the Fe isotopic composition is much altered. We propose that the 58Fe richness that is very evident in the three SiC grains analyzed to date is the result of ion implantation during the grain’s rapid radial motion through the shocked and decelerated overlying supernova gas that is 58Fe-rich. We point to other likely applications of this same idea and speculate that only the dominant isotopes of the SiC grains, namely 28Si and 12C, can be safely assumed to be initial thermal condensate. We conclude that a violent period of implantation plus sputtering has overprinted the initial thermal condensate. If correct, this points to a new technique for sampling the velocity mixing within young supernova remnants
s-Process Nucleosynthesis in Advanced Burning Phases of Massive Stars
We present a detailed study of s-process nucleosynthesis in massive stars of
solar-like initial composition and masses 15, 20,25, and 30 Msun. We update our
previous results of s-process nucleosynthesis during the core He-burning of
these stars and then focus on an analysis of the s-process under the physical
conditions encountered during the shell-carbon burning. We show that the recent
compilation of the Ne22(alpha,n)Mg25 rate leads to a remarkable reduction of
the efficiency of the s-process during core He-burning. In particular, this
rate leads to the lowest overproduction factor of Kr80 found to date during
core He-burning in massive stars. The s-process yields resulting from shell
carbon burning turn out to be very sensitive to the structural evolution of the
carbon shell. This structure is influenced by the mass fraction of C12 attained
at the end of core helium burning, which in turn is mainly determined by the
C12(alpha,gamma)O16 reaction. The still present uncertainty in the rate for
this reaction implies that the s-process in massive stars is also subject to
this uncertainty. We identify some isotopes like Zn70 and Rb87 as the
signatures of the s-process during shell carbon burning in massive stars. In
determining the relative contribution of our s-only stellar yields to the solar
abundances, we find it is important to take into account the neutron exposure
of shell carbon burning. When we analyze our yields with a Salpeter Initial
Mass Function, we find that massive stars contribute at least 40% to s-only
nuclei with mass A 90, massive stars
contribute on average ~7%, except for Gd152, Os187, and Hg198 which are ~14%,
\~13%, and ~11%, respectively.Comment: 52 pages, 16 figures, accepted for publication in Ap
White paper on nuclear astrophysics and low energy nuclear physics Part 1: Nuclear astrophysics
This white paper informs the nuclear astrophysics community and funding agencies about the scientific directions and priorities of the field and provides input from this community for the 2015 Nuclear Science Long Range Plan. It summarizes the outcome of the nuclear astrophysics town meeting that was held on August 21–23, 2014 in College Station at the campus of Texas A&M University in preparation of the NSAC Nuclear Science Long Range Plan. It also reflects the outcome of an earlier town meeting of the nuclear astrophysics community organized by the Joint Institute for Nuclear Astrophysics (JINA) on October 9–10, 2012 Detroit, Michigan, with the purpose of developing a vision for nuclear astrophysics in light of the recent NRC decadal surveys in nuclear physics (NP2010) and astronomy (ASTRO2010). The white paper is furthermore informed by the town meeting of the Association of Research at University Nuclear Accelerators (ARUNA) that took place at the University of Notre Dame on June 12–13, 2014. In summary we find that nuclear astrophysics is a modern and vibrant field addressing fundamental science questions at the intersection of nuclear physics and astrophysics. These questions relate to the origin of the elements, the nuclear engines that drive life and death of stars, and the properties of dense matter. A broad range of nuclear accelerator facilities, astronomical observatories, theory efforts, and computational capabilities are needed. With the developments outlined in this white paper, answers to long standing key questions are well within reach in the coming decade
Heavy Element Nucleosynthesis
This contribution deals with the important subject of the nucleosynthesis of heavy elements in the Galaxy. After an overview of several observational features, the physical processes responsible mainly for the formation of heavy elements will be described and linked to possible stellar sites and to galactic chemical evolution. In particular, we focus on the neutron-capture processes, namely the s-process (slow neutron capture) and the r-process (rapid neutron capture) and discuss some problems in connection with their sites and their outcome. The aim is to give a brief overview on the exciting subject of the heavy element nucleosynthesis in the Galaxy, emphasizing its importance to trace the galactic chemical evolution and illustrating the challenge of this subject