129 research outputs found
Nucleosynthesis of Elements in Low to Intermediate Mass Stars through the AGB Phase
We present a review of the main phases of stellar evolution with particular
emphasis on the nucleosynthesis and mixing mechanisms in low- and
intermediate-mass stars. In addition to explicit studies of the effects of the
first, second and third dredge-up, we also discuss cool bottom processing and
hot bottom burning.Comment: 30 pages, latex, 18 figures, uses style files aipproc.cls aipproc.sty
epsf.sty ; to be published in (refereed) conference proceedings
"Astrophysical Implications of the Laboratory Study of Presolar Materials",
ed. T. Bernatowitz and E. Zinner (AIP: Sunnyside, NY), in press; also
available at http://www.maths.monash.edu.au/~boothroy
The treatment of mixing in core helium burning models -- III. Suppressing core breathing pulses with a new constraint on overshoot
Theoretical predictions for the core helium burning phase of stellar
evolution are highly sensitive to the uncertain treatment of mixing at
convective boundaries. In the last few years, interest in constraining the
uncertain structure of their deep interiors has been renewed by insights from
asteroseismology. Recently, Spruit (2015) proposed a limit for the rate of
growth of helium-burning convective cores based on the higher buoyancy of
material ingested from outside the convective core. In this paper we test the
implications of such a limit for stellar models with a range of initial mass
and metallicity. We find that the constraint on mixing beyond the Schwarzschild
boundary has a significant effect on the evolution late in core helium burning,
when core breathing pulses occur and the ingestion rate of helium is fastest.
Ordinarily, core breathing pulses prolong the core helium burning lifetime to
such an extent that models are at odds with observations of globular cluster
populations. Across a wide range of initial stellar masses (), applying the Spruit constraint reduces the core
helium burning lifetime because core breathing pulses are either avoided or
their number and severity reduced. The constraint suggested by Spruit therefore
helps to resolve significant discrepancies between observations and theoretical
predictions. Specifically, we find improved agreement for , the observed
ratio of asymptotic giant branch to horizontal branch stars in globular
clusters; the luminosity difference between these two groups; and in
asteroseismology, the mixed-mode period spacing detected in red clump stars in
the \textit{Kepler} field.Comment: Accepted for publication in MNRAS; 11 pages, 6 figure
Compulsory Deep Mixing of 3He and CNO Isotopes in the Envelopes of low-mass Red Giants
Three-dimensional stellar modeling has enabled us to identify a deep-mixing
mechanism that must operate in all low mass giants. This mixing process is not
optional, and is driven by a molecular weight inversion created by the
3He(3He,2p)4He reaction. In this paper we characterize the behavior of this
mixing, and study its impact on the envelope abundances. It not only eliminates
the problem of 3He overproduction, reconciling stellar and big bang
nucleosynthesis with observations, but solves the discrepancy between observed
and calculated CNO isotope ratios in low mass giants, a problem of more than 3
decades' standing. This mixing mechanism, which we call `-mixing',
operates rapidly (relative to the nuclear timescale of overall evolution, ~
10^8 yrs) once the hydrogen burning shell approaches the material homogenized
by the surface convection zone. In agreement with observations, Pop I stars
between 0.8 and 2.0\Msun develop 12C/13C ratios of 14.5 +/- 1.5, while Pop II
stars process the carbon to ratios of 4.0 +/- 0.5. In stars less than
1.25\Msun, this mechanism also destroys 90% to 95% of the 3He produced on the
main sequence.Comment: Final accepted version (submitted to Astrophys J in Jan 2007...
Deep Mixing of He-3: Reconciling Big Bang and Stellar Nucleosynthesis
Low-mass stars, ~1-2 solar masses, near the Main Sequence are efficient at
producing He-3, which they mix into the convective envelope on the giant branch
and should distribute into the Galaxy by way of envelope loss. This process is
so efficient that it is difficult to reconcile the low observed cosmic
abundance of He-3 with the predictions of both stellar and Big Bang
nucleosynthesis. In this paper we find, by modeling a red giant with a fully
three-dimensional hydrodynamic code and a full nucleosynthetic network, that
mixing arises in the supposedly stable and radiative zone between the
hydrogen-burning shell and the base of the convective envelope. This mixing is
due to Rayleigh-Taylor instability within a zone just above the
hydrogen-burning shell, where a nuclear reaction lowers the mean molecular
weight slightly. Thus we are able to remove the threat that He-3 production in
low-mass stars poses to the Big Bang nucleosynthesis of He-3.Comment: Accepted by Science, and available from Science Express onlin
The Sixth Torino Workshop
Nuclear astrophysics is about 50 years old. It grew from nuclear physics and was nurtured by those with an interest in astrophysics. A milestone was the publication of the seminal paper of Geoffrey Burbdige, Margaret Burbidge, Willy Fowler and Fred Hoyle in 1957. Since then, it has become a mainstay of modern astrophysics, and has a natural and especially close link to stellar astrophysics. But there has been a revolution in recent years, which has seen the isolation and analysis of circumstellar dust grains, recovered from meteorites. These 'pre-solar'grains (indicating that they predate the solar system, and survived the various processes associated with its formation) have provided incomparable and revolutionary data of exceptional quality. It is now common to have access to isotopic abundance ratios for major elements as well as for trace elements, and these can be magnificent indications for processes active in the parent star. Much of the presolar grain analysis has been performed on samples of the Murchison meteorite, which fell around the little town of Murchison, near Melbourne, in 1969. The time was right for a workshop dedicated to the astrophysics learned from this analysis, and we are pleased to present some of the papers in this issue of PASA. This workshop was also the sixth in a series of workshops on nuclear astrophysics, initiated at Torino University. We are proud to continue the name and the tradition of these workshops. This was only the second Torino workshop held outside of Italy. The first was organised by Manuel Forestini, in France. Tragically, Manuel suffered a fatal heart attack on March 11, 2003. Manuel worked in many areas of stellar evolution, and was the author of a textbook on the subject. But his main area was AGB stars, and he has contributed much to our understanding of these complex objects, and especially their nucleosynthesis. Manuel did everything with passion and good humour, and was a very dear friend and a valued collaborator. We wish to dedicate this meeting to his memory, his science and his friendship
Super-AGB Stars and their role as Electron Capture Supernova progenitors
We review the lives, deaths and nucleosynthetic signatures of intermediate
mass stars in the range approximately 6.5-12 Msun, which form super-AGB stars
near the end of their lives. We examine the critical mass boundaries both
between different types of massive white dwarfs (CO, CO-Ne, ONe) and between
white dwarfs and supernovae and discuss the relative fraction of super-AGB
stars that end life as either an ONe white dwarf or as a neutron star (or an
ONeFe white dwarf), after undergoing an electron capture supernova. We also
discuss the contribution of the other potential single-star channels to
electron-capture supernovae, that of the failed massive stars. We describe the
factors that influence these different final fates and mass limits, such as
composition, the efficiency of convection, rotation, nuclear reaction rates,
mass loss rates, and third dredge-up efficiency. We stress the importance of
the binary evolution channels for producing electron-capture supernovae. We
discuss recent nucleosynthesis calculations and elemental yield results and
present a new set of s-process heavy element yield predictions. We assess the
contribution from super-AGB star nucleosynthesis in a Galactic perspective, and
consider the (super-)AGB scenario in the context of the multiple stellar
populations seen in globular clusters. A brief summary of recent works on dust
production is included. Lastly we conclude with a discussion of the
observational constraints and potential future advances for study into these
stars on the low mass/high mass star boundary.Comment: 28 pages, 11 figures. Invited review for Publications of the
Astronomical Society of Australia, to be published in special issue on
"Electron Capture Supernovae". Submitte
- …