1,152 research outputs found
Multidimensional hydrodynamic simulations of the hydrogen injection flash
The injection of hydrogen into the convection shell powered by helium burning
during the core helium flash is commonly encountered during the evolution of
metal-free and extremely metal-poor low-mass stars. With specifically designed
multidimensional hydrodynamic simulations, we aim to prove that an entropy
barrier is no obstacle for the growth of the helium-burning shell convection
zone in the helium core of a metal-rich Pop I star, i.e. convection can
penetrate into the hydrogen-rich layers for these stars, too. We further study
whether this is also possible in one-dimensional stellar evolutionary
calculations. Our hydrodynamical simulations show that the helium-burning shell
convection zone in the helium core moves across the entropy barrier and reaches
the hydrogen-rich layers. This leads to mixing of protons into the hotter
layers of the core and to a rapid increase of the nuclear energy production at
the upper edge of the helium-burning convection shell - the hydrogen injection
flash. As a result a second convection zone appears in the hydrogen-rich
layers. Contrary to 1D models, the entropy barrier separating the two
convective shells from each other is largely permeable to chemical transport
when allowing for multidimensional flow, and consequently, hydrogen is
continuously mixed deep into the helium core. We find it difficult to achieve
such a behavior in one-dimensional stellar evolutionary calculations.Comment: 8 pages, 8 figures - accepted for publication in Astronomy and
Astrophysics. Animations related to the manuscript can be downloaded from
http://www-astro.ulb.ac.be/~mocak/index.php/Main/AnimationsHeFlas
Simulations of Stellar Collisions Involving Pre-Main Sequence Stars
In this paper, we present the results of smoothed particle hydrodynamic (SPH)
simulations of collisions between pre-main sequence stars and a variety of
other kinds of stars. Simulations over a range of impact parameters and
velocities were performed. We find that pre-main sequence stars tend to ``wrap
themselves'' around their impactor. We discuss the probable evolutionary state
of products of collisions between pre-main sequence stars and pre-main
sequence, main sequence, giant branch, and compact stars. The nature of the
collision product does not depend strongly on the impact parameter or the
velocity of the collision.Comment: Accepted by Ap
A new stellar mixing process operating below shell convection zones following off-center ignition
During most stages of stellar evolution the nuclear burning of lighter to
heavier elements results in a radial composition profile which is stabilizing
against buoyant acceleration, with light material residing above heavier
material. However, under some circumstances, such as off-center ignition, the
composition profile resulting from nuclear burning can be destabilizing, and
characterized by an outwardly increasing mean molecular weight. The potential
for instabilities under these circumstances, and the consequences that they may
have on stellar structural evolution, remain largely unexplored. In this paper
we study the development and evolution of instabilities associated with
unstable composition gradients in regions which are initially stable according
to linear Schwarzschild and Ledoux criteria. In particular, we explore the
mixing taking place under various conditions with multi-dimensional
hydrodynamic convection models based on stellar evolutionary calculations of
the core helium flash in a 1.25 \Msun star, the core carbon flash in a
9.3\,\Msun star, and of oxygen shell burning in a star with a mass of
23\,\Msun. The results of our simulations reveal a mixing process associated
with regions having outwardly increasing mean molecular weight that reside
below convection zones. The mixing is not due to overshooting from the
convection zone, nor is it due directly to thermohaline mixing which operates
on a timescale several orders of magnitude larger than the simulated flows.
Instead, the mixing appears to be due to the presence of a wave field induced
in the stable layers residing beneath the convection zone which enhances the
mixing rate by many orders of magnitude and allows a thermohaline type mixing
process to operate on a dynamical, rather than thermal, timescale. We discuss
our results in terms of related laboratory phenomena and associated theoretical
developments.Comment: accepted for publication in Astrophysical Journal, 9 pages, 8 figure
Hydrodynamic simulations of shell convection in stellar cores
Shell convection driven by nuclear burning in a stellar core is a common
hydrodynamic event in the evolution of many types of stars. We encounter and
simulate this convection (i) in the helium core of a low-mass red giant during
core helium flash leading to a dredge-down of protons across an entropy
barrier, (ii) in a carbon-oxygen core of an intermediate-mass star during core
carbon flash, and (iii) in the oxygen and carbon burning shell above the
silicon-sulfur rich core of a massive star prior to supernova explosion. Our
results, which were obtained with the hydrodynamics code HERAKLES, suggest that
both entropy gradients and entropy barriers are less important for stellar
structure than commonly assumed. Our simulations further reveal a new dynamic
mixing process operating below the base of shell convection zones.Comment: 8 pages, 3 figures .. submitted to a proceedings of conference about
"Red Giants as Probes of the Structure and Evolution of the Milky Way" which
has taken place between 15-17 November 2010 in Rom
Thermohaline mixing in evolved low-mass stars
Thermohaline mixing has recently been proposed to occur in low-mass red
giants, with large consequence for the chemical yields of low-mass stars. We
investigate the role of thermohaline mixing during the evolution of stars
between 1Msun and 3Msun, in comparison to other mixing processes acting in
these stars. We use a stellar evolution code which includes rotational mixing,
internal magnetic fields and thermohaline mixing. We confirm that during the
red giant stage, thermohaline mixing has the potential to decrease the
abundance of ^3He which is produced earlier on the main sequence. In our models
we find that this process is working on the RGB only in stars with initial mass
M \simle 1.5Msun. Moreover we report that thermohaline mixing is present also
during core helium burning and beyond, and has the potential to change the
surface abundances of AGB stars. While we find rotational and magnetic mixing
to be negligible compared to the thermohaline mixing in the relevant layers,
the interaction of thermohaline motions with the differential rotation may be
essential to establish the time scale of thermohaline mixing in red giants. To
explain the surface abundances observed at the bump in the luminosity function,
the speed of the mixing process needs to be more than two orders of magnitude
higher than in our models. However it is not clear if thermohaline mixing is
the only physical process responsible for these surface abundance anomalies.
Therefore, at this stage, it is not possible to calibrate the efficiency of
thermohaline mixing against the observations.Comment: 10 pages - Accepted for publication in A&
A Survey for Low-Mass Stars and Brown Dwarfs in the Eta Cha and Eps Cha Young Associations
I present the results of a search for new low-mass stars and brown dwarfs in
the Eta Cha and Eps Cha young associations. Within radii of 1.5 and 0.5 deg
surrounding Eta Cha and Eps Cha, respectively, I have constructed
color-magnitude diagrams from DENIS and 2MASS photometry and have obtained
spectra of the candidate low-mass members therein. The five candidates in Eta
Cha are classified as four field M dwarfs and one carbon star. No new members
are found in this survey, which is complete for M_sun=0.015-0.15 according to
the evolutionary models of Chabrier and Baraffe. Thus, an extended population
of low-mass members is not present in Eta Cha out to four times the radius of
the known membership. Meanwhile, the three candidate members of Eps Cha are
classified as young stars, and thus likely members of the association, based on
Li absorption and gravity-sensitive absorption lines. These new sources have
spectral types of M2.25, M3.75, and M5.75, corresponding to masses of 0.45,
0.25, and 0.09 M_sun by the models of Chabrier and Baraffe. For one of these
stars, intense H(alpha) emission, forbidden line emission, and strong K-band
excess emission suggest the presence of accretion, an outflow, and a disk,
respectively. This young star is also much fainter than expected for an
association member at its spectral type, which could indicate that it is seen
in scattered light. No brown dwarfs are detected in Eps Cha down to the
completeness limit of 0.015 M_sun. The absence of brown dwarfs in these
associations is statistically consistent with the mass functions measured in
star-forming regions, which exhibit only ~2 and ~1 brown dwarfs for stellar
samples at the sizes of the Eta Cha and Eps Cha associations.Comment: 19 pages, The Astrophysical Journal, 2004, v616 (December 1
The nuclear processes responsible for the CNO synthesis
The abundances of the isotopes of the elements C, N and O are mainly affected
by the cold CNO cycles in non-explosive stellar situations, or by the hot CNO
chains that can develop in certain explosive sites, like classical novae.
Helium burning phases can modify the composition of the ashes of the CNO
transmutations through several -capture reactions, the most famed one
being 12C(a,g)16O. This contribution presents a short review of the purely
nuclear physics limitations imposed on the accuracy of the predicted C, N and O
yields from H-burning in non-explosive stars or novae. This analysis makes
largely use of the NACRE compilation for the rates of the reactions on stable
targets making up the cold CNO cycle. Some more recent rate determinations are
also considered. The analysis of the impact of the rate uncertainties on the
abundance predictions is conducted in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently. A limited use of detailed stellar models is
also made for the purpose of some specific illustrations.Comment: 9 pages including 5 figures. Conference proceedings for "CNO in the
Universe", St Luc, Switzerland, 10-14 September 2002 C. Charbonnel, D.
Schaerer & G. Meynet (eds) ASP Conference Seri
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