83 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
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
Beyond Mixing-length Theory: a step toward 321D
We examine the physical basis for algorithms to replace mixing-length theory
(MLT) in stellar evolutionary computations. Our 321D procedure is based on
numerical solutions of the Navier-Stokes equations. These implicit large eddy
simulations (ILES) are three-dimensional (3D), time-dependent, and turbulent,
including the Kolmogorov cascade. We use the Reynolds-averaged Navier-Stokes
(RANS) formulation to make concise the 3D simulation data, and use the 3D
simulations to give closure for the RANS equations. We further analyze this
data set with a simple analytical model, which is non-local and time-dependent,
and which contains both MLT and the Lorenz convective roll as particular
subsets of solutions. A characteristic length (the damping length) again
emerges in the simulations; it is determined by an observed balance between (1)
the large-scale driving, and (2) small-scale damping.
The nature of mixing and convective boundaries is analyzed, including
dynamic, thermal and compositional effects, and compared to a simple model.
We find that
(1) braking regions (boundary layers in which mixing occurs) automatically
appear {\it beyond} the edges of convection as defined by the Schwarzschild
criterion,
(2) dynamic (non-local) terms imply a non-zero turbulent kinetic energy flux
(unlike MLT),
(3) the effects of composition gradients on flow can be comparable to thermal
effects, and
(4) convective boundaries in neutrino-cooled stages differ in nature from
those in photon-cooled stages (different P\'eclet numbers).
The algorithms are based upon ILES solutions to the Navier-Stokes equations,
so that, unlike MLT, they do not require any calibration to astronomical
systems in order to predict stellar properties. Implications for solar
abundances, helioseismology, asteroseismology, nucleosynthesis yields,
supernova progenitors and core collapse are indicated.Comment: 22 pages, 4 figures, 2 tables; significantly re-written, critique of
Pasetto, et al. model added, accepted for publication by Ap
Quality information system SQS in plant Skoda Auto India Private Limited
katedra: KPE; přílohy: 1 CD-ROM; rozsah: 80 s., 3 s. obr. přílohTato diplomová práce se zabývá optimalizací informačního systému kvality SQS, sběru kvalitativních dat o vozech v závodě Škoda Auto India Private Limited. Úvod této práce čtenáře stručně seznamuje s historií společnosti Škoda Auto, dále pak s pojmy kvalita, řízení, informační systémy a informační technologie. Následuje část, která je věnovaná popisu struktury a fungování informačního systému SQS ve společnosti ŠKODA AUTO a.s. a charakteristice projektu výroby rozložených vozů. Poslední část práce se zabývá analýzou sběru kvalitativních dat v závodě SAIPL v Indii, popisem změn uživatelského prostředí pro zadávání závad, spojenou s implementací nové verze aplikace a podává hodnocení variant jednotlivých nově navržených kroků evidence závad.This thesis deals with optimizing of quality information system SQS and qualitative car data collection process in plant Skoda Auto India Private Limited. The beginning of this thesis briefly introduces to history and present period of the Škoda Auto Company, conceptions of quality, management, information system and information technology. The following part contains descriptions of the structure and operation of Škoda Auto´s information system SQS and of the knocked-down vehicle production project. The final part contains the analysis of quality data collection in the SAIPL plant in India, description of modifications of user defect data input environment in the new implementation of the application, and the evaluation of variants of different newly proposed defect registration phases
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
Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars II. s-process nucleosynthesis during the core He flash
Models of primordial and hyper-metal-poor stars with masses similar to the
Sun experience an ingestion of protons into the hot core during the core helium
flash phase at the end of their red giant branch evolution. This produces a
concurrent secondary flash powered by hydrogen burning that gives rise to
further nucleosynthesis in the core. We perform post-process nucleosynthesis
calculations on a one-dimensional stellar evolution calculation of a star of 1
solar mass and metallicity [Fe/H] = -6.5 that suffers a proton ingestion
episode. Our network includes 320 nuclear species and 2,366 reactions and
treats mixing and burning simultaneously. The mixing and burning of protons
into the hot convective core leads to the production of 13C, which then burns
via the 13C(alpha,n)16O reaction releasing a large number of free neutrons.
During the first two years of neutron production the neutron poison 14N
abundance is low, allowing the prodigious production of heavy elements such as
strontium, barium, and lead via slow neutron captures (the s process). These
nucleosynthetic products are later mixed to the stellar surface and ejected via
stellar winds. We compare our results with observations of the hyper-metal-poor
halo star HE 1327-2326, which shows a strong Sr overabundance. Our model
provides the possibility of self-consistently explaining the Sr overabundance
in HE 1327-2326 together with its C, N, and O overabundances (all within a
factor of ~4) if the material were heavily diluted, for example, via mass
transfer in a wide binary system. The model produces at least 18 times too much
Ba than observed, but this may be within the large modelling uncertainties. In
this scenario, binary systems of low mass must have formed in the early
Universe. If true then this puts constraints on the primordial initial mass
function.Comment: Accepted for publication on Astronomy & Astrophysics Letter
Shock-heating of stellar envelopes: A possible common mechanism at the origin of explosions and eruptions in massive stars
Observations of transient phenomena in the Universe reveal a spectrum of
mass-ejection properties associated with massive stars, covering from Type
II/Ib/Ic core-collapse supernovae (SNe) to giant eruptions of Luminous Blue
Variables (LBV) and optical transients. Here, we hypothesize that a fraction of
these phenomena may have an explosive origin, the distinguishing ingredient
being the ratio of the prompt energy release E_dep to the envelope binding
energy E_binding. Using one-dimensional one-group radiation hydrodynamics and a
set of 10-25Msun, massive-star models, we explore the dynamical response of a
stellar envelope subject to a strong, sudden, and deeply-rooted energy release.
Following energy deposition, a shock systematically forms, crosses the
progenitor envelope on a day timescale, and breaks-out with a signal of
hour-to-days duration and a 10^5-10^11 Lsun luminosity. For E_dep > E_binding,
full envelope ejection results with a SN-like bolometric luminosity and kinetic
energy, modulations being commensurate to the energy deposited and echoing the
diversity of Type II-Plateau SNe. For E_dep ~ E_binding, partial envelope
ejection results with a small expansion speed, and a more modest but year-long
luminosity plateau, reminiscent of LBV eruptions or so-called SN impostors. For
E_dep < E_binding, we obtain a "puffed-up" star, secularly relaxing back to
thermal equilibrium. In parallel with gravitational collapse and Type II SNe,
we argue that the thermonuclear combustion of merely a few 0.01Msun of C/O
could power a wide range of explosions/eruptions in loosely-bound massive
stars, as those in the 8-12Msun range, or in more massive ones owing to their
proximity to the Eddington limit and/or critical rotation.Comment: 20 pages, 16 figures, 2 tables; accepted to MNRA
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