221 research outputs found
The core helium flash revisited III. From Pop I to Pop III stars
Degenerate ignition of helium in low-mass stars at the end of the red giant
branch phase leads to dynamic convection in their helium cores. One-dimensional
(1D) stellar modeling of this intrinsically multi-dimensional dynamic event is
likely to be inadequate. Previous hydrodynamic simulations imply that the
single convection zone in the helium core of metal-rich Pop I stars grows
during the flash on a dynamic timescale. This may lead to hydrogen injection
into the core, and a double convection zone structure as known from
one-dimensional core helium flash simulations of low-mass Pop III stars. We
perform hydrodynamic simulations of the core helium flash in two and three
dimensions to better constrain the nature of these events. To this end we study
the hydrodynamics of convection within the helium cores of a 1.25 \Msun
metal-rich Pop I star (Z=0.02), and a 0.85 \Msun metal-free Pop III star (Z=0)
near the peak of the flash. These models possess single and double convection
zones, respectively. We use 1D stellar models of the core helium flash computed
with state-of-the-art stellar evolution codes as initial models for our
multidimensional hydrodynamic study, and simulate the evolution of these models
with the Riemann solver based hydrodynamics code Herakles which integrates the
Euler equations coupled with source terms corresponding to gravity and nuclear
burning. The hydrodynamic simulation of the Pop I model involving a single
convection zone covers 27 hours of stellar evolution, while the first
hydrodynamic simulations of a double convection zone, in the Pop III model,
span 1.8 hours of stellar life. We find differences between the predictions of
mixing length theory and our hydrodynamic simulations. The simulation of the
single convection zone in the Pop I model shows a strong growth of the size of
the convection zone due to turbulent entrainment. Hence we predict that for the
Pop I model a hydrogen injection phase (i.e. hydrogen injection into the helium
core) will commence after about 23 days, which should eventually lead to a
double convection zone structure known from 1D stellar modeling of low-mass Pop
III stars. Our two and three-dimensional hydrodynamic simulations of the double
(Pop III) convection zone model show that the velocity field in the convection
zones is different from that predicted by stellar evolutionary calculations.
The simulations suggest that the double convection zone decays quickly, the
flow eventually being dominated by internal gravity waves.Comment: 16 pages, 18 figures, submitted to Aa
Turbulent convection in stellar interiors. III. Mean-field analysis and stratification effects
We present 3D implicit large eddy simulations (ILES) of the turbulent
convection in the envelope of a 5 Msun red giant star and in the oxygen-burning
shell of a 23 Msun supernova progenitor. The numerical models are analyzed in
the framework of 1D Reynolds-Averaged Navier-Stokes (RANS) equations. The
effects of pressure fluctuations are more important in the red giant model,
owing to larger stratification of the convective zone. We show how this impacts
different terms in the mean-field equations. We clarify the driving sources of
kinetic energy, and show that the rate of turbulent dissipation is comparable
to the convective luminosity. Although our flows have low Mach number and are
nearly adiabatic, our analysis is general and can be applied to photospheric
convection as well. The robustness of our analysis of turbulent convection is
supported by the insensitivity of the mean-field balances to linear mesh
resolution. We find robust results for the turbulent convection zone and the
stable layers in the oxygen-burning shell model, and robust results everywhere
in the red giant model, but the mean fields are not well converged in the
narrow boundary regions (which contain steep gradients) in the oxygen-burning
shell model. This last result illustrates the importance of unresolved physics
at the convective boundary, which governs the mixing there.Comment: 26 pages, 20 figures, Accepted for publication in Ap
The core helium flash revisited: II. Two and three-dimensional hydrodynamic simulations
We study turbulent convection during the core helium flash close to its peak
by comparing the results of two and three-dimensional hydrodynamic simulations.
We use a multidimensional Eulerian hydrodynamics code based on
state-of-the-art numerical techniques to simulate the evolution of the helium
core of a Pop I star.
Our three-dimensional hydrodynamic simulations of the evolution of a star
during the peak of the core helium flash do not show any explosive behavior.
The convective flow patterns developing in the three-dimensional models are
structurally different from those of the corresponding two-dimensional models,
and the typical convective velocities are smaller than those found in their
two-dimensional counterparts. Three-dimensional models also tend to agree
better with the predictions of mixing length theory. Our hydrodynamic
simulations show the presence of turbulent entrainment that results in a growth
of the convection zone on a dynamic time scale. Contrary to mixing length
theory, the outer part of the convection zone is characterized by a
sub-adiabatic temperature gradient.Comment: 19 pages, 18 figure
Hydrodynamic simulations of the core helium flash
We describe and discuss hydrodynamic simulations of the core helium flash
using an initial model of a 1.25 M_sol star with a metallicity of 0.02 near at
its peak. Past research concerned with the dynamics of the core helium flash is
inconclusive. Its results range from a confirmation of the standard picture,
where the star remains in hydrostatic equilibrium during the flash (Deupree
1996), to a disruption or a significant mass loss of the star (Edwards 1969;
Cole & Deupree 1980). However, the most recent multidimensional hydrodynamic
study (Dearborn 2006) suggests a quiescent behavior of the core helium flash
and seems to rule out an explosive scenario. Here we present partial results of
a new comprehensive study of the core helium flash, which seem to confirm this
qualitative behavior and give a better insight into operation of the convection
zone powered by helium burning during the flash. The hydrodynamic evolution is
followed on a computational grid in spherical coordinates using our new version
of the multi-dimensional hydrodynamic code HERAKLES, which is based on a direct
Eulerian implementation of the piecewise parabolic method.Comment: 6 pages, 5 figures. IAUS 252 Conference Proceeding (Sanya, China):
"The art of modeling stars in the 21st century
3D simulations of a neon burning convective shell in a massive star
© 2024 The Author(s). Published by Oxford University Press on behalf of Royal Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/The treatment of convection remains a major weakness in the modelling of stellar evolution with one-dimensional (1D) codes. The ever-increasing computing power makes now possible to simulate in three-dimensional (3D) part of a star for a fraction of its life, allowing us to study the full complexity of convective zones with hydrodynamics codes. Here, we performed state-of-the-art hydrodynamics simulations of turbulence in a neon-burning convective zone, during the late stage of the life of a massive star. We produced a set of simulations varying the resolution of the computing domain (from 1283 to 10243 cells) and the efficiency of the nuclear reactions (by boosting the energy generation rate from nominal to a factor of 1000). We analysed our results by the mean of Fourier transform of the velocity field, and mean-field decomposition of the various transport equations. Our results are in line with previous studies, showing that the behaviour of the bulk of the convective zone is already well captured at a relatively low resolution (2563), while the details of the convective boundaries require higher resolutions. The different boosting factors used show how various quantities (velocity, buoyancy, abundances, and abundance variances) depend on the energy generation rate. We found that for low boosting factors, convective zones are well mixed, validating the approach usually used in 1D stellar evolution codes. However, when nuclear burning and turbulent transport occur on the same time-scale, a more sophisticated treatment would be needed. This is typically the case when shell mergers occur.Peer reviewe
Carbon-rich RR Lyr type stars
We have derived CNO abundances in 12 RR Lyrae stars. Four stars show [C/Fe]
near 0.0 and two stars show [C/Fe] = 0.52 and 0.65. Red giant branch stars,
which are known to be the predecessors of RR Lyrae stars, generally show a
deficiency of carbon due to proton captures during their evolution from the
main sequence up the giant branch. We suggest that the enhancement of carbon is
due to production during the helium flash combined with mixing to the surface
by vigorous convection induced by the flash itself.Comment: accepted for publication in ApJ
Anodic stripping voltammetry with graphite felt electrodes for the trace analysis of silver
Graphite felt (GF) is a mass produced porous carbon electrode material commonly used in redox flow batteries. Previous studies have suggested GF may have valuable applications in electroanalysis as a low cost disposable carbon electrode material, although most GF sensors have used flow cell arrangements. In this work, an elegant wetting technique is employed that allows GF electrodes to be used in quiescent solution to detect trace levels of silver in water via anodic stripping voltammetry. GF electrodes display good repeatability and a limit of detection of 25 nM of Ag+ in 0.1 M HNO3, with a linear range spanning two orders of magnitude. This compares to a value of around 140 nM when using conventional carbon electrodes. Combined with their low cost and disposable nature, the results suggest GF electrodes can make a valuable contribution to electroanalysis
Drug Facilitated Sexual Assault: Detection and Stability of Benzodiazepines in Spiked Drinks Using Gas Chromatography-Mass Spectrometry
Benzodiazepines are detected in a significant number of drug facilitated sexual assaults (DFSA). Whilst blood and urine from the victim are routinely analysed, due to the delay in reporting DFSA cases and the short half lives of most of these drugs in blood and urine, drug detection in such samples is problematic. Consideration of the drinks involved and analysis for drugs may start to address this. Here we have reconstructed the ‘spiking’ of three benzodiazepines (diazepam, flunitrazepam and temazepam) into five drinks, an alcopop (flavoured alcoholic drink), a beer, a white wine, a spirit, and a fruit based non-alcoholic drink (J2O) chosen as representative of those drinks commonly used by women in 16–24 year old age group. Using a validated GC-MS method for the simultaneous detection of these drugs in the drinks we have studied the storage stability of the benzodiazepines under two different storage conditions, uncontrolled room temperature and refrigerator (4°C) over a 25 day period. All drugs could be detected in all beverages over this time period. Diazepam was found to be stable in all of the beverages, except the J2O, under both storage conditions. Flunitrazepam and temazepam were found not to be stable but were detectable (97% loss of temazepam and 39% loss of flunitrazepam from J2O). The recommendations from this study are that there should be a policy change and that drinks thought to be involved in DFSA cases should be collected and analysed wherever possible to support other evidence types
Sarcosine oxidase composite screen-printed electrode for sarcosine determination in biological samples
As the prostate cancer (PCa) progresses, sarcosine levels increase both in tumor cells and urine samples, suggesting that this metabolite measurements can help in the creation of non-invasive diagnostic methods for this disease. In this work, a biosensor device was developed for the quantification of sarcosine via electrochemical detection of H2O2 (at 0.6 V) generated from the catalyzed oxidation of sarcosine. The detection was carried out after the modification of carbon screen printed electrodes (SPEs) by immobilization of sarcosine oxidase (SOX) on the electrode surface. The strategies used herein included the activation of the carbon films by an electrochemical step and the formation of an NHS/EDAC layer to bond the enzyme to the electrode, the use of metallic or semiconductor nanoparticles layer previously or during the enzyme immobilization. In order to improve the sensor stability and selectivity a polymeric layer with extra enzyme content was further added. The proposed methodology for the detection of sarcosine allowed obtaining a limit of detection (LOD) of 16 nM, using a linear concentration range between 10 and 100 nM. The biosensor was successfully applied to the analysis of sarcosine in urine samples
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