Boundaries Of Convection Zone Research Articles

Radiation burnthrough measurements to infer opacity at conditions close to the solar radiative zone–convective zone boundary

Recent measurements at the Sandia National Laboratory of the x-ray transmission of iron plasma have inferred opacities much higher than predicted by theory, which casts doubt on modeling of iron x-ray radiative opacity at conditions close to the solar convective zone-radiative zone boundary. An increased radiative opacity of the solar mixture, in particular iron, is a possible explanation for the disagreement in the position of the solar convection zone-radiative zone boundary as measured by helioseismology and predicted by modeling using the most recent photosphere analysis of the elemental composition. Here, we present data from radiation burnthrough experiments, which do not support a large increase in the opacity of iron at conditions close to the base of the solar convection zone and provide a constraint on the possible values of both the mean opacity and the opacity in the x-ray range of the Sandia experiments. The data agree with opacity values from current state-of-the-art opacity modeling using the CASSANDRA opacity code.

A proposal to measure iron opacity at conditions close to the solar convective zone-radiative zone boundary

A major problem in stellar modelling is the discrepancy between solar models and helioseismology data in the position of the convective zone-radiative zone boundary in the sun. This could be explained by a large uncertainty in the calculated opacity data and recent experimental data on iron using the Sandia National Laboratory Z facility have shown large differences, up to a factor of 4, between measurement and prediction at plasma conditions close to the convective zone-radiative zone boundary. This paper describes a proposal for a radiative burn-through experiment to be fielded on NIF to observe if a radiation wave transit through a Fe2O3 sample is consistent with the factor of 2–4 change in the iron opacity seen in the Z experiments. A target design and the diagnostic method are described. A detailed radiation-hydrodynamic model has been used to generate synthetic results and explore the sensitivities and experimental accuracy needed for the proposed measurement.

Computation of Atomic Astrophysical Opacities

The revision of the standard Los Alamos opacities in the 1980–1990s by a group from the Lawrence Livermore National Laboratory (OPAL) and the Opacity Project (OP) consortium was an early example of collaborative big-data science, leading to reliable data deliverables (atomic databases, monochromatic opacities, mean opacities, and radiative accelerations) widely used since then to solve a variety of important astrophysical problems. Nowadays the precision of the OPAL and OP opacities, and even of new tables (OPLIB) by Los Alamos, is a recurrent topic in a hot debate involving stringent comparisons between theory, laboratory experiments, and solar and stellar observations in sophisticated research fields: the standard solar model (SSM), helio and asteroseismology, non-LTE 3D hydrodynamic photospheric modeling, nuclear reaction rates, solar neutrino observations, computational atomic physics, and plasma experiments. In this context, an unexpected downward revision of the solar photospheric metal abundances in 2005 spoiled a very precise agreement between the helioseismic indicators (the radius of the convection zone boundary, the sound-speed profile, and helium surface abundance) and SSM benchmarks, which could be somehow reestablished with a substantial opacity increase. Recent laboratory measurements of the iron opacity in physical conditions similar to the boundary of the solar convection zone have indeed predicted significant increases (30–400%), although new systematic improvements and comparisons of the computed tables have not yet been able to reproduce them. We give an overview of this controversy, and within the OP approach, discuss some of the theoretical shortcomings that could be impairing a more complete and accurate opacity accounting.

The Effect of Ionic Correlations on Radiative Properties in the Solar Interior and Terrestrial Experiments

Abstract With the aim of solving the decade-old problem of solar opacity, we report substantial photoabsorption uncertainty due to the effect of ion–ion correlations. By performing detailed opacity calculations of the solar mixture, we find that taking into account the ionic structure changes the Rosseland opacity near the convection zone by ∼10%. We also report a ∼15% difference in the Rosseland opacity for iron, which was recently measured at the Sandia Z facility, where the temperature reached that prevailing in the convection zone boundary while the density was 2.5 times lower. Finally, we propose a method to measure opacities at solar temperatures and densities that have never been reached in the past via laboratory radiation flow experiments, by using plastic foams doped with permilles of dominant photon absorbers in the Sun. The method is advantageous for an experimental study of solar opacities that may lead to a resolution of the solar abundance problem.

Solar Models in Light of New High Metallicity Measurements from Solar Wind Data

Abstract We study the impact of new metallicity measurements, from solar wind data, on the solar model. The “solar modeling problem” refers to the persisting discrepancy between helioseismological observations and predictions of solar models computed implementing state-of-the-art photospheric abundances. We critically reassess the problem, in particular considering the new set of abundances of von Steiger & Zurbuchen, determined through the in situ collection of solar wind samples from polar coronal holes. This new set of abundances indicates a solar metallicity , significantly higher than the currently established value. The new values hint at an abundance of volatile elements (i.e., C, N, O, Ne) close to previous results of Grevesse and Sauval, whereas the abundance of refractory elements (i.e., Mg, Si, S, Fe) is considerably increased. Using the Linear Solar Model formalism, we determine the variation of helioseismological observables in response to the changes in elemental abundances, in order to explore the consistency of these new measurements with constraints from helioseismology. We find that for observables that are particularly sensitive to the abundance of volatile elements, in particular the radius of the convective zone boundary (CZB) and the sound speed around the radius of CZB, improved agreement over previous models is obtained. Conversely, the high abundance of refractories correlates with a higher core temperature, resulting in an overproduction of neutrinos and a huge increase in the surface helium abundance. We conclude that the “solar modeling problem” remains unsolved.

Proper use of Schwarzschild Ledoux criteria in stellar evolution computations

The era of detailed asteroseismic analyses opened by space missions such as CoRoT and Kepler has highlighted the need for stellar models devoid of numerical inaccuracies, in order to be able to diagnose which physical aspects are being ignored or poorly treated in standard stellar modeling. We tackle here the important problem of fixing convective zone boundaries in the frame of the local mixing length theory. First we show that the only correct way to locate a convective zone boundary is to find, at each iteration step, through interpolations or extrapolations from points within the convective zone, the mass where the radiative luminosity is equal to the total luminosity. We then discuss two misuses of the boundary condition and the ways they affect stellar modeling and stellar evolution. The first consists in applying the neutrality condition for convective instability on the radiative side of the convective boundary. The second way of misusing the boundary condition comes from the process of fixing the convective boundary through the search for a change of sign of a possibly discontinuous function. We show that these misuses can lead to completely wrong estimates of convective core sizes with important consequences for the following evolutionary phases. We point out the advantages of using a double mesh point at each convective zone boundary. The specific problem of a convective shell is discussed and some remarks concerning overshooting are given.

LOW MACH NUMBER MODELING OF CORE CONVECTION IN MASSIVE STARS

This work presents three-dimensional simulations of core convection in a 15 M☉ star halfway through its main sequence lifetime. To perform the necessary long-time calculations, we use the low Mach number code MAESTRO, with initial conditions taken from a one-dimensional stellar model. We first identify several key factors that the one-dimensional initial model must satisfy to ensure efficient simulation of the convection process. We then use the three-dimensional simulations to examine the effects of two common modeling choices on the resulting convective flow: using a fixed composition approximation and using a reduced domain size. We find that using a fixed composition model actually increases the computational cost relative to using the full multi-species model because the fixed composition system takes longer to reach convection that is in a quasi-static state. Using a reduced (octant rather than full sphere) simulation domain yields flow with statistical properties that are within a factor of two of the full sphere simulation values. Both the octant and full sphere simulations show similar mixing across the convection zone boundary that is consistent with the turbulent entrainment model. However, the global character of the flow is distinctly different in the octant simulation, showing more rapid changes in the large-scale structure of the flow and thus a more isotropic flow on average.

Design of dynamic Hohlraum opacity samples to increase measured sample density on Z

We are attempting to measure the transmission of iron on Z at plasma temperatures and densities relevant to the solar radiation and convection zone boundary. The opacity data published by us to date has been taken at an electron density about a factor of 10 below the 9×10(22)/cm(3) electron density of this boundary. We present results of two-dimensional (2D) simulations of the heating and expansion of an opacity sample driven by the dynamic Hohlraum radiation source on Z. The aim of the simulations is to design foil samples that provide opacity data at increased density. The inputs or source terms for the simulations are spatially and temporally varying radiation temperatures with a Lambertian angular distribution. These temperature profiles were inferred on Z with on-axis time-resolved pinhole cameras, x-ray diodes, and bolometers. A typical sample is 0.3 μm of magnesium and 0.078 μm of iron sandwiched between 10 μm layers of plastic. The 2D LASNEX simulations indicate that to increase the density of the sample one should increase the thickness of the plastic backing.

Irradiance and solar cycle variability: clues in cycle phase properties

We see solar cycle-related variations in almost every coronal, chromospheric, and photospheric measurement obtained over a several-years timescale. With such an abundant observational foundation, we should expect our physical models of the solar cycle to be overconstrained. In fact this is not the case and the “standard dynamo model” is not predictive. This paper describes some new thoughts on how solar irradiance variations may provide clues to the solar cycle regulation mechanism near the radiative–convection zone boundary.

Dynamics of the fast solar tachocline

One possible scenario for the origin of the solar known as the \fast tachocline, assumes that the turbulent diusivity exceeds > 10 9 cm 2 s 1 . In this case the dynamics will be governed by the dynamo- generated oscillatory magnetic eld on relatively short timescales. Here, for the rst time, we present detailed numerical models for the fast solar tachocline with all components of the magnetic eld calculated explicitly, assuming axial symmetry and a constant turbulent diusivity and viscosity . We nd that a suciently strong oscillatory poloidal eld with dipolar latitude dependence at the tachocline{convective zone boundary is able to conne the tachocline. Exploring the three-dimensional parameter space dened by the viscosity in the range log = 9{11, the magnetic Prandtl number in the range Prm =0 :1 10, and the meridional flow amplitude ( 3t o +3 cm s 1 ), we also nd that the conning eld strength Bconf, necessary to reproduce the observed thickness of the increases with viscosity , with magnetic Prandtl number =, and with equatorward meridional flow speed. Nevertheless, the resulting Bconf values remain quite reasonable, in the range 10 3 10 4 G, for all parameter combinations considered here. The thickness of the tachocline shows a marked dependence on both time and latitude. The latitude dependence is similar to that inferred by helioseismology, while the time dependence is within the observational errors.