Solar Opacities Research Articles

Retardation effects on radiative transitions at solar interior conditions

Existing discrepancies between helioseismic data and solar models have questioned the accuracy of theoretical opacities for the solar interior. The opacities were further challenged by recent estimates claiming significant retardation effects, often neglected in opacity calculations, for radiative transitions relevant to the solar problem. Contrary to these claims, it is shown that the retardation effects, in agreement with past results, are negligible in radiative transitions relevant to solar opacities.

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.

LINE BROADENING AND THE SOLAR OPACITY PROBLEM

ABSTRACT The calculation of line widths constitutes theoretical and computational challenges in the calculation of opacities of hot, dense plasmas. Opacity models use line broadening approximations that are untested at stellar interior conditions. Moreover, calculations of atomic spectra of the Sun indicate a large discrepancy in the K-shell line widths between several atomic codes and the Opacity-Project (OP). In this work, the atomic code STAR is used to study the sensitivity of solar opacities to line broadening. Variations in the solar opacity profile due to an increase of the Stark widths resulting from discrepancies with OP, are compared, in light of the solar opacity problem, with the required opacity variations of the present day Sun, as imposed by helioseismic and neutrino observations. The resulting variation profile is much larger than the discrepancy between different atomic codes, agrees qualitatively with the missing opacity profile, recovers about half of the missing opacity nearby the convection boundary, and has a little effect in the internal regions. Since it is hard to estimate quantitatively the uncertainty in the Stark widths, we show that an increase of all line widths by a factor of about ∼100 recovers quantitatively the missing opacity. These results emphasize the possibility that photoexcitation processes are not modeled properly, and more specifically, highlight the need for a better theoretical characterization of the line broadening phenomena at stellar interior conditions, and of the uncertainty due to the way it is implemented by atomic codes.

SOLAR MIXTURE OPACITY CALCULATIONS USING DETAILED CONFIGURATION AND LEVEL ACCOUNTING TREATMENTS

An opacity model (OPAS) combining detailed configuration and level accounting treatments has been developed to calculate radiative opacity of plasmas in local thermodynamic equilibrium. The model is presented and used to compute spectral opacities of a solar mixture. Various density-temperature couples have been considered from the solar center up to the vicinity of the radiative/convective zone interface. For a given solar thermodynamic path, OPAS calculations are compared to Opacity Project (OP) and OPAL data. Rosseland mean opacity values are in very good agreement over all the considered solar thermodynamic path, while OPAS and OP spectral opacities of each element may vary considerably. Main sources of discrepancy are discussed.

Solar abundances of oxygen and neon derived from solar wind observations

Context. Recently, a revision of the solar abundances of C, N, and O to substantially lower values has led to a controversy on solar opacities in the solar standard model and to the suggestion to revise the solar abundance of neon upward by as much as a factor of 1.6 leading to enhanced solar neon/oxygen abundance ratios by a factor of 3. Neon and oxygen are neighboring elements with easily defined charged states in the solar wind, and they have been well identified and measured for over two decades in the solar wind under many circumstances and with several instruments. The solar wind Ne/O ratio is 0.14 with a conservative error estimate of ±0.03, consistent with the coronal value derived from solar energetic particle measurements. Aims. We investigate, whether solar wind observations are consistent with the newly proposed elemental solar abundances of neon and oxygen. Methods. The solar helium abundance has been derived from helioseismological observations. Helium and neon abundances in the solar wind have been well determined with the Apollo Foil experiments and, more recently, confirmed with the Genesis sample return mission. With these observations and the neon/oxygen solar wind abundance ratio determined by in-situ mass-spectrometry and using a simple theoretical model of Coulomb-drag fractionation for the solar wind, we estimate solar abundances for neon and oxygen. Results. From the variability of the helium/oxygen and the helium/neon ratio in the solar wind and from theoretical considerations, we conclude that the helium/neon ratio in the outer solar convective zone is 900 ± 110. Our best estimates of the solar neon and oxygen abundances in logarithmic dex-units are [Ne] = 7.96 ± 0.13, and [O] = 8.87 ± 0.11. Conclusions. Our solar neon/oxygen abundance ratio is consistent with the ratio derived from EUV-spectra from SOHO/CDS. However, our absolute abundance for oxygen is only marginally compatible with the newly derived value for oxygen, and our neon value is clearly incompatible with the recently proposed enhancement of the solar neon abundance.

The sensitivity of the seismic solar model to Newton's constant

New experimental determinations of Newton’s gravitational constant have been recently given. There are significant disagreements between these new measurements. We show that the high accuracy obtained with current standard solar models is such that the uncertainties in core temperature profiles arising from the uncertainties in G can be comparable to the range in predictions by the solar standard model of the observed acoustic modes and solar neutrino fluxes. Solar models that have been computed with a relatively high value of Newton’s constant, in agreement with the new measurements, produce an increase in the predicted sound speed and density profiles, with significant changes in the solar structure within the central region. These solar models predict a value of the active 8 B neutrino flux slightly closer to the Sudbury Neutrino Observatory and Super-Kamiokande measurements than the value obtained with the previous solar standard model (with G from CODATA 1986). Furthermore, we find that solar models with G between 6.65 × 10 −8 and 6.685 × 10 −8 cm 3 g −1 s −2 will have a reasonable agreement with the seismic solar model. Current uncertainties in solar physics and opacities prevent any stronger conclusions at present. However, we may be able potentially to use the Sun to determine an independent value of Newton’s constant, representing an alternative measurement to the value that is determined in Earth laboratories.

Our Sun. V. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars

The relatively warm temperatures required on early Earth and Mars have been difficult to account for via warming from greenhouse gases. We tested whether this problem can be resolved for both Earth and Mars by a young Sun that is brighter than predicted by the standard solar model. We computed high-precision solar evolutionary models with slightly increased initial masses of M_i = 1.01 to 1.07 M_sun; for each mass, we considered three different mass loss scenarios. We then tested whether these models were consistent with the current high-precision helioseismic observations. The relatively modest mass loss rates in these models are consistent with observational limits from young stars and estimates of the past solar wind obtained from lunar rocks, and do not significantly affect the solar lithium depletion. For appropriate initial masses, all three mass loss scenarios are capable of yielding a solar flux 3.8 Gyr ago high enough to be consistent with water on ancient Mars. We find that all of our mass-losing solar models are consistent with the helioseismic observations. The early solar mass loss of a few percent does indeed leave a small fingerprint on the Sun's internal structure. However, for helioseismology to significantly constrain early solar mass loss would require higher accuracy in the observed solar parameters and input physics, namely, by a factor of about 3 for the observed solar surface composition, and a factor of 2 for the solar interior opacities, the pp nuclear reaction rate, and the diffusion constants for gravitational settling.

Solar opacities

Solar opacities are presented from the center of the Sun to the photosphere. The temperatures, densities and hydrogen mass fractions are taken from the standard solar model. For the heavy element abundance the Grevesse mixture is used. In the solar interior photoabsorption is dominated by free–free absorption and we compare two sets of opacities based on two different models for the inverse bremsstrahlung. The radiative luminosity calculated from the two sets of opacities are compared with those predicted by previous models of the standard solar model and also with the known luminosity of the Sun. Pressures, specific heats and the speed of sound in the solar plasma are also presented.

Solar models with helium and heavy-element diffusion

Helium and heavy-element diffusion are both included in precise calculations of solar models. In addition, improvements in the input data for solar interior models are described for nuclear reaction rates, the solar luminosity, the solar age, heavy-element abundances, radiative opacities, helium and metal diffusion rates, and neutrino interaction cross sections. The effects on the neutrino fluxes of each change in the input physics are evaluated separately by constructing a series of solar models with one additional improvement added at each stage. The effective 1 σ uncertainties in the individual input quantities are estimated and used to evaluate the uncertainties in the calculated neutrino fluxes and the calculated event rates for solar neutrino experiments. The calculated neutrino event rates, including all of the improvements, are 9.3-1.4+1.2 SNU for the 37Cl experiment and 137-7+8 SNU for the 71Ga experiments. The calculated flux of 7Be neutrinos is 5.1 (1.00-0.07+0.06)×10^9 cm^-2 s^-1 and the flux of 8B neutrinos is 6.6(1.00-0.17+0.14)×10^6 cm^-2 s^-1. The primordial helium abundance found for this model is Y=0.278. The present-day surface abundance of the model is Ys=0.247, in agreement with the helioseismological measurement of Ys=0.242±0.003 determined by Hernandez and Christensen-Dalsgaard (1994). The computed depth of the convective zone is R=0.712R⊙, in agreement with the observed value determined from p-mode oscillation data of R=0.713±0.003R⊙ found by Christensen-Dalsgaard et al. (1991). Although the present results increase the predicted event rate in the four operating solar neutrino experiments by almost 1 σ (theoretical uncertainty), they only slightly increase the difficulty of explaining the existing experiments with standard physics (i.e., by assuming that nothing happens to the neutrinos after they are created in the center of the sun). For an extreme model in which all diffusion (helium and heavy-element diffusion) is neglected, the event rates are 7.0-1.0+0.9 SNU for the 37Cl experiment and 126-6+6 SNU for the 71Ga experiments, while the 7Be and 8B neutrino fluxes are, respectively, 4.5(1.00-0.07+0.06)×10^9 cm^-2 s^-1 and 4.9(1.00-0.17+0.14)×10^6 cm^-2 s^-1. For the no-diffusion model, the computed value of the depth of the convective zone is R=0.726R⊙, which disagrees with the observed helioseismological value. The calculated surface abundance of helium, Ys=0.268, is also in disagreement with the p-mode measurement. The authors conclude that helioseismology provides strong evidence for element diffusion and therefore for the somewhat larger solar neutrino event rates calculated in this paper.

A new model for Mars atmospheric dust based upon analysis of ultraviolet through infrared observations from Mariner 9, Viking, and Phobos

We propose key modifications to the Toon et al. (1977) model of the particle size distribution and composition of Mars atmospheric dust, based on a variety of spacecraft and wavelength observations of the dust. A much broader (reffvariance ∼ 0.8 μm), smaller particle size (rmode ∼ 0.02 μm) distribution coupled with a “palagonite‐like” composition is argued to fit the complete ultraviolet‐to‐30‐μm absorption properties of the dust better than the montmorillonite‐basalt, reffvariance = 0.4 μm, rmode = 0.40 dust model of Toon et al. Mariner 9 (infrared interferometer spectrometer) IRIS spectra of high atmospheric dust opacities during the 1971–1972 Mars global dust storm are analyzed in terms of the Toon et al. dust model, and a Hawaiian palagonite sample (Roush et al., 1991) with two different size distribution models incorporating smaller dust particle sizes. Viking Infrared Thermal Mapper (IRTM) emission‐phase‐function (EPF) observations at 9 μm are analyzed to retrieve 9‐μm dust opacities coincident with solar band dust opacities obtained from the same EPF sequences (Clancy and Lee, 1991). These EPF dust opacities provide an independent measurement of the visible/9‐μm extinction opacity ratio (≥2) for Mars atmospheric dust, which is consistent with a previous measurement by Martin (1986). Model values for the visible/9‐μm opacity ratio and the ultraviolet and visible single‐scattering albedos are calculated for the palagonite model with the smaller particle size distributions and compared to the same properties for the Toon et al. model of dust. The montmorillonite model of the dust is found to fit the detailed shape of the dust 9‐μm absorption well. However, it predicts structured, deep absorptions at 20 μm which are not observed and requires a separate ultraviolet‐visible absorbing component to match the observed behavior of the dust in this wavelength region. The modeled palagonite does not match the 8‐ to 9‐μm absorption presented by the dust in the IRIS spectra, probably due to its low SiO2 content (31%). However, it does provide consistent levels of ultraviolet/visible absorption, 9‐ to 12‐μm absorption, and a lack of structured absorption at 20 μm. The ratios of dust extinction opacities at visible, 9 μm, and 30 μm are strongly affected by the dust particle size distribution. The Toon et al. dust size distribution (rmode = 0.40, reffvariance = 0.4 μm, rcwμ = 2.7 μm) predicts the correct ratio of the 9‐ to 30‐μm opacity, but underpredicts the visible/9‐μm opacity ratio considerably (1 versus ≥2). A similar particle distribution width with smaller particle sizes (rmode = 0.17, reffvariance = 0.4 μm, rcwμ = 1.2 μm) will fit the observed visible/9‐μm opacity ratio, but overpredicts the observed 9‐μm/30‐μm opacity ratio. A smaller and much broader particle size distribution (rmode = 0.02, reffvariance = 0.8 μm, rcwμ = 1.8 μm) can fit both dust opacity ratios. Overall, the nanocrystalline structure of palagonite coupled with a smaller, broader distribution of dust particle sizes provides a more consistent fit than the Toon et al. model of the dust to the IRIS spectra, the observed visible/9‐μm dust opacity ratio, the Phobos occultation measurements of dust particle sizes (Chassefiere et al., 1992), and the weakness of surface near IR absorptions expected for clay minerals (Clark, 1992; Bell and Crisp, 1993).

Solar opacities based on the ion-sphere and ion-correlation models

Solar opacities based on the ion-sphere and ion-correlation models are given for the Grevesse mixture. Uncertainties connected with these models are pointed out. Although at low densities both may be realistic, at high densities when the interionic distances are comparable with the wavelengths of the bound-state wave functions, they have to be improved. Neither can give an accurate many-body representation of the plasma, and the question of which is more realistic can be answered only by more accurate computer simulation.

Solar Opacities Constrained by Solar Neutrinos and Solar Oscillations

AbstractThis review discusses the current situation for opacities at the solar center, the solar surface, and for the few million kelvin temperatures that occur below the convection zone. The solar center conditions are important because they are crucial for the neutrino production, which continues to be predicted about 4 times that observed. The main extinction effects there are free-free photon absorption in the electric fields of the hydrogen, helium and the CNO atoms, free electron scattering of photons, and the bound-free and bound-bound absorption of photons by iron atoms with two electrons in the 1s bound level. An assumption that the iron is condensed-out below the convection zone, and the opacity in the central regions is thereby reduced, results in about a 25 percent reduction in the central opacity but only a 5 percent reduction at the base of the convection zone. Furthermore, the p-mode solar oscillations are changed with this assumption, and do not fit the observed ones as well as for standard models. A discussion of the large effective opacity reduction by weakly interacting massive particles (WIMPs or Cosmions) also results in poor agreement with observed p-mode oscillation frequencies. The much larger opacities for the solar surface layers from the Los Alamos Astrophysical Opacity Library instead of the widely used Cox and Tabor values show small improvements in oscillation frequency predictions, but the largest effect is in the discussion of p-mode stability. Solar oscillation frequencies can serve as an opacity experiment for the temperatures and densities, respectively, of a few million kelvin and between 0.1 and 10g/cm3. Current oscillation frequency calculations indicate that possibly the Opacity Library values need an increase of typically 15 percent just at the bottom of the convection zone at 3×106K. Opacities have uncertainties at the photosphere and deeper than the convection zone ranging from 10 to 25 percent. The equation of state that supplies data for the opacity calculations fortunately has pressure uncertainties of only about 1 percent, but opacity uncertainties will always be much larger. A discussion is given about opacity experiments that the stars provide. Opacities in the envelopes of the Hyades G stars, the Cepheids,δScuti variables, and theβCephei variables indicate that significantly larger opacities, possibly caused by iron lines, seem to be required.

The standard solar model - Composition, opacities, and seismology

High-precision standard solar models are used to investigate systematically the implications of current uncertainties in the interior radiative opacities for both the derivation of the helium content Y and the modeling of p-mode oscillation frequencies of the sun. The differences between the Cox-Stewart solar opacities and the Los Alamos Opacity Library solar opacities are found to be due principally to differences in the absorption coefficients themselves. The uncertainty of Ne in the solar chemical composition is shown to result in only a small uncertainty in Y. In the standard model framework, the uncertainty in Y is between 0.29 and 0.27. Only a small dependence of the asymptotic spacing of p-modes on chemical composition is detected. The effects of dependence on Y of the adiabatic exponent Gamma(1) and of the density of the structure of the outer layers is not monotonic because both these variables depend on the complex interplay of H and He ionization. The greatest sensitivity of sound speed to chemical composition is found below the surface convection zone. 18 refs.