Radiative Opacity Research Articles

Equation of State and Opacities for Hydrogen Atmospheres of Magnetars

The equation of state and radiative opacities of partially ionized, strongly magnetized hydrogen plasmas, presented in a previous paper for the magnetic field strengths, 8 × 1011 G B 3 × 1013 G, are extended to the field strengths, 3 × 1013 G B ≤ 1015 G, relevant for magnetars. The first- and second-order thermodynamic functions and radiative opacities are calculated and tabulated for 5 × 105 K ≤ T ≤ 4 × 107 K in a wide range of densities. We show that bound-free transitions give an important contribution to the opacities in the considered range of B in the outer neutron star atmosphere layers. Unlike the case of weaker fields, bound-bound transitions are unimportant.

Radiative opacities and configuration interaction effects of hot iron plasma using a detailed term accounting model

We have calculated the radiative opacities of iron plasma in local thermodynamic equilibrium using a detailed term accounting model. The extensive atomic data are obtained by multiconfiguration Hartree-Fock (MCHF) method, with Breit-Pauli relativistic corrections. Extensive configuration interaction (CI) has been included based on LS coupling to obtain energy levels and the bound-bound transition cross sections. A detailed configuration accounting model is applied to evaluate the bound-free absorption cross sections. We simulate two experimental transmission spectra [G. Winhart et al., Phys. Rev. E 53, R1332 (1996); P. T. Springer et al., J. Quant. Spectrosc. Radiat. Transf. 58, 927 (1997)] to verify our calculation model, one is at a temperature of 22 eV and a density of 10(-2) g/cm(3) and the other is at a temperature of 20 eV and a lower density of 10(-4) g/cm(3). It is shown that the strong CI can effectively change the oscillator strengths in contrast to the single configuration HF method. For both of the two simulated transmission spectra good agreement is obtained between the present MCHF results and the experimental data. Spectrally resolved opacities and Planck and Rosseland mean opacities are also calculated. For the isothermal sequence of T=20 eV, when the density decreases from 10(-2) to 10(-5) g/cm(3), the linewidth also decreases so that the iron transition arrays show more discrete line structures and the linewidth becomes very important to the Rosseland mean opacity.

Thermal Structure and Cooling of Superfluid Neutron Stars with Accreted Magnetized Envelopes

We study the thermal structure of neutron stars with magnetized envelopes composed of accreted material, using updated thermal conductivities of plasmas in quantizing magnetic fields, as well as the equation of state and radiative opacities for partially ionized hydrogen in strong magnetic fields. The relation between the internal and local surface temperatures is calculated and fitted by an analytic function of the internal temperature, magnetic field strength, angle between the field lines and the normal to the surface, surface gravity, and the mass of the accreted material. The luminosity of a neutron star with a dipole magnetic field is calculated for various values of the accreted mass, internal temperature, and magnetic field strength. Using these results, we simulate cooling of superfluid neutron stars with magnetized accreted envelopes. We consider slow and fast cooling regimes, paying special attention to very slow cooling of low-mass, superfluid neutron stars. In the latter case, the cooling is strongly affected by the combined effect of magnetized accreted envelopes and neutron superfluidity in the stellar crust. Our results are important for the interpretation of observations of the isolated neutron stars hottest for their age, such as RX J0822-43 and PSR B1055-52.

STARBURST AND AGN CONNECTIONS AND MODELS

There is accumulating evidence for a strong link between nuclear starbursts and AGN. Molecular gas in the central regions of galaxies plays a critical role in fueling nuclear starburst activity and feeding central AGN. The dense molecular ISM is accreted to the nuclear regions by stellar bars and galactic interactions. Here we describe recent observational results for the OB star forming regions in M51 and the nuclear star burst in Arp 220 - both of which have approximately the same rate of star formation per unit mass of ISM. We suggest that the maximum efficiency for forming young stars is an Eddington-like limit imposed by the radiation pressure of newly formed stars acting on the interstellar dust. This limit corresponds to approximately 500 <TEX>$L_{\bigodot} / M_{\bigodot}$</TEX> for optically thick regions in which the radiation has been degraded to the NIR. Interestingly, we note that some of the same considerations can be important in AGN where the source of fuel is provided by stellar evolution mass-loss or ISM accretion. Most of the stellar mass-loss occurs from evolving red giant stars and whether their mass-loss can be accreted to a central AGN or not depends on the radiative opacity of the mass-loss material. The latter depends on whether the dust survives or is sublimated (due to radiative heating). This, in turn, is determined by the AGN luminosity and the distance of the mass-loss stars from the AGN. Several AGN phenomena such as the broad emission and absorption lines may arise in this stellar mass-loss material. The same radiation pressure limit to the accretion may arise if the AGN fuel is from the ISM since the ISM dust-to-gas ratio is the same as that of stellar mass-loss.

Quantum Molecular Dynamics calculations of radiative opacities

We show that Quantum Molecular Dynamics provides a powerful tool to extend and benchmark current opacities libraries into the complex regime of warm dense matter. In this regime, the medium can be constituted of electrons, protons, atoms and molecules, while plasma and many-body effects can not be treated as perturbations. Among the most notable features of this new approach for calculating Rosseland mean opacities is the ability to obtain a consistent set of material (equation-of-state), optical and electrical properties for various mixtures from the same simulation.

Temperature Diagnostics for iron Plasmas by Means ofTransmission Spectrum Obtained by Accurate Atomic Data

Accurate atomic data such as fine-structure energy levels and oscillatorstrengths are calculated to obtain the radiative opacity andtransmission of iron plasmas. At a temperature of 22 eV and a densityof 0.01 g/cm3, the calculated transmission spectrum is in betteragreement with the experiment than that by other opacity codes such asOPAL. The transmission spectrum is very sensitive to the temperature ofplasmas, meaning that it is an ideal diagnostic tool for the temperatureof plasmas. Practical calculations show that the uncertainty of thetemperature diagnostics obtained by theoretical predictions is betterthan ±2 eV.

Equation of State and Opacities for Hydrogen Atmospheres of Neutron Stars with Strong Magnetic Fields

We present an equation of state and radiative opacities for a strongly magnetized hydrogen plasma at magnetic fields B, temperatures T, and densities ρ typical for atmospheres of isolated neutron stars. The first- and second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for 8 × 1011 G ≤ B ≤ 3 × 1013 G, 2 × 105 K ≤ T ≤ 107 K, and a wide range of ρ. We show that bound-bound and bound-free transitions give an important contribution to the opacities at T (1-5) × 106 K in the considered range of B in the outer neutron star atmosphere layers, which may substantially modify the X-ray spectrum of a typical magnetized neutron star. In addition, we reevaluate opacities due to free-free transitions, taking into account the motion of both interacting particles, electron and proton, in a strong magnetic field. Compared to the previous neutron star atmosphere models, the free-free absorption is strongly suppressed at photon frequencies below the proton cyclotron frequency. The latter result holds for any field strength, which prompts a revision of existing theoretical models of X-ray spectra of magnetar atmospheres.

Low-Mass AGB Stellar Models for 0.003 ≤ Z ≤ 0.02: Basic Formulae for Nucleosynthesis Calculations

AbstractWe have extended our published set of low-mass AGB stellar modelsto lower metallicities. Different mass-loss rates have been explored. We provide interpolation formulae for the luminosity, effective temperature, core mass, mass of dredge up material and maximum temperature in the convective zone generated by thermal pulses. Finally, we discuss the resultant modification of these quantities when we use an appropriate treatment of the inward propagation of the convective instability, as caused by the steeprise in radiative opacity when the convective envelope penetratesthe H-depleted region.

Calculation of the radiative opacity of laser-produced plasmas using a relativistic-screened hydrogenic model

In this work, we use a relativistic-screened hydrogenic model to compute the radiative opacity of laser-produced plasmas. The model is based on a set of screening charges which allow one to easily calculate atomic properties of isolated ions. These screened charges have been fitted to a fourth-order polynomial depending on the nuclear charge Z for ground and single excited states of ions belonging to the isoelectronic sequences comprised between He-like to U-like. In the opacity model used, ionic populations are obtained by solving the Saha equation including degeneracy corrections. Bound–bound transitions are determined using a Voigt profile for line shape, which includes natural, collisional, Doppler and UTA widths. Bound–free and free–free opacities are evaluated using the Kramer cross-sections with appropriate corrections. Scattering processes are computed through the use of the Thomson formula with corrections. The results are compared with other screened hydrogenic models and more sophisticated self-consistent codes.

A Survey of Proper-Motion Stars. XVI. Orbital Solutions for 171 Single-lined Spectroscopic Binaries

We report 25,563 radial velocity measurements for 1359 single-lined stars in the Carney-Latham sample of 1464 stars selected for high proper motion. For 171 of these, we present spectroscopic orbital solutions. We find no obvious difference between the binary characteristics in the halo and the disk populations. The observed frequency is the same, and the period distributions are consistent with the hypothesis that the two sets of binaries were drawn from the same parent population. This suggests that metallicity in general, and radiative opacities in particular, have little influence over the fragmentation process that leads to short-period binaries. All the binaries with periods shorter than 10 days have nearly circular orbits, while the binaries with periods longer than 20 days exhibit a wide range of eccentricities and a median value of 0.37. For the metal-poor high-velocity halo binaries in our sample, the transition from circular to eccentric orbits appears to occur at about 20 days, supporting the conclusion that tidal circularization on the main sequence is important for the oldest binaries in the Galaxy.

Detailed-term-accounting approximation calculations of the radiative opacity of aluminum plasmas: a systematic study.

The spectrally resolved radiative opacity and the Rosseland and Planck mean opacities are calculated by using the detailed-term-accounting approximation for aluminum plasmas with varieties of density and temperature. The results are presented along a 40 eV isothermal sequence, a 0.01 g/cm(3) isodense sequence, and a sequence with average ionization degree Z* approximately 7.13. Particular attention is given to the influence of the detailed treatment of spectral lines on the Rosseland mean opacity under different thermodynamic conditions. The results show that at densities of 0.004 g/cm(3) and higher, the opacities are not very sensitive to the spectral linewidth within a reasonable range. As examples, the Rosseland mean opacity, which is most sensitive to the detailed linewidth, at 40 eV and 0.004 g/cm(3) changes no more than 15%, when we change the electron impact spectral linewidth artificially by reducing it by 50% or increasing it twice, and at 40 eV and 0.1 g/cm(3) it changes less than 5%. For comparison, we also carried out calculations by using an average atom model. For the Rosseland mean opacities, the two models show quite large differences, in particular at low densities, while for the Planck mean opacities the results of the two models are much closer.

A Pseudo Detailed Term Accounting Method for Radiative Opacityof Laser-Produced Plasmas

A statistical method is introduced to deal with the two-electron energies of aconfiguration in the detailed term accounting (DTA) approximation for thespectral line absorption in the radiative opacity of laser-produced plasmasin local thermodynamics equilibrium. The relativistic Hartree-Fock-Slaterself-consistent field average atom model is used to select the reference ionwith most probability and providing basically atomic data. In this way,fully frequency-dependent and spectral resolved opacity for heavy elementscan be obtained readily for the primary design of radiation transfer in theinertial confinement fusion simulation. Opacities of Au plasmas are given.

Detailed-term-accounting-approximation calculations of the radiative opacity of laser-produced Al plasmas.

An extensive configuration interaction (CI) scheme and the R-matrix method are combined to calculate the radiative opacity for laser-produced aluminum plasma in local thermodynamic equilibrium using the detailed-term-accounting (DTA) approximation. The CI scheme is used to obtain the absorption oscillator strengths of the electric dipole allowed transitions for evaluating the bound-bound absorption cross sections, and the R-matrix method is used to obtain the bound-free absorption (photoionization) cross sections. For an aluminum plasma at a temperature of 20 eV and the density of 0.01 g/cm(3), the Rosseland and Planck mean opacities are calculated to be 4184 and 24891 cm(2)/g, respectively, by integrating the spectrally resolved opacities with Rosseland and Planck weighting functions. The two mean opacities are also obtained by using the average atom model, and they are 22520 and 30402 cm(2)/g, respectively. The optical transmission from the photon energy of 70-250 eV, which was experimentally measured by Winhart et al. [G. Winhart et al. Phys. Rev. E 53, R1332 (1996)], is also calculated. Generally good agreement is found between our DTA and experimental transmission. Our theoretical result reproduces all structures shown in the experiment, whereas some of the structures near the higher energy edge did not show up in some other opacity models. These structures are attributed to the detailed treatment of the photoionization process.

Atmospheres and spectra of strongly magnetized neutron stars

We construct atmosphere models for strongly magnetized neutron stars with surface fields B � 10 12 10 15 G and effective temperatures Teff � 10 6 10 7 K. The atmospheres directly determine the characteristics of thermal emission from isolated neutron stars, including radio pulsars, soft gamma-ray repeaters, and anomalous Xray pulsars. In our models, the atmosphere is composed of pure hydrogen or helium and is assumed to be fully ionized. The radiative opacities include free-free absorption and scattering by both electrons and ions computed for the two photon polarization modes in the magnetized electron-ion plasma. Since the radiation emerges from deep layers in the atmosphere with � > 10 2 g/cm 3 , plasma effects can significantly modify the photon opacities by changing the properties of the polarization modes. In the case where the magnetic field and the surface normal are parallel, we solve the full, angle-dependent, coupled radiative transfer equations for both polarization modes. We also construct atmosphere models for general field orientations based on the diffusion approximation of the transport equations and compare the results with models based on full radiative transport. In general, the emergent thermal radiation exhibits significant deviation from blackbody, with harder spectra at high energies. The spectra also show a broad feature (�E/EBi � 1) around the ion cyclotron resonance EBi = 0.63(Z/A)(B/10 14 G) keV, where Z and A are the atomic charge and atomic mass of the ion, respectively; this feature is particularly pronounced when EBi > 3kTeff. Detection of the resonance feature would provide a direct measurement of the surface magnetic fields on magnetars.

The radiative opacity at the Sun centre—a code comparison study

The results of a recent code comparison study are presented. In the study different codes were used to calculate the opacity of a plasma mixture whose composition, temperature and density were close to those believed to occur at the Sun's centre. The codes showed relatively small variation about an average value that was very close to that given by the most recent calculations of solar opacity which are used widely by the astrophysics community. The average result is also very close to the value calculated previously which had been used widely prior to this. We conclude that the study underwrites current opacity calculations for these extreme conditions.

Plasma non-ideality effects on the photon–electron scattering contribution to radiative opacities

Photon–electron scattering (Compton and Thomson scattering) can be a significant contribution to plasma radiative opacities under conditions where the atoms in the plasma are fully ionized. For high plasma densities and low temperatures, plasma non-ideality effects due to strong coupling and electron degeneracy can lead to a reduction in the amount of photon scattering expected as compared to the ideal Compton scattering case. In the present work, we will examine some of the consequences of these non-idealities and evaluate their effects on the frequency dependent and Rosseland mean opacity. We will examine electron screening and other plasma collective effects as well as electron degeneracy as it effects both initial and final scattering states. We will examine lowest order corrections to existing scattering formulae and discuss the consequences of the next highest order corrections. Comparisons will be made with existing Los Alamos National Laboratory opacity tables for hydrogen for some specific cases where these effects produce significant changes in the Rosseland means.

Observational Tests and Predictive Stellar Evolution

We compare 18 binary systems with precisely determined radii and masses from 23 to 1.1 M☉ and stellar evolution models produced with our newly revised code TYCHO. Overshooting and rotational mixing were suppressed in order to establish a baseline for isolating these and other hydrodynamic effects. Acceptable coeval fits are found for 16 pairs without optimizing for heavy-element or helium abundance. The precision of these tests is limited by the accuracies of the observed effective temperatures. High-dispersion spectra and detailed atmospheric modeling should give more accurate effective temperatures and heavy-element abundances. PV Cas, a peculiar early A system, EK Cep B, a known post-T Tauri star, and RS Cha, a member of a young OB association, are matched by pre-main-sequence models. Predicted mass loss agrees with upper limits from IUE for CW Cep A and B. Relatively poor fits are obtained for binaries having at least one component in the mass range 1.7 < M/M☉ < 2.6, whose evolution is sensitive to mixing. These discrepancies are robust and consistent with additional mixing in real stars. The predicted apsidal motion implies that massive star models are systematically less centrally condensed than the real stars. If these effects are due to overshooting, then the overshooting parameter αOV increases with stellar mass. The apsidal motion constants are controlled by radiative opacity under conditions close to those directly measured in laser experiments, making this test more stringent than possible before.

Detailed Spectral Line Effects on the Radiative Opacity ofLaser-Produced Al Plasmas

The detailed term accounting (DTA) approximation for the line absorptionand the average atom (AA) model for bound-free and free-free absorptionare used to calculate the radiative opacity of high-powerlaser-produced plasmas in local thermodynamic equilibrium (LTE).The spectral resolved opacities of Al plasmas are calculatedat the temperatures and densities of 40 eV and 0.0135 g/cm3,and 58 eV and 0.02 g/cm3. Rosseland and Planck mean opacities areobtained by integrating the spectral resolved opacities withRosseland and Planck weighting functions. The results show thatthe Rosseland mean opacity is very sensitive to the detailed profile.Radiative opacities under the same plasma conditionsare also obtained by using a completely pure AA model. Theresults from the DTA and pure AA models are then compared.

Scaling law of radiative opacities for ICF elements

This work is focused on the determination of Rosseland and Planck mean analytical formulas for several single elements used in ICF targets. A scaling law of these mean opacities is given as a function of the plasma parameters: electron temperature and plasma density. These opacities have been tested with numerical results from other codes and with available experimental results.

Effects of non-equilibrium radiative transfer and opacity on the implosion of capsule filled DT gas

Implosion behavior of plastic capsules filled with DT gas has been investigated numerically using two different approximations for the radiative transfer equation coupled with energy equations of electron and ion. The effects of the non-equilibrium radiative transfer and opacity on the implosion of capsule are given.