Opacity Project Data Research Articles

Verification of atomic data for solar oxygen abundance models

ABSTRACT Recently, Bergemann et al. reported a re-analysis of O i lines, 777 and 630 nm, using their new atomic models of O and Ni, and presented solar photospheric oxygen abundance differing from others. We discuss the accuracy of atomic data employed in their models, and demonstrate that the photoionization cross-sections (σPI) of the two levels ${\rm1s^22s^22p^33s (^5S^o_2)}$ and 1s22s22p33p(5P1), which they present as illustrations of their data in the non-local-thermodynamic-equilibrium (NLTE) model, are incorrect by large factors. For example, their σPI at the threshold of level ${\rm ^5S^o_2}$ is larger by a factor of 1000 than that of Nahar and the Opacity Project data base, TOPbase, and has an incorrect energy behaviour. They included a blending of the 630 nm line with N i for which they computed σPI in the hydrogenic approximation. The approximation does not include electron–electron interaction and hence is not valid for Ni i. They noted that the use of oscillator strengths of O i by Hibbert et al., which agree very well with TOPbase and Nahar, gave an abundance close to the existing values, but reported using averaged values of two sources. Therefore, their oxygen abundance from the new atomic models is likely to be in question.

New nickel opacities and their impact on stellar models

Context. The chemical element nickel is of particular interest in stellar physics. In the layers in which the Fe-peak elements dominate the mean opacity (the so-called Z-bump), Ni is the second contributor to the Rosseland opacity after iron, according to the Opacity Project data. Reliable nickel cross sections are therefore mandatory for building realistic stellar models, especially for main-sequence pulsators such as β Cep and slowly pulsating B stars, whose oscillations are triggered by the κ-mechanism of the Fe-peak elements. Unfortunately, the Opacity Project data for Ni were extrapolated from those of Fe, and previous studies have shown that they were underestimated in comparison to detailed calculations. Aims. We investigate the impact of newly computed monochromatic cross sections on the Rosseland mean opacity of Ni and on the structure of main-sequence massive pulsators. We compare our results with the widely used Opacity Project and OPAL data. Methods. Monochromatic cross sections for Ni were obtained with the SCO-RCG code. The Toulouse-Geneva evolution code was used to build the stellar models. Results. With the new data, the Rosseland opacities of Ni are roughly the same as those of the Opacity Project or OPAL at high temperatures (log T > 6). At lower temperatures, significant departures are observed; the ratios are up to six times higher with SCO-RCG. These discrepancies span a wider temperature range in the comparison with OPAL than in comparison with the Opacity Project. For massive star models, the results of the comparison with a structure computed with Opacity Project data show that the Rosseland mean of the global stellar mixture is only marginally altered in the Z-bump. The maximum opacity is shifted towards slightly more superficial layers. A new maximum appears in the temperature derivative of the mean opacity, and the driving of the pulsations should be affected.

IPOPv2 online service for the generation of opacity tables

In the framework of the present phase – IPOPv2 – of the international Opacity Project (OP), a new web service has been implemented based on the latest release of the OP opacities. The user may construct online opacity tables to be conveniently included in stellar evolution codes in the format most commonly adopted by stellar physicists, namely the OPAL format. This facility encourages the use and comparison of both the OPAL and OP data sets in applications. The present service allows for the calculation of multi-element mixtures containing the 17 species (H, He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni) considered by the OP, and underpins the latest release of OP opacities. This new service provides tables of Rosseland mean opacites using OP atomic data. We provide an alternative to the OPAL opacity services allowing direct comparison as well as study of the effect of uncertainties in stellar modeling due to mean opacities.

Radiative Levitation in Massive Stars: A self-consistent approach

AbstractIn B stars, the transportation of metals such as iron due to radiative levitation is thought to trigger sub-surface convection, which may lead to significant stellar pulsations through the iron-induced kappa mechanism (Pamyatnykh et al. 2004). The main goal of this work is to model the evolution of these stars by treating radiative levitation in an accurate and efficient manner in order to enable rapid asteroseismic analysis using the Opacity Project data and codes (Seaton 2005) with the GARching STellar Evolution Code (Weiss & Schlattl 2008).

Testing the opacity and equation of state of LTE and non-LTE model atmospheres with OPAL and OP data for early-type stars

Context. Complex investigations of stars are made by studying their atmospheres, evolutionary states and oscillation properties, but the opacities and equation of state are still uncertain. Aims. Compatibility of model atmospheres with internal structure of early-type stars are investigated at photospheric and subphotospheric regions. Methods. The problem was studied quantitatively by means of diagrams that involve the Rosseland-mean opacities, temperatures, densities, and gradients of radiation pressure as functions of depth using stellar atmosphere and envelope models. Results. Two new grids of radiative-equilibrium models of atmospheres were calculated assuming the local thermodynamic equilibrium (LTE). The first one is based on Kurucz’s ATLAS12 computer code implemented with the occupation probability formalism, which accounts for the destruction of loosely bound states of atoms by interactions with particles in the plasma. The second grid of models is based on monochromatic opacities and equations of state taken from the well-known international opacity project (OP). Non-LTE model atmospheres are also calculated and some aspects of the modeling are pointed out to improve agreement with ATLAS12, OP data, as well as with results of the Rogers & Iglesias OPAL opacity computing code (OPAL). The model atmospheres are also discussed by analyzing emerging fluxes of radiation. Conclusions. Commonly used LTE and non-LTE models of stellar atmospheres of early-type stars differ markedly from each other and do not fit OPAL or OP stellar envelope models at great optical depths. The results of the OP project distributed as the OPCD v 3.3 base are useful for calculating not only stellar envelopes but also fully line-blanketed LTE model atmospheres. These models have diagnostic values for studying the atomic physics used for modeling of photospheric and sub-photospheric regions of stars. The OPAL and OP opacities are markedly underestimated in comparison with the Rosseland-mean opacities taken from the Castelli & Kurucz (2003, IAU Symp., 210, 10) atmosphere models in which a new opacity bump appears at lgT ≈ 5.06. This additional opacity bump affects the OPAL- and OP-driving zone for stellar pulsations and therefore the new envelope models may markedly change predicted spectra of unstable oscillation modes. Differences between current non-LTE and LTE models based on the TLUSTY200 and ATLAS codes, respectively, cannot be assigned entirely to non-LTE effects, but for B stars they reflect mainly opacity effects.

Opacity Project data on CD for mean opacities and radiative accelerations

Opacity Project data on CD for mean opacities and radiative accelerations

Opacity Project data on CD for mean opacities and radiative accelerations

All monochromatic opacity data from the Opacity Project (OP), together with all codes required for the calculation of mean opacities and radiative accelerations for any required chemical mixture, temperature and mass density, are being put on a 700-MB CD which will be made generally available. The present letter gives a concise summary of the contents of the CD. More complete documentation will be provided on the CD itself.

Atomic data from the Iron Project

We present new calculations for transition probabilities of Fe IV, with much more extensive datasets than heretofore available, for a variety of astrophysical applications. The large-scale close coupling R-matrix calculations yield 1,798 LS bound states with and , and corresponding 138 121 dipole allowed transitions in the form of oscillator strengths f, line strengths S, and A-coeffficients. This represents the largest R-matrix dataset in LS coupling for any ion under either the Opacity Project or the Iron Project. Through algebraic transformation of the LS multiplets, a total of 712 120 dipole allowed fine structure transitions for Fe IV are obtained. Observed transition energies, whenever available, are used together with the energy independent line strengths to derive the f- and the A-values for improved accuray; the adopted algorithm used calculated energies for the remainder. Present results show significantly better accuracy for the important low-lying states than previous calculations. Monochromatic and mean opacities for Fe IV are computed and compared with those obtained using the Opacity Project data. We find differences which could have important consequences for several astrophysical applications involving low ionization stages of iron.

Comparison of Radiative Accelerations Obtained with Atomic Data from OP and OPAL

Microscopic diffusion processes (such as radiative levitation, gravitational settling, and thermal diffusion) in the outer layers of stars are important because they may give rise to surface abundance anomalies. Here we compare radiative accelerations (grad) derived from the new Opacity Project (OP) data with those computed from OPAL and some previous data from OP. For the case in which we have full data from OPAL (carbon, five points in the ρ-T plane), the differences in the Rosseland mean opacities between OPAL and the new OP data are within 12% and are less than 30% between new OP and previous OP data (OP1). The radiative accelerations grad differ at up to the 17% level when compared to OPAL and up to the 38% level when compared to OP1. The comparison with OP1 on a larger ρ-T space gives a difference of up to 40% for grad(C) and increases for heavier elements, reaching 60% for Si and 65% for S and Fe. We also constructed four representative stellar models in order to compare the new OP accelerations with prior published results that used OPAL data. The Rosseland means overall agree better than 10% for all our cases. For the accelerations, the comparisons with published values yield larger differences in general. The published OPAL accelerations for carbon are even larger relative to OP compared to those that our direct comparisons indicate. Potential reasons for this puzzling behavior are discussed. In light of the significant differences in the inferred acceleration rates, theoretical errors should be taken into account when comparing models with observations. The implications for stellar evolution are briefly discussed. The sensitivity of grad to the atomic physics may provide a useful test of different opacity sources.

Resolution and accuracy of resonances in R-matrix cross sections

We investigate the effect of resonances in photoionization and recombination cross sections computed using the R-matrix method. Photoionization and recombination rates derived from high-resolution cross sections for oxygen ions are compared with earlier works with less resolution and accuracy, such as in the widely used Opacity Project data. We find significant differences in photoionization rates for O II metastable states, averaged over Planck functions corresponding to ionizing radiation fields, with respect to the intrinsic accuracy of the calculations and improved resolution. Furthermore, for highly charged ions other physical effects are also important. Recombination rate coefficients, averaged over a Maxwellian distribution, are extremely sensitive to the position and resolution of near-threshold resonances and radiation damping, in (e + O VII) ⟷ O VI + hν. Surprisingly, however, the effect on the monochromatic and the mean Rosseland and Planck bound-free opacities is relatively small, but may be potentially significant.

L Shell Photoionisation of N2+ and O3+ Isoelectronic Ions

We present absolute measurements for photoionisation of multiply-charged ions N2+ and O3+ in the ground and metastable states. Multiconfiguration Dirac Fock calculations were performed for N2+ and are tested here by comparison with experimental results. Our experimental and theoretical results are also compared to the theoretical data available in the Opacity Project data base.

Transitions in Na III

The work of Blackford and Hibbert(1994 At. Data Nucl. Data Tables 58 101-64)has been extended to provide oscillator strengths and transition probabilitiesfor J→J' transitions amongst n = 2 and n = 3 levels of Na III. Configuration interaction wavefunctions,obtained with the code CIV3 in the Breit-Pauli approximation,and with additional correlation effects, have been constructed.Further semi-empirical corrections to the wavefunctions (`fine-tuning') are made so as to bring the calculated energydifferences into agreement with the corresponding experimental values.It is demonstrated that in LS coupling there is excellent agreement between lengthand velocity forms of the oscillator strengths, and for most multiplet transitions the length form agrees wellboth with the earlier calculations of Blackfordand Hibbert and with the results of the Opacity Project. The recent claims (Kelleher et al 1999 Contributed papers, Sixth Int. Coll. on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas pp 83-86)of major discrepanciesbetween the CIV3 calculations and the Opacity Project (OP) data are shown to be due largely to the use of simple algebraictransformations of the LS-coupled OP data to produce line oscillator strengths in making the comparison. However, it is shown thatfor certain transitions, fine-tuning makes a substantial difference to the calculated oscillator strengths, so that the presentwork is a significant improvement over the previous calculations.

Photoionization cross sections and oscillator strengths for oxygen ions: O I-O VII

Ab initio close-coupling calculations are reported using the R-matrix method for photoionization cross sections $({\ensuremath{\sigma}}_{\mathrm{PI}}),$ oscillator strengths (f values), and energy levels (E) for oxygen ions: O I, O II, O III, O IV, O V, O VI, and O VII. Total, partial, and state-specific photoionization cross sections are computed and investigated in detail. Important features are found that should affect overall photoionization and recombination, especially at high temperatures and energies. Although these ions have been previously studied individually, present work aims at studying them as an isonuclear sequence and obtaining more accurate and complete results for applications. More extended eigenfunction expansions than in previous works, including $n=3$ states, show correlation effects primarily manifested as additional groups of resonances in photoionization of oxygen ions such as O VI and O VII that are of importance in extreme ultraviolet and x-ray laboratory and astrophysical plasmas. Lifetimes of excited bound states are obtained from the present oscillator strengths and compared with experiments. The computed radiative atomic parameters, energy levels, oscillator strengths, and photoionization cross sections constitute a larger dataset than the Opacity Project data [M. J. Seaton, J. Phys. B 20, 6363 (1987)], with which comparisons are made.

Consistent Solar Evolution Model Including Diffusion and Radiative Acceleration Effects

The solar evolution has been calculated including all the effects of the diffusion of helium and heavy elements. Monochromatic opacities are used to calculate radiative accelerations and Rosseland opacities at each evolution time step, taking into account the local abundance changes of all important (21) chemical elements. The OPAL monochromatic data are used for the opacities and the radiative accelerations. The Opacity Project data are needed to calculate how chemical species and electrons share the momentum absorbed from the radiation flux. A detailed evaluation of the impact of atomic diffusion on solar models is presented. On some elements thermal diffusion adds approximately 50% to the gravitational settling velocity. While gravitational settling had been included in previous solar models, this is the first time that the impact of radiative accelerations is considered. Radiative accelerations can be up to 40% of gravity below the solar convection zone and thus affect chemical element diffusion significantly, contrary to current belief. Up to the solar age, the abundances of most metals change by 7.5% if complete ionization is assumed, but by 8.5%-10% if detailed ionization of each species is taken into account. If radiative accelerations are included, intermediate values are obtained. Diffusion leads to a change of up to 8% in the Rosseland opacities, compared to those of the original mixture. Most of this effect can be taken into account by using tables with several values of Z. If one isolates the effects of radiative accelerations, the abundance changes they cause alter the Rosseland opacity by up to 0.5%; the density is affected by up to 0.2%; the sound speed is affected by at most 0.06%. The inclusion of radiative accelerations leads to a reduction of 3% of neutrino fluxes measured with 37Cl detectors and 1% measured with 71Ga detectors. The partial transformation of C and O into N by nuclear reactions in the core causes a ~1% change in the opacities that cannot be modeled by a change in Z alone. The evolution is allowed to proceed to 1010 yr in order to determine the impact at the end of the main-sequence life of solar-type stars. It is found that immediately below the convection zone, the radiative acceleration on some iron peak elements is within a few percent of gravity. The abundance anomalies reach 18% for He in the convection zone but are kept within 12% and 15% for most because of grad. They would have reached 18% in the absence of grad.

Radiative accelerations on iron using Opacity Project data

view Abstract Citations (37) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiative Accelerations on Iron Using Opacity Project Data Alecian, G. ; Michaud, G. ; Tully, J. Abstract Radiative accelerations have been calculated in detail for iron in main-sequence models with T(eff) = 6700, 8000, and 10,000 K and log g = 4.0 and 4.3 and a number of Fe abundances. Atomic data for Fe IX to Fe XVII were taken from the Opacity Project, and this allowed us to calculate the accelerations over an important fraction of the envelope and to calibrate formulae that were obtained earlier using data from the first few states of ionization. We have derived approximate formulas that allow our results to be used in stellar evolution calculations. From a comparison of radiative acceleration and gravity, one may conclude that only small Fe abundance anomalies are to be expected in those stars with T(eff) = 6700 K where atomic diffusion is important. For stars with lower effective temperatures, only small underabundances are to be expected. However, in hotter stars, large Fe overabundances are expected in part of the envelope. These would appear at the same time as He underabundances. Iron could then completely dominate the opacity over part of the envelope. Only detailed evolutionary calculations, with the simultaneous inclusion of Fe and He diffusion, will determine where the Fe overabundances appear and how large they are. Publication: The Astrophysical Journal Pub Date: July 1993 DOI: 10.1086/172892 Bibcode: 1993ApJ...411..882A Keywords: Acceleration (Physics); Iron; Main Sequence Stars; Opacity; Radiation Effects; Stellar Evolution; Heavy Elements; Stellar Interiors; Stellar Structure; Stellar Temperature; Astrophysics; DIFFUSION; ATOMIC DATA; STARS: ABUNDANCES; STARS: INTERIORS full text sources ADS |