We present a re—ned treatment of H, He I, and He II recombination in the early universe. The diUer- ence from previous calculations is that we use multilevel atoms and evolve the population of each level with redshift by including all bound-bound and bound-free transitions. In this framework we follow several hundred atomic energy levels for H, He I, and He II combined. The main improvements of this method over previous recombination calculations are (1) allowing excited atomic level populations to depart from an equilibrium distribution, (2) replacing the total recombination coefficient with recombi- nation to and photoionization from each level calculated directly at each redshift step, and (3) correct treatment of the He I atom, including the triplet and singlet states. We —nd that is approximately 10% smaller at redshifts than in previous calcu- x e (4 n e /n H ) (800 lations, as a result of the nonequilibrium of the excited states of H that is caused by the strong but cool radiation —eld at those redshifts. In addition, we —nd that He I recombination is delayed compared with previous calculations and occurs only just before H recombination. These changes in turn can aUect the predicted power spectrum of microwave anisotropies at the few percent level. Other improvements, such as including molecular and ionic species of H, including complete heating and cooling terms for the evolution of the matter temperature, including collisional rates, and including feedback of the secondary spectral distortions on the radiation —eld, produce negligible change to the ionization fraction. The lower at low z found in this work aUects the abundances of H molecular and ionic species by 10%¨25%. x e However, this diUerence is probably not larger than other uncertainties in the reaction rates. Subject headings: atomic processescosmic microwave backgroundcosmology: theory ¨ early universe

https://iopscience.iop.org/article/10.1086/313388/pdf

How exactly did the universe become neutral

We present a refined treatment of H, He I, and He II recombination in the early Universe. The difference from previous calculations is that we use multi-level atoms and evolve the population of each level with redshift by including all bound-bound and bound-free transitions. In this framework we follow several hundred atomic energy levels for H, He I, and He II combined. The main improvements of this method over previous recombination calculations are: (1) allowing excited atomic level populations to depart from an equilibrium distribution; (2) replacing the total recombination coefficient with recombination to and photoionization from each level directly at each redshift step; and (3) correct treatment of the He I atom, including the triplet and singlet states. We find that the ionization fraction x_e = n_e/n_H is approximately 10% smaller at redshifts <~800 than in previous calculations, due to the non-equilibrium of the excited states of H, which is caused by the strong but cool radiation field at those redshifts. In addition we find that He I recombination is delayed compared with previous calculations, and occurs only just before H recombination. These changes in turn can affect the predicted power spectrum of microwave anisotropies at the few percent level. Other improvements such as including molecular and ionic species of H, including complete heating and cooling terms for the evolution of the matter temperature, including collisional rates, and including feedback of the secondary spectral distortions on the radiation field, produce negligible change to x_e. The lower x_e at low z found in this work affects the abundances of H molecular and ionic species by 10-25%. However this difference is probably not larger than other uncertainties in the reaction rates.

How exactly did the Universe become neutral

Anew air-radiationmodel is presented for the calculation of the radiative flux from lunar-return shock layers. For modeling atomic lines, the data from a variety of theoretical and experimental sources are compiled and reviewed. A line model is chosen that consists of oscillator strengths from the National Institute of Standards and Technology database and theOpacity Project (formany lines not listedby theNational Institute of Standards andTechnology), as well as Stark broadening widths obtained from the average of available values. Uncertainties for the oscillator strengths and Stark broadening widths are conservatively chosen from the reviewed data, and for the oscillator strengths, the chosen uncertainties are found to be larger than those listed in the National Institute of Standards and Technology database. This new atomic line model is compared with previous models for equilibrium constantproperty layers chosen to approximately represent a lunar-return shock layer. It is found that the new model increases the emission resulting from the 1–6-eV spectral range by up to 50%.This increase is due to both the increase in oscillator strengths for some important lines and to the addition of lines from the Opacity Project, which are not commonly treated in shock-layer radiation predictions. Detailed theoretical atomic bound–free cross sections obtained from the Opacity Project’s TOPbase are applied for nitrogen and oxygen. An efficient method of treating these detailed cross sections is presented.The emission fromnegative ions is considered and shown to contribute up to 10% to the total radiative flux. The modeling of the molecular-band systems using the smeared-rotational-band approach is reviewed. The validity of the smeared-rotational-band approach for both emitting and absorbing-band systems is shown through comparisons with the computationally intensive line-by-line approach. The absorbingband systems are shown to reduce the radiative flux by up to 10%, whereas the emitting-band systems are shown to contribute less than a 5% increase in the flux. The combined models chosen for the atomic line, atomic bound–free, negative-ion, and molecular-band components result in a computationally efficient model that is ideal for coupled solutions with a Navier–Stokes flowfield. It is recommended that the notable increases shown, relative to previous models, for the atomic line and negative-ion continuum should be included in future radiation predictions for lunarreturn vehicles.

Spectrum Modeling for Air Shock-Layer Radiation at Lunar-Return Conditions

We have calculated the net emission coefficient of air plasmas at atmospheric pressure in the temperature range between 300 and 40?000?K, in the assumption of local thermodynamic equilibrium and isothermal plasmas. This calculation takes into account the radiation due to the atomic continuum, the molecular continuum, the molecular bands (several systems for O2, N2, NO and N2+) and the atomic lines. Special attention has been devoted in this paper to the description of the molecular bands radiation. The results show that in the high temperature range where molecules are dissociated, the radiation properties of air plasmas are mainly due to those of nitrogen plasmas, the resonance atomic lines playing an important role in spite of their strong self-absorption. At low temperature (T<6000?K) the role of the molecular bands of oxygen (O2) and NO is predominant.

https://iopscience.iop.org/article/10.1088/0022-3727/35/22/306/pdf

Net emission coefficient of air thermal plasmas

Aims. The aim of this work is to present and discuss the observations of the iron peak (Fe, Ni) and neutron-capture element (Y, Zr, Ba, La, Ce, Nd, Sm, and Eu) abundances for 276 FGK dwarfs, located in the Galactic disk with metallicity −1 < [Fe/H] < +0.3. Methods. Atmospheric parameters and chemical composition of the studied stars were determined from an high resolution, high signal-to-noise echelle spectra obtained with the echelle spectrograph ELODIE at the Observatoire de Haute-Provence (France). Effective temperatures were estimated by the line depth ratio method and from the Hα line-wing fitting. Surface gravities (log g )w ere determined by parallaxes and the ionization balance of iron. Abundance determinations were carried out using the LTE approach, taking the hyperfine structure for Eu into account, and the abundance of Ba was computed under the NLTE approximation. Results. We are able to assign most of the stars in our sample to the substructures of the Galaxy thick disk, thin disk, or Hercules stream according to their kinematics. The classification of 27 stars is uncertain. For most of the stars in the sample, the abundances of neutron-capture elements have not been measured earlier. For all of them, we provide the chemical composition and discuss the contribution from different nucleosynthesis processes. Conclusions. The [Ni/Fe] ratio shows a flat value close to the solar one for the whole metallicity range, with a small scatter, pointing to a nearly solar Ni/Fe ratio for the ejecta of both core-collapse SN and SNIa. The increase in the [Ni/Fe] for metallicity higher than solar is confirmed, and it is due to the metallicity dependence of 56 Ni ejecta from SNIa. Under large uncertainty in the age determination of observed stars, we verified that there is a large dispersion in the AMR in the thin disk, and no clear trend as in the thick disk. That may be one of the main reasons for the dispersion, observed for the s-process elements in the thin disk (e.g., Ba and La), whereas much narrower dispersion can be seen for r-process elements (e.g., Eu). Within the current uncertainties, we do not see a clear decreasing trend of [Ba/ Fe] or [La/Fe] with metallicity in the thin disk, except maybe for super-solar metallicities. We cannot confirm an increase in the mentioned ratios with decreasing stellar age.

/pdf/abundances-of-neutron-capture-elements-in-stars-of-the-tyi6ix0twx.pdf

D. Hofsaess

Papers

Photoionization cross sections calculated by the scaled Thomas-Fermi method (hv ≤ 50 eV)☆