On inelastic hydrogen atom collisions in stellar atmospheres

The influence of inelastic hydrogen atom collisions on non-LTE spectral line formation has been, and remains to be, a significant source of uncertainty for stellar abundance analyses, due to the difficulty in obtaining accurate data for low-energy atomic collisions either experimentally or theoretically. For lack of a better alternative, the classical "Drawin formula" is often used. Over recent decades, our understanding of these collisions has improved markedly, predominantly through a number of detailed quantum mechanical calculations. In this paper, the Drawin formula is compared with the quantum mechanical calculations both in terms of the underlying physics and the resulting rate coefficients. It is shown that the Drawin formula does not contain the essential physics behind direct excitation by H atom collisions, the important physical mechanism being quantum mechanical in character. Quantitatively, the Drawin formula compares poorly with the results of the available quantum mechanical calculations, usually significantly overestimating the collision rates by amounts that vary markedly between transitions.Comment: 9 pages, 6 figures, accepted for A&

Manufacture of silicide coatings for the protection of niobium alloys against high temperature oxidation

At the moment, silicide coatings provide the best protection against high temperature oxidation for niobium alloys. These are envisaged for replacing nickel base alloys currently used in the hot section of turbo-engines. Silicides confer higher protectiveness to Nb base system when compared to the environmental resistance presented by Nb alloys coated with aluminides. One major advantage of silicides is probably the great number of possibilities for modifying their composition as well as their crystallographic structure. Thus, many elemental substitutions were performed over the past 20 years in order to optimize their performances in terms of oxidation resistance. The works performed at the University of Nancy focused currently on the M3M\u273CrSi6 phase which has Nb3Fe3CrSi6 as prototype [i]. Initially, the coatings were developed for the protection of niobium alloys, strengthened by solid solution and in which the niobium content was rather high, in the range of 95 weight %. The first stage of the works was devoted to thermodynamic studies leading to the determination of phase equilibria in the Nb-Fe-Cr-Si system. Then, the obtained results were used to determine the compositions of masteralloys and the conditions of the pack-cementation processs (temperature, gas atmosphere and time) for depositing the Nb3Fe3CrSi6 phase as superficial layer, in avoiding the growth of brittle silicides as well as silicide sensitive to pest phenomeno

Analysis of a plasma test cell including non-neutrality and complex collision mechanisms

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97097/1/AIAA2012-3736.pd

Non-LTE calculations for neutral Na in late-type stars using improved atomic data

Neutral sodium is a minority species in the atmospheres of late-type stars, and line formation in local thermodynamic equilibrium (LTE) is often a poor assumption, in particular for strong lines. We present an extensive grid of non-LTE calculations for several NaI lines in cool stellar atmospheres, including metal-rich and metal-poor dwarfs and giants. For the first time, we constructed a Na model atom that incorporates accurate quantum mechanical calculations for collisional excitation and ionisation by electrons as well as collisional excitation and charge exchange reactions with neutral hydrogen. Similar to LiI, the new rates for hydrogen impact excitation do not affect the statistical equilibrium calculations, while charge exchange reactions have a small but non-negligible influence. The presented LTE and non-LTE curves-of-growth can be interpolated to obtain non-LTE abundances and abundance corrections for arbitrary stellar parameter combinations and line strengths. The typical corrections for weak lines are -0.1...-0.2dex, whereas saturated lines may overestimate the abundance in LTE by more than 0.5dex. The non-LTE Na abundances appear very robust with respect to uncertainties in the input collisional data.Comment: 9 pages, 8 figures, accepted for publication in A&

Lithium abundances of halo dwarfs based on excitation temperatures. II : Non-local thermodynamic equilibrium

Original article can be found at: http://www.aanda.org/ Copyright The European Southern Observatory (ESO)Context. The plateau in the abundance of 7Li in metal-poor stars was initially interpreted as an observational indicator of the primordial lithium abundance. However, this observational value is in disagreement with that deduced from calculations of Big Bang nucleosynthesis (BBN), when using the Wilkinson microwave anisotropy probe (WMAP) baryon density measurements. One of the most important factors in determining the stellar lithium abundance is the effective temperature. In a previous study by the authors, new effective temperatures (Teff) for sixteen metal-poor halo dwarfs were derived using a local thermodynamic equilibrium (LTE) description of the formation of Fe lines. This new Teff scale reinforced the discrepancy. Aims. For six of the stars from our previous study we calculate revised temperatures using a non-local thermodynamic equilibrium (NLTE) approach. These are then used to derive a new mean primordial lithium abundance in an attempt to solve the lithium discrepancy. Methods. Using the code MULTI we calculate NLTE corrections to the LTE abundances for the Fe i lines measured in the six stars, and determine new Teff's. We keep other physical parameters, i.e. log g, [Fe/H] and ξ, constant at the values calculated in Paper I. With the revised Teff scale we derive new Li abundances. We compare the NLTE values of Teff with the photometric temperatures of Ryan et al. (1999, ApJ, 523, 654), the infrared flux method (IRFM) temperatures of Meléndez & Ramírez (2004, ApJ, 615, L33), and the Balmer line wing temperatures of Asplund et al. (2006, ApJ, 644, 229). Results. We find that our temperatures are hotter than both the Ryan et al. and Asplund et al. temperatures by typically ~110–160 K, but are still cooler than the temperatures of Meléndez & Ramírez by typically ~190 K. The temperatures imply a primordial Li abundance of 2.19 dex or 2.21 dex, depending on the magnitude of collisions with hydrogen in the calculations, still well below the value of 2.72 dex inferred from WMAP + BBN. We discuss the effects of collisions on trends of7Li abundances with [Fe/H] and Teff, as well as the NLTE effects on the determination of log g through ionization equilibrium, which imply a collisional scaling factor SH > 1 for collisions between Fe and H atoms. [please see original online abstract for correct notation]Peer reviewe

Lithium abundance in the globular cluster M4: from the Turn-Off to the RGB Bump

We present Li and Fe abundances for 87 stars in the GC M4,obtained with GIRAFFE high-resolution spectra. The targets range from the TO up to the RGB Bump. The Li abundance in the TO stars is uniform, with an average value A(Li)=2.30+-0.02 dex,consistent with the upper envelope of Li content measured in other GCs and in the Halo stars,confirming also for M4 the discrepancy with the primordial Li abundance predicted by WMAP+BBNS. The iron content of M4 is [Fe/H]=-1.10+-0.01 dex, with no systematic offsets between dwarf and giant stars.The behaviour of the Li and Fe abundance along the entire evolutionary path is incompatible with models with atomic diffusion, pointing out that an additional turbulent mixing below the convective region needs to be taken into account,able to inhibit the atomic diffusion.The measured A(Li) and its homogeneity in the TO stars allow to put strong constraints on the shape of the Li profile inside the M4 TO stars. The global behaviour of A(Li) with T_{eff} can be reproduced with different pristine Li abundances, depending on the kind of adopted turbulent mixing.One cannot reproduce the global trend starting from the WMAP+BBNS A(Li) and adopting the turbulent mixing described by Richard et al.(2005) with the same efficiency used by Korn et al.(2006) to explain the Li content in NGC6397. Such a solution is not able to well reproduce simultaneously the Li abundance observed in TO and RGB stars.Otherwise, theWMAP+BBNS A(Li) can be reproduced assuming a more efficient turbulent mixing able to reach deeper stellar regions where the Li is burned. The cosmological Li discrepancy cannot be easily solved with the present,poor understanding of the turbulence in the stellar interiors and a future effort to well understand the true nature of this non-canonical process is needed.Comment: Accepted for publication in the MNRA

Fundamental Parameters and Abundances of Metal-Poor Stars: The SDSS Standard BD +17 4708

The atmospheric parameters and iron abundance of the Sloan Digital Sky Survey (SDSS) spectrophotometric standard star BD +17 4708 are critically examined using up-to-date Kurucz model atmospheres, LTE line formation calculations, and reliable atomic data. We find Teff = 6141+-50 K, log g = 3.87+-0.08, and [Fe/H]=-1.74+-0.09. The line-of-sight interstellar reddening, bolometric flux, limb-darkened angular diameter, stellar mass, and the abundances of Mg, Si, and Ca are also obtained. This star is a unique example of a moderately metal-poor star for which the effective temperature can be accurately constrained from the observed spectral energy distribution (corrected for reddening). Such analysis leads to a value that is higher than most spectroscopic results previously reported in the literature (~5950 K). We find that the ionization balance of Fe lines is satisfied only if a low Teff (~5950 K) is adopted. With our preferred Teff (6141 K), the mean iron abundance we obtain from the FeII lines is lower by about 0.15 dex than that from the FeI lines, and therefore, the discrepancy between the mean iron abundance from FeI and FeII lines cannot be explained by overionization by UV photons as the main non-LTE effect. We also comment on non-LTE effects and the importance of inelastic collisions with neutral H atoms in the determination of oxygen abundances in metal-poor stars from the 777 nm OI triplet. (Abridged)Comment: A&A in pres

Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity

We present a detailed study of the non-Local Thermodynamic Equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in MARCS model atmospheres representative of late-type stars with spectral types F to K. We carried out the calculations using the statistical equilibrium code MULTI, including estimates of the impact on elemental abundance analysis. The atomic model employed includes, in particular, recent quantum-mechanical electron collision data. We confirm that the O I triplet lines form under non-LTE conditions in late-type stars, suffering negative abundance corrections with respect to LTE. At low metallicity, large line opacity stems from triplet-quintet intersystem electron collisions, a form of coupling previously not considered or seriously underestimated. The non-LTE effects become generally severe for models (both giants and dwarfs) with higher T_eff. Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance corrections tend to rapidly become more severe towards lower metallicity. When neglecting H collisions, they amount to as much as ~ 0.9 dex and ~ 1.2 dex, respectively at [Fe/H]=-3 and [Fe/H]=-3.5. Even when such collisions are included, the LTE abundance remains a serious overestimate, correspondingly by ~ 0.5 dex and ~ 0.9 dex at such low metallicities. Although the poorly known inelastic hydrogen collisions thus remain an important uncertainty, the large metallicity-dependent non-LTE effects seem to point to a resulting "low" (compared to LTE) [O/Fe] in metal-poor halo stars.Comment: 19 pages, 10 figures, aa.cls v6.1 included. Accepted for publication in A&

Optical emission spectroscopy of electron-cyclotron-resonance-heated helium mirror plasmas

In this experiment emission spectroscopy in the 3000–5000 Å range has been utilized to determine the electron temperature (15–60 eV) and ion density (2–5 x 10 11 cm −3 ) of helium plasmas produced by the Michigan mirror machine (1) (MIMI). The plasma is generated and heated by whistler-mode electron-cyclotron resonance (ECR) waves at 7.43 GHz with 400–900 W power in 80-ms-long pulses. Gas fueling is provided at the midplane region by a leak valve with a range in pressure of 3 x 10 to 2 x 10 4 Torr. Emission line intensities are interpreted using a model of the important collisional and radiative processes occurring in the plasma. The model examines secondary processes such as radiation trapping, excitation transfer between levels of the carne principle quantum number, and excitation front metastable states for plasmas in the parameter range of MIMI ( n c = 1−6 x 10 11 cm −3 ). Front the analysis of line intensity ratios for neutral helium, the electron temperature is measured and its dependence upon the gas pressure and microwave power is determined. These temperatures agree with those obtained by Langmuir probe measurements. Art analysis of the line intensity ratio between singly ionized helium and neutral helium yields a measurement of the ion density which is in good agreement with electron density measurements made by a microwave interferometer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45473/1/11090_2005_Article_BF01447032.pd