Na atom excitation in low energy H+Na collisions

Fleck, I.; Großer, J.; Schnecke, A.; Steen, W. M.; Voigt, Hans-Michael

doi:10.1088/0953-4075/24/18/015

Cited by 36 publications

(40 citation statements)

References 8 publications

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“…Laboratory measurements for the Na  resonance line at H beam energies above 10 eV indicate efficiencies as low as S H ≈ 0.01 (Fleck et al 1991). We are convinced that S H = 3 is not an artefact of our modelling, as the same code results in much lower efficiencies for simpler atoms like Mg or Al (Baumüller & Gehren 1996;Zhao et al 1998).…”

Section: Multmentioning

confidence: 54%

Kinetic equilibrium of iron in the atmospheres of cool stars

Korn

Shi

Gehren³

2003

A&A

117

144

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Abstract. Non-LTE line formation calculations of Fe  are performed for a small number of reference stars to investigate and quantify the efficiency of neutral hydrogen collisions. Using the atomic model that was described in previous publications, the final discrimination with respect to hydrogen collisions is based on the condition that the surface gravities as determined by the Fe /Fe  ionization equilibria are in agreement with their astrometric counterparts obtained from H parallaxes.High signal-to-noise, high-resolutionéchelle spectra are analysed to determine individual profile fits and differential abundances of iron lines. Depending on the choice of the hydrogen collision scaling factor S H , we find deviations from LTE in Fe  ranging from 0.00 (S H = ∞) to 0.46 dex (S H = 0 for HD 140283) in the logarithmic abundances while Fe  follows LTE. With the exception of Procyon, for which a mild temperature correction is needed to fulfil the ionization balance, excellent consistency is obtained for the metal-poor reference stars if Balmer profile temperatures are combined with S H = 3. This value is much higher than what is found for simple atoms like Li or Ca, both from laboratory measurements and inference of stellar analyses. The correct choice of collisional damping parameters ("van-der-Waals" constants) is found to be generally more important for these little evolved metal-poor stars than considering departures from LTE. For the Sun the calibrated value for S H leads to average Fe  non-LTE corrections of 0.02 dex and a mean abundance from Fe  lines of log ε(Fe) = 7.49 ± 0.08.We confront the deduced stellar parameters with comparable spectroscopic analyses by other authors which also rely on the iron ionization equilibrium as a gravity indicator. On the basis of the H astrometry our results are shown to be an order of magnitude more precise than published data sets, both in terms of offset and star-to-star scatter.

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Section: Multmentioning

confidence: 54%

Kinetic equilibrium of iron in the atmospheres of cool stars

Korn

Shi

Gehren³

2003

A&A

117

144

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“…Motivated by Steenbock and Holweger's work, Fleck et al (1991) performed an experimental study of Na(3s) + H → Na(3p) + H at low energies (15-1500 eV), though due to experimental difficulties, not down to near the threshold (2.1 eV), which is the relevant regime for the temperatures of interest. The results were compared with theory by Mo & Riera (1992) at energies above 200 eV.…”

Section: Quantum Collision Calculationsmentioning

confidence: 99%

On inelastic hydrogen atom collisions in stellar atmospheres

Barklem

Belyaev²,

Guitou³

et al. 2011

A&A

109

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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.

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“…In some cases, where comparisons with experimental data or theoretical results can be made, the Drawin recipe overestimates the cross-sections by one to six orders of magnitude (e.g. Fleck et al 1991;Barklem et al 2003). Corrections to the Drawin cross-sections are suggested by Lambert (1993) to compensate for these differences.…”

Section: Transition Ratesmentioning

confidence: 99%

Lithium abundances of halo dwarfs based on excitation temperatures

Hosford

Pérez

Collet³

et al. 2010

A&A

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Context. The plateau in the abundance of 7 Li 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 (T eff ) for sixteen metal-poor halo dwarfs were derived using a local thermodynamic equilibrium (LTE) description of the formation of Fe lines. This new T eff 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 T eff '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 T eff scale we derive new Li abundances. We compare the NLTE values of T eff 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 of 7 Li abundances with [Fe/H] and T eff , as well as the NLTE effects on the determination of log g through ionization equilibrium, which imply a collisional scaling factor S H > 1 for collisions between Fe and H atoms.

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Na atom excitation in low energy H+Na collisions

Abstract: Experimental results for the integral cross sxtions forthe emission of Na(3p-3s) and Na(4p-3s) light after H + N a collisions at energies from 15 to 1500eVare reponed. The data are compared with model predictions.

Cited by 36 publications

References 8 publications

Kinetic equilibrium of iron in the atmospheres of cool stars

Kinetic equilibrium of iron in the atmospheres of cool stars

On inelastic hydrogen atom collisions in stellar atmospheres

Lithium abundances of halo dwarfs based on excitation temperatures

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