scholarly journals Fe I/Fe II ionization equilibrium in cool stars: NLTE versus LTE

2009 ◽  
Vol 5 (S265) ◽  
pp. 197-200 ◽  
Author(s):  
Lyudmila Mashonkina ◽  
Thomas Gehren ◽  
Jianrong Shi ◽  
Andreas Korn ◽  
Frank Grupp
Keyword(s):  
Local Thermodynamic Equilibrium ◽  
The Sun ◽  
Line Formation ◽  
Formation Model ◽  
Hydrogen Atoms ◽  
Cool Stars ◽  
Solar Metallicity ◽  
Non Local ◽  
The Difference ◽  
Model Atom

AbstractNon-local thermodynamic equilibrium (NLTE) line formation for neutral and singly-ionized iron is considered through a range of stellar parameters characteristic of cool stars. A comprehensive model atom for Fe I and Fe II is presented. Our NLTE calculations support the earlier conclusions that the statistical equilibrium (SE) of Fe I shows an underpopulation of Fe I terms. However, the inclusion of the predicted high-excitation levels of Fe I in our model atom leads to a substantial decrease in the departures from LTE. As a test and first application of the Fe I/II model atom, iron abundances are determined for the Sun and four selected stars with well determined stellar parameters and high-quality observed spectra. Within the error bars, lines of Fe I and Fe II give consistent abundances for the Sun and two metal-poor stars when inelastic collisions with hydrogen atoms are taken into account in the SE calculations. For the close-to-solar metallicity stars Procyon and β Vir, the difference (Fe II - Fe I) is about 0.1 dex independent of the line formation model, either NLTE or LTE. We evaluate the influence of departures from LTE on Fe abundance and surface gravity determination for cool stars.

scholarly journals Non-local thermodynamic equilibrium line formation for Si i–ii–iii in A–B stars and the origin of Si ii emission lines in ι Her

2020 ◽  
Vol 493 (4) ◽  
pp. 6095-6108 ◽  
Author(s):  
Lyudmila Mashonkina
Keyword(s):  
Thermodynamic Equilibrium ◽  
Local Thermodynamic Equilibrium ◽  
Emission Lines ◽  
Atomic Data ◽  
Line Formation ◽  
Atmospheric Parameters ◽  
B Stars ◽  
Non Local ◽  
Effective Temperatures ◽  
Model Atom

ABSTRACT A comprehensive model atom was developed for Si i–ii–iii using the most up-to-date atomic data available so far. Based on non-local thermodynamic equilibrium (NLTE) line formation for Si i, Si ii and Si iii and high-resolution observed spectra, we determined the NLTE abundances for a sample of nine unevolved A9–B3 type stars with well-determined atmospheric parameters. For each star, NLTE reduces the line-to-line scatter for Si ii substantially compared with the LTE case and leads to consistent mean abundances from lines of different ionization stages. In the hottest star of our sample, ι Her, Si ii is subject to overionization that drives emission in the lines arising from the high-excitation doublet levels. Our NLTE calculations reproduced 10 emission lines of Si ii observed in ι Her. The same overionization effect leads to greatly weakened Si ii lines, which are observed in absorption in ι Her. Large positive NLTE abundance corrections (up to 0.98 dex for 5055 Å) were useful for achieving consistent mean abundances from lines of the two ionization stages, Si ii and Si iii. It was found that NLTE effects are overestimated for the Si ii 6347, 6371 Å doublet in ι Her, while the new model atom works well for cooler stars. At this stage, we failed to understand this problem. We computed a grid of the NLTE abundance corrections for lines of Si i, Si ii and Si iii in model atmospheres with effective temperatures and surface gravities characteristic of unevolved A–B type stars.

scholarly journals The 3D non-LTE solar nitrogen abundance from atomic lines

2020 ◽  
Vol 636 ◽  
pp. A120 ◽  
Author(s):  
A. M. Amarsi ◽  
N. Grevesse ◽  
J. Grumer ◽  
M. Asplund ◽  
P. S. Barklem ◽  
...  
Keyword(s):  
Local Thermodynamic Equilibrium ◽  
Neutral Hydrogen ◽  
Inelastic Collisions ◽  
Line Formation ◽  
Free Electrons ◽  
Nitrogen Abundance ◽  
Non Local ◽  
Atomic Lines ◽  
Galactic Astronomy ◽  
Model Atom

Nitrogen is an important element in various fields of stellar and Galactic astronomy, and the solar nitrogen abundance is crucial as a yardstick for comparing different objects in the cosmos. In order to obtain a precise and accurate value for this abundance, we carried out N I line formation calculations in a 3D radiative-hydrodynamic STAGGER model solar atmosphere in full 3D non-local thermodynamic equilibrium (non-LTE). We used a model atom that includes physically motivated descriptions for the inelastic collisions of N I with free electrons and with neutral hydrogen. We selected five N I lines of high excitation energy to study in detail, based on their strengths and on their being relatively free of blends. We found that these lines are slightly strengthened from non-LTE photon losses and from 3D granulation effects, resulting in negative abundance corrections of around − 0.01 dex and − 0.04 dex, respectively. Our advocated solar nitrogen abundance is log ɛN = 7.77, with the systematic 1σ uncertainty estimated to be 0.05 dex. This result is consistent with earlier studies after correcting for differences in line selections and equivalent widths.

scholarly journals Cobalt-Hydrogen Atomic and Ionic Collisional Data

Atoms ◽  
2020 ◽  
Vol 8 (3) ◽  
pp. 34
Author(s):  
Svetlana A. Yakovleva ◽  
Andrey K. Belyaev ◽  
Maria Bergemann
Keyword(s):  
Local Thermodynamic Equilibrium ◽  
Ion Pair ◽  
Pair Formation ◽  
Stellar Atmospheres ◽  
Rate Coefficients ◽  
Line Formation ◽  
Non Local ◽  
Excitation Processes ◽  
Inelastic Processes ◽  
Ionized Systems

Rate coefficients for inelastic processes in low-energy Co + H, Co + + H − , Co + + H , and Co 2 + + H − collisions are estimated using the quantum simplified model. Considerations include 44 triplet and 55 quintet molecular states of CoH, as well as 91 molecular states of CoH + . The estimations provide the rate coefficients for the 4862 partial processes (mutual neutralization, ion-pair formation, excitation, and de-excitation) in the neutral CoH system, and for the 8190 partial processes in the ionized CoH + system, 13 , 052 processes in total. At T = 6000 K, the rate coefficients with the largest values around 6 × 10 − 8 cm 3 s − 1 correspond to the mutual neutralization processes into the Co ( e 2 F ) + H and Co + ( g 5 F ) + H final channels in the neutral and ionized systems, respectively. Among the excitation and de-excitation processes in Co + H and in Co + + H collisions, at T = 6000 K, the largest rate coefficients have values around 7 × 10 − 9 cm 3 s − 1 and correspond to the processes Co ( y 2 S ∘ ) + H → Co ( e 2 F ; v 4 D ∘ ) + H and Co + ( h 3 P ) + H → Co + ( g 3 P ; g 5 P ; g 5 F ) + H , respectively. The calculations single out inelastic processes important for non-local thermodynamic equilibrium (NLTE) modelling of Co I and Co II spectra in stellar atmospheres. The test NLTE calculations are carried out, and it is found that the new collision rates have a strong effect on the line formation and NLTE abundance corrections.

Convection and the solar abundances: Does the sun have a sub-solar metallicity?

2006 ◽  
Vol 2 (S239) ◽  
pp. 122-129
Author(s):  
Martin Asplund
Keyword(s):  
Convection Zone ◽  
Model Atmosphere ◽  
Solar Photosphere ◽  
The Sun ◽  
Line Formation ◽  
Solar Convection ◽  
Solar Metallicity ◽  
Solar Abundances ◽  
Hydrodynamical Simulations ◽  
Mixing Length Theory

AbstractIn the Sun, the convection zone reaches up to the solar photosphere and can thus directly influence the emergent spectrum. Traditionally, the effects of convection has been modelled with the local mixing length theory in theoretical 1D hydrostatic model atmospheres. In a different approach, we have performed realistic time-dependent, 3D, radiative-hydrodynamical simulations of the outer layers of the solar convection zone, including the photosphere. Both the different mean stratification and the presence of atmospheric inhomogeneities in 3D impact the spectral line formation. In a series of papers, we have applied our 3D solar model atmosphere to the problem of the solar chemical composition. Furthermore, we have adopted the best possible atomic and molecular line data and taken into account departures from LTE in the line formation when necessary. The inferred C, N, O and Ne abundances are all significantly lower than estimated from previous 1D modelling by 0.2-0.3 dex. These results have significant implications for a range of topics in contemporary astrophysics, including causing a severe headache for helioseismology.

scholarly journals Ca line formation in late-type stellar atmospheres

2019 ◽  
Vol 623 ◽  
pp. A103 ◽  
Author(s):  
Y. Osorio ◽  
K. Lind ◽  
P. S. Barklem ◽  
C. Allende Prieto ◽  
O. Zatsarinny
Keyword(s):  
Local Thermodynamic Equilibrium ◽  
State Of The Art ◽  
Stellar Atmospheres ◽  
Atomic Data ◽  
The Sun ◽  
Stellar Spectra ◽  
Wide Wavelength Range ◽  
Infra Red ◽  
Ionised Calcium ◽  
Model Atom

Context. Departures from local thermodynamic equilibrium (LTE) distort the calcium abundance derived from stellar spectra in various ways, depending on the lines used and the stellar atmospheric parameters. The collection of atomic data adopted in non-LTE (NLTE) calculations must be sufficiently complete and accurate. Aims. We derive NLTE abundances from high-quality observations and reliable stellar parameters using a model atom built afresh for this work, and check the consistency of our results over a wide wavelength range with transitions of atomic and singly ionised calcium. Methods. We built and tested Ca I and Ca II model atoms with state-of-the-art radiative and collisional data, and tested their performance deriving the Ca abundance in three benchmark stars: Procyon, the Sun, and Arcturus. We have excellent-quality observations and accurate stellar parameters for these stars. Two methods to derive the LTE/NLTE abundances were used and compared. The LTE/NLTE centre-to-limb variation (CLV) of Ca lines in the Sun was also investigated. Results. The two methods used give similar results in all three stars. Several discrepancies found in LTE do not appear in our NLTE results; in particular the agreement between abundances in the visual and infra-red (IR) and the Ca I and Ca II ionisation balance is improved overall, although substantial line-to-line scatter remains. The CLV of the calcium lines around 6165 Å can be partially reproduced. We suspect differences between our modelling and CLV results are due to inhomogeneities in the atmosphere that require 3D modelling.

scholarly journals Statistical equilibrium of silicon in the atmospheres of cool stars

2013 ◽  
Vol 9 (S298) ◽  
pp. 437-437
Author(s):  
J. R. Shi ◽  
T. Gehren ◽  
L. Mashonkina ◽  
G. Zhao
Keyword(s):  
Thermodynamic Equilibrium ◽  
Local Thermodynamic Equilibrium ◽  
Statistical Equilibrium ◽  
Cool Stars ◽  
Non Local

AbstractThe statistical equilibrium of neutral and ionized silicon in the atmospheres of cool stars is discussed. Non-local thermodynamic equilibrium effects (NLTE) are investigated. It is found that the NLTE effects for Si are important, in particular for warm metal-poor stars. For warm metal-poor stars, the NLTE abundance correction reaches ~0.2 dex relative to standard LTE calculations.

scholarly journals Influence of Departures from LTE on Oxygen Abundance Determination in the Atmospheres of A – K stars

2013 ◽  
Vol 9 (S298) ◽  
pp. 387-393
Author(s):  
Tatyana Sitnova ◽  
Lyudmila Mashonkina ◽  
Gang Zhao ◽  
Tatiana Ryabchikova ◽  
Yury Pakhomov
Keyword(s):  
Solar Physics ◽  
Neutral Hydrogen ◽  
Lower Surface ◽  
Model Atmosphere ◽  
Visible Spectral Range ◽  
Inelastic Collisions ◽  
Hydrogen Atoms ◽  
Oxygen Abundance ◽  
Cool Stars ◽  
The Difference

AbstractSolar oxygen abundance is a key parameter for the studies of solar physics. Oxygen abundances of cool stars with different metallicities are important for understanding the galactic chemical evolution. We present non-LTE calculations for O I with the classical plane-parallel (1D) model atmospheres for a set of stellar parameters corresponding to stars of spectral types from A to K. Non-LTE leads to strengthening the O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The departures from LTE grow toward higher effective temperature and lower surface gravity. In the entire temperature range and log g = 4, the non-LTE correction does not exceed 0.05 dex in absolute value for lines of O I in the visible spectral range. The non-LTE corrections are significantly larger for the infrared O I 7771-5, 8446 Å lines and reach an order of magnitude for A-type stars. To differentiate the effects of inelastic collisions with electrons and neutral hydrogen atoms on the statistical equilibrium (SE) of O I, we derived the oxygen abundance for the five well studied A-type stars. For each star, non-LTE largely removes the difference between the infrared and visible lines found in LTE. In the case of cool stars (Sun and Procyon), inelastic collisions with H I affect the SE of O I, and agreement between the abundances from different lines is achieved when using the Drawin's formalism for collisional rates calculations. The solar mean oxygen abundance from the six lines is ϵ = 8.74 ± 0.05, when using the MAFAGS-OS solar model atmosphere and ϵ = 8.78 ± 0.03, when applying the 3D corrections taken from the literature. The non-LTE abundances of oxygen are derived for the sample of cool dwarfs with various metallicities on high-resolution spectra observed in the Lick observatory.

scholarly journals Spectroscopic distances to late-type stars

2012 ◽  
Vol 8 (S289) ◽  
pp. 83-86
Author(s):  
Maria Bergemann ◽  
Aldo Serenelli ◽  
Gregory Ruchti
Keyword(s):  
Stellar Evolution ◽  
Large Scale ◽  
Local Thermodynamic Equilibrium ◽  
Systematic Errors ◽  
Spectroscopic Techniques ◽  
The Sun ◽  
Late Type ◽  
Non Local ◽  
Ionization Balance

AbstractA common approach to determining distances to stars without astrometric information is to compare stellar evolution models with parameters obtained from spectroscopic techniques. This method is routinely applied in the context of large-scale stellar surveys out to distances of several kpc. However, systematic errors may arise because of inaccurate spectroscopic parameters. We explore the effects of non-local thermodynamic equilibrium (NLTE) on the determination of surface gravities and metallicities for a large sample of metal-poor stars within approximately 10 kpc of the Sun. Using the improved Teff scale, we then show that stellar parameters estimated based on the widely used method of 1D LTE excitation-ionization balance of Fe results in distances which are systematically in error. For metal-poor giants, [Fe/H] ~ −2 dex, the distances can be overestimated by up to 70%. We compare the results with those from the Radial Velocity Experiment Survey catalogue (rave) for the stars in common, and find similar offsets.

scholarly journals NLTE abundances of C, O, Ca, Ti, and Fe in the reference BAF-type stars

2017 ◽  
Vol 13 (S334) ◽  
pp. 360-361
Author(s):  
Tatyana Sitnova ◽  
Tatyana Ryabchikova ◽  
Sofya Alexeeva ◽  
Lyudmila Mashonkina
Keyword(s):  
Local Thermodynamic Equilibrium ◽  
Signal To Noise Ratio ◽  
Model Atmosphere ◽  
High Signal ◽  
Complete Model ◽  
Near Ir ◽  
Hydrostatic Model ◽  
Non Local ◽  
Model Atom ◽  
Abundance Determination

AbstractWe present accurate methods of abundance determination based on the non-local thermodynamic equilibrium (NLTE) line formation for carbon, oxygen, calcium, titanium, and iron in the atmospheres of BAF-type stars. For C I-II, O I, Ca I-II, and Ti I-II, their comprehensive model atoms were described in our previous papers. A fairly complete model atom of Fe I-II is first applied in this study. We determine the NLTE abundances of the nine BAF-type stars with well-determined atmospheric parameters, using high-resolution and high signal-to-noise ratio spectral observations in the broad wavelength range, from the UV to the IR. For C, Ca, Ti, and Fe, NLTE leads to consistent abundances from the lines of the two ionisation stages. The C I, Ca II, and Fe II emission lines were detected in the near IR spectrum of the late B-type subgiant star HD 160762. They are well reproduced in the classical hydrostatic model atmosphere, when applying our NLTE methods.

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