non-LTE Abundance Corrections Research Articles

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.

The light elements in the light of 3D and non-LTE effects

AbstractIn this review we discuss possible systematic errors inherent in classical 1D LTE abundance analyses of late-type stars for the light elements (here: H, He, Li, Be and B). The advent of realistic 3D hydrodynamical model atmospheres and the availability of non-LTE line formation codes place the stellar analyses on a much firmer footing and indeed drastically modify the astrophysical interpretations in many cases, especially at low metallicities. For the Teff-sensitive hydrogen lines both stellar granulation and non-LTE are likely important but the combination of the two has not yet been fully explored. A fortuitous near-cancellation of significant but opposite 3D and non-LTE effects leaves the derived 7Li abundances largely unaffected but new atomic collisional data should be taken into account. We also discuss the impact on 3D non-LTE line formation on the estimated lithium isotopic abundances in halo stars in light of recent claims that convective line asymmetries can mimic the presence of 6Li. While Be only have relatively minor non-LTE abundance corrections, B is sensitive even if the latest calculations imply smaller non-LTE effects than previously thought.

Departures from LTE for neutral Li in late-type stars

We perform non-LTE calculations of lithium in late-type stars for a wide range of stellar parameters, including quantum mechanical cross-sections for collisions with neutral hydrogen and the negative hydrogen ion. Non-LTE abundance corrections for the lithium resonance line at 670.7nm and the subordinate line at 610.3nm, are calculated using 1D MARCS model atmospheres spanning a grid Teff=[4000,8000]K, log(g)=[1.0,5.0], and [Fe/H]=[0.0,-3.0], for lithium abundances in the range A(Li)=[-0.3,4.2]. The competing effects of ultraviolet over-ionization and photon losses in the resonance line govern the behaviour of the non-LTE effects with stellar parameters and lithium abundance. The size and sign of the non-LTE abundance corrections vary significantly over the grid for the 670.7nm line, but are typically positive and below 0.15dex for the 610.3nm, line. The new collisional data play a significant role in determining the abundance corrections.

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.

The non-LTE line formation of neutral carbon in late-type stars

Aims. We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line formation of neutral carbon in late-type stars in order to remove some of the potential systematic errors in stellar abundance analyses employing C i features. Methods. The statistical equilibrium code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric models. Results. Within the parameter space explored, the high-excitation C i lines studied are stronger in non-LTE due to the combined effect of line source function drop and increased line opacity due to overpopulation of the lower level for the transitions considered; the relative importance of the two effects depends on the particular combination of Teff ,l ogg ,[ Fe/H] and [C/Fe] and on the analysed C i line. As a consequence, the non-LTE abundance corrections are negative and can be substantially so, for example ∼−0.4 dex in halo turn-off stars at [Fe/H] ∼− 3. The magnitude of the non-LTE corrections is rather insensitive to whether inelastic H collisions are included or not. Conclusions. Our results have implications on studies of nucleosynthetic processes and on Galactic chemical evolution models. When applying our calculated corrections to recent observational data, the upturn in [C/O] at low metallicity might still be present (thus apparently still necessitating contributions from massive Pop. III stars for the carbon production), but at a lower level and possibly with a rather shallow trend of ∼−0. 2d ex/dex below [O/H] ∼− 1.

Effects of line-blocking on the non-LTE Fe I spectral line formation

The effects of background line opacity (line-blocking) in statistical equilibrium calculations for Fe in late-type stellar atmospheres have been investigated using an extensive and up-to-date model atom with radiative data primarily from the IRON Project. The background metal line opacities have been computed using data from the MARCS stellar model atmospheres. While accounting for this line opacity is important at solar metallicity, the differences between calculations including and excluding line-blocking at low metallicity are insignificant for the non-local thermodynamic equilibrium (non-LTE) abundance corrections for Fe I lines. The line-blocking has no impact on the non-LTE effects of Fe II lines. The dominant uncertainty in Fe non-LTE calculations for metal-poor stars is still the treatment of the inelastic H I collisions, which have here been included using scaling factors to the classical Drawin formalism, and whether or not thermalisation of the high Fe I levels to Fe II ground state should be enforced. Without such thermalisation, the Fe I non-LTE abundance corrections are substantial in metal-poor stars: about 0.3 dex with efficient (i.e. Drawin-like) H I collisions and about 0.5 dex without. Without both thermalisation and H I collisions, even Fe II lines show significant non-LTE effects in such stars.

CI non–LTE spectral line formation in late-type stars

We present non-Local Thermodynamic Equilibrium (non-LTE) calculations for neutral carbon spectral line formation, carried out for a grid of model atmospheres covering the range of late-type stars. The results of our detailed calculations suggest that the carbon non-LTE corrections in these stars are higher than usually adopted, remaining substantial even at low metallicity. For the most metal-poor stars in the sample of Akerman et al. (2004), the non-LTE abundance corrections are of the order of -0.35...-0.45 dex (when neglecting H collisions). Applying our results to those observations, the apparent [C/O] upturn seen in their LTE analysis is no longer present, thus revealing no need to invoke contributions from Pop. III stars to the carbon nucleosynthesis.

The Impact of Non‐LTE Effects and Granulation Inhomogeneities on the Derived Iron and Oxygen Abundances in Metal‐Poor Halo Stars

This paper presents the results of a detailed theoretical investigation of the impact of non-LTE (NLTE) effects and of granulation inhomogeneities on the derived iron and oxygen abundances in the metal-poor halo subgiant HD 140283. Our analysis is based on both the classical one-dimensional stellar atmosphere models and on the new generation of three-dimensional hydrodynamic models. The NLTE calculations presented here have been carried out without inelastic collisions with neutral hydrogen atoms. We find that if NLTE effects are taken into account when synthesizing the Fe I spectrum in both types of atmospheric models, then the derived iron abundance turns out to be very similar in both cases. The emergent spectral line profiles in both models are very much weaker in NLTE than in LTE because the UV overionization mechanism produces a very strong underpopulation of the Fe I levels, in particular in the granular regions of the three-dimensional model. As a result, the NLTE effects on the derived iron abundance are very important, amounting to ~0.9 and to ~0.6 dex in the three- and one-dimensional cases, respectively. On the other hand, we find that NLTE and three-dimensional effects have to be taken into account for a reliable determination of the iron abundance from weak Fe II lines, because the significant overexcitation of their upper levels in the granular regions tend to produce emission features. As a result, such Fe II lines are weaker than in LTE and the abundance correction amounts to ~0.4 dex for the three-dimensional case. We also derive the oxygen-to-iron abundance ratio in the metal-poor star HD 140283 by using the O I triplet at 7772-7775 A and the forbidden [O I] line at 6300 A. Our results for the oxygen abundance confirm the values reported in some recent investigations. While the oxygen abundance derived from the O I IR triplet is not very sensitive to the presence of granulation inhomogeneities, such three-dimensional effects amount to ~-0.2 dex for the [O I] line. The NLTE abundance correction for the O I IR triplet turns out to be -0.2 dex, approximately. Interestingly, when both NLTE and three-dimensional effects are taken into account there still remain significant discrepancies in the iron abundances derived from Fe I and Fe II lines, as well as in the oxygen abundances inferred from the O I and [O I] lines. We conclude that the discrepancies could be due to uncertainties in the stellar parameters of this metal-poor star. We argue that adopting Teff ≈ 5600 K (instead of Teff ≈ 5700 K) and [Fe/H] ≈ -2.0 (instead of [Fe/H] ≈ -2.5) substantially reduces the discrepancies in the abundances of iron and oxygen inferred from several spectral lines. Under such circumstances, we find [O/Fe] ≈ 0.5 at [Fe/H] = -2. Obviously, our tentative conclusion that the metallicity of this type of metal-poor star is significantly larger than previously thought may have far-reaching implications in stellar astrophysics.

Oxygen line formation in late-F through early-K disk/halo stars

In order to investigate the formation of O i 7771–5 and [O i] 6300/6363 lines, extensive non-LTE calculations for neutral atomic oxygen were carried out for wide ranges of model atmosphere parameters, which are applicable to early-K through late-F halo/disk stars of various evolutionary stages. The formation of the triplet O i lines was found to be well described by the classical two-level-atom scattering model, and the non-LTE correction is practically determined by the parameters of the line-transition itself without any significant relevance to the details of the oxygen atomic model. This simplifies the problem in the sense that the non-LTE abundance correction is essentially determined only by the line-strength (), if the atmospheric parameters of Teff, , and ξ are given, without any explicit dependence of the metallicity; thus allowing a useful analytical formula with tabulated numerical coefficients. On the other hand, our calculations lead to the robust conclusion that LTE is totally valid for the forbidden [O i] lines. An extensive reanalysis of published equivalent-width data of O i 7771–5 and [O i] 6300/6363 taken from various literature resulted in the conclusion that, while a reasonable consistency of O i and [O i] abundances was observed for disk stars (), the existence of a systematic abundance discrepancy was confirmed between O i and [O i] lines in conspicuously metal-poor halo stars () without being removed by our non-LTE corrections, i.e., the former being larger by ~0.3 dex at . An inspection of the parameter-dependence of this discordance indicates that the extent of the discrepancy tends to be comparatively lessened for higher stars, suggesting the preference of dwarf (or subgiant) stars for studying the oxygen abundances of metal-poor stars.

A method for modeling the formation of caii lines in the spectra of irradiated stellar atmospheres

We have developed a method for calculating deviations from LTE of level populations and profiles of selected spectral lines in stellar atmospheres in the presence of external radiation. The influence of Thomson scattering at the frequencies of the external radiation is considered. The method used to calculate model irradiated atmospheres in a semi-grey approximation has been improved. We have modified the NONLTE3 code used to determine the level populations to make it suitable for irradiated atmospheres. A model for the CaII atom including 42 energy levels of CaII, the ground state of CaIII, and 80 linearized transitions was constructed for these calculations. This atomic model takes into account the effect of all relevant collisional processes and radiative processes at the frequencies of the internal and external radiation. We investigated the correctness of the non-LTE calculations for the CaII ion by analyzing 16 lines of ionized calcium in the solar spectrum. The influence of uncertainties in the atomic data on the non-LTE level populations and CaII line profiles was also analyzed, and the van der Waals broadening coefficients C6 were refined. The scaling coefficient in the Dravin formula was taken to be 0.1. We found the non-LTE abundance corrections for most lines to be significant (Δlogɛ(Ca)=0.05−0.15dex), even under the conditions for the solar atmosphere. The lines of the λ=8498, 8542, 8662 A infrared triplet can be adequately described. Differences in the mean calcium abundance obtained using different model atmospheres are smaller than 0.02 dex. Our final estimate of the mean calcium abundance in the solar atmosphere is logɛ(Ca)=6.31, in good agreement with the meteoritic abundance, logɛ(Ca)=6.32.

On the Abundance of Potassium in Metal-Poor Stars

Abstract Based on extensive statistical-equilibrium calculations, we performed a non-LTE analysis of the K i 7699 equivalent-width data of metal-deficient stars for the purpose of clarifying the behavior of the photospheric potassium abundance in disk/halo stars. While the resulting non-LTE abundance corrections turned out to be considerably large, amounting to 0.2–0.7 dex, their effect on the [K/Fe] vs. [Fe/H] relation is not very important, since these corrections do not show any significant dependence on the metallicity. Hence, we again confirmed the results of previous LTE studies, that [K/Fe] shows a gradual systematic increase toward a lowered metallicity up to [K/Fe] $sim$ 0.3–0.5 at [Fe/H] $sim$-1 to -2, such as in the case of $ \alpha$ elements.

Non-LTE line formation for Ni/ii: Abundances and stellar parameters

A comprehensive model atom for non-LTE line formation calculations for neutral and singly-ionized nitrogen is presented. Highly accurate radiative and collisional atomic data are incorporated, recently determined for astrophysical and fusion research using the R-matrix method in the close-coupling approximation. As a test and rst application of the model, nitrogen abundances are determined on the basis of line-blanketed LTE model atmospheres for ve stars, the main sequence object Vega (A0 V) and the supergiants Leo (A0 Ib), HD 111613 (A2 Iabe), HD 92207 (A0 Iae) and Ori (B8 Iae), using high S=N and high-resolution spectra at visual and near-IR wavelengths. The computed non-LTE line proles t the observations excellently for a given nitrogen abundance in each object. Moreover, the ionization equilibrium of N i/ii proves to be a sensitive temperature indicator for late B-type and early A-type supergiants { even at low metallicities { due to the apparent nitrogen overabundance in these objects. All supergiants within our sample show an enrichment of nitrogen on the order of0.3{0.6 dex, indicating the mixing of CN-cycled material into atmospheric layers, with the sum of the CNO abundances staying close to solar. This nding is in accordance with recent stellar evolution models accounting for mass-loss and rotation. For Vega, an underabundance of nitrogen by 0.25 dex is found, in good agreement with the similar underabundance of other light elements. The dependence of the non-LTE eects on the atmospheric parameters is discussed with special emphasis on the supergiants where a strong radiation eld at low particle densities favours deviations from LTE. Non-LTE eects systematically strengthen the N i/ii lines. For some N i lines in supergiants non-LTE abundance corrections in excess of 1 dex are found and they react sensitively to modications of the collisional excitation data. The influence of microturbulence on the statistical-equilibrium calculations is also investigated: the line-strengths of the strong N i features show some sensitivity due to modications of the line-formation depths and the departure coecients, while the { in this parameter range { weak N ii lines remain unaected.

Statistical equilibrium and photospheric abundance of silicon in the Sun and in Vega

Based on detailed non-LTE calculations, an updated determination of the abundance of silicon in the Sun and Vega is presented. The model atom includes neutral and singly ionized stages of silicon with 115 energy levels and 84 line transitions. Non-LTE eects are found to be quite small in the Sun. The mean non-LTE abundance correction is 0:010 dex with respect to standard LTE calculations, leading to a solar abundance of logNLTE =7 :550 0:056. For the prototype A0 V star Vega the non-LTE eects are small, too. With a non-LTE abundance correction of log = 0:054, a silicon abundance of logNLTE =6 :951 0:100 is derived, implying a deciency of 0:599 dex with respect to the Sun. This conrms the classication of Vega as a mild Boo star.

Non-LTE analysis of the formation of EuII lines in the atmospheres of solar-type stars

A method to analyze the statistical equilibrium of the EuII ion based on a 36-level model atom has been developed. The formation of EuII lines without assuming local thermodynamic equilibrium (LTE) is considered for Teff=5500–7000 K, logg=4.0, and metallicities [A] from 0 to −1.5. Non-LTE effects in the level populations are primarily due to radiative pumping of excited states from the ground and low-lying levels, which leads to over-population of upper relative to lower levels. As a result, the studied λ4129 and λ6645 A lines are weaker than in the LTE case. However, due to the small energy differences between even low-lying EuII levels, collisional coupling is strong, and deviations from LTE in EuII lines are modest: for the Sun, non-LTE corrections to the abundance are only 0.04 dex. The non-LTE effects grow with an increase in the effective temperature and with a decrease in the metallicity, so that non-LTE abundance corrections can reach 0.12 dex for Teff=5500K, logg=4.0, [A]=−1.5 and 0.1 dex for Teff=7000K, logg=4.0, [A]=0. The effect of inaccuracy in the atomic parameters for EuII on the non-LTE calculations is examined. Analysis of the profiles of the solar EuII λ4129 and λ6645 A lines is used to empirically refine estimates of the efficiency of collisional processes in forbidden transitions in establishing the distribution of EuII ions over excited states.

Oxygen Abundances in Late-B through F Supergiants

A spectral-synthesis analysis was performed for twenty-six late-B through F supergiants (including Cepheids) in order to determine their photospheric oxygen abundances from the O I 6156–8 feature, while taking into account the non-LTE effect, which is progressively Teff-dependent from ≲ 0.1 dex for F-type supergiants to ∼ 0.4 dex for late-B ones in terms of the non-LTE abundance correction. The resulting oxygen abundances show a moderate underabundance relative to the Sun $([\textrm O/\textrm H]{\sim -0.3} $) along with a remarkably small scatter (within ± 0.1 dex) over this wide temperature range. Considering the recent evidence that the solar oxygen abundance is mildly enhanced by 0.2−0.3 dex relative to that of the galactic gas which forms young stars, we conclude that the original O-composition in the atmosphere of these supergiants had suffered almost no appreciable alteration during their past evolutionary history, even though signs of significant mixing of CN-cycle or NeNa-cycle products are evident. This excludes the possibility of any global-mixing mechanism, such that producing a significant reduction of oxygen due to a deep dredge-up of considerable ON-processed material, whatever the details of the envelope-mixing process for these relatively massive stars may be.

Oxygen-Line Formation in Population-II A-Type Stars

The non-LTE formation of neutral oxygen lines in the visible (λ 6156–8) and near-IR (λ 7771–5) region was studied, and extensive grids of theoretical non-LTE equivalent widths and abundance corrections were computed in the parameter range of −2 ≤ [Fe/H] ≤ 0, 7000 K ≤Teff ≤ 14000 K, and 3.0 ≤ log g ≤ 4.0, with the intention of applying them to A-type stars, especially Population-II stars such as horizontal-branch objects. It was found that the inclusion of UV Lyman lines (such as Lyman β overlapping with a strong OI resonance transition) in the background opacity may be important, which tends to suppress the non-LTE effect, especially for the metal-deficient case. Nevertheless, the resulting non-LTE correction for OI 7771–5 is still of considerable amount (≳ 0.5–1.0 dex), even in the weak-line cases of low abundance, and should inevitably be taken into account when determining oxygen abundance from this near-IR triplet.