Formation of Zr I and II lines under non-LTE conditions of stellar atmospheres

The formation of Zr I and Zr II lines in stellar atmospheres under non-LTE conditions has been considered for the first time. A model zirconium atom has been composed using 148 Zr I levels, 772 Zr II levels, and the ground Zr III state. Non-LTE calculations have been performed for model atmospheres with T _eff = 5500 and 6000 K, log g = 2.0 and 4.0, [M/H] = −3, −2, −1, 0. In the entire investigated range of parameters, the Zr I levels are shown to be underpopulated relative to their LTE populations in the line formation region. In contrast, the excited Zr II levels are overpopulated, while the ground state and lower excited levels of Zr II retain their LTE populations. Since the non-LTE effects cause the Zr I and Zr II spectral lines being investigated to weaken, the non-LTE corrections to the abundance derived from Zr I and Zr II lines are positive. For Zr II lines, they increase with decreasing metallicity and surface gravity up to 0.34 dex for the model with T _eff = 5500, log g = 2.0, and [M/H] = −2. The non-LTE effects depend weakly on temperature. The non-LTE corrections for Zr I lines reach 0.33 dex for solar-metallicity models. Zr I and Zr II lines in the solar spectrum have been analyzed. The non-LTE zirconium abundances derived from lines in the two ionization stages are shown to agree between themselves within the error limits, while the LTE abundance difference is 0.28 dex. The zirconium abundance in the solar atmosphere (averaged over Zr I and Zr II lines) is log ɛ_Zr,⊙ = 2.63 ± 0.07.     

Influence of inelastic collisions with hydrogen atoms on the formation of AlI and SiI lines in stellar spectra

We have performed calculations by abandoning the assumption of local thermodynamic equilibrium (within the so-called non-LTE approach) for Al I and Si I with model atmospheres corresponding to stars of spectral types F–G–Kwith differentmetal abundances. To take into account inelastic collisions with hydrogen atoms, for the first time we have applied the cross sections calculated by Belyaev et al. using model approaches within the formalism of the Born–Oppenheimer quantum theory. We show that for Al I non-LTE leads to higher ionization (overionization) than in LTE in the spectral line formation region and to a weakening of spectral lines, which is consistent with earlier non-LTE studies. However, our results, especially for the subordinate lines, differ quantitatively from the results of predecessors. Owing to their large cross sections, the ion-pair production and mutual neutralization processes Al I(nl) + HI(1s) ? Al II(3s ²) + H^? provide a close coupling of highly excited Al I levels with the Al II ground state, which causes the deviations from the equilibrium level population to decrease compared to the calculations where the collisions only with electrons are taken into account. For three moderately metal-deficient dwarf stars, the aluminum abundance has been determined from seven Al I lines in different models of their formation. Under the assumption of LTE and in non-LTE calculations including the collisions only with electrons, the Al I 3961 ?A resonance line gives a systematically lower abundance than the mean abundance from the subordinate lines, by 0.25–0.45 dex. The difference for each star is removed by taking into account the collisions with hydrogen atoms, and the rms error of the abundance derived from all seven Al I lines decreases by a factor of 1.5–3 compared to the LTE analysis. We have calculated the non- LTE corrections to the abundance for six subordinate Al I lines as a function of the effective temperature (4500 K ≤ T _eff ≤ 6500 K), surface gravity (3.0 ≤ log g ≤ 4.5), and metal abundance ([M/H] = 0, ?1, ?2, and ?3). For Si I including the collisions with HI leads to the establishment of equilibrium populations in the spectral line formation region even in hot metal-deficient models and to vanishingly small departures from LTE in spectral lines.     

Influence of departures from LTE on oxygen abundance determination in the atmospheres of A-K stars

We have performed non-LTE calculations for O I with a multilevel model atom using currently available atomic data for a set of parameters corresponding to stars of spectral types from A to K. Departures from LTE lead to a strengthening of O I lines, and the difference between the non-LTE and LTE abundances (non-LTE correction) is negative. The non-LTE correction does not exceed 0.05 dex in absolute value for visible O I lines for main-sequence stars in the entire temperature range. For the infrared O I 7771 Å line, the non-LTE correction can reach ?1.9 dex. The departures from LTE are enhanced with increasing temperature and decreasing surface gravity. We have derived the oxygen abundance for three A-type mainsequence stars with reliably determined parameters (Vega, Sirius, HD 32115). For each of the stars, allowance for the departures from LTE leads to a decrease in the difference between the abundances from infrared and visible lines, for example, for Vega from 1.17 dex in LTE to 0.14 dex when abandoning LTE. In the case of Procyon and the Sun, inelastic collisions with HI affect the statistical equilibrium of OI, and agreement between the abundances from different lines is achieved when using Drawin’s classical formalism. Based on the O I 6300, 6158, 7771-5, and 8446 Å lines of the solar spectrum, we have derived the mean oxygen abundance log ? = 8.74 ± 0.05 using a classical plane-parallel model solar atmosphere and log ? _+3D = 8.78 ± 0.03 by applying the 3D corrections taken from the literature.     

Observational restrictions on sodium and aluminium abundance variations in evolution of the galaxy

In this paper we construct and analyze the uniform non-LTE distributions of the aluminium ([Al/Fe]-[Fe/H]) and sodium ([Na/Fe]-[Fe/H]) abundances in the sample of 160 stars of the disk and halo of our Galaxy with metallicities within ?4.07 ≤ [Fe/H] ≤ 0.28. The values of metallicity [Fe/H] and microturbulence velocity ξ _turb indices are determined from the equivalent widths of the Fe II and Fe I lines. We estimated the sodium and aluminium abundances using a 21-level model of the Na I atom and a 39-level model of the Al I atom. The resulting LTE distributions of [Na/Fe]-[Fe/H] and [Al/Fe]-[Fe/H] do not correspond to the theoretical predictions of their evolution, suggesting that a non-LTE approach has to be applied to determine the abundances of these elements. The account of non-LTE corrections reduces by 0.05–0.15 dex the abundances of sodium, determined from the subordinate lines in the stars of the disk with [Fe/H] ≥ ?2.0, and by 0.05–0.70 dex (with a strong dependence on metallicity) the abundances of [Na/Fe], determined by the resonance lines in the stars of the halo with [Fe/H] ≤ ?2.0. The non-LTE corrections of the aluminium abundances are strictly positive and increase from 0.0–0.1 dex for the stars of the thin disk (?0.7 ≤ [Fe/H] ≤ 0.28) to 0.03–0.3 dex for the stars of the thick disk (?1.5 ≤ [Fe/H] ≤ ?0.7) and 0.06–1.2 dex for the stars of the halo ([Fe/H] ≤ ?2.0). The resulting non-LTE abundances of [Na/Fe] reveal a scatter of individual values up to Δ[Na/Fe] = 0.4 dex for the stars of close metallicities. The observed non-LTE distribution of [Na/Fe]-[Fe/H] within 0.15 dex coincides with the theoretical distributions of Samland and Kobayashi et al. The non-LTE aluminium abundances are characterized by a weak scatter of values (up to Δ[Al/Fe] = 0.2 dex) for the stars of all metallicities. The constructed non-LTE distribution of [Al/Fe]-[Fe/H] is in a satisfactory agreement to 0.2 dex with the theoretical data of Kobayashi et al., but strongly differs (up to 0.4 dex) from the predictions of Samland.     

Influence of departures from LTE on calcium,titanium, and iron abundance determinations in cool giants of different metallicities

We have performed statistical equilibrium calculations for Ca I–Ca II, Ti I–Ti II, and Fe I–Fe II by taking into account the nonequilibrium line formation conditions (the non-LTE approach) in model atmospheres of giant stars with effective temperatures 4000 K ≤ T _eff ≤ 5000 K and metal abundances ?4 ≤ [Fe/H] ≤ 0. The dependence of departures from LTE on atmospheric parameters has been analyzed. We present the non-LTE abundance corrections for 28 Ca I lines, 42 Ti I lines, 54 Ti II lines, and 262 Fe I lines and a three-dimensional interpolation code to obtain the non-LTE correction online for an individual line and specified atmospheric parameters.     

Globular Clusters in the Large Magellanic Cloud: An Impact from GAIA Photometry

The goal of this work is to assess the expected scientific output from the photometric studies of globular clusters in the Large Magellanic Cloud with ESA's astrometric space mission GAIA. For this purpose we simulate GAIA photometry of individual stars in synthetic cluster populations, covering a large range of cluster ages and metallicities. We find that accurate effective temperatures (Δ T _eff<10%) can be obtained from GAIA photometry down to V ∼ 18 for stars in populations within the studied metallicity range ([M/H] = -0.4 ... -1.7). GAIA will also provide photometric metallicities (Δ [M/H] ≲ 0.3 dex) for the cluster giants brighter than V ∼ 17.5. The knowledge of the effective temperature sand metallicities will allow to obtain accurate ages of stellar populations younger than about 1 Gyr using the usual procedure of main sequence turn-off point fitting. Ages of older stellar populations (≳ 1 Gyr) may be constrained from the isochrone fits to the giant branches in the observed CMDs. We conclude that GAIA will provide excellent opportunities for studying star formation histories far beyond the Milky Way, providing means for better understanding of stellar and galactic evolution in different astrophysical environments. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spectroscopy of B and Be stars in the diffuse stellar clusters NGC 6871 and NGC 6913

This is a study of the population of B and Be stars in the young, relatively poor, diffuse stellar clusters NGC 6871 and NGC 6913. High resolution spectra are used to study the Hα line of eleven stars in order to detect emission. Emission profiles were found for three stars in the cluster NGC 6871; one of these is a known WR-star and the Be-star BD +35°3956 demonstrates the transition from the B to the Be phase. Spectra of seven of the B stars revealed no traces of emission in the Hα line. During the time of our observations, the Be star V1322 Cyg in the cluster NGC 6913 had a strong emission Hα line profile with substantial variability in intensity and equivalent width. Moderate resolution spectra of seven stars in the cluster NGC 6871 over wavelengths of 4420-4960 ? and ten stars in the cluster NGC 6913 over wavelengths of 4050-5100 ? are used to classify the series of B and Be stars spectrally and to estimate their T _eff and log g. It was found that three of the stars are not members of the clusters NGC 6871 or NGC 6913. Translated from Astrofizika, Vol. 52, No. 2, pp. 257–274 (May 2009).     

Disentangling Metallicity and Age for Turnoff Stars

This is a progress report on calculations of near-ultraviolet spectrawith Ben Dorman at NASA/Goddard, for the ultimate purpose of extractingage and metallicity from extragalactic spectra. We are calculating fromfirst principles a grid of spectra covering 2200–3400 Å using theKurucz program SYNTHE, beginning with stars of metallicity less thanone-fifth solar ([Fe/H] < ?0.7). For these stars, LTE calculationsusing known opacities and line lists including only transitions measuredin the laboratory, coupled with standard line-blanketed LTE models, providea satisfactory match to the spectra of turnoff stars of temperaturesT _eff = 5750 K – 6250 K. For more metal-rich stars, two problemsarise: lines without a laboratory identification become increasinglyinfluential, and the cores of all strong lines become too strong. Theseproblems must be addressed to match near-UV spectra of turnoff stars ofsolar metallicity or higher.     

Distances to galaxies from the brightest stars in the Universe

Blue Supergiants (BSGs) are the brightest stars in the universe at visual light with absolute magnitudes up to M _V =−10 mag. They are ideal stellar objects for the determination of extragalactic distances, in particular, because the perennial uncertainties troubling most of the other stellar distance indicators, interstellar extinction and metallicity, do not affect them. The quantitative spectral analysis of low resolution spectra of individual BSGs provides accurate stellar parameters and chemical composition, which are then used to determine accurate reddening and extinction from photometry for each individual object. Accurate distances can be determined from stellar gravities and effective temperatures using the “Flux Weighted Gravity–Luminosity Relationship (FGLR)”.     

Oxygen Abundances in Solar-Type Stars

The absolute solar oxygen abundance,ε ⊙ = 8.80± 0.06, has been determined from various oxygen abundance indicators in different solar atlases, and a new method is proposed to test collision rate coefficientsfor the NLTE model of OI.Using effective temperatures derived from Balmer lines, oxygen abundances from O triplets in 83 solar-type stars within the solar neighborhood spanning a metallicity range of [Fe/H] = −2.3 ... +0.4 have been determined.NLTE effects are not negligible, especially in warm stars(T_eff ≥ 5800) with [Fe/H] ≥ −0.5. This revised version was published online in July 2006 with corrections to the Cover Date.     

GAIA Photometric System: Evaluation of Performance

We present a method and results of evaluation of the photometric systems (PSs) proposed for the GAIA mission. The method, however, can be applied for the analysis of virtually any multicolour PS designed for the global investigation of the Galactic stellar populations. Performance of the 1F, 2A and 3G PSs is evaluated taking into account their ability to simultaneously determine the main stellar parameters,T _eff, log g, [M/H] and E _B-V, for a large variety of stars down to G ∼ 20. The representative Galactic stellar populations were constructed and employed for evaluation of the PSs. Despite the fact that the 2A and 3G PSs perform significantly better than the 1F (presently adopted as a baseline PS for GAIA), we conclude that still there is no photometric system proposed to date, which would allow to achieve the scientific objectives of the GAIA mission. This revised version was published online in July 2006 with corrections to the Cover Date.     

Determining the fundamental parameters of F and G supergiants

A technique for determining the effective temperature T _eff and the acceleration of gravity log g of F and G supergiants is discussed using four bright stars as examples, specifically two F supergiants, α Lep(F0 Ib) and π Sgr (F2 II), and two G supergiants, β Aqr (G0 Ib) and α Aqr (G2 Ib). In all four cases the parameter log g was derived from the high precision parallaxes recently obtained by van Leeuwen in a new reduction of data from Hipparcos. Because of this, the accuracy of the determinations of log g is much greater than before. Estimates of the parameter T _eff were checked using accurate values of T _eff obtained previously by the infrared flux method (IRFM). In the case of the early F supergiants, this method confirms the good accuracy of the T _eff values derived from the Balmer lines and the β-index. Measurements of the Balmer lines for the G supergiants are difficult because of strong blending, so the indices [c ₁] and β serve as indicators of T _eff . It is shown that the indices [c ₁] and β yield a systematic difference in the values of T _eff ; the IRFM confirms that deriving T _eff from the index [c ₁] is more accurate. Based on the values of T _eff and log g that have been found here, with the aid of the evolutionary tracks, we estimate the mass M and age t of each star. The Fe II lines, which are insensitive to departures from LTE, have been used to determine the microturbulence parameter V _t and the iron abundance. The latter is close to the solar iron abundance. Some problems concerning the chemical composition of these stars are discussed using the supergiant α Lep as an example. Translated from Astrofizika, Vol. 52, No. 2, pp. 237–257 (May 2009).     

The Stellar Populations of the Bars of Barred Spirals Through Evolutionary Synthesis: First Results

As part of an ongoing project aimed at studying the age and metallicity gradients of the stellar populations along the bars of a sample of barred spirals of different morphological types, we present our first results on NGC 4314 (SBa). We have obtained optical and NIR colours and spectral indices along the bar and we interpret some of these results here and discuss their uncertainties on the basis of single stellar population models. In a preliminary analysis, we constrain the limits for the age and metallicity of the nucleus and two selected regions in the star formation ring of NGC 4314, characterizing both as metal rich (Z<Z _solar) stellar populations, and finding a difference in the mean luminosity-weighted age of at least ∼ 3–4 Gyr. This revised version was published online in July 2006 with corrections to the Cover Date.     

The resolved red giant branches of E/S0 galaxies

Formation paradigms for massive galaxies have long centered around two antipodal hypotheses – the monolithic-collapse and the accretion/merger scenarios. Empirical data on the stellar contents of galaxy halos is crucial in order to develop galaxy formation and assembly scenarios which have their root in observations, rather than in numerical simulations. The Hubble Space Telescope (HST) has enabled us to study directly individual stars in the nearby E/S0 galaxies Cen A, NGC 3115, NGC 5102, and NGC 404. We here present and discuss HST single-star photometry in V and I bands. Using color-magnitude diagrams and stellar luminosity functions, we gauge the galaxies' stellar contents. This can be done at more than one position in the halo, but data with deeper limiting magnitudes are desired to quantify the variation of metallicity with galactocentric radius. We here compare the color distributions of red giant stars with stellar isochrones, and we intercompare the galaxies' halo populations, noting that their total absolute V magnitudes cover the range from about –21.5 to –17.5. In the future, we plan to model the stellar metallicity distributions with the aim to constrain chemical enrichment scenarios, a step towards unravelling the evolutionary history of elliptical and lenticular galaxies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Chemical Evolution of Gas-Rich Galaxy Mergers

We investigate numerically the chemodynamical evolution of major disc–disc galaxy mergers in order to explore the origin of the mass-dependent chemical, photometric and spectroscopic properties observed in elliptical galaxies. We investigate especially the dependence of the fundamental properties on merger progenitor disc mass (M _d). Three main results are obtained in this study:– More massive (luminous) ellipticals formed by galaxy mergers between more massive spirals have higher metallicity (Z) and thus show redder colours; the typical metallicity ranges from ∼ 1.0 solar abundance (Z∼ 0.02) for ellipticals formed by mergers with M _d = 10¹⁰ M _⊙to ∼ 2.0 solar (Z∼ 0.04) for those with M _d= 10¹² M _⊙.– Both the Mg₂ line index in the central part of ellipticals (R ≤ 0.1 R _e) and the radial gradient of Mg₂ (δ Mg₂ / δ log R) are more likely to be larger for massive ellipticals. δ Mg₂ / δ log R correlates reasonably well with the central Mg₂ in ellipticals. For most of the present merger models, ellipticals show a positive radial gradient of the H_β line index. – Both M/L _B and M/L _K (where M, L _B, and L _K are the total stellar mass of galaxy mergers, the B-band and the K-band luminosities, respectively) depend on galactic mass in such a way that more massive ellipticals have larger M/L _B and smaller M/L _K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Abundance Ratios in Metal-Rich Halo and Thick-Disk Stars

Recent determinations of precise abundance ratios for nearby halo and thick disk stars in the metallicity range −1.3 < [Fe/H] < −0.5 have revealed a significant cosmic spread in the abundances of oxygen, magnesium, sodium, nickel, s-process and r-process elements relative to iron. Possible explanations of these variations are reviewed. In particular, it is discussed if the differences in abundance ratios are correlated with the kinematics of the stars, and hence can be used to identify stellar populations in the Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

Statistical properties of the <Emphasis Type="Italic">GALEX</Emphasis> spectroscopic stellar sample

The GALEX General Data Release 4/5 includes 174 spectroscopic tiles, obtained from slitless grism observations, for a total of more than 60 000 ultraviolet spectra. We have determined statistical properties of the sample of GALEX stars. We have defined a suitable system of spectroscopic indices, which measure the main mid-UV features at the GALEX low spectral resolution and we have employed it to determine the atmospheric parameters of stars in the range 4500≲T _eff≲9000 K. Our preliminary results indicate that the majority of the sample is formed by main sequence F- and G-type stars, with metallicity [M/H]≳−1 dex.     

Physical parameters of multiple systems like the trapezium of early spectral types, derived fromuvbyβ photometry. II

The effective temperatures T_eff, gravitational force g at the stellar surface, and Fe/H have been calculated for 50 components of 15 Trapezia of early spectral types from electrophotometric observations in the six-color uvbyβ photometric system. The results of the calculation showed that the log T_eff of Trapezia components lie in the range 4.03–4.54, while their log g lie in the range 2.58–4.02. The atmospheric parameters T_eff and g were used to determine the stars’ masses and ages based on Hejlesen’s calculated evolutionary model sequences. All the Trapezia components have several times more mass than the sun and almost half of them have over 10 times that mass. Over 70% of the components are less than 40.10⁶ years old. Translated from Astrofizika, Vol. 41. No. 2. pp. 235–244, April-June, 1998.     

α-process Elements in the Galaxy: A Possible GAIA Contribution

The sensitivity of stellar spectra to α/Fe abundance changes is investigated with the aim to be detected photometrically and employed for the scientific goals of the GAIA mission. A grid of plane parallel, line blanketed, flux constant, LTE model atmospheres with different [α/Fe] ratios was calculated. As a first step, the modeled stellar energy fluxes for solar-type stars and giants were computed and intercompared. The spectral sensitivity to α/Fe abundance changes is noticeable and has to be taken into account when selecting photometric filters for GAIA. The Ca II H and K lines and Mg I b triplet are the most sensitive direct indicators ofα/Fe abundance changes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of redistribution with dipole scattering on line source functions

The partial frequency redistribution function for zero natural line width with dipole scattering (R_I) has been considered in obtaining the simultaneous solution of the statistical equilibrium and line transfer equations in the comoving frame of the expanding gas. We have considered a non-LTE two level atom in an expanding spherical medium whose outer radii are 3, 10 and 20 times the stellar radius with a total optical depthT ≃ 2 × 10³. In all the cases, we have calculated the population ratio of the two levels N₂/N₁ and compared these results with those obtained by using different expansion velocities and geometrical extensions. Initially, the upper level population (N₂) is set equal to zero. The converged simultaneous solution shows that the upper level population is enhanced considerably from the initial value. Variation in velocity gradients seem to have little effect on the ratio N₂/N₁ when the geometrical thickness of the medium is 3 or 10 times the stellar radius. However, when the thickness is increased to 20 times the central radius, the velocity gradients change the ratio N₂/N₁ considerably in the region where log T ≤ 2. The effect of variation of geometrical thickness is to reduce the N₂/N₁ ratio atτ = 0.