Non-LTE effects in Al I lines

We present the theoretical analysis of the Al I line formation in the spectra of late-type stars ignoring the assumption of local thermodynamic equilibrium (LTE). The calculations were based on the 39-level aluminum atom model for one-dimensional hydrostatic stellar atmosphere models with the parameters: T _eff from 4000 to 9000 K, log g = 0.0–4.5, and metallicity [A] = 0.0;–1.0;–2.0;–3.0;–4.0. The aluminum atom model and the method of calculations were tested by the study of line profiles in the solar spectrum. We refined the oscillator strengths and Van-der-Vaals broadening constants C ₆ of the investigated transitions. We conclude that the Al I atom is in the overionization state: the 3p level is underpopulated in the line formation region. This leads to the line weakening, as compared with the LTE results. The overionization effect becomes more pronounced with increasing temperature and decreasing metallicity. We show that the use of various atomic data (ionization cross-sections) for the low levels of Al I does not change the behavior of non-LTE deviations, whereas the value of these deviations varies essentially. For nine selected Al I lines we calculated the grids of theoretical non-LTE corrections (ΔX _NLTE = logɛ _NLTE − log ɛ _LTE) to the Al abundances determinedwith the LTE assumption. The non-LTE corrections are positive and significant for the stars with temperatures T _eff > 6000 K. These corrections weakly depend on log g, and increase with declining stellar metallicity.     

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.     

Metallicity measurements using atomic lines in M and K dwarf stars

We report the first survey of chemical abundances in M and K dwarf stars using atomic absorption lines in high-resolution spectra. We have measured Fe and Ti abundances in 35 M and K dwarf stars using equivalent widths measured from  λ/Δλ≈ 33 000  spectra. Our analysis takes advantage of recent improvements in model atmospheres of low-temperature dwarf stars. The stars have temperatures between 3300 and 4700 K, with most cooler than 4100 K. They cover an iron abundance range of  −2.44 < [Fe/H] < +0.16  . Our measurements show [Ti/Fe] decreasing with increasing [Fe/H], a trend similar to that measured for warmer stars, where abundance analysis techniques have been tested more thoroughly. This study is a step towards the observational calibration of procedures to estimate the metallicity of low-mass dwarf stars using photometric and low-resolution spectral indices.     

Strömgren 4-Color Photometry of Very Metal-Poor Stars

Strömgren uvby photometry has been observed for an additional 140 very metal-poor stars from the survey of Beers et al. (1992). These Galactic stars of very-low metallicity provide crucial information for the investigation of the formation and evolution of the Galaxy, as well as on the nature of the early Universe. The Strömgren uvby-β system allows the measurement of stellar atmospheric parameters as a prelude to detailed abundance studies which will make use of high-resolution spectroscopy and the new generation of large telescopes. The photometric techniques developed by Schuster et al. (1996) are used not only to classify these very metal-poor stars but also to derive effective temperatures, surface gravities, and improved estimates for their interstellar reddenings. In particular, photometric diagrams such as [c ₁], [m ₁] and c ₀, (b-y)₀ are used to classify these stars, especially those near the main-sequence turnoff, where contamination from slightly-evolved subgiants, lower surface-gravity horizontal-branch stars, and even a few supergiant or AGB candidates is found.     

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.     

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.     

Spectrum of the star BM Ori at minimum light

Two spectra of the star BM Ori were obtained with the 2.6-m Crimean Astrophysical Observatory telescope near its maximum eclipse phase. The light detector was a CCD array. The wavelength range 5305–5373 Å was chosen in such a way that it contained no strong primary lines. Optimum filtration of the spectra yielded a signal-to-noise ratio of ～300. Eighteen secondary lines are seen in the spectrum. Atmospheric parameters of the secondary star were determined: T _eff=5740 K and logg=3.0; the secondary was classified by these parameters as being of spectral type G2 III. The best agreement between observed and synthetic spectra is achieved for metallicity [M/H]=?0.5 and microturbulence ξ_t=0 km s^?1. The projected rotational velocity is Vsini=60 km s^?, in agreement with the synchronous velocity in the hypothesis that assumes a total eclipse by the secondary star. Atmospheric elemental abundances in the secondary are estimated. Nickel, chromium, and iron exhibit an underabundance of ～1 dex.     

Toward Understanding the Formation of Molecular Clouds

Based on a comparison of similar colour indices we investigate the effects of luminosity and metallicity that influence the Strömgren and Vilnius photometric quantities for different spectral types. We notice significant differences for similar indices, apparently due to small deviations of central wavelengths and band-widths of the filters. For the G-K type stars of moderate luminosity we interpret the difference introduced by v and X pass-bands as an log g effect, reflecting the presence of H&;K Ca II stellar lines in these spectral types. We discuss a possible influence of the H&;K Ca II lines on the luminosities derived from the Vilnius classification diagrams.     

High Resolution Optical Spectroscopy of an Intriguing High-Latitude B-Type Star HD119608

We present an LTE analysis of high resolution echelle optical spectra obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph for a B1Ib high galactic latitude supergiant HD119608. A fresh determination of the atmospheric parameters using line-blanketed LTE model atmospheres and spectral synthesis provided T_eff = 23 300 ± 1000 K, log g = 3.0 ± 0.3, and the microturbulent velocity ξ = 6.0 ± 1.0 kms^?1 and [Fe/H] = 0.16. The rotational velocity of the star was derived fromC, O, N, Al, and Fe lines as v sin i = 55.8 ± 1.3 kms^?1. Elemental abundances were obtained for 10 different species. He, Al, and P abundances of the star were determined for the first time. In the spectra, hot post-AGB status as well as the Pop I characteristics of the star were examined. The approximately solar carbon and oxygen abundances, along with mild excess in helium and nitrogen abundances do not stipulate a CNO processed surface composition, hence a hot post-AGB status. The LTE abundances analysis also indicates solar sulphur and moderately enriched magnesium abundances. The average abundances of B dwarfs of well studied OB associations and Population I stars show a striking resemblance to abundances obtained for HD119608 in this study. This may imply a runaway status for the star.     

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.     

Sample of Wolf-Rayet galaxies from the SLOAN digital sky survey

We have analyzed the spectra of blue compact dwarf galaxies from the SLOAN Digital Sky Survey (SDSS) Data Release 7 and created a sample of 271 galaxies with Wolf-Rayet (WR) spectral features produced by high-velocity stellar winds. A blue WR feature (bump) is a blend of the N V λλ 460.5 and 462.0 nm, N III λλ 463.4 and 464.0 nm, C III λ 465.0 nm, C IV λ 465.8 nm, and He II λ 468.6 nm emission lines. A red WR feature (bump) is the broad C IV λ 580.8 nm emission. The blue WR bump is mainly due to emissions of nitrogen WR (WN) stars, while the red bump is fully produced by emissions of carbon WR (WC) stars. All the sample spectra show the blue WR bumps, whereas the red WR bumps are only identified in 50% of sample spectra. We have derived the numbers of early-type WC stars (WCE) and late-type WN stars (WNL) in the galaxies using the luminosities of single WC and WN stars in the red and blue bumps, respectively. The number of O stars is estimated using the H_β luminosity. The ratio of the overall number of WR stars of different types to the number of all massive stars N(WR)/N(O + WR) decreases with decreasing metallicity, corresponding to the evolution population synthesis models.     

The Wings of Ca?ii H and K as Photospheric Diagnostics and the Reliability of One-Dimensional Photosphere Modeling

The extended wings of the Ca?ii H and K lines provide excellent diagnostics of the temperature stratification of the photosphere of the Sun and of other cool stars, thanks to their LTE opacities and source functions and their large span in formation height. The aim of this study is to calibrate the usage of the H and K wings in a one-dimensional interpretation of spatially averaged spectra and in deriving per-pixel stratifications from resolved spectra. I use multi-dimensional simulations of solar convection to synthesize the H and K wings, derive one-dimensional models from these wings as if they were observed, and compare the resulting models to the actual simulation input. I find that spatially averaged models constructed from the synthesized wings generally match the simulation averages well, except for the deepest layers of the photosphere where large thermal inhomogeneities and Planck-function non-linearity gives large errors. The larger the inhomogeneity, the larger the error. The presence of strong network fields increases such inhomogeneity. For quiet photospheric conditions the temperature excesses reach about 200?K. One-dimensional stratification fits of discrete structures such as granulation and small-scale magnetic concentrations give satisfactory results with errors that are primarily due to steep temperature gradients and abrupt changes of temperature with depth. I conclude that stratification modeling using the H and K wings is a useful technique for the interpretation of solar high-resolution observations.     

Europium abundances in cool dwarf stars of the galactic thick and thin disks

Abundances of europium for 112 FGK dwarf stars of thick and thin disks have been determined in the metallicity range of ?1.0 < [Fe/H] < +0.3. Spectra of the studied stars have been obtained using the 1.93-m telescope of Haute-Provence Observatory (France) with spectral resolution R = 42000 and signal-to-noise ratio S/N = 100?300. Eu content has been calculated with assumption of LTE using the synthetic spectrum approach with detailed consideration of superfine structure. Analysis of europium abundances as a function of metallicity in kinematically selected stars of the Galactic thick and thin disks revealed different values in the disks. Comparison of europium abundances with magnesium abundances makes it possible to assume that at [Fe/H] < ?0.2 dex the origins of these elements are similar and at [Fe/H] > ?0.2 dex they are, probably, different.     

High Resolution Optical Spectroscopy of Hot Post-AGB Star Candidates LS IV-04 1 and LB3116

We present LTE analysis of high resolution optical spectra for B-type hot PAGB stars LS IV-04 1 and LB3116 (LSE 237). The spectra of these high Galactic latitude stars were obtained with the 3.9-m Anglo-Australian Telescope (AAT) and the UCLES spectrograph. The standard 1D LTE analysis with line-blanketed LTE model atmospheres and spectral synthesis provided fundamental atmospheric parameters of T_eff= 15 000±1000 K, log g= 2.5±0.2, ξ = 5.0±1.0 km s^?1, [M/H] = ?1.81 dex, and v sin i= 5 km s^?1 for LSIV-04 1 and T_eff= 16 000±1000 K, log g= 2.5±0.1, v sin i= 25 km s^?1, and [Fe/H] = ?0.93 dex for LB 3116. Chemical abundances of ten different elements were obtained. For LS IV-04 1, its derived model temperature contradicts with previous analysis results. The upper limits for its nitrogen and oxygen abundances were reported for the first time. The magnesium, silicon and calcium were overabundant (i.e. [Mg/Fe] = 0.8 dex, [Si/Fe] = 0.5 dex, [Ca/Fe] = 0.9 dex). With its metal-poor photosphere and V_LSR ≈ 96 km s^?1, LSIV-04 1 is likely a population II star and most probably a PAGB star. LTE abundances of LB 3116 were reported for the first time. The spectrum of this helium rich star shows 0.9 dex enhancement in the nitrogen. The photosphere of the star is slightly deficient in Mg, Si, and S. (i.e. [Mg/Fe] = ?0.2 dex, [Si/Fe] = ?0.4 dex, [S/Fe] = ?0.2 dex). The Al is slightly enhanced. The phosphorus is overabundant, i.e. [P/Fe] ≈ 1.7 ± 0.47 dex, hence LB3116 may be the first example of a PAGB star which is rich in phosphorus. With its high radial velocity (i.e.V_LSR = 73 km s^?1), and the deficiencies observed in C, Mg, Si, and S indicate that LB 3116 is likely a hot PAGB star at high galactic latitude.     

Non-LTE models of the accretion disks of UX Ori stars

The results of a non-LTE analysis of a number of spectral lines formed in the accreting envelopes of UX Ori stars are given. The accretion rate is estimated from an analysis of the first three lines of the Balmer series: M _a = 10^?8 ?10^?9 M _⊙ The gas temperature in this region is about 10,000 K. In the immediate vicinity of the star there is a hotter region, with T > 15,000 K, in which the 5876 Å line of neutral helium, observed in the spectra of these stars, is formed. The region of formation of this line has a small geometrical thickness, covers a small fraction of the star’s visible disk, and evidently consists of the site of contact of the accreting gas with the stellar surface. The low gas rotation rates in this region (150–200 km/sec) may mean that rapid rotation of the accreting gas is damped by the star’s magnetic field, which is strong enough to affect the gas stream. We estimate the magnetic field strength in this region to be about 150 G.     

A NLTE model atmosphere analysis of the pulsating sdO star SDSS J1600+0748

We started a program to construct several grids of suitable model atmospheres and synthetic spectra for hot subdwarf O stars computed, for comparative purposes, in LTE, NLTE, with and without metals. For the moment, we use our grids to perform fits on our spectrum of SDSS J160043.6+074802.9 (J1600+0748 for short), this unique pulsating sdO star. Our best fit is currently obtained with NLTE model atmospheres including carbon, nitrogen and oxygen in solar abundances, which leads to the following parameters for SDSS J1600+0748 : T _eff=69060±2080 K, log?g=6.00±0.09 and log?N(He)/N(H)=?0.61±0.06. Improvements are needed, however, particularly for fitting the available He ii lines. It is hoped that the inclusion of Fe will help remedy the situation.     

Extremely metal‐poor stars in SDSS fields

Some insight on the first generation of stars can be obtained from the chemical composition of their direct descendants, extremely metal‐poor stars (EMP), with metallicity less than or equal to 1/1000 of the solar metallicity. Such stars are exceedingly rare, the most successful surveys, for this purpose, have so far provided only about 100 stars with 1/1000 the solar metallicity and 4 stars with about 1/10000 of the solar metallicity. The Sloan Digital Sky Survey has the potential to provide a large number of candidates of extremely low metallicity. X‐shooter has the unique capability of performing the necessary follow‐up spectroscopy providing accurate metallicities and abundance ratios for several elements (Mg, Al, Ca, Ti, Cr, Sr,...) for EMP candidates. We here report on the results for the first two stars observed in the course of our Franco‐Italian X‐shooter GTO. The two stars were targeted to be of metallicity around –3.0, the analysis of the X‐shooter spectra showed them to be of metallicity around –2.0, but with a low α to iron ratio, which explains the underestimate of the metallicity from the SDSS spectra. The efficiency of X‐shooter allows an in situ study of the outer halo, for the two stars studied here we estimate distances of 3.9 and 9.1 kpc, these are likely the most distant dwarf stars studied in detail to date (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The study of s-process nucleosynthesis based on barium stars, CEMP-s and CEMP-r/s stars

In order to get a broader view of the s-process nucleosynthesis we study the abundance distribution of heavy elements of 35 barium stars and 24 CEMP-stars, including nine CEMP-s stars and 15 CEMP-r/s stars. The similar distribution of [Pb/hs] between CEMP-s and CEMP-r/s stars indicate that the s-process material of both CEMP-s and CEMP-r/s stars should have a uniform origin, i.e. mass transfer from their predominant AGB companions. For the CEMP-r/s stars, we found that the r-process should provide similar proportional contributes to the second s-peak and the third s-peak elements, and also be responsible for the higher overabundance of heavy elements than those in CEMP-s stars. Which hints that the r-process origin of CEMP-r/s stars should be closely linked to the main r-process. The fact that some small r values exist for both barium and CEMP-s stars, implies that the single exposure event of the s-process nucleosynthesis should be general in a wide metallicity range of our Galaxy. Based on the relation between C _r and C _s, we suggest that the origin of r-elements for CEMP-r/s stars have more sources. A common scenario is that the formation of the binary system was triggered by only one or a few supernova. In addition, accretion-induced collapse(AIC) or SN 1.5 should be the supplementary scenario, especially for these whose pre-AGB companion with higher mass and smaller orbit radius, which support the higher values of both C _r and C _s.     

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.     

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.