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.     

Composition of Coronal Streamers from the SOHO Ultraviolet Coronagraph Spectrometer

The Ultraviolet Coronagraph Spectrometer on the SOHO satellite covers the 940–1350 Å range as well as the 470–630 Å range in second order. It has detected coronal emission lines of H, N, O, Mg, Al, Si, S, Ar, Ca, Fe, and Ni, particularly in coronal streamers. Resonance scattering of emission lines from the solar disk dominates the intensities of a few lines, but electron collisional excitation produces most of the lines observed. Resonance, intercombination and forbidden lines are seen, and their relative line intensities are diagnostics for the ionization state and elemental abundances of the coronal gas. The elemental composition of the solar corona and solar wind vary, with the abundance of each element related to the ionization potential of its neutral atom (First Ionization Potential–FIP). It is often difficult to obtain absolute abundances, rather than abundances relative to O or Si. In this paper, we study the ionization state of the gas in two coronal streamers, and we determine the absolute abundances of oxygen and other elements in the streamers. The ionization state is close to that of a log T = 6.2 plasma. The abundances vary among, and even within, streamers. The helium abundance is lower than photospheric, and the FIP effect is present. In the core of a quiescent equatorial streamer, oxygen and other high-FIP elements are depleted by an order of magnitude compared with photospheric abundances, while they are depleted by only a factor of 3 along the edges of the streamer. The abundances along the edges of the streamer (‘legs’) resemble elemental abundances measured in the slow solar wind, supporting the identification of streamers as the source of that wind component.     

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.     

Spectroscopy of planetary nebulae in the region of Canis Major

We present the results of a pilot project of spectroscopic observations for planetary nebulae (PNe) and PN candidates in Canis Major, a sky region where the remnant of a disrupted dwarf galaxy cannibalized by the Milky Way may be located. The spectra of seven objects were taken while testing the SALT spectrograph (South African Astronomical Observatory). All elemental abundances have been obtained by the T _e method, where the electron temperature is calculated directly using the measured weak auroral [OIII] ?? 4363 ? and/or [NII] ?? 5755 ? lines. We have measured the intensities of all the detected emission lines and determined the abundances of oxygen and several other elements (N, Ne, S, Cl, C, and He) in all PNe. The radial velocity for one PN has been measured for the first time and the velocities for all of the remaining PNe have been measured with a considerably better accuracy than that of the previously published ones. The elemental abundances for three PNe have been calculated for the first time and the accuracies of determining the abundances for three others have been improved. The measured heavy-element abundance ratios (S/O, Ne/O, Cl/O) are in good agreement with their typical values for HII regions. Among the PNe studied, ESO 428-05 is the first and so far the most likely candidate for belonging to the remnants of a possible dwarf galaxy disrupted by the tidal interaction with the Milky Way.     

Carbon,nitrogen, and oxygen abundances in Procyon,Sun, and Arcturus

Ci, Ni, andOi permitted and forbidden lines in the near-infrared spectra of Procyon (F5 IV–V), Sun (G2 V), and Arcturus (K2 IIIp) have been analyzed to determine their carbon, nitrogen, and oxygen abundances. Procyon is found to have normal abundances while Arcturus is found to have an enhanced nitrogen abundance by a factor of 1.03 dex relative to the Sun. The dependence of the derived abundances on the effective temperature and luminosity is investigated.Much of the work reported here was contained in the Ph.D. thesis of the author submitted to the Istanbul University, Turkey, in 1986.     

New Views on the Early Evolution of Oxygen in the Galaxy

We discuss results on the oxygen abundance in a sample of 23 metal-poor (?3.0≤ [Fe/H] ≤ ?0.3) unevolved stars and one giant. High resolutionspectroscopy of OH lines in the near UV allowed us to trace the early evolution of oxygenversus metallicity. Contrary to previous expectations, we find that oxygen abundances derived from these low excitation lines agree well withthose derived from the high excitation lines of the OI IR triplet and from the [OI] λ 6300 Å line. Our new oxygen abundances show a smooth extension of previouslyknown trends of [O/Fe] versus [Fe/H] in disk stars to much lower metallicities, with a slope of ?0.31± 0.11. The [O/Fe] ratio increasesfrom 0.6 to 1 between [Fe/H] =?1.5 and ?3.0. Comparison with oxygen abundances in giant stars of the same metallicity imply that the lattermay have suffered a process of oxygen depletion. We briefly discussthe impact of these results on the yields of Type II SNe in the early Galaxy and on the age of globular clusters.     

A comprehensive study of reported high-metallicity giant H ii regions – I. Detailed abundance analysis

We present long-slit observations in the optical and near-infrared of 14 H  ii regions in the spiral galaxies NGC 628, 925, 1232 and 1637, all of them reported to have solar or oversolar abundances according to empirical calibrations. For seven of the observed regions, ion-weighted temperatures from optical forbidden auroral to nebular line ratios are obtained and, for six of them, the oxygen abundances derived by standard methods turn out to be significantly lower than solar. The other one, named CDT1 in NGC 1232, shows an oxygen abundance of     , and constitutes, to the best of our knowledge, the first high-metallicity H  ii region for which accurate line temperatures, and hence elemental abundances, have been derived.
For the rest of the regions no line temperature measurements could be made, and the metallicity has been determined by means of both detailed photoionization modelling and the sulphur abundance parameter S ₂₃. Only one of these regions shows values of O ₂₃ and S ₂₃ implying a solar or oversolar metallicity.
According to our analysis, only two of the observed regions can therefore be considered as of high metallicity. These two fit the trends previously found in other high-metallicity H  ii regions, i.e., N/O and S/O abundance ratios seem to be higher and lower than solar respectively.     

Chemical composition of the Orion nebula derived from echelle spectrophotometry

We present echelle spectroscopy in the 3500- to 7060-... range for two positions of the Orion nebula. The data were obtained using the 2.1-m telescope at Observatorio Astronómico Nacional in San Pedro Mártir, Baja California. We have measured the intensities of about 220 emission lines, in particular 81 permitted lines of C⁺, N⁺, N⁺⁺, O⁰, O⁺, Ne⁰, Si⁺, Si⁺⁺ and S⁺, some of them produced by recombination only and others mainly by fluorescence. We have determined electron temperatures, electron densities and ionic abundances using different continuum and line intensity ratios. We derived the He, C and O abundances from recombination lines and find that the C/H and O/H values are very similar to those derived from B stars of the Orion association, and that these nebular values are independent of the temperature structure. We have also derived abundances from collisionally excited lines. These abundances depend on the temperature structure; accurate t ² values have been derived comparing the O II recombination lines with the [O III ] collisionally excited lines. The gaseous abundances of Mg, Si and Fe show significant depletions, implying that a substantial fraction of these atoms is tied up in dust grains. The derived depletions are similar to those found in warm clouds of the Galactic disc, but are not as large as those found in cold clouds. A comparison of the solar and Orion chemical abundances is made.     

Properties of the ionised gas of circumnuclear star-forming regions in early-type spirals

A study of circumnuclear star-forming regions (CNSFRs) in several early-type spirals has been carried out in order to investigate their main properties: stellar and gas kinematics, dynamical masses, ionising stellar masses, chemical abundances and other properties of the ionised gas. Both high resolution (R～20,000) and moderate resolution (R～5000) have been used. In some cases, these regions (about 100–150 pc in size) are composed of several individual star clusters with sizes between 1.5 and 4.9 pc, estimated from Hubble Space Telescope images. Stellar and gas velocity dispersions are found to differ by about 20 to 30 km?s^?1, with the Hβ emission lines being narrower than both the stellar lines and the [Oiii]λ5007 Å lines. The twice ionised oxygen, on the other hand, shows velocity dispersions comparable to those of stars. We have applied the virial theorem to estimate dynamical masses of the clusters, assuming that the systems are gravitationally bounded and spherically symmetric, and using previously measured sizes. The measured values of the stellar velocity dispersions yield dynamical masses of the order of 10⁷ to 10⁸ M_⊙ for the full CNSFRs. We obtain oxygen abundances which are comparable to those found in high-metallicity disc Hii regions from direct measurements of electron temperatures and consistent with solar values within the errors. The region with the highest oxygen abundance is R3+R4 in NGC3504, 12+log(O/H)=8.85, about 1.5 times solar. The derived N/O ratios are, on average, larger than those found in high-metallicity disc Hii regions, and they do not seem to follow the trend of N/O vs. O/H which marks the secondary behaviour of nitrogen. On the other hand, the S/O ratios span a very narrow range—between 0.6 and 0.8 times solar. Compared to high-metallicity disc Hii regions, CNSFRs show values of the O₂₃ and the N2 parameters whose distributions are shifted to lower and higher values, respectively. Hence, even though their derived oxygen and sulphur abundances are similar, higher values would in principle be obtained for the CNSFRs if pure empirical methods were used to estimate abundances. CNSFRs also exhibit lower ionisation parameters than their disc counterparts, as derived from [Sii]/[Siii]. Their ionisation structure also seems to be different, with CNSFRs showing radiation-field properties more similar to Hii galaxies than to disc high-metallicity Hii regions.     

Hydrostatic gas distributions: global estimates of temperature and abundance

Estimating the temperature and metal abundance of the intracluster and the intragroup media is crucial to determine their global metal content and to determine fundamental cosmological parameters. When a spatially resolved temperature or abundance profile cannot be recovered from observations (e.g. for distant objects), or deprojection is difficult (e.g. due to a significant non-spherical shape), only global average temperature and abundance are derived. After introducing a general technique to build hydrostatic gaseous distributions of prescribed density profile in potential wells of any shape, we compute the global mass-weighted and emission-weighted temperature and abundance for a large set of barotropic equilibria and an observationally motivated abundance gradient. We also compute the spectroscopic-like temperature that is recovered from a single temperature fit of observed spectra. The derived emission-weighted abundance and temperatures are higher by 50 to 100 per cent than the corresponding mass-weighted quantities, with overestimates that increase with the gas mean temperature. Spectroscopic temperatures are intermediate between mass and luminosity-weighted temperatures. Dark matter flattening does not lead to significant differences in the values of the average temperatures or abundances with respect to the corresponding spherical case (except for extreme cases).     

Deep high spectral resolution spectroscopy and chemical composition of ionized nebulae

High spectral resolution spectroscopy has proved to be very useful for the advancement of chemical abundances studies in photoionized nebulae, such as H II regions and planetary nebulae (PNe). Classical analyses make use of the intensity of bright collisionally excited lines (CELs), which have a strong dependence on the electron temperature and density. By using high resolution spectrophotometric data, our group has led the determination of chemical abundances of some heavy element ions, mainly O⁺⁺, O⁺, and C⁺⁺ from faint recombination lines (RLs), allowing us to deblend them from other nearby emission lines or sky features. The importance of these lines is that their emissivity depends weakly on the temperature and density structure of the gas. The unresolved issue in this field is that recombination lines of heavy element ions give abundances that are about 2–3 times higher than those derived from CELs – in H II regions – for the same ion, and can even be a factor of 70 times higher in some PNe. This uncertainty puts into doubt the validity of face values of metallicity that we use as representative not only for ionized nebulae in the Local Universe, but also for star‐forming dwarf and spiral galaxies at different redshifts. Additionally, high‐resolution data can allow us to detect and deblend faint lines of neutron capture element ions in PNe. This information would introduce further restrictions to evolution models of AGBs and would help to quantify the chemical enrichment in s‐elements produced by low and intermediate mass stars. The availability of an échelle spectrograph at the E‐ELT will be of paramount interest to: (a) extend the studies of heavyelement recombination lines to low metallicity objects, (b) to extend abundance determinations of s‐elements to planetary nebulae in the extragalactic domain and to bright Galactic and extragalactic H II regions. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diffusion of elements in the interstellar medium in early-type galaxies

The role of diffusion in the redistribution of elements in the hot interstellar medium of earlytype galaxies is considered. It is well known that gravitational sedimentation can affect significantly the abundances of helium and heavy elements in the hot intergalactic gas of massive galaxy clusters. The universal temperature profile in cool-core clusters and the theoretical mass–temperature relation suggest that the maximum effect of sedimentation must take place in the most massive virialized objects in the Universe. However, observational data from the Chandra and XMM-Newton observatories demonstrate more complex scaling relations between the masses of early-type galaxies and other parameters, such as the mass fraction and temperature of the interstellar gas. An important fact is that the radial temperature profile can have both falling and rising patterns. We have calculated the diffusion based on the observed gas density and temperature distributions for 13 early-type galaxies that have different envelope types and cover a wide range of X-ray luminosities. To estimate the maximum effect of sedimentation and thermal diffusion, we have solved the full set of Burgers equations for a non-magnetized interstellar plasma. The results obtained demonstrate a considerable increase of the He/H ratio within one effective radius for all galaxies of our sample. For galaxies with a falling or constant temperature profile the average increase of the helium abundance is 60% in one billion years of diffusion. The revealed effect can introduce a significant bias into the metal abundance estimate based on X-ray spectroscopy and can affect the evolution of stars that could be formed from a gas with a high helium abundance.     

Oxygen and nitrogen abundances in cold H II regions from Sloan Digital Sky Survey

Oxygen and nitrogen abundances for a selection of 113607 cold objects from the Sloan Digital Sky Survey (SDSS) spectra are derived. Values of oxygen abundances 12 + log(O/H) are obtained using three parametric N calibrations, one of which is constructed in this work. Values of nitrogen abundances 12 + log(N/H) are found with two N calibrations. The values of oxygen abundances that are derived using three parametric N calibrations are consistent within 0.05 dex for the vast majority of the objects. It is not improbable that there are systematic differences between the values of oxygen abundances which do not exceed 0.05 dex. The differences between the values of nitrogen abundances which are obtained using ON and NS calibrations reach 0.1 dex. We show that the vast majority of cold SDSS objects are in the same area of the diagram O/H—N/O as cold H₂ regions of nearby galaxies, for which the oxygen and nitrogen abundances are derived through the T _e -method. This is indirect evidence that the values of oxygen and nitrogen abundances that are found with parametric N calibrations are reliable.     

Study of galaxies in the Lynx-Cancer void. II. Element abundances

In the framework of study of the evolutionary status of galaxies in the nearby Lynx-Cancer void, we present the results of the SAO RAS 6-m telescope spectroscopy for 20 objects in this region. The principal faint line [Oiii]λ4363 Å, used to determine the electron temperature and oxygen abundance (O/H) by the classicalmethod, is clearly detected in only about 2/3 of the studied objects. For the remaining galaxies this line is either faint or undetected. To obtain the oxygen abundances in these galaxies we as well apply the semi-empirical method by Izotov and Thuan, and/or the empirical methods of Pilyugin et al., which are only employing the intensities of sufficiently strong lines. We also present our O/H measurements for 22 Lynx-Cancer void galaxies, for which the suitable Sloan Digital Sky Survey (SDSS) spectra are available. In total, we present the combined O/H data for 48 Lynx-Cancer void galaxies, including the data adopted from the literature and our own earlier results. We make a comparison of their locations on the (O/H)-M_B diagram with those of the dwarf galaxies of the Local Volume in the regions with denser environment. We infer that the majority of galaxies from this void on the average reveal an about 30% lower metallicity. In addition, a substantial fraction (not less than 10%) of the void dwarf galaxies have a much larger O/H deficiency (up to a factor of 5). Most of them belong to the tiny group of objects with the gas metallicity Z ⊙/20 or 12+log(O/H)?7.35. The surface density of very metal-poor galaxies (Z ⊙/10) in this region of the sky is 2–2.5 times higher than that, derived from the emission-line galaxy samples in the Hamburg-SAO and the SDSS surveys. We discuss possible implications of these results for the galaxy evolution models.     

Spectroscopy of candidate metal-deficient galaxies with the 6-m telescope

We present the first results of our program of search for the most metal-deficient blue compact galaxies (BCGs) carried out with the 6-m Special Astrophysical Observatory telescope. The results of spectrophotometry are presented and discussed for ten galaxies from the Case and Hamburg/SAO surveys. The selection of candidates, observations, and data reduction are described in detail. For all the galaxies studied, we measured the intensity of the [O III] λ4363 Å emission line, which allows us to properly determine the temperatures of H II regions and to deduce elemental abundances. We measured the intensities of all the detected emission lines in H II regions of the galaxies under study and determined the abundances of oxygen and neon in them and in some of these galaxies, of other elements (N, S, He, Ar, and Fe). The oxygen abundance log (O/H)+12 in six galaxies was derived with an error ≤0.1 dex. Six of the ten galaxies studied turned out to be metal-poor with an oxygen abundance ≤1/10 of its solar value [i.e., 12+log(O/H)≤7.92]. HS 0837+4717 with 12+log(O/H) ≤7.7 is one of the most metal-poor galaxies in this sample and one of the candidates for young galaxies. Low-contrast, broad emission components of the nebular [O III]λ4959 and 5007 Å lines were detected in its spectrum, suggesting high velocities of gas motions in this galaxy.     

He,C, N,and O abundances in an ensemble of galactic planetary nebulae

The He, C, N, and O abundances in more than 120 planetary nebulae (PNe) of our Galaxy and the Magellanic Clouds have been redetermined by analyzing new PNe observations. The characteristics of PNe obtained by modeling their spectra have been used to compile a new catalog of parameters for Galactic and extragalactic PNe, which is accessible at http://www.astro.spbu.ru/staff/afk/GalChemEvol.html. The errors in the parameters of PNe and their elemental abundances related to inaccuracies in the observational data have been analyzed. The He abundance is determined with an accuracy of 0.06 dex, while the errors in the C, N, and O abundances are 0.1–0.2 dex. Taking into account the inaccuracies in the corrections for the ionization stages of the elements whose lines are absent in the PNe spectra increases the errors in the He abundance to 0.1 dex and in the C, N, and O abundances to 0.2–0.3 dex. The elemental abundances in PNe of various Galactic subsystems and the Magellanic Clouds have been analyzed. This analysis suggests that the Galactic bulge objects are similar to type II PNe in Peimbert’s classification, whose progenitor stars belong to the thin-disk population with ages of at least 4–6 Gyr. A similarity between the elemental abundances in PNe of the Magellanic Clouds and the Galactic halo has been established.     

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.     

On the maximum value of the cosmic abundance of oxygen and the oxygen yield

We search for the maximum oxygen abundance in spiral galaxies. Because this maximum value is expected to occur in the centres of the most luminous galaxies, we have constructed the luminosity – central metallicity diagram for spiral galaxies, based on a large compilation of existing data on oxygen abundances of H  ii regions in spiral galaxies. We found that this diagram shows a plateau at high luminosities  (−22.3 ≲ M _B ≲−20.3)  , with a constant maximum value of the gas-phase oxygen abundance  12 + log (O/H) ∼ 8.87  . This provides strong evidence that the oxygen abundance in the centres of the most luminous metal-rich galaxies reaches the maximum attainable value of oxygen abundance. Since some fraction of the oxygen (about 0.08 dex) is expected to be locked into dust grains, the maximum value of the true gas + dust oxygen abundance in spiral galaxies is 12 + log(O/H) ∼ 8.95. This value is a factor of ∼2 higher than the recently estimated solar value. Based on the derived maximum oxygen abundance in galaxies, we found the oxygen yield to be about 0.0035, depending on the fraction of oxygen incorporated into dust grains.     

The WC10 central stars CPD−56° 8032 and He 2–113 –III. Wind electron temperatures and abundances

We present a direct spectroscopic measurement of the wind electron temperatures and a determination of the stellar wind abundances of the WC10 central stars of planetary nebulae CPD−56° 8032 and He 2–113, for which high-resolution (0.15-Å) UCLES echelle spectra have been obtained using the 3.9-m Anglo-Australian Telescope.
The intensities of dielectronic recombination lines, originating from autoionizing resonance states situated in the C²⁺+e⁻ continuum, are sensitive to the electron temperature through the populations of these states, which are close to their LTE values. The high-resolution spectra allow the intensities of fine-structure components of the dielectronic multiplets to be measured. New atomic data for the autoionization and radiative transition probabilities of the resonance states are presented, and used to derive wind electron temperatures in the two stars of 21 300 K for CPD−56°8032 and 16 400 K for He 2–113. One of the dielectronic lines is shown to have an autoionization width in agreement with the theoretical predictions. Wind abundances of carbon with respect to helium are determined from bound–bound recombination lines, and are found to be C/He=0.44 for CPD−56° 8032 and C/He=0.29 for He 2–113 (by number). The oxygen abundances are determined to be O/He=0.24 for CPD−56° 8032 and 0.26 for He 2–113.
The effect of optical depth on the temperature and abundance determinations is investigated by means of a Sobolev escape-probability model. We conclude that the optically thicker recombination lines can still be used for abundance determinations, provided that their upper levels are far from LTE.     

Influence of Inelastic Collisions with Hydrogen Atoms on Non-LTE Oxygen Abundance Determinations

We present the results of our modeling of the O I line formation under non-LTE conditions in the atmospheres of FG stars. The statistical equilibrium of O I has been calculated using Barklem’s quantum-mechanical rates of inelastic collisions with hydrogen atoms. We have determined the non-LTE oxygen abundance from atomic O I lines for the Sun and 46 FG stars in a wide metallicity range, ?2.6 < [Fe/H] < 0.2. The application of accurate atomic data has led to an increase in the departures from LTE and a decrease in the oxygen abundance compared to the use of Drawin’s theoretical approximation. The change in the non-LTE abundance from the infrared O I 7771-5 Å triplet lines is 0.11 dex for solar atmospheric parameters and diminishes in absolute value with decreasing metallicity. We have revised the [O/Fe]–[Fe/H] relationship derived by us previously. The change in [O/Fe] is small in the [Fe/H] range from ?1.5 to 0.2. For stars with [Fe/H] < ?1 the [O/Fe] ratio has increased so that [O/Fe] = 0.60 at [Fe/H] = ?0.8 and rises to [O/Fe] = 0.75 at [Fe/H] = ?2.6.