Hydrodynamical Model Atmospheres Research Articles

Abundance of strontium in the Galactic globular cluster 47 Tuc

Aims. We have determined Sr abundance in a sample of 31 red giant branch stars located in the Galactic globular cluster 47 Tuc with the aim to identify potential differences in the Sr abundance between first population (1P, Na-poor) and second population (2P, Na-rich) stars. Methods. We derived the Na and Sr abundances from the archival spectra obtained with the UVES spectrograph. To do this, we used 1D ATLAS9 model atmospheres and a 1D local thermodynamic equilibrium spectral synthesis method. Particular attention was paid to assessing the potential impact of CN line blending on the obtained Sr abundances. Furthermore, we evaluated the potential influence of convection on the Sr line formation by using 3D hydrodynamical model atmospheres computed with the CO5 BOLD code. Results. Our results suggest a weak correlation between the abundances of Sr and Na. Together with a similar correlation between the abundances of Zr and Na determined in our previous study, our analysis of Sr suggests that polluters that have enriched 2P stars with light elements may have produced some s-process elements as well. The mean Sr abundance determined in 31 red giant branch stars of 47 Tuc is ⟨[Sr/Fe]⟩ = 0.18 ± 0.08 (the error denotes the standard deviation due to the star-to-star abundance scatter). This value is within the range of the Sr abundance variation that is observed in Galactic field stars of similar metallicity. The mean [Sr/Zr] abundance ratio in our sample stars suggests that the two s-process elements could have been synthesized by either low-mass asymptotic giant branch stars (M = 1 − 4 M⊙) or massive (M = 10 − 20 M⊙) fast-rotating (vrot = 200 − 300 km s−1) stars.

Center–Limb Variation of Solar Photospheric Microturbulence

Microturbulence (\xi) is a key parameter introduced in stellar spectroscopy to explain the strength of saturated lines by formally incorporating an additional thermal broadening term in the line opacity profile. Although our Sun can serve as an important testing bench to check the usual assumption of constant \xi, the detailed behavior of how \xi varies from the disk center through the limb seems to have never been investigated so far. In order to fill this gap, local \xi values on the solar disk were determined from the equivalent widths of 46 Fe I lines at 32 points from the center to the limb by requiring the consistency between the abundances derived from lines of various strengths. The run of \xi with \theta (angle between line of sight and the surface normal) was found to be only gradual from ~1.0km/s (at sin\theta = 0: disk center) to ~1.3km/s (at sin\theta ~ 0.7: two-thirds of radial distance); but thereafter increasing more steeply up to ~2km/s (at sin\theta = 0.97: limb). This result further suggests that the microturbulence derived from the flux spectrum of the disk-integrated Sun is by ~20% larger than that of the disk-center value, which is almost consistent with the prediction from 3D hydrodynamical model atmospheres.

TITANS metal-poor reference stars

Context. Several large stellar spectroscopic surveys are producing enormous amounts of data that can be used to determine stellar atmospheric parameters and chemical abundances. Nonetheless, the accuracy achieved in the derived astrophysical parameters is still insufficient, mainly because of the paucity of adequate calibrators, particularly in the metal-poor regime ([Fe/H] ≤−1.0). Aims. Our aim is to increase the number of metal-poor stellar calibrators that have accurate parameters. Here, we introduce the TITANS metal-poor reference stars: a sample of 41 dwarf and subgiant stars with accurate, but model-dependent, parameters. Methods. Effective temperatures (Teff) were derived by fitting observed Hα profiles with synthetic lines computed using three-dimensional (3D) hydrodynamic model atmospheres that take local thermodynamic equilibrium departures into account (non-LTE effects). Surface gravities (log g) were computed using evolutionary tracks and parallaxes from Gaia early-data release 3. Results. The same methods recover the Teff values of the Gaia benchmark stars, which are mostly based on interferometric measurements, with a 1σ dispersion of ±50 K. We assume this to be the accuracy of the Hα profiles computed from 3D non-LTE models for metal-poor dwarfs and subgiants, although this is likely an upper-bound estimate dominated by the uncertainty of the standard Teff values. We achieved an internal precision typically between 30 and 40 K; these errors are dominated by instrumental effects. The final total uncertainty for the Teff values of the TITANS are thus estimated to be of the order of 1%. The typical error for log g is ≤0.04 dex. In addition, we identified a few members of Gaia-Enceladus, of Sequoia, and of the Helmi stream in our sample. These stars can pave the way for the accurate chemical characterisation of these Galactic substructures. Conclusions. Using the TITANS as a reference, large stellar surveys will be able to improve the internal calibration of their astrophysical parameters. Ultimately, this sample will help users of data from Gaia and large surveys to reach their goal of redefining our understanding of stars, stellar systems, and the Milky Way.

The solar gravitational redshift from HARPS-LFC Moon spectra

Context. The general theory of relativity predicts the redshift of spectral lines in the solar photosphere as a consequence of the gravitational potential of the Sun. This effect can be measured from a solar disk-integrated flux spectrum of the Sun’s reflected light on Solar System bodies. Aims. The laser frequency comb (LFC) calibration system attached to the HARPS spectrograph offers the possibility of performing an accurate measurement of the solar gravitational redshift (GRS) by observing the Moon or other Solar System bodies. Here, we analyse the line shift observed in Fe absorption lines from five high-quality HARPS-LFC spectra of the Moon. Methods. We selected an initial sample of 326 photospheric Fe lines in the spectral range between 476–585 nm and measured their line positions and equivalent widths (EWs). Accurate line shifts were derived from the wavelength position of the core of the lines compared with the laboratory wavelengths of Fe lines. We also used a CO5BOLD 3D hydrodynamical model atmosphere of the Sun to compute 3D synthetic line profiles of a subsample of about 200 spectral Fe lines centred at their laboratory wavelengths. We fit the observed relatively weak spectral Fe lines (with EW< 180 mÅ) with the 3D synthetic profiles. Results. Convective motions in the solar photosphere do not affect the line cores of Fe lines stronger than about ∼150 mÅ. In our sample, only 15 Fe I lines have EWs in the range 150< EW(mÅ) < 550, providing a measurement of the solar GRS at 639 ± 14 m s−1, which is consistent with the expected theoretical value on Earth of ∼633.1 m s−1. A final sample of about 97 weak Fe lines with EW < 180 mÅ allows us to derive a mean global line shift of 638 ± 6 m s−1, which is in agreement with the theoretical solar GRS. Conclusions. These are the most accurate measurements of the solar GRS obtained thus far. Ultrastable spectrographs calibrated with the LFC over a larger spectral range, such as HARPS or ESPRESSO, together with a further improvement on the laboratory wavelengths, could provide a more robust measurement of the solar GRS and further testing of 3D hydrodynamical models.

Carbon, oxygen, and iron abundances in disk and halo stars

The abundances of carbon, oxygen, and iron in late-type stars are important parameters in exoplanetary and stellar physics, as well as key tracers of stellar populations and Galactic chemical evolution. However, standard spectroscopic abundance analyses can be prone to severe systematic errors, based on the assumption that the stellar atmosphere is one-dimensional (1D) and hydrostatic, and by ignoring departures from local thermodynamic equilibrium (LTE). In order to address this, we carried out three-dimensional (3D) non-LTE radiative transfer calculations for C I and O I, and 3D LTE radiative transfer calculations for Fe II, across the STAGGER-grid of 3D hydrodynamic model atmospheres. The absolute 3D non-LTE versus 1D LTE abundance corrections can be as severe as − 0.3 dex for C I lines in low-metallicity F dwarfs, and − 0.6 dex for O I lines in high-metallicity F dwarfs. The 3D LTE versus 1D LTE abundance corrections for Fe II lines are less severe, typically less than + 0.15 dex. We used the corrections in a re-analysis of carbon, oxygen, and iron in 187 F and G dwarfs in the Galactic disk and halo. Applying the differential 3D non-LTE corrections to 1D LTE abundances visibly reduces the scatter in the abundance plots. The thick disk and high-α halo population rise in carbon and oxygen with decreasing metallicity, and reach a maximum of [C/Fe] ≈ 0.2 and a plateau of [O/Fe] ≈ 0.6 at [Fe/H] ≈ −1.0. The low-α halo population is qualitatively similar, albeit offset towards lower metallicities and with larger scatter. Nevertheless, these populations overlap in the [C/O] versus [O/H] plane, decreasing to a plateau of [C/O] ≈ −0.6 below [O/H] ≈ −1.0. In the thin-disk, stars having confirmed planet detections tend to have higher values of C∕O at given [O/H]; this potential signature of planet formation is only apparent after applying the abundance corrections to the 1D LTE results. Our grids of line-by-line abundance corrections are publicly available and can be readily used to improve the accuracy of spectroscopic analyses of late-type stars.

A Hot Ultraviolet Flare on the M Dwarf Star GJ 674

Abstract As part of the Mega-Measurements of the Ultraviolet Spectral Characteristics of Low-Mass Exoplanetary Systems Hubble Space Telescope (HST) Treasury program, we obtained time-series ultraviolet spectroscopy of the M2.5V star, GJ 674. During the far-ultraviolet (FUV) monitoring observations, the target exhibited several small flares and one large flare (E FUV = 1030.75 erg) that persisted over the entirety of an HST orbit and had an equivalent duration >30,000 s, comparable to the highest relative amplitude event previously recorded in the FUV. The flare spectrum exhibited enhanced line emission from chromospheric, transition region, and coronal transitions and a blue FUV continuum with an unprecedented color temperature of TC ≃ 40,000 ± 10,000 K. In this Letter, we compare the flare FUV continuum emission with parameterizations of radiative hydrodynamic model atmospheres of M star flares. We find that the observed flare continuum can be reproduced using flare models but only with the ad hoc addition of a hot, dense emitting component. This observation demonstrates that flares with hot FUV continuum temperatures and significant extreme-ultraviolet/FUV energy deposition will continue to be of importance to exoplanet atmospheric chemistry and heating, even as the host M dwarfs age beyond their most active evolutionary phases.

Abundances of Mg and K in the atmospheres of turn-off starsin Galactic globular cluster 47 Tucanae

Aims. We determined abundances of Mg and K in the atmospheres of 53 (Mg) and 75 (K) turn-off (TO) stars of the Galactic globular cluster 47 Tuc. The obtained abundances, together with those of Li, O, and Na that we had earlier determined for the same sample of stars, were used to search for possible relations between the abundances of K and other light elements, Li, O, Na, and Mg, as well as the connections between the chemical composition of TO stars and their kinematical properties. Methods. Abundances of Mg and K were determined using archival high resolution VLT FLAMES/GIRAFFE spectra, in combination with the one-dimensional (1D) non-local thermodynamic equilibrium (NLTE) spectral synthesis methodology. Spectral line profiles were computed with the MULTI code, using 1D hydrostatic ATLAS9 stellar model atmospheres. We also utilized three-dimensional (3D) hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres for computing 3D–1D LTE abundance corrections for the spectral lines of Mg and K, in order to assess the influence of convection on their formation in the atmospheres of TO stars. Results. The determined average abundance-to-iron ratios and their root mean square variations due to star-to-star abundance spreads were 〈[Mg/Fe]〉1D NLTE = 0.47 ± 0.12, and 〈[K/Fe]〉1D NLTE = 0.39 ± 0.09. Although the data suggest the possible existence of a weak correlation in the [K/Fe]–[Na/Fe] plane, its statistical significance is low. No statistically significant relations between the abundance of K and other light elements were detected. Also, we did not find any significant correlations or anti-correlations between the [Mg/Fe] and [K/Fe] ratios and projected distance from the cluster center. Similarly, no relations between the absolute radial velocities of individual stars and abundances of Mg and K in their atmospheres were detected. The 3D–1D abundance corrections were found to be small (≤0.1 dex) for the lines of Mg and K used in this study, thus indicating that the influence of convection on their formation is small.

Global hot-star wind models for stars from Magellanic Clouds

We provide mass-loss rate predictions for O stars from Large and Small Magellanic Clouds. We calculate global (unified, hydrodynamic) model atmospheres of main sequence, giant, and supergiant stars for chemical composition corresponding to Magellanic Clouds. The models solve radiative transfer equation in comoving frame, kinetic equilibrium equations (also known as NLTE equations), and hydrodynamical equations from (quasi-)hydrostatic atmosphere to expanding stellar wind. The models allow us to predict wind density, velocity, and temperature (consequently also the terminal wind velocity and the mass-loss rate) just from basic global stellar parameters. As a result of their lower metallicity, the line radiative driving is weaker leading to lower wind mass-loss rates with respect to the Galactic stars. We provide a formula that fits the mass-loss rate predicted by our models as a function of stellar luminosity and metallicity. On average, the mass-loss rate scales with metallicity as Ṁ ~ Z0.59. The predicted mass-loss rates are lower than mass-loss rates derived from Hα diagnostics and can be reconciled with observational results assuming clumping factor Cc = 9. On the other hand, the predicted mass-loss rates either agree or are slightly higher than the mass-loss rates derived from ultraviolet wind line profiles. The calculated P v ionization fractions also agree with values derived from observations for LMC stars with Teff ≤ 40 000 K. Taken together, our theoretical predictions provide reasonable models with consistent mass-loss rate determination, which can be used for quantitative study of stars from Magellanic Clouds.

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars

Although observational data unequivocally point out to the presence of chromospheres in red giant stars, no attempts have been made so far to model them using 3D hydrodynamical model atmospheres. We therefore compute an exploratory 3D hydrodynamical model atmosphere for a cool red giant in order to study the dynamical and thermodynamic properties of its chromosphere, as well as the influence of the chromosphere on its observable properties. 3D radiation hydrodynamics simulations are carried out with the CO5BOLD model atmosphere code for a star with the atmospheric parameters (Teff=4010 K, log g=1.5, [M/H]=0.0), which are similar to those of the K-type giant star Aldebaran (alpha Tau). ... we compute the emergent continuum intensity maps at different wavelengths, spectral line profiles of Ca II K, the Ca II infrared triplet line at 854.2nm, and H alpha, as well as the spectral energy distribution (SED) of the emergent radiative flux. The initial model quickly develops a dynamical chromosphere characterised by propagating and interacting shock waves. The peak temperatures in the chromospheric shock fronts reach values on the order of up to 5000 K although the shock fronts remain quite narrow. Like for the Sun, the gas temperature distribution in the upper layers is composed of a cool component due to adiabatic cooling in the expanding post-shock regions and a hot component due to shock waves. For this red giant model, the hot component is a rather flat high-temperature tail, which nevertheless affects the resulting average temperatures significantly. The simulations show that the atmospheres of red giant stars are dynamic and intermittent. Consequently, many observable properties cannot be reproduced with one-dimensional static models but demand for advanced 3D HD modelling. Furthermore, including a chromosphere in the models might produce significant contributions to the emergent UV flux.

Lithium abundance and6Li/7Li ratio in the active giant HD 123351

Current three-dimensional (3D) hydrodynamical model atmospheres together with NLTE spectrum synthesis, permit to derive reliable atomic and isotopic chemical abundances from high-resolution stellar spectra. Not much is known about the presence of the fragile 6Li isotope in evolved solar-metallicity RGB stars, not to mention its production in magnetically active targets like HD123351. From fits of the observed CFHT spectrum with synthetic line profiles based on 1D and 3D model atmospheres, we seek to estimate the abundance of the 6Li isotope and to place constraints on its origin. We derive A(Li) and the 6Li/7Li isotopic ratio by fitting different synthetic spectra to the Li-line region of a high-resolution CFHT spectrum (R=120 000, S/R=400). The synthetic spectra are computed with four different line lists, using in parallel 3D hydrodynamical CO5BOLD and 1D LHD model atmospheres and treating the line formation of the lithium components in non-LTE (NLTE). We find A(Li)=1.69+/-0.11 dex and 6Li/7Li=8.0+/-4.4 % in 3D-NLTE, using the line list of Mel\'endez et al. (2012), updated with new atomic data for V I, which results in the best fit of the lithium line profile of HD123351. Two other line lists lead to similar results but with inferior fit qualities. Our 2-sigma detection of the 6Li isotope is the result of a careful statistical analysis and the visual inspection of each achieved fit. Since the presence of a significant amount of 6Li in the atmosphere of a cool evolved star is not expected in the framework of standard stellar evolution theory, non-standard, external lithium production mechanisms, possibly related to stellar activity or a recent accretion of rocky material, need to be invoked to explain the detection of 6Li in HD123351.

Abundances of Na, Mg, and K in the atmospheres of red giant branch stars of Galactic globular cluster 47 Tucanae

We study the abundances of Na, Mg, and K in the atmospheres of 32 RGB stars in the Galactic globular cluster (GGC) 47 Tuc, with the goal to investigate the possible existence of Na-K and Mg-K correlations/anti-correlations, similar to those that were recently discovered in two other GGCs, NGC 2419 and 2808. The abundances of K, Na, and Mg were determined using high-resolution 2df spectra obtained with the AAT. The 1D NLTE abundance estimates were obtained using 1D hydrostatic ATLAS9 model atmospheres and spectral line profiles synthesized with the MULTI package. We also used 3D hydrodynamical CO5BOLD and 1D hydrostatic LHD model atmospheres to compute 3D-1D LTE abundance corrections, $\Delta_{\rm 3D-1D~LTE}$, for the spectral lines of Na, Mg, and K used in our study. These abundance corrections were used to understand the role of convection in the formation of spectral lines, as well as to estimate the differences in the abundances obtained with the 3D hydrodynamical and 1D hydrostatic model atmospheres. The average element-to-iron abundance ratios and their RMS variations due to star-to-star abundance spreads determined in our sample of RGB stars were $\langle{\rm [Na/Fe]}\rangle^{\rm 1D~NLTE}=0.42\pm0.13$, $\langle{\rm [Mg/Fe]}\rangle^{\rm 1D~NLTE}=0.41\pm0.11$, and $\langle{\rm [K/Fe]}\rangle^{\rm 1D~NLTE}=0.05\pm0.14$. We found no statistically significant relations between the abundances of the three elements studied here. Also, there were no abundance trends with the distance from the cluster center, nor any statistically significant relations between the abundance/abundance ratios and absolute radial velocities of individual stars. All these facts suggest the similarity of K abundance in stars that belong to different generations in 47 Tuc which, in turn, may hint that evolution of K in this particular cluster was unrelated to the nucleosynthesis of Na and/or Mg.

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars

Although oxygen is an important tracer of the early Galactic evolution, its abundance trends with metallicity are still relatively poorly known at [Fe/H] < -2.5. This is in part due to a lack of reliable oxygen abundance indicators in the metal-poor stars, in part due to shortcomings in 1D LTE abundance analyses. In this study we determined the oxygen abundance in the metal-poor halo giant HD 122563 using a 3D hydrodynamical CO5BOLD model atmosphere. Our main goal was to understand whether a 3D LTE analysis may help to improve the reliability of oxygen abundances determined from OH UV lines in comparison to those obtained using standard 1D LTE methodology. The oxygen abundance in HD 122563 was determined using 71 OH UV lines located in the wavelength range between 308-330 nm. The analysis was done using a high-resolution VLT UVES spectrum with a 1D LTE spectral line synthesis performed using the SYNTHE package and classical ATLAS9 model atmosphere. Subsequently, a 3D hydrodynamical CO5BOLD, and 1D hydrostatic LHD model atmospheres were used in order to compute 3D-1D abundance corrections. For this, the microturbulence velocity used with the 1D LHD model atmosphere was derived from the hydrodynamical CO5BOLD model atmosphere. As in previous studies, we found trends of the 1D LTE oxygen abundances determined from OH UV lines with line parameters, such as the line excitation potential and the line strength. These trends become significantly less pronounced in 3D LTE. Using OH UV lines we determined a 3D LTE oxygen abundance in HD 122563 of A(O) = 6.23 +/- 0.13. This is in fair agreement with the oxygen abundance obtained from OH IR lines, A(O) = 6.39 +/- 0.11, but it is noticeably lower than that determined using the forbidden [OI] line, A(O) = 6.53 +/- 0.15. While the exact cause for this discrepancy remains unclear, it is very likely that non-LTE effects may play a decisive role here.

An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres

Context. Tighter constraints on metal-poor stars we observe are needed to better understand the chemical processes of the early Universe. Computing a stellar spectrum in 3D allows one to model complex stellar behaviours, which cannot be replicated in 1D. Aims. We examine the effect that the intrinsic CNO abundances have on a 3D model structure and the resulting 3D spectrum synthesis. Methods. Model atmospheres were computed in 3D for three distinct CNO chemical compositions using the CO5BOLD model atmosphere code, and their internal structures were examined. Synthetic spectra were computed from these models using Linfor3D and they were compared. New 3D abundance corrections for the G-band and a selection of UV OH lines were also computed. Results. The varying CNO abundances change the metal content of the 3D models. This had an effect on the model structure and the resulting synthesis. However, it was found that the C/O ratio had a larger effect than the overall metal content of a model. Conclusions. Our results suggest that varying the C/O ratio has a substantial impact on the internal structure of the 3D model, even in the hot turn-off star models explored here. This suggests that bespoke 3D models, for specific CNO abundances should be sought. Such effects are not seen in 1D at these temperature regimes.

An in-depth spectroscopic examination of molecular bands from 3D hydrodynamical model atmospheres

Recent developments in the three-dimensional (3D) spectral synthesis code Linfor3D have meant that, for the first time, large spectral wavelength regions, such as molecular bands, can be synthesised with it in a short amount of time. A detailed spectral analysis of the synthetic G-band for several dwarf turn-off-type 3D atmospheres (5850 <= T_eff [K] <= 6550, 4.0 <= log g <= 4.5, -3.0 <= [Fe/H] <= -1.0) was conducted, under the assumption of local thermodynamic equilibrium. We also examine carbon and oxygen molecule formation at various metallicity regimes and discuss the impact it has on the G-band. Using a qualitative approach, we describe the different behaviours between the 3D atmospheres and the traditional one-dimensional (1D) atmospheres and how the different physics involved inevitably leads to abundance corrections, which differ over varying metallicities. Spectra computed in 1D were fit to every 3D spectrum to determine the 3D abundance correction. Early analysis revealed that the CH molecules that make up the G-band exhibited an oxygen abundance dependency; a higher oxygen abundance leads to weaker CH features. Nitrogen abundances showed zero impact to CH formation. The 3D corrections are also stronger at lower metallicity. Analysis of the 3D corrections to the G-band allows us to assign estimations of the 3D abundance correction to most dwarf stars presented in the literature. The 3D corrections suggest that A(C) in CEMP stars with high A(C) would remain unchanged, but would decrease in CEMP stars with lower A(C). It was found that the C/O ratio is an important parameter to the G-band in 3D. Additional testing confirmed that the C/O ratio is an equally important parameter for OH transitions under 3D. This presents a clear interrelation between the carbon and oxygen abundances in 3D atmospheres through their molecular species, which is not seen in 1D.

Hydrodynamical model atmospheres: Their impact on stellar spectroscopy and asteroseismology of late‐type stars

AbstractHydrodynamical, i.e. multi‐dimensional and time‐dependent, model atmospheres of late‐type stars have reached a high level of realism. They are commonly applied in high‐fidelity work on stellar abundances but also allow the study of processes that are not modelled in standard, one‐dimensional hydrostatic model atmospheres. Here, we discuss two observational aspects that emerge from such processes, the photometric granulation background and the spectroscopic microturbulence. We use CO5BOLD hydrodynamical model atmospheres to characterize the total granular brightness fluctuations and characteristic time scale for FGK stars. Emphasis is put on the diagnostic potential of the granulation background for constraining the fundamental atmospheric parameters. We find a clear metallicity dependence of the granulation background. The comparison between the model predictions and available observational constraints at solar metallicity shows significant differences, that need further clarification. Concerning microturbulence, we report on the derivation of a theoretical calibration based on CO5BOLD models, which shows good correspondence with the measurements for stars in the Hyades. We emphasize the importance of a consistent procedure when determining the microturbulence, and point to limitations of the commonly applied description of microturbulence in hydrostatic model atmospheres.

Non-LTE line formation of Fe in late-type stars – III. 3D non-LTE analysis of metal-poor stars

As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D hydrodynamic Stagger model atmospheres, and employ a new model atom that includes new quantum-mechanical neutral hydrogen collisional rate coefficients. With the exception of the red giant HD122563, we find that the 3D non-LTE models achieve Fe i/Fe ii excitation and ionization balance as well as not having any trends with equivalent width to within modelling uncertainties of 0.05 dex, all without having to invoke any microturbulent broadening; for HD122563 we predict that the current best parallax-based surface gravity is over-estimated by 0.5 dex. Using a 3D non-LTE analysis, we infer iron abundances from the 3D model atmospheres that are roughly 0.1 dex higher than corresponding abundances from 1D MARCS model atmospheres; these differences go in the same direction as the non-LTE effects themselves. We make available grids of departure coefficients, equivalent widths and abundance corrections, calculated on 1D MARCS model atmospheres and horizontally- and temporally-averaged 3D Stagger model atmospheres.

Lithium spectral line formation in stellar atmospheres

Different simplified approaches are used to account for the non-local thermodynamic equilibrium (NLTE) effects with 3D hydrodynamical model atmospheres. In certain cases, chemical abundances are derived in 1D NLTE and corrected for the 3D effects by adding 3D-1D LTE abundance corrections (3D+NLTE approach). Alternatively, average <3D> model atmospheres are sometimes used to substitute for the full 3D hydrodynamical models. We tested whether the results obtained using these simplified schemes (i.e., 3D+NLTE, <3D> NLTE) may reproduce those derived using the full 3D NLTE computations. The tests were made using 3D hydrodynamical CO5BOLD model atmospheres of the main sequence (MS), main sequence turn-off (TO), subgiant (SGB), and red giant branch (RGB) stars, all at [M/H]=0.0 and -2.0. Our goal was to investigate the role of 3D and NLTE effects on the formation of the 670.8 nm lithium line by assessing strengths of synthetic 670.8 nm line profiles, computed using 3D/1D NLTE/LTE approaches. Our results show that Li 670.8 nm line strengths obtained using different methodologies differ only slightly in most of the models at solar metallicity. However, the line strengths predicted with the 3D NLTE and 3D+NLTE approaches become significantly different at subsolar metallicities. At [M/H]=-2.0, this may lead to (3D NLTE)-(3D+NLTE) differences in the predicted lithium abundance of ~0.46 and ~0.31 dex in the TO and RGB stars, respectively. On the other hand, NLTE line strengths computed with the average <3D> and 1D model atmospheres are similar to those obtained with the full 3D NLTE approach for MS, TO, SGB, and RGB stars, at all metallicities; 3D-<3D> and 3D-1D differences in the predicted abundances are always less than ~0.04 dex and ~0.08 dex, respectively. However, neither of the simplified approaches can reliably substitute 3D NLTE spectral synthesis when precision is required.

Non-LTE oxygen line formation in 3D hydrodynamic model stellar atmospheres

The OI 777 nm lines are among the most commonly used diagnostics for the oxygen abundances in the atmospheres of FGK-type stars. However, they form in conditions that are far from local thermodynamic equilibrium (LTE). We explore the departures from LTE of atomic oxygen, and their impact on OI lines, across the Stagger-grid of three-dimensional hydrodynamic model atmospheres. For the OI 777 nm triplet we find significant departures from LTE. These departures are larger in stars with larger effective temperatures, smaller surface gravities, and larger oxygen abundances. We present grids of predicted 3D non-LTE based equivalent widths for the OI616nm, [OI] 630 nm, [OI] 636 nm, and OI 777 nm lines, as well as abundance corrections to 1D LTE based results.

The photospheric solar oxygen project

The solar photospheric oxygen abundance is still widely debated. Adopting the solar chemical composition based on the "low" oxygen abundance, as determined with the use of three-dimensional (3D) hydrodynamical model atmospheres, results in a well-known mismatch between theoretical solar models and helioseismic measurements that is so far unresolved. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the OI IR triplet lines at 777 nm in different sets of spectra with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTETD. The idea is to simultaneously derive the oxygen abundance,A(O), and the scaling factor SH that describes the cross-sections for inelastic collisions with neutral hydrogen relative the classical Drawin formula. The best fit of the center-to-limb variation of the triplet lines achieved with the CO5BOLD 3D solar model is clearly of superior quality compared to the line profile fits obtained with standard 1D model atmospheres. Our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 +/- 0.02, with the scaling factor SH in the range between 1.2 and 1.8. All 1D non-LTE models give much lower oxygen abundances, by up to -0.15 dex. This is mainly a consequence of the assumption of a $\mu$-independent microturbulence.

The Galactic chemical evolution of oxygen inferred from 3D non-LTE spectral-line-formation calculations

Abstract We revisit the Galactic chemical evolution of oxygen, addressing the systematic errors inherent in classical determinations of the oxygen abundance that arise from the use of one-dimensional (1D) hydrostatic model atmospheres and from the assumption of local thermodynamic equilibrium (LTE). We perform detailed 3D non-LTE radiative-transfer calculations for atomic oxygen lines across a grid of 3D hydrodynamic stagger model atmospheres for dwarfs and subgiants. We apply our grid of predicted line strengths of the [O i] 630 nm and O i 777 nm lines using accurate stellar parameters from the literature. We infer a steep decay in [O/Fe] for [Fe/H] ≳ −1.0, a plateau [O/Fe] ≈ 0.5 down to [Fe/H] ≈ −2.5, and an increasing trend for [Fe/H] ≲ −2.5. Our 3D non-LTE calculations yield overall concordant results from the two oxygen abundance diagnostics.