| 1. |
- Fabbian, D., et al.
(författare)
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Neutral oxygen spectral line formation revisited with new collisional data : large departures from LTE at low metallicity
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 500, s. 1221-1238
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Tidskriftsartikel (refereegranskat)abstract
- Aims: A detailed study is presented, including estimates of the impact on elemental abundance analysis, of the non-local thermodynamic equilibrium (non-LTE) formation of the high-excitation neutral oxygen 777 nm triplet in model atmospheres representative of stars with spectral types F to K. Methods: We have applied the statistical equilibrium code MULTI to a number of plane-parallel MARCS atmospheric models covering late-type stars (4500 ≤ T_eff ≤ 6500 K, 2 ≤ log g ≤ 5 [cgs], and -3.5 ≤ [Fe/H] ≤ 0). The atomic model employed includes, in particular, recent quantum-mechanical electron collision data. Results: We confirm that the O i triplet lines form under non-LTE conditions in late-type stars, suffering negative abundance corrections with respect to LTE. At solar metallicity, the non-LTE effect, mainly attributed in previous studies to photon losses in the triplet itself, is also driven by an additional significant contribution from line opacity. At low metallicity, the very pronounced departures from LTE are due to overpopulation of the lower level (3s ^5S^o) of the transition. Large line opacity stems from triplet-quintet intersystem electron collisions, a form of coupling previously not considered or seriously underestimated. The non-LTE effects generally become severe for models (both giants and dwarfs) with higher T_eff. Interestingly, in metal-poor turn-off stars, the negative non-LTE abundance corrections tend to rapidly become more severe towards lower metallicity. When neglecting H collisions, they amount to as much as |Δlog ɛ_O| ~ 0.9 dex and ~1.2 dex, respectively at [Fe/H] = -3 and [Fe/H] = -3.5. Even when such collisions are included, the LTE abundance remains a serious overestimate, correspondingly by |Δlog ɛ_O| ~ 0.5 dex and ~0.9 dex at such low metallicities. Although the poorly known inelastic hydrogen collisions thus remain an important uncertainty, the large metallicity-dependent non-LTE effects seem to point to a resulting “low” (compared to LTE) [O/Fe] in metal-poor halo stars. Conclusions: Our results may be important in solving the long-standing [O/Fe] debate. When applying the derived non-LTE corrections, the LTE oxygen abundance inferred from the 777 nm permitted triplet will be decreased substantially at low metallicity. If the classical Drawin formula is employed for O+H collisions, the derived [O/Fe] trend becomes almost flat below [Fe/H] ~ -1, in better agreement with recent literature estimates generally obtained from other oxygen abundance indicators. A value of [O/Fe] ⪉ +0.5 may therefore be appropriate, as suggested by standard theoretical models of type II supernovae nucleosynthetic yields. If neglecting impacts with H atoms instead, [O/Fe] decreases towards lower [Fe/H], which would open new questions. Our tests using ATLAS model atmospheres show that, though non-LTE corrections for metal-poor dwarfs are smaller (by ~0.2 dex when adopting efficient H collisions) than in the MARCS case, our main conclusions are preserved, and that the LTE approach tends to seriously overestimate the O abundance at low metallicity. However, in order to finally reach consistency between oxygen abundances from the different available spectral features, it is of high priority to reduce the large uncertainty regarding H collisions, to undertake a full investigation of the interplay of non-LTE and 3D effects, and to clarify the issue of the temperature scale at low metallicity.
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| 2. |
- Pereira, T. M. D., et al.
(författare)
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Oxygen lines in solar granulation. : II. Centre-to-limb variation, NLTE line formation, blends, and the solar oxygen abundance
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 508, s. 1403-1416
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Tidskriftsartikel (refereegranskat)abstract
- Context: There is a lively debate about the solar oxygen abundance and the role of 3D models in its recent downward revision. These models have been tested using high-resolution solar atlases of flux and disk-centre intensity. Further testing can be done using centre-to-limb variations. Aims: Using high-resolution and high S/N observations of neutral oxygen lines across the solar surface, we seek to test that the 3D and 1D models reproduce their observed centre-to-limb variation. In particular we seek to assess whether the latest generation of 3D hydrodynamical solar model atmospheres and NLTE line formation calculations are appropriate to derive the solar oxygen abundance. Methods: We use our recent observations of O i 777 nm, O i 615.81 nm, [O i] 630.03 nm, and nine lines of other elements for five viewing angles 0.2≤μ≤ 1 of the quiet solar disk. We compared them with the predicted line profiles from the 3D and 1D models computed with the most up-to-date line formation codes and line data and allowing for departures of LTE. The centre-to-limb variation of the O i 777 nm lines is also used to obtain an empirical correction for the poorly known efficiency of the inelastic collisions with H i. Results: The 3D model generally reproduces the centre-to-limb observations of the lines very well, particularly the oxygen lines. From the O i 777 nm lines we find that the classical Drawin recipe slightly overestimates H i collisions (S_H≈ 0.85 agrees best with the observations). The limb observations of the O i 615.82 nm line allow us to identify a previously unknown contribution of molecules for this line, prevalent at the solar limb. A detailed treatment of the [O i] 630.03 nm line that includes the recent nickel abundance shows that the 3D modelling closely agrees with the observations. The derived oxygen abundances with the 3D model are 8.68 (777 nm lines), 8.66 (630.03 nm line), and 8.62 (615.82 nm line). Conclusions: These additional tests have reinforced the trustworthiness of the 3D model and line formation for abundance analyses. SST spectra are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/508/1403
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| 3. |
- Pereira, T. M. D., et al.
(författare)
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Oxygen lines in solar granulation. : I. Testing 3D models against new observations with high spatial and spectral resolution
- 2009
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 507, s. 417-432
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Tidskriftsartikel (refereegranskat)abstract
- Aims: We seek to provide additional tests of the line formation of theoretical 3D solar photosphere models. In particular, we set out to test the spatially-resolved line formation at several viewing angles, from the solar disk-centre to the limb and focusing on atomic oxygen lines. The purpose of these tests is to provide additional information on whether the 3D model is suitable to derive the solar oxygen abundance. We also aim to empirically constrain the NLTE recipes for neutral hydrogen collisions, using the spatially-resolved observations of the O i 777 nm lines. Methods: Using the Swedish 1-m Solar Telescope we obtained high-spatial-resolution observations of five atomic oxygen lines (as well as several lines for other species, mainly Fe i) for five positions on the solar disk. These observations have a high spatial (sub-arcsecond) and spectral resolution, and a continuum intensity contrast up to 9% at 615 nm. The theoretical line profiles were computed using the 3D model, with a full 3D NLTE treatment for oxygen and LTE for the other lines. Results: At disk-centre we find an excellent agreement between predicted and observed line shifts, strengths, FWHM and asymmetries. At other viewing angles the agreement is also good, but the smaller continuum intensity contrast makes a quantitative comparison harder. We use the disk-centre observations we constrain SH, the scaling factor for the efficiency of collisions with neutral hydrogen. We find that SH=1 provides the best match to the observations, although this method is not as robust as the centre-to-limb line variations to constrain SH. Conclusions: Overall there is a very good agreement between predicted and observed line properties over the solar granulation. This further reinforces the view that the 3D model is realistic and a reliable tool to derive the solar oxygen abundance. 2D spectrograms are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/507/417
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| 4. |
- Pereira, T. M. D., et al.
(författare)
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Testing 3D solar models against observations . : Center-to-limb variations of oxygen lines, spatially-resolved line formation and probing for departures from LTE
- 2009
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 80, s. 650-
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Tidskriftsartikel (refereegranskat)abstract
- We present results from a series of observational tests to 3D and 1D solar models. In particular, emphasis is given to the line formation of atomic oxygen lines, used to derive the much debated solar oxygen photospheric abundance. Using high-quality observations obtained with the Swedish Solar Telescope (SST) we study the center-to-limb variation of the O I lines, testing the models and line formation (LTE and non-LTE). For the O I 777 nm triplet, the center-to-limb variation sets strong constraints in the non-LTE line formation, and is used to derive an empirical correction factor (SH) to the classical Drawin recipe for neutral hydrogen collisions. Taking advantage of the spatially-resolved character of the SST data, an additional framework for testing the 3D model and line formation is also studied. From the tests we confirm that the employed 3D model is realistic and its predictions agree very well with the observations.
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| 5. |
- Asplund, Martin, et al.
(författare)
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Line-blanketed model atmospheres for R Coronae Borealis stars and hydrogen-deficient carbon stars
- 1997
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Ingår i: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 318:2, s. 521-534
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Tidskriftsartikel (refereegranskat)abstract
- We have constructed line-blanketed model atmospheres for the hydrogen-deficient and carbon-rich R Coronae Borealis (RCrB) stars, as well as for the similar hydrogen-deficient carbon (HdC) stars and the cool extreme helium (EHe) stars. Improved continuum o
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| 6. |
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| 7. |
- Canocchi, G., et al.
(författare)
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3D non-LTE modeling of the stellar center-To-limb variation for transmission spectroscopy studies
- 2024
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Ingår i: Astronomy and Astrophysics. - 0004-6361. ; 683
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Tidskriftsartikel (refereegranskat)abstract
- Context. Transmission spectroscopy is one of the most powerful techniques used to characterize transiting exoplanets, since it allows for the abundance of the atomic and molecular species in the planetary atmosphere to be measured. However, stellar lines may bias the determination of such abundances if their center-To-limb variations (CLVs) are not properly accounted for. Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and the assumption of non-local ther-modynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D1 and K I resonance lines on transmission spectra. Methods. We modeled the CLV of the Na I D1 and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra were compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between μ = 0.1 and μ = 1.0. Results. Our 3D non-LTE modeling of the Na I D1 resonance line at 5896 A and the K I 7699 A resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. Moreover, the simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from the one obtained from a 1D atmosphere. The latter does indeed tend to overestimate the amplitude of the transmission curve by a factor that is on the same order of magnitude as a planetary absorption depth (i.e., up to 0.2%). Conclusions. This work highlights the fact that to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra ought to be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. Moreover, since different spectral lines show different CLV curves for the same geometry of the planet-star system, it is fundamental to model the CLV individually for each line of interest. The work will be extended to other lines and FGK-Type stars, allowing for synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, for example, those drawn from JWST.
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| 8. |
- Fabbian, D., et al.
(författare)
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The non-LTE line formation of neutral carbon in late-type stars
- 2006
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 458:3, s. 899-914
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Tidskriftsartikel (refereegranskat)abstract
- Aims.We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line formation of neutral carbon in late-type stars in order to remove some of the potential systematic errors in stellar abundance analyses employing C i features.Methods: .The statistical equilibrium code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric models.Results: .Within the parameter space explored, the high-excitation C i lines studied are stronger in non-LTE due to the combined effect of line source function drop and increased line opacity due to overpopulation of the lower level for the transitions considered; the relative importance of the two effects depends on the particular combination of T{eff}, log g, [Fe/H] and [C/Fe] and on the analysed C i line. As a consequence, the non-LTE abundance corrections are negative and can be substantially so, for example ˜ -0.4 dex in halo turn-off stars at [Fe/H]˜ -3. The magnitude of the non-LTE corrections is rather insensitive to whether inelastic H collisions are included or not.Conclusions: .Our results have implications on studies of nucleosynthetic processes and on Galactic chemical evolution models. When applying our calculated corrections to recent observational data, the upturn in [C/O] at low metallicity might still be present (thus apparently still necessitating contributions from massive Pop. III stars for the carbon production), but at a lower level and possibly with a rather shallow trend of ˜ -0.2 dex/dex below [O/H]˜ -1.
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| 9. |
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| 10. |
- Gustafsson, Bengt, et al.
(författare)
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KI emission from envelopes around N-type stars - Spectroscopic observations and interpretations
- 1997
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Ingår i: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 318:2, s. 535-542
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Tidskriftsartikel (refereegranskat)abstract
- Circumstellar envelopes around three bright N-type stars, R Scl, X TrA, and V Aql have been detected in emission in resonance lines from K I. This radiation, which is most probably scattered photospheric radiation, was first found spectroscopically, but h
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