Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity

Fabbian, D.; Asplund, Martin; Barklem, P. S.; Carlsson, Mats; Kiselman, Dan

doi:10.1051/0004-6361/200809640

Cited by 89 publications

(146 citation statements)

References 76 publications

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“…2), and hence we were able to account for the intersystem collisional transitions. The latter transitions are thought to play an important role in defining the statistical equilibrium of oxygen (especially at low metallicities, see Fabbian et al 2009) and must be taken into account in realistic NLTE simulations. While the work of Barklem (2007) provides data for a total of 171 bound-bound transitions, we have only used 48 of them, based on the magnitude of their rate coefficient.…”

Section: The Oxygen Model Atommentioning

confidence: 99%

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The photospheric solar oxygen project

Steffen

Prakapavičius

Caffau

et al. 2015

A&A

108

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Context. 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. Aims. We carry out an independent redetermination of the solar oxygen abundance by investigating the center-to-limb variation of the O i IR triplet lines at 777 nm in different sets of spectra.Methods. The high-resolution and high signal-to-noise solar center-to-limb spectra are analyzed with the help of detailed synthetic line profiles based on 3D hydrodynamical CO5BOLD model atmospheres and 3D non-LTE line formation calculations with NLTE3D. The idea is to exploit the information contained in the observations at different limb angles to simultaneously derive the oxygen abundance, A(O), and the scaling factor S H that describes the cross-sections for inelastic collisions with neutral hydrogen relative to the classical Drawin formula. Using the same codes and methods, we compare our 3D results with those obtained from the semiempirical Holweger-Müller model atmosphere as well as from different one-dimensional (1D) reference models. Results. With the CO5BOLD 3D solar model, the best fit of the center-to-limb variation of the triplet lines is obtained when the collisions by neutral hydrogen atoms are assumed to be efficient, i.e., when the scaling factor S H is between 1.2 and 1.8, depending on the choice of the observed spectrum and the triplet component used in the analysis. The line profile fits achieved with standard 1D model atmospheres (with fixed microturbulence, independent of disk position μ) are clearly of inferior quality compared to the 3D case, and give the best match to the observations when ignoring collisions with neutral hydrogen (S H = 0). The results derived with the Holweger-Müller model are intermediate between 3D and standard 1D. Conclusions. The analysis of various observations of the triplet lines with different methods yields oxygen abundance values (on a logarithmic scale where A(H) = 12) that fall in the range 8.74 < A(O) < 8.78, and our best estimate of the 3D non-LTE solar oxygen abundance is A(O) = 8.76 ± 0.02. 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 μ-independent microturbulence. An independent determination of the relevant collisional cross-sections is essential to substantially improve the accuracy of the oxygen abundance derived from the O i IR triplet.

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Section: The Oxygen Model Atommentioning

confidence: 99%

“…In this case, an effective minimum oscillator strength of f = 0.001 is adopted for all forbidden transitions, including the transitions between the triplet and quintuplet system, as suggested by Fabbian et al (2009). Scaling by S H is restricted to the allowed transitions.…”

Section: Alternative Implementations Of the Drawin Formulamentioning

confidence: 99%

The photospheric solar oxygen project

Steffen

Prakapavičius

Caffau

et al. 2015

A&A

108

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“…For the four targets for which this was possible, we also determined the oxygen abundance based on the NIR triplet lines at 777.1 nm and 777.4 nm, finding very good agreement with the 630 nm line in LTE. Fabbian et al (2009) order of −0.05... − 0.08 dex for our targets, adopting hydrogen collisions with S H = 1.0 (Pereira et al 2009). Applying this correction would slightly lessen the agreement with the forbidden line.…”

Section: Observations and Analysismentioning

confidence: 99%

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

Lind

Charbonnel

Decressin

et al. 2011

A&A

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Context. The chemical compositions of globular clusters provide important information on the star formation that occurred at very early times in the Galaxy. In particular the abundance patterns of elements with atomic number z ≤ 13 may shed light on the properties of stars that early on enriched parts of the star-forming gas with the rest-products of hydrogen-burning at high temperatures. Aims. We analyse and discuss the chemical compositions of a large number of elements in 21 red giant branch stars in the metal-poor globular cluster NGC 6397. We compare the derived abundance patterns with theoretical predictions in the framework of the "wind of fast rotating massive star"-scenario. Methods. High-resolution spectra were obtained with the FLAMES/UVES spectrograph on the VLT. We determined non-LTE abundances of Na, and LTE abundances for the remaining 21 elements, including O (from the [OI] line at 630 nm), Mg, Al, α, iron-peak, and neutron-capture elements, many of which had not been previously analysed for this cluster. We also considered the influence of possible He enrichment in the analysis of stellar spectra. Results. We find that the Na abundances of evolved, as well as unevolved, stars in NGC 6397 show a distinct bimodality, which is indicative of two stellar populations: one primordial stellar generation of composition similar to field stars, and a second generation that is polluted with material processed during hydrogen-burning, i.e., enriched in Na and Al and depleted in O and Mg. The red giant branch exhibits a similar bimodal distribution in the Strömgren colour index c y = c 1 − (b − y), implying that there are also large differences in the N abundance. The two populations have the same composition for all analysed elements heavier than Al, within the measurement uncertainty of the analysis, with the possible exception of [Y/Fe]. Using two stars with almost identical stellar parameters, one from each generation, we estimate the difference in He content, ΔY = 0.01 ± 0.06, given the assumption that the mass fraction of iron is the same for the stars. Conclusions. NGC 6397 hosts two stellar populations that have different chemical compositions of N, O, Na, Mg, and probably Al. The cluster is dominated (75%) by the second generation. We show that massive stars of the first generation can be held responsible for the abundance patterns observed in the second generation long-lived stars of NGC 6397. We estimate that the initial mass of this globular cluster is at least ten times higher than its present-day value.

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“…However, non-local thermal equilibrium (non-LTE) effects on oxygen lines may explain the O/Fe discrepancy. Including corrections for non-LTE effects, smaller [O/Fe] (Fabbian et al 2006;Fabbian et al 2009b).…”

Section: Uncertainties In Iron Productionmentioning

confidence: 99%

The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?

Mattsson

2010

A&A

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Multi-zone chemical evolution models (CEMs), differing in the nucleosynthesis prescriptions (yields) and prescriptions of star formation, have been computed for the Milky Way. All models fit the observed O/H and Fe/H gradients well and reproduce the main characteristics of the gas distribution, but they are also designed to do so. For the C/H gradient the results are inconclusive with regards to yields and star formation. The C/Fe and O/Fe vs. Fe/H, as well as C/O vs. O/H trends predicted by the models for the solar neighbourhood zone were compared with stellar abundances from the literature. For O/Fe vs. Fe/H all models fit the data, but for C/O vs. O/H, only models with increased carbon yields for zero-metallicity stars or an evolving initial mass function provide good fits. Furthermore, a steep star formation threshold in the disc can be ruled out since it predicts a steep fall-off in all abundance gradients beyond a certain galactocentric distance (∼13 kpc) and cannot explain the possible flattening of the C/H and Fe/H gradients in the outer disc seen in observations. Since in the best-fit models the enrichment scenario is such that carbon is primarily produced in low-mass stars, it is suggested that in every environment where the peak of star formation happened a few Gyr back in time, winds of carbon-stars are responsible for most of the carbon enrichment. However, a significant contribution by zero-metallicity stars, especially at very early stages, and by winds of high-mass stars, which are increasing in strength with metallicity, cannot be ruled out by the CEMs presented here. In the solar neighbourhood, as much as 80%, or as little as 40% of the carbon may have been injected to the interstellar medium by low-and intermediate-mass stars. The stellar origin of carbon remains an open question, although production in low-and intermediate-mass stars appears to be the simplest explanation of observed carbon abundance trends.

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Neutral oxygen spectral line formation revisited with new collisional data: large departures from LTE at low metallicity

Cited by 89 publications

References 76 publications

The photospheric solar oxygen project

The photospheric solar oxygen project

Tracing the evolution of NGC 6397 through the chemical composition of its stellar populations

The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?

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