Analysis of Stellar Spectra with 3-D and NLTE Models

Bergemann, M.

doi:10.1007/978-3-319-06956-2_17

Cited by 2 publications

(2 citation statements)

References 69 publications

(80 reference statements)

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“…Accurate chemical analysis of cool stellar spectra (T eff 8000K) rely on a deep understanding of radiative transfer physics. As extensively explored over many astronomical applications in the review by Nissen & Gustafsson (2018), currently the two main challenges for abundance derivation involve the three-dimensional (3D) modelling of hydrodynamics of stellar atmospheres and the modelling of line formation in conditions of non-local thermodynamic equilibrium (NLTE) (see Nordlund & Stein 2009;Bergemann & Nordlander 2014;Bergemann 2014, for the physics principles). Among the large number of publications on NLTE effects (each generally focused on one particular element and on a limited number of lines; see review by Mashonkina 2014), some elements have been particularly studied.…”

Section: Introductionmentioning

confidence: 99%

Probing 3D and NLTE models using APOGEE observations of globular cluster stars

Masseron

Osorio

García-Hernández

et al. 2021

A&A

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Context. Hydrodynamical (or 3D) and non-local thermodynamic equilibrium (NLTE) effects are known to affect abundance analyses. However, there are very few observational abundance tests of 3D and NLTE models. Aims. We developed a new way of testing the abundance predictions of 3D and NLTE models, taking advantage of large spectroscopic survey data. Methods. We use a line-by-line analysis of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) spectra (H band) with the Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS). We compute line-by-line abundances of Mg, Si, Ca, and Fe for a large number of globular cluster K giants in the APOGEE survey. We compare this line-by-line analysis against NLTE and 3D predictions. Results. While the 1D–NLTE models provide corrections in the right direction, there are quantitative discrepancies between different models. We observe a better agreement with the data for the models including reliable collisional cross-sections. The agreement between data and models is not always satisfactory when the 3D spectra are computed in LTE. However, we note that for a fair comparison, 3D corrections should be computed with self-consistently derived stellar parameters, and not on 1D models with identical stellar parameters. Finally, we focus on 3D and NLTE effects on Fe lines in the H band, where we observe a systematic difference in abundance relative to the value from the optical. Our results suggest that the metallicities obtained from the H band are more accurate in metal-poor giants. Conclusions. Current 1D–NLTE models provide reliable abundance corrections, but only when the atom data and collisional cross-sections are accurate and complete. Therefore, we call for more atomic data for NLTE calculations. In contrast, we show that 3D corrections in LTE conditions are often not accurate enough, thus confirming that 3D abundance corrections are only valid when NLTE is taken into account. Consequently, more extended self-consistent 3D–NLTE computations need to be made. The method we have developed for testing 3D and NLTE models could be extended to other lines and elements, and is particularly suited for large spectroscopic surveys.

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Section: Introductionmentioning

confidence: 99%

Probing 3D and NLTE models using APOGEE observations of globular cluster stars

Masseron

Osorio

García-Hernández

et al. 2021

A&A

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“…The fundamental parameters of stars such as their effective temperature, surface gravity, and chemical composition are not observable quantities: rather, they must be inferred using model stellar atmospheres (Bergemann 2014). Three dimensional (3D) hydrodynamic 'box-in-a-star' models (Nordlund 1982) are increasingly being used in this context (Ludwig et al 2009;Magic et al 2013a;Trampedach et al 2013).…”

Section: Introductionmentioning

confidence: 99%

On line contribution functions and examining spectral line formation in 3D model stellar atmospheres

Amarsi

2015

Mon. Not. R. Astron. Soc.

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Line contribution functions are useful diagnostics for studying spectral line formation in stellar atmospheres. I derive an expression for the contribution function to the absolute flux depression that emerges from three-dimensional 'box-in-a-star' model stellar atmospheres. I illustrate the result by comparing the local thermodynamic equilibrium (LTE) spectral line formation of the high-excitation permitted OI 777 nm lines with the non-LTE case.

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Analysis of Stellar Spectra with 3-D and NLTE Models

Cited by 2 publications

References 69 publications

Probing 3D and NLTE models using APOGEE observations of globular cluster stars

Probing 3D and NLTE models using APOGEE observations of globular cluster stars

On line contribution functions and examining spectral line formation in 3D model stellar atmospheres

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