Three-dimensional hydrodynamical simulations of surface convection in red giant stars: impact on spectral line formation and abundance analysis

By R. Collet, Martin Asplund, R. Trampedach. 2007

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The information about the chemical compositions of stars is encoded in their spectra. Accurate determinations of these compositions are crucial for our understanding of stellar nucleosynthesis and Galactic chemical evolution. The determination of elemental abundances in stars requires models for the stellar atmospheres and the processes of line formation. Nearly all spectroscopic analyses of late-type stars carried out today are based on one-dimensional (1D), hydrostatic model atmospheres and on the assumption of local thermodynamic equilibrium (LTE). This approach can lead to large systematic errors in the predicted stellar atmospheric structures and line-strengths, and, hence, in the derived stellar abundances. In this thesis, examples of departures from LTE and from hydrostatic equilibrium are explored. The effects of background line opacities (line-blocking) due to atomic lines on the statistical equilibrium of Fe are investigated in late-type stars. Accounting for this line opacity is important at solar metallicity, where line-blocking significantly reduces the rates of radiatively induced ionizations of Fe. On the contrary, the effects of line-blocking in metal-poor stars are insignificant. In metal-poor stars, the dominant uncertainty in the statistical equilibrium of Fe is the treatment of inelastic H+Fe collisions. Substantial departures of Fe abundances from LTE are found at low metallicities: about 0.3 dex with efficient H+Fe collisions and about 0.5 dex without. The impact of three-dimensional (3D) hydrodynamical model atmospheres on line formation in red giant stars is also investigated. Inhomogeneities and correlated velocity fields in 3D models and differences between the mean 3D stratifications and corresponding 1D model atmospheres can significantly affect the predicted line strengths and derived abundances, in particular at very low metallicities. In LTE, the differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are in the range -0.5 dex to -1.0 dex at [Fe/H]=-3. Large negative corrections (about -0.8 dex) are also found in LTE for weak low-excitation neutral Fe lines. We also investigate the impact of 3D hydrodynamical model stellar atmospheres on the determination of elemental abundances in the carbon-rich, hyper iron-poor stars HE 0107-5240 and HE 1327-2326. The lower temperatures of the line-forming regions of the 3D models compared with 1D models cause changes in the predicted spectral line strengths. In particular we find the 3D abundances of C, N, and O to be lower by about -0.8 dex (or more) than estimated from a 1D analysis. The 3D abundance of Fe is decreased but only by -0.2 dex. Departures from LTE for Fe might actually be very large for these stars and dominate over the effects due to granulation.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/dc869a60298147d5b33df506747bdfcdd4926564

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Citations in the corpus

The Stagger-grid: A grid of 3D stellar atmosphere models - I. Methods and general properties (2013, methodology): “However, in the upper layers of the metal-poor〈3D〉 models, the effect of the non-vanishing adiabatic cooling rate is to shift the balance with radiative heating to lower temperatures due to a scarcity and weakness of spectral lines at lower metallicities (Asplundet al. 1999a; Collet et al. 2007).”
Gaia FGK benchmark stars: Metallicity (2013, methodology): “In addition, some groups have started to use three-dimensional (3D) hydrodynamical atmospheric models which can lead to different stellar parameters compared to when using onedimensional (1D) hydrostatic models(e.g, Collet et al. 2007).”
FUNDAMENTAL PARAMETERS AND CHEMICAL COMPOSITION OF ARCTURUS (2011, background): “It has been shown that these so-called 3D effects are very important for molecular features in cool giants, an effect that is further enhanced by a low metallicity (e.g., Collet et al. 2007, 2009).”
Non‐LTE line formation of Fe in late‐type stars – I. Standard stars with 1D and 〈3D〉 model atmospheres (2012, result): “We note, however, that direct comparison of our results with that of, e.g., Collet et al. (2007), is not meaningful since they performed LTE line formation calculations with full 3D hydrodynamic model atmospheres.”
Stellar Abundances for the Galactic Archeology (SAGA) Database — Compilation of the Characteristics of Known Extremely Metal-Poor Stars (2008, background): “However, it is beyond the scope of this paper and is discussed in the other extensive works for non-LTE abundances (see, e.g, Andrievsky et al. 2007) and for 3D model atmosphere (Asplund & Garćıa Pérez 2001; Collet et al. 2007).”
Atomic Diffusion and Mixing in Old Stars. I. Very Large Telescope FLAMES-UVES Observations of Stars in NGC 6397 (2007, methodology): “We use here the surface convection simulations of a red giant (Collet et al. 2007) and of a turnoff star (Asplund and Garćıa Pérez 2001) at a metallicity of [Fe/H] = −2 as time-dependent 3D hydrodynamic model atmospheres to calculate Mg, Ti, and Fe spectral lines and estimate the impact of 3D…”
CARBON-ENHANCED METAL-POOR STAR FREQUENCIES IN THE GALAXY: CORRECTIONS FOR THE EFFECT OF EVOLUTIONARY STATUS ON CARBON ABUNDANCES (2014, background): “It has been suggested (e.g., Collet et al. 2007) that these effects can lead to an overestimate of [C/Fe] = +0.5 to +0.9 for the CH feature for red giants at [Fe/H] ∼ −3.0.”
A SEARCH FOR STARS OF VERY LOW METAL ABUNDANCE. VI. DETAILED ABUNDANCES OF 313 METAL-POOR STARS (2014, methodology): “Molecular formation in cool stellar atmospheres is sensitive to the temperature and density, especially the presence and degree of granulation found in three-dimensional hydrodynamical models (e.g., Collet et al. 2007).”
The s-Process in Low Metallicity Stars. II. Interpretation of High-Resolution Spectroscopic Observations with AGB models (2011, background): “Spectroscopic observations of C and N in CEMP stars are affected by high uncertainties due to strong molecular bands, NLTE corrections or 3D hydrodynamical model atmospheres (Asplund 2005, Collet et al. 2007; Grevesse et al. 2007; Caffau et al. 2009).”
Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC 6397 , (2009, background): “However, 3D modelling of stellar atmospheres show that especially low excitation lines may give strongly overestimated abundances in 1D models of metal-poor stars (Asplund et al. 1999; Collet et al. 2007).”
Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars (2008, background): “…2005), we note that, according to simulations of slightly less evolved red giants, the 3D abundance corrections for the species considered herein are expected to be modest:|∆ logǫ| <∼ 0.1 dex at [Fe/H] ≥ 0, although slightly larger at the lowest metallicities of our targets (Collet et al. 2007).”
First stars XII. Abundances in extremely metal-poor turnoff stars,and comparison with the giants (2009, background): “The real effect of the overshooting is the overcooling of the outer layers with respect to what is predicted in radiative equilibrium models (Asplund et al. 1999; Collet et al. 2007; Caffau & Ludwig 2007; González Hernández et al. 2008, Paper XI).”
Asteroseismology of 16,000 Kepler Red Giants: Global Oscillation Parameters, Masses, and Radii (2018, background): “Collet et al. (2007) performed 3D hydrodynamical simulations of red giants with [Fe/H] from -3.0 through 0.0, and found that more metal-rich stars have larger granules (see their Figure 4) due to increased opacity.”