Remo Collet

Non‐LTE line formation of Fe in late‐type stars – I. Standard stars with 1D and 〈3D〉 model atmospheres

We investigate departures from LTE in the line formation of Fe for a number of well-studied late-type stars in different evolutionary stages. A new model of Fe atom was constructed from the most up-to-date theoretical and experimental atomic data available so far. Non-local thermodynamic equilibrium (NLTE) line formation calculations for Fe were performed using 1D hydrostatic MARCS and MAFAGS-OS model atmospheres, as wellas the spatial and temporal average stratifications from full 3D hydrodynamical s imulations of stellar convection computed using the Stagger code. It is shown that the Fe I/Fe II ionization balance can be well established with the 1D and mean 3D models under NLTE including calibrated inelastic collisions with H I calculated from the Drawin’s (1969) form ulae. Strong low-excitation Fe I lines are very sensitive to the atmospheric structure; clas sical 1D models fail to provide consistent excitation balance, particularly so for cool metal-po or stars. A better agreement between Fe I lines spanning a range of excitation potentials is obtai ned with the mean 3D models. Mean NLTE metallicities determined for the standard stars using the 1D and mean 3D models are fully consistent. Also, the NLTE spectroscopic effective temperatures and gravities from ionization balance agree with that determined by other methods, e.g., infrared flux method and parallaxes, if one of the stellar parameters is constrai ned independently.

A probable stellar solution to the cosmological lithium discrepancy

The measurement of the cosmic microwave background has strongly constrained the cosmological parameters of the Universe. When the measured density of baryons (ordinary matter) is combined with standard Big Bang nucleosynthesis calculations, the amounts of hydrogen, helium and lithium produced shortly after the Big Bang can be predicted with unprecedented precision. The predicted primordial lithium abundance is a factor of two to three higher than the value measured in the atmospheres of old stars. With estimated errors of 10 to 25%, this cosmological lithium discrepancy seriously challenges our understanding of stellar physics, Big Bang nucleosynthesis or both. Certain modifications to nucleosynthesis have been proposed, but found experimentally not to be viable. Diffusion theory, however, predicts atmospheric abundances of stars to vary with time, which offers a possible explanation of the discrepancy. Here we report spectroscopic observations of stars in the metal-poor globular cluster NGC 6397 that reveal trends of atmospheric abundance with evolutionary stage for various elements. These element-specific trends are reproduced by stellar-evolution models with diffusion and turbulent mixing. We thus conclude that diffusion is predominantly responsible for the low apparent stellar lithium abundance in the atmospheres of old stars by transporting the lithium deep into the star.

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

We investigate the impact of 3D hydrodynamical model atmospheres of red giant stars at different metallicities on the formation of spectral lines of a number of ions and molecules. We carry out realistic 3D simulations of surface convection in red giant stars with varying stellar parameters. We use the simulations as time-dependent hydrodynamical model stellar atmospheres to compute atomic (Li, O, Na, Mg, Ca, Fe) and molecular (CH, NH, OH) spectral lines under the assumption of local thermodynamic equilibrium (LTE). We compare the line strengths computed in 3D with the results of analogous line formation calculations for 1D, hydrostatic, plane-parallel MARCS model atmospheres in order to estimate the impact of 3D models on the derivation of elemental abundances. The temperature and density inhomogeneities and correlated velocities in 3D models, as well as the differences between the 1D and mean 3D structures significantly affect the predicted line strengths. Under the assumption of LTE, the low atmospheric temperatures of very metal-poor 3D model atmospheres cause the lines from neutral species and molecules to appear stronger than in 1D. Therefore, elemental abundances derived from these lines using 3D models are significantly lower than according to 1D analyses. Differences between 3D and 1D abundances of C, N, and O derived from CH, NH, and OH weak low-excitation lines are found to be in the range -0.5 dex to -1.0 dex for the the red giant stars at [Fe/H]=-3 considered here. At this metallicity, large negative corrections (about -0.8 dex) are also found for weak low-excitation Fe I lines. We caution, however, that departures from LTE might be significant for these and other elements and comparable to the effects due to stellar granulation.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.

ATOMIC DIFFUSION AND MIXING IN OLD STARS. I. VERY LARGE TELESCOPE FLAMES-UVES OBSERVATIONS OF STARS IN NGC 6397

We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ -2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar parameter differences between groups of stars, are in good agreement with broadband and Stromgren photometry calibrated on the infrared flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 30% less abundant in the turnoff point stars than in the red giants. An abundance difference in lithium is seen between the turnoff point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti, and Fe) are compared with stellar structure models including the effects of atomic diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that atomic diffusion and turbulent mixing are largely responsible for the subprimordial stellar lithium abundances of warm halo stars. Other consequences of atomic diffusion in old metal-poor stars are also discussed.

The Stagger-grid: A grid of 3D stellar atmosphere models - I. Methods and general properties

Aims. We present the Stagger-grid, a comprehensive grid of time-dependent, three-dimensional (3D), hydrodynamic model atmospheres for late-type stars with realistic treatment of rad iative transfer, covering a wide range in stellar parameter s. This grid of 3D models is intended for various applications besides studies of stellar convection and atmospheres per se, including stellar parameter determination, stellar spectroscopy and abundance analysis, asteroseismology, calibration of stellar evolution mo dels, interferometry, and extrasolar planet search. In this introductory paper, w e describe the methods we applied for the computation of the grid and discuss the general properties of the 3D models as well as of their temporal and spatial averages (here denotedh 3Di models). Methods. All our models were generated with the Stagger-code, using realistic input physics for the equation of sta te (EOS) and for continuous and line opacities. Our∼ 220 grid models range in effective temperature, Teff, from 4000 to 7000 K in steps of 500 K, in surface gravity, log g, from 1.5 to 5.0 in steps of 0.5 dex, and metallicity, [Fe/H], from−4.0 to +0.5 in steps of 0.5 and 1.0 dex. Results. We find a tight scaling relation between the vertical velocit y and the surface entropy jump, which itself correlates with the constant entropy value of the adiabatic convection zone. The range in intensity contrast is enhanced at lower metallici ty. The granule size correlates closely with the pressure scale height sampled at the depth of maximum velocity. We compare theh 3Di models with currently widely applied one-dimensional (1D) atmosphere models, as well as with theoretical 1D hydrostatic models generated with the same EOS and opacity tables as the 3D models, in order to isolate the effects of using self-consistent and hydrodynamic modeling of convection, rather than the classical mixing length theory (MLT) approach. For the first time, we are able to quantify s ystematically over a broad range of stellar parameters the uncertainties of 1D models arising from the simplified treatment of physics, in particular convective energy transport. In agreement with previous fin dings, we find that the di fferences can be rather significant, especially for metal-poor stars.

HE 1327-2326, An Unevolved Star With Fe/H < -5.0. II. New 3D-1D Corrected Abundances From A Very Large Telescope UVES Spectrum

We present a new abundance analysis of HE 1327?2326, which is currently the most iron-poor star, based on observational data obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES). We correct the one-dimensional (1D) LTE abundances for three-dimensional (3D) effects to provide an abundance pattern that supersedes previous works and should be used to observationally test current models of the chemical yields of the first-generation supernovae (SNe). Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D?1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 3360 ? is greatly improved due to the application of the 3D?1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected; the new Li upper limit is extremely low, -->A(Li) < 0.62, and in stark contrast with results not only from the Wilkinson Microwave Anisotropy Probe (WMAP) but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327?2326 is being reproduced by individual and integrated yields of SNe.

Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres - numerical methods and application to the quiet, non-magnetic, surface of a solar-type star

Aims. We present the implementation of a radiative transfer solver with coherent scattering in the new BIFROST code for radiative magneto-hydrodynamical (MHD) simulations of stellar surface convection. The code is fully parallelized using MPI domain decomposition, which allows for large grid sizes and improved resolution of hydrodynamical structures. We apply the code to simulate the surface granulation in a solar-type star, ignoring magnetic fields, and investigate the importance of coherent scattering for the atmospheric structure. Methods. A scattering term is added to the radiative transfer equation, requiring an iterative computation of the radiation field. We use a short-characteristics-based Gauss-Seidel acceleration scheme to compute radiative flux divergences for the energy equation. The effects of coherent scattering are tested by comparing the temperature stratification of three 3D time-dependent hydrodynamical atmosphere models of a solar-type star: without scattering, with continuum scattering only, and with both continuum and line scattering. Results. We show that continuum scattering does not have a significant impact on the photospheric temperature structure for a star like the Sun. Including scattering in line-blanketing, however, leads to a decrease of temperatures by about 350 K below log10 τ5000 < −4. The effect is opposite to that of 1D hydrostatic models in radiative equilibrium, where scattering reduces the cooling effect of strong LTE lines in the higher layers of the photosphere. Coherent line scattering also changes the temperature distribution in the high atmosphere, where we observe stronger fluctuations compared to a treatment of lines as true absorbers.

The elemental composition of the Sun I. The intermediate mass elements Na to Ca

The chemical composition of the Sun is an essential piece of reference data for astronomy, cosmology, astroparticle, space and geophysics: elemental abundances of essentially all astronomical objects are referenced to the solar composition, and basically every process involving the Sun depends on its composition. This article, dealing with the intermediate-mass elements Na to Ca, is the first in a series describing the comprehensive re-determination of the solar composition. In this series we severely scrutinise all ingredients of the analysis across all elements, to obtain the most accurate, homogeneous and reliable results possible. We employ a highly realistic 3D hydrodynamic model of the solar photosphere, which has successfully passed an arsenal of observational diagnostics. For comparison, and to quantify remaining systematic errors, we repeat the analysis using three different 1D hydrostatic model atmospheres (marcs, miss and Holweger & Muller 1974, Sol. Phys., 39, 19) and a horizontally and temporally-averaged version of the 3D model (〈3D〉). We account for departures from local thermodynamic equilibrium (LTE) wherever possible. We have scoured the literature for the best possible input data, carefully assessing transition probabilities, hyperfine splitting, partition functions and other data for inclusion in the analysis. We have put the lines we use through a very stringent quality check in terms of their observed profiles and atomic data, and discarded all that we suspect to be blended. Our final recommended 3D+NLTE abundances are: log e = 6:21 ± 0:04, log e = 7:59 ± 0:04, log e = 6:43 ± 0:04, log e = 7:51 ± 0:03, log e = 5:41 ± 0:03, log e = 7:13 ± 0:03, log e = 5:04 ± 0:05 and log e = 6:32 ± 0:03. The uncertainties include both statistical and systematic errors. Our results are systematically smaller than most previous ones with the 1D semi-empirical Holweger & Muller model, whereas the 〈3D〉 model returns abundances very similar to the full 3D calculations. This analysis provides a complete description and a slight update of the results presented in Asplund et al. (2009, ARA&A, 47, 481) for Na to Ca, and includes full details of all lines and input data used.

A GRID OF THREE-DIMENSIONAL STELLAR ATMOSPHERE MODELS OF SOLAR METALLICITY. I. GENERAL PROPERTIES, GRANULATION, AND ATMOSPHERIC EXPANSION

Present grids of stellar atmosphere models are the workhorses in interpreting stellar observations and determining their fundamental parameters. These models rely on greatly simplified models of convection, however, lending less predictive power to such models of late-type stars. We present a grid of improved and more reliable stellar atmosphere models of late-type stars, based on deep, three-dimensional (3D), convective, stellar atmosphere simulations. This grid is to be used in general for interpreting observations and improving stellar and asteroseismic modeling. We solve the Navier Stokes equations in 3D and concurrent with the radiative transfer equation, for a range of atmospheric parameters, covering most of stellar evolution with convection at the surface. We emphasize the use of the best available atomic physics for quantitative predictions and comparisons with observations. We present granulation size, convective expansion of the acoustic cavity, and asymptotic adiabat as functions of atmospheric parameters.

Chemical Homogeneity in Collinder 261 and Implications for Chemical Tagging

This paper presents abundances for 12 red giants of the old open cluster Collinder 261 based on spectra from the Very Large Telescope UVES. Abundances were derived for Na, Mg, Si, Ca, Mn, Fe, Ni, Zr, and Ba. We find that the cluster has a solar-level metallicity of [Fe/H] = -0.03 dex. However, most α- and s-process elements were found to be enhanced. The star-to-star scatter was consistent with the expected measurement uncertainty for all elements. The observed rms scatter is as follows: Na = 0.07, Mg = 0.05, Si = 0.06, Ca = 0.05, Mn = 0.03, Fe = 0.02, Ni = 0.04, Zr = 0.12, and Ba = 0.03 dex. The intrinsic scatter was estimated to be less than 0.05 dex. Such high levels of homogeneity indicate that chemical information remains preserved in this old open cluster. We use the chemical homogeneity we have now established in Cr 261, the Hyades, and the HR 1614 moving group to examine the uniqueness of the individual cluster abundance patterns, i.e., chemical signatures. We demonstrate that the three studied clusters have unique chemical signatures and discuss how other such signatures may be searched for in the future. Our findings support the prospect of chemically tagging disk stars to common formation sites in order to unravel the dissipative history of the Galactic disk.