arXiv subjects

Maria Bergemann

Publications and source records attributed to Maria Bergemann.

88 records · Page 5Linked to original sources

arXiv2012

Red Supergiant Stars as Cosmic Abundance Probes: II. NLTE Effects in J-band Silicon Lines

Medium resolution J-band spectroscopy of individual red supergiant stars is a promising tool to investigate the chemical composition of the young stellar population in star forming galaxies. As a continuation of recent work on iron and titanium, detailed non-LTE calculations are presented to investigate the influence of NLTE on the formation of silicon lines in the J-band spectra of red supergiants. Substantial effects are found resulting in significantly stronger absorption lines of neutral silicon in non-LTE. As a consequence, silicon abundances determined in non-LTE are significantly smaller than in LTE with the non-LTE abundance corrections varying smoothly between -0.4 dex to -0.1 dex for effective temperatures between 3400K to 4400K. The effects are largest at low metallicity. The physical reasons behind the non-LTE effects and the consequences for extragalactic J-band abundance studies are discussed.

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arXiv2012

Unveiling systematic biases in 1D LTE excitation-ionisation balance of Fe for FGK stars. A novel approach to determination of stellar parameters

We present a comprehensive analysis of different techniques available for the spectroscopic analysis of FGK stars, and provide a recommended methodology which efficiently estimates accurate stellar atmospheric parameters for large samples of stars. Our analysis includes a simultaneous equivalent width analysis of Fe I and Fe II spectral lines, and for the first time, utilises on-the-fly NLTE corrections of individual Fe I lines. We further investigate several temperature scales, finding that estimates from Balmer line measurements provide the most accurate effective temperatures at all metallicites. We apply our analysis to a large sample of both dwarf and giant stars selected from the RAVE survey. We then show that the difference between parameters determined by our method and that by standard 1D LTE excitation-ionisation balance of Fe reveals substantial systematic biases: up to 400 K in effective temperature, 1.0 dex in surface gravity, and 0.4 dex in metallicity for stars with [Fe/H] ~ -2.5. This has large implications for the study of the stellar populations in the Milky Way.

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arXiv2012

NLTE line formation of Fe in late-type stars II: 1D spectroscopic stellar parameters

We investigate departures from local thermodynamic equilibrium (NLTE) in the line formation of neutral and singly ionised iron lines and their impact on spectroscopic stellar parameters. The calculations are performed for an extensive grid of 1D MARCS models of metal-rich and metal-poor late-type dwarfs and giants. We find that iron abundances derived from FeI lines are increasingly underestimated in hotter, lower surface-gravity, and more metal-poor stars, in a simple and well-defined pattern, while LTE is usually a realistic approximation for FeII lines. For the vast majority of dwarfs and giants, the perturbed ionisation balance of FeI and FeII is the main relevant NLTE effect to consider in the determination of spectroscopic parameters, while for extremely metal-poor stars and hot giant stars significant impact is seen also on the excitation balance and on the microturbulence determination from FeI lines.

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arXiv2012

NLTE line formation of Fe for 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 well as the spatial and temporal average stratifications from full 3D hydrodynamical simulations 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) formulae. Strong low-excitation Fe I lines are very sensitive to the atmospheric structure; classical 1D models fail to provide consistent excitation balance, particularly so for cool metal-poor stars. A better agreement between Fe I lines spanning a range of excitation potentials is obtained 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 constrained independently.

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arXiv2012

The remarkable solar twin HIP 56948: a prime target in the quest for other Earths

We study HIP 56948, the best solar twin known to date, to determine with an unparalleled precision how similar is to the Sun in its physical properties, chemical composition and planet architecture. We explore whether the abundances anomalies may be due to pollution from stellar ejecta or to terrestrial planet formation. We perform a differential abundance analysis (both in LTE and NLTE) using high resolution (R = 100,000) high S/N (600) Keck HIRES spectra of the Sun and HIP 56948. We use precise radial velocity data from the McDonald and Keck observatories to search for planets around this star. We achieve a precision of sigma = 0.003 dex for several elements. Including errors in stellar parameters the total uncertainty is as low as sigma = 0.005 dex (1 %), which is unprecedented in elemental abundance studies. The similarities between HIP 56948 and the Sun are astonishing. HIP 56948 is only 17+/-7 K hotter than the Sun, and log g, [Fe/H] and microturbulence are only +0.02+/-0.02 dex, +0.02+/-0.01 dex and +0.01+/-0.01 km/s higher than solar, respectively. HIP 56948 has a mass of 1.02+/-0.02M_Sun and is 1 Gyr younger than the Sun. Both stars show a chemical abundance pattern that differs from most solar twins. The trend with T_cond in differential abundances (twins - HIP56948) can be reproduced very well by adding 3 M_Earth of a mix of Earth and meteoritic material, to the convection zone of HIP 56948. From our radial velocity monitoring we find no indications of giant planets interior to or within the habitable zone of HIP 56948. We conclude that HIP 56948 is an excellent candidate to host a planetary system like our own, including the possible presence of inner terrestrial planets. Its striking similarity to the Sun and its mature age makes HIP 56948 a prime target in the quest for other Earths and SETI endeavors.

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arXiv2012

Red Supergiant Stars as Cosmic Abundance Probes: NLTE Effects in J-band Iron and Titanium Lines

Detailed non-LTE calculations for red supergiant stars are presented to investigate the influence of NLTE on the formation of atomic iron and titanium lines in the J-band. With their enormous brightness at J-band red supergiant stars are ideal probes of cosmic abundances. Recent LTE studies have found that metallicities accurate to 0.15 dex can be determined from medium resolution spectroscopy of individual red supergiants in galaxies as distant as 10 Mpc. The non-LTE results obtained in this investigation support these findings. Non-LTE abundance corrections for iron are smaller than 0.05 dex for effective temperatures between 3400K to 4200K and 0.1 dex at 4400K. For titanium the non-LTE abundance corrections vary smoothly between -0.4 dex and +0.2 dex as a function of effective temperature. For both elements, the corrections also depend on stellar gravity and metallicity. The physical reasons behind the non-LTE corrections and the consequences for extragalactic J-band abundance studies are discussed.

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arXiv2011

How to Make an Atomic Blog in Your Own Kitchen. Summary of the Workshop: Uncertainties in Atomic Data and How They Propagate in Chemical Abundances

This workshop brought together scientists (including atomic physicists, theoretical astrophysicists and astronomers) concerned with the completeness and accuracy of atomic data for astrophysical applications. The topics covered in the workshop included the evaluation of uncertainties in atomic data, the propagation of such uncertainties in chemical abundances, and the feedback between observations and calculations. On a different level, we also discussed communication issues such as how to ensure that atomic data are correctly understood and used, and which forum is the best one for a fluid interaction between all communities involved in the production and use of atomic data. This paper reports on the discussions held during the workshop and introduces AstroAtom, a blog created as a platform for timely and open discussions on the needs and concerns over atomic data, and their effects on astronomical research. The complete proceedings will be published on http://astroatom.wordpress.com/.

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arXiv2011

NLTE effects on Fe I/II in the atmospheres of FGK stars and application to abundance analysis of their spectra

We describe the first results from our project aimed at large-scale calculations of NLTE abundance corrections for important astrophysical atoms and ions. In this paper, the focus is on Fe which is a proxy of stellar metallicity and is commonly used to derive effective temperature and gravity. We present a small grid of NLTE abundance corrections for Fe I lines and discuss how NLTE effects influence determination of effective temperature, surface gravity, and metallicity for late-type stars.

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arXiv2011

Role of input atomic data in spectroscopic analyses of the Sun and metal-poor stars

Analysis of high-resolution stellar spectra relies heavily upon atomic data. These include energy levels, wavelengths, cross-sections for various types of interactions between particles and photons, such as photoionization and collision induced transitions. Quantum-mechanical calculations provide largest part of these data. In this paper I describe atomic data necessary to compute model atmospheres and line formation for the late-type stars both in the assumption of local thermodynamic equilibrium (LTE) and in a more general case of non-LTE. I will focus on transition metals with 21 < Z < 29 and discuss whether (and where) more complete and/or accurate atomic data are necessary.

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arXiv2011

Ionization balance of Ti in the photospheres of the Sun and four late-type stars

In this paper we investigate statistical equilibrium of Ti in the atmospheres of late-type stars. The Ti I/Ti II level populations are computed with available experimental atomic data, except for photoionization and collision induced transition rates, for which we have to rely on theoretical approximations. For the Sun, the NLTE line formation with adjusted H I inelastic collision rates and MAFAGS-OS model atmosphere solve the long-standing discrepancy between Ti I and Ti II lines. The NLTE abundances determined from both ionization stages agree within $0.01$ dex with each other and with the Ti abundance in C I meteorites. The Ti NLTE model does not perform similarly well for the metal-poor stars, overestimating NLTE effects in the atmospheres of dwarfs, but underestimating overionization for giants. Investigating different sources of errors, we find that only [Ti/Fe] ratios based on Ti II and Fe II lines can be safely used in studies of Galactic chemical evolution. To avoid spurious abundance trends with metallicity and dwarf/giant discrepancies, it is strongly recommended to disregard Ti I lines in abundance analyses, as well as in determination of surface gravities.

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arXiv2010

NLTE abundances of Cr in the Sun and metal-poor stars

We investigate statistical equilibrium of Cr in the atmospheres of late-type stars. The main goal is to ascertain the reason for a systematic abundance discrepancy between Cr I and Cr II lines, which is often encountered in spectroscopic analyses of metal-poor stars. Up to now, all these studies relied on the assumption of local thermodynamic equilibrium (LTE) in the spectrum modelling. For the first time, we perform NLTE calculations in subdwarfs and subgiants of different metallicities. We show that the LTE assumption is inadequate to describe excitation-ionization equilibrium of Cr I/Cr II in stellar atmospheres and, as a result, leads to large errors in abundances. In particular, the NLTE abundance corrections to Cr I lines range from $+0.3$ to $+0.5$ dex at low [Fe/H]. The NLTE [Cr/Fe] trend in the halo and the disk is flat and can be reproduced by most of the models of Galactic chemical evolution with standard prescriptions for Cr and Fe nucleosynthesis.

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arXiv2010

Chromium: NLTE abundances in metal-poor stars and nucleosynthesis in the Galaxy

Aims. We investigate statistical equilibrium of Cr in the atmospheres of late-type stars to show whether the systematic abundance discrepancy between Cr I and Cr II lines, as often encountered in the literature, is due to deviations from LTE. Furthermore, we attempt to interpret the NLTE trend of [Cr/Fe] with [Fe/H] using chemical evolution models for the solar neighborhood. Methods. NLTE calculations are performed for the model of Cr atom, comprising 340 levels and 6806 transitions in total. We make use of the quantum-mechanical photoionization cross-sections of Nahar (2009) and investigate sensitivity of the model to uncertain cross-sections for H I collisions. NLTE line formation is performed for the MAFAGS-ODF model atmospheres of the Sun and 10 metal-poor stars with -3.2 < [Fe/H] < -0.5, and abundances of Cr are derived by comparison of the synthetic and observed flux spectra. Results. We achieve good ionization equilibrium of Cr for the models with different stellar parameters, if inelastic collisions with H I atoms are neglected. The solar NLTE abundance based on Cr I lines is 5.74 dex with σ = 0.05 dex; it is \sim 0.1 higher than the LTE abundance. For the metal-poor stars, the NLTE abundance corrections to Cr I lines range from +0.3 to +0.5 dex. The resulting [Cr/Fe] ratio is roughly solar for the range of metallicities analyzed here, which is consistent with current views on production of these iron peak elements in supernovae. Conclusions. The tendency of Cr to become deficient with respect to Fe in metal-poor stars is an artifact due to neglect of NLTE effects in the line formation of Cr I, and it has no relation to peculiar physical conditions in the Galactic ISM or deficiencies of nucleosynthesis theory.

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arXiv2010

Manganese Abundances in the Globular Cluster Omega Centauri

We present manganese abundances in 10 red-giant members of the globular cluster Omega Centauri; 8 stars are from the most metal-poor population (RGB MP and RGB MInt1) while two targets are members of the more metal rich groups (RGB MInt2 and MInt3). This is the first time Mn abundances have been studied in this peculiar stellar system. The LTE values of [Mn/Fe] in Omega Cen overlap those of Milky Way stars in the metal poor Omega Cen populations ([Fe/H] ~ -1.5 to -1.8), however unlike what is observed in Milky Way halo and disk stars, [Mn/Fe] declines in the two more metal-rich RGB MInt2 and MInt3 targets. Non-LTE calculations were carried out in order to derive corrections to the LTE Mn abundances. The non-LTE results for Omega Cen in comparison with the non-LTE [Mn/Fe] versus [Fe/H] trend obtained for the Milky Way confirm and strengthen the conclusion that the manganese behavior in Omega Cen is distinct. These results suggest that low-metallicity supernovae (with metallicities < -2) of either Type II or Type Ia dominated the enrichment of the more metal-rich stars in Omega Cen. The dominance of low-metallicity stars in the chemical evolution of Omega Cen has been noted previously in the s-process elements where enrichment from metal-poor AGB stars is indicated. In addition, copper, which also has metallicity dependent yields, exhibits lower values of [Cu/Fe] in the RGB MInt2 and MInt3 Omega Cen populations.

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arXiv2009

Fe-peak element abundances in disk and halo stars

At present none of Galactic chemical evolution (GCE) models provides a self-consistent description of observed trends for all iron-peak elements with metallicity simultaneously. The question is whether the discrepancy is due to deficiencies of GCE models, such as stellar yields, or due to erroneous spectroscopically-determined abundances of these elements in metal-poor stars. The present work aims at a critical reevaluation of the abundance trends for several odd and even-Z Fe-peak elements, which are important for understanding explosive nucleosynthesis in supernovae.

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arXiv2008

NLTE abundances of Mn in a sample of metal-poor stars

Following our solar work, we perform NLTE calculations of the Mn abundance for fourteen stars with [Fe/H] from 0 to -2.5, mainly to show how NLTE affects Mn abundances in cool stars of different metallicities.The spectrum synthesis and Mn abundances are based on statistical equilibrium calculations using various estimates for the influence of hydrogen collisions.The NLTE abundances of Mn in all studied stars are systematically higher than the LTE abundances. At low metallicities, the NLTE abundance corrections may run up to 0.5 - 0.7 dex. Instead of a strong depletion of Mn relative to Fe in metal-poor stars as found by the other authors, we only find slightly subsolar values of [Mn/Fe] throughout the range of metallicities analyzed here. The [Mn/Fe] trend in metal-poor stars is inconsistent with the predictions of galactic chemical evolution models, where Mn is less produced than Fe.

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arXiv2007

Formation of MnI lines in the solar atmosphere

We present a detailed NLTE analysis of 39 MnI lines in the solar spectrum. The influence of NLTE effects on the line formation and element abundance is investigated. Our goal is the derivation of solar log gfe values for manganese lines, which will later be used in differential abundance analysis of metal-poor stars. The method of spectrum synthesis is employed, which is based on a solar model atmosphere with initially specified element abundances. A manganese abundance of 5.47 dex is used with the theoretical line-blanketed model atmosphere. Statistical equilibrium calculations are carried out for the model atom, which comprises 245 and 213 levels for MnI and MnII, respectively. Photoionization cross-sections are assumed hydrogenic. For line synthesis van der Waals broadening is calculated according to Anstee & O'Mara's formalism. It is shown that hyperfine structure of the Mn lines also has strong broadening effects, and that manganese is prone to NLTE effects in the solar atmosphere. The nature of the NLTE effects and the validity of the LTE approach are discussed in detail. The role of photoionization and collisional interaction is investigated.

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