arXiv subjects

Martin Asplund

Publications and source records attributed to Martin Asplund.

At least 109 records · Page 6Linked to original sources

arXiv2013

The Stagger-grid - A Grid of 3D Stellar Atmosphere Models

Theoretical atmosphere models provide the basis for a variety of applications in astronomy. In simplified one-dimensional (1D) atmosphere models, convection is usually treated with the mixing length theory despite its well-known insufficiency, therefore, the superadiabatic regime is poorly rendered. Due to the increasing computational power over the last decades, we are now capable to compute large grids of realistic three-dimensional (3D) hydrodynamical model atmospheres with the realistic treatment of the radiative transfer. We have computed the Stagger-grid, a comprehensive grid of 3D atmosphere models for late-type stars. In the presented contribution, we discuss initial results of the grid by exploring global properties and mean stratifications of the 3D models. Furthermore, we also depict the differences to classic 1D atmosphere models.

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arXiv2013

The Stagger-grid: A Grid of 3D Stellar Atmosphere Models - II. Horizontal and Temporal Averaging and Spectral Line Formation

We study the implications of averaging methods with different reference depth scales for 3D hydrodynamical model atmospheres computed with the Stagger-code. The temporally and spatially averaged (hereafter denoted as <3D>) models are explored in the light of local thermodynamic equilibrium (LTE) spectral line formation by comparing spectrum calculations using full 3D atmosphere structures with those from <3D> averages. We explore methods for computing mean <3D> stratifications from the Stagger-grid time-dependent 3D radiative hydro- dynamical atmosphere models by considering four different reference depth scales (geometrical depth, column-mass density, and two optical depth scales). Furthermore, we investigate the influence of alternative averages (logarithmic or enforced hydrostatic equilibrium, flux-weighted temperatures). For the line formation we compute curves of growth for Fe i and Fe ii lines in LTE . The resulting <3D> stratifications for the four reference depth scales can be considerably different. We find typically that in the upper atmosphere and in the superadiabatic region just below the optical surface, where the temperature and density fluctuations are highest, the differences become considerable and increase for higher Teff, lower logg, and lower [Fe/H]. The differential comparison of spectral line formation shows distinctive differences depending on which <3D> model is applied. The averages over layers of constant column-mass density yield the best mean <3D> representation for LTE line formation, while the averages on layers at constant geometrical height are the least appropriate. Unexpectedly, the usually preferred averages over layers of constant optical depth are prone to the increasing interference of the reversed granulation towards higher effective temperature, in particular at low metallicity.

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arXiv2013

High Precision Abundances of the Old Solar Twin HIP 102152: Insights on Li Depletion from the Oldest Sun

We present the first detailed chemical abundance analysis of the old 8.2 Gyr solar twin, HIP 102152. We derive differential abundances of 21 elements relative to the Sun with precisions as high as 0.004 dex ($\lesssim$1%), using ultra high-resolution (R = 110,000), high S/N UVES spectra obtained on the 8.2-m Very Large Telescope. Our determined metallicity of HIP 102152 is [Fe/H] = -0.013 $\pm$ 0.004. The atmospheric parameters of the star were determined to be 54 K cooler than the Sun, 0.09 dex lower in surface gravity, and a microturbulence identical to our derived solar value. Elemental abundance ratios examined vs. dust condensation temperature reveal a solar abundance pattern for this star, in contrast to most solar twins. The abundance pattern of HIP 02152 appears to be the most similar to solar of any known solar twin. Abundances of the younger, 2.9 Gyr solar twin, 18 Sco, were also determined from UVES spectra to serve as a comparison for HIP 102152. The solar chemical pattern of HIP 102152 makes it a potential candidate to host terrestrial planets, which is reinforced by the lack of giant planets in its terrestrial planet region. The following non-local thermodynamic equilibrium Li abundances were obtained for HIP 102152, 18 Sco, and the Sun: log $ε$ (Li) = 0.48 $\pm$ 0.07, 1.62 $\pm$ 0.02, and 1.07 $\pm$ 0.02, respectively. The Li abundance of HIP 102152 is the lowest reported to date for a solar twin, and allows us to consider an emerging, tightly constrained Li-age trend for solar twin stars.

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arXiv2013

High Precision Differential Abundance Measurements in Globular Clusters: Chemical Inhomogeneities in NGC 6752

We report on a strictly differential line-by-line analysis of high quality UVES spectra of bright giants in the metal-poor globular cluster NGC 6752. We achieved high precision differential chemical abundance measurements for Fe, Na, Si, Ca, Ti, Cr, Ni, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy with uncertainties as low as ~0.01 dex (~2%). We obtained the following main results. (1) The observed abundance dispersions are a factor of ~2 larger than the average measurement uncertainty. (2) There are positive correlations, of high statistical significance, between all elements and Na. (3) For any pair of elements, there are positive correlations of high statistical significance, although the amplitudes of the abundance variations are small. Removing abundance trends with effective temperature and/or using a different pair of reference stars does not alter these results. These abundance variations and correlations may reflect a combination of (a) He abundance variations and (b) inhomogeneous chemical evolution in the pre- or proto-cluster environment. Regarding the former, the current constraints on Delta Y from photometry likely preclude He as being the sole explanation. Regarding the latter, the nucleosynthetic source(s) must have synthesised Na, alpha, Fe-peak and neutron-capture elements and in constant amounts for species heavier than Si; no individual object can achieve such nucleosynthesis. We speculate that other, if not all, globular clusters may exhibit comparable abundance variations and correlations to NGC 6752 if subjected to a similarly precise analysis.

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arXiv2013

The lithium isotopic ratio in very metal-poor stars

Un-evolved, very metal-poor stars are the most important tracers of the cosmic abundance of lithium in the early universe. Combining the standard Big Bang nucleosynthesis model with Galactic production through cosmic ray spallation, these stars at [Fe/H]<-2 are expected to show an undetectably small 6Li/7Li isotopic signature. Evidence to the contrary may necessitate an additional pre-galactic production source or a revision of the standard model of Big Bang nucleosynthesis. We revisit the isotopic analysis of four halo stars, two with claimed 6Li-detections in the literature, to investigate the influence of improved model atmospheres and line formation treatment. For the first time, a combined 3D, NLTE (non-local thermodynamic equilibrium) modelling technique for Li, Na, and Ca lines is utilised to constrain the intrinsic line-broadening and to determine the Li isotopic ratio. We discuss the influence of 3D NLTE effects on line profile shapes and assess the realism of our modelling using the Ca excitation and ionisation balance. By accounting for NLTE line formation in realistic 3D hydrodynamical model atmospheres, we can model the Li resonance line and other neutral lines with a consistency that is superior to LTE, with no need for additional line asymmetry caused by the presence of 6Li. Contrary to the results from 1D and 3D LTE modelling, no star in our sample has a significant 2-sigma detection of the lighter isotope in NLTE. Over a large parameter space, NLTE modelling systematically reduces the best-fit Li isotopic ratios by up to five percentage points. As a bi-product, we also present the first ever 3D NLTE Ca and Na abundances of halo stars, which reveal significant departures from LTE. The observational support for a significant and non-standard 6Li production source in the early universe is substantially weakened by our findings.

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arXiv2013

How realistic are solar model atmospheres?

Recently, new solar model atmospheres have been developed to replace classical 1D LTE hydrostatic models and used to for example derive the solar chemical composition. We aim to test various models against key observational constraints. In particular, a 3D model used to derive the solar abundances, a 3D MHD model (with an imposed 10 mT vertical magnetic field), 1D models from the PHOENIX project, the 1D MARCS model, and the 1D semi-empirical model of Holweger & Müller. We confront the models with observational diagnostics of the temperature profile: continuum centre-to-limb variations (CLV), absolute continuum fluxes, and the wings of hydrogen lines. We also test the 3D models for the intensity distribution of the granulation and spectral line shapes. The predictions from the 3D model are in excellent agreement with the continuum CLV observations, performing even better than the Holweger & Müller model (constructed largely to fulfil such observations). The predictions of the 1D theoretical models are worse, given their steeper temperature gradients. For the continuum fluxes, predictions for most models agree well with the observations. No model fits all hydrogen lines perfectly, but again the 3D model comes ahead. The 3D model also reproduces the observed continuum intensity fluctuations and spectral line shapes very well. The excellent agreement of the 3D model with the observables reinforces the view that its temperature structure is realistic. It outperforms the MHD simulation in all diagnostics, implying that recent claims for revised abundances based on MHD modelling are premature. Several weaknesses in the 1D models are exposed. The differences between the PHOENIX LTE and NLTE models are small. We conclude that the 3D hydrodynamical model is superior to any of the tested 1D models, which gives further confidence in the solar abundance analyses based on it.

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arXiv2013

A Grid of 3D 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, 3D, convective, stellar atmosphere simulations. This grid is to be used in general for interpreting observations, and improve 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 use of the best available atomic physics for quantitative predictions and comparisons with observations. We present granulation size, convective expansion of the acoustic cavity, asymptotic adiabat, as function of atmospheric parameters. These and other results are also available in electronic form.

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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

The detection and treatment of distance errors in kinematic analyses of stars

We present a new method for detecting and correcting systematic errors in the distances to stars when both proper motions and line-of-sight velocities are available. The method, which is applicable for samples of 200 or more stars that have a significant extension on the sky, exploits correlations between the measured U, V and W velocity components that are introduced by distance errors. We deliver a formalism to describe and interpret the specific imprints of distance errors including spurious velocity correlations and shifts of mean motion in a sample. We take into account correlations introduced by measurement errors, Galactic rotation and changes in the orientation of the velocity ellipsoid with position in the Galaxy. Tests on pseudodata show that the method is more robust and sensitive than traditional approaches to this problem. We investigate approaches to characterising the probability distribution of distance errors, in addition to the mean distance error, which is the main theme of the paper. Stars with the most overestimated distances bias our estimate of the overall distance scale, leading to the corrected distances being slightly too small. We give a formula that can be used to correct for this effect. We apply the method to samples of stars from the SEGUE survey, exploring optimal gravity cuts, sample contamination, and correcting the used distance relations.

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arXiv2011

The chemical evolution of the Galactic Bulge seen through micro-lensing events

Galactic bulges are central to understanding galaxy formation and evolution. Here we report on recent studies using micro-lensing events to obtain spectra of high resolution and moderately high signal-to-noise ratios of dwarf stars in the Galactic bulge. Normally this is not feasible for the faint turn-off stars in the Galactic bulge, but micro-lensing offers this possibility. Elemental abundance trends in the Galactic bulge as traced by dwarf stars are very similar to those seen for dwarf stars in the solar neighbourhood. We discuss the implications of the ages and metallicity distribution function derived for the micro-lensed dwarf stars in the Galactic bulge.

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arXiv2011

The StaggerGrid Project: a Grid of 3D Model Atmospheres for High-Precision Spectroscopy

In this contribution, we present the StaggerGrid, a collaborative project for the construction of a comprehensive grid of time-dependent, three-dimensional (3D), hydrodynamic model atmospheres of solar- and late-type stars with different effective temperatures, surface gravities, and chemical compositions. We illustrate the main characteristics of these 3D models and their effects on the predicted strengths, wavelength-shifts, and shapes of spectral lines, highlighting the differences with respect to calculations based on classical, one-dimensional, hydrostatic models, and discuss some of their possible applications to elemental abundance analysis of stellar spectra in the context of large observational surveys.

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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

Is the solar spectrum latitude dependent? An investigation with SST/TRIPPEL

Context: In studies of the solar spectrum relative to spectra of solar twin stars, it has been found that the chemical composition of the Sun seems to depart systematically from those of the twins. One possible explanation is that the effect is due to the special aspect angle of the Sun when observed from Earth, as compared with the aspect angles of the twins. Thus, a latitude dependence of the solar spectrum, even with the heliocentric angle constant, could lead to effects of the type observed. Aim: We explore a possible variation in the strength of certain spectral lines, used in the comparisons between the composition of the Sun and the twins, at loci on the solar disk with different latitudes but at constant heliocentric angle. Methods: We use the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope on La Palma to record spectra in five spectral regions in order to compare different locations on the solar disk at a heliocentric angle of 45 deg. Equivalent widths and other parameters are measured for fifteen different lines representing nine atomic species. Results: The relative variations in equivalent widths at the equator and at solar latitude 45 deg are found to be less than 1.5 % for all spectral lines studied. Translated to elemental abundances as they would be measured from a terrestrial and a hypothetical pole-on observer, the difference is estimated to be within 0.005 dex in all cases. Conclusion: It is very unlikely that latitude effects could cause the reported abundance difference between the Sun and the solar twins. The accuracy obtainable in measurements of small differences in spectral line strengths between different solar disk positions is very high.

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arXiv2011

New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey

We present a re-analysis of the Geneva-Copenhagen survey, which benefits from the infrared flux method to improve the accuracy of the derived stellar effective temperatures and uses the latter to build a consistent and improved metallicity scale. Metallicities are calibrated on high-resolution spectroscopy and checked against four open clusters and a moving group, showing excellent consistency. The new temperature and metallicity scales provide a better match to theoretical isochrones, which are used for a Bayesian analysis of stellar ages. With respect to previous analyses, our stars are on average 100 K hotter and 0.1 dex more metal rich, which shift the peak of the metallicity distribution function around the solar value. From Stromgren photometry we are able to derive for the first time a proxy for alpha elements, which enables us to perform a tentative dissection of the chemical thin and thick disc. We find evidence for the latter being composed of an old, mildly but systematically alpha-enhanced population that extends to super solar metallicities, in agreement with spectroscopic studies. Our revision offers the largest existing kinematically unbiased sample of the solar neighbourhood that contains full information on kinematics, metallicities, and ages and thus provides better constraints on the physical processes relevant in the build-up of the Milky Way disc, enabling a better understanding of the Sun in a Galactic context.

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arXiv2011

On the alleged duality of the Galactic halo

We examine the kinematics of the Galactic halo based on SDSS/SEGUE data by Carollo et al. (2007, 2010). We find that their claims of a counter-rotating halo are the result of substantial biases in distance estimates (of order 50%): the claimed retrograde component, which makes up only a tiny fraction of the entire sample, prone to contaminations, is identified as the tail of distance overestimates. The strong overestimates also result in a lift in the vertical velocity component, which explains the large altitudes those objects were claimed to reach. Errors are worst for the lowest metallicity stars, which explains the metal-poor nature of the artificial component. We also argue that measurement errors were not properly accounted for and that the use of Gaussian fitting on intrinsically non-Gaussian Galactic components invokes the identification of components that are distorted or even artificial. Our evaluation of the data leads to a revision of the estimated velocity ellipsoids and does not yield any reliable evidence for a counterrotating halo component. If a distinct counterrotating halo component exists it must be far weaker than claimed by Carollo et al. Finally we note that their revised analysis presented in Beers et al. (2011) does not alleviate our main concerns.

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arXiv2010

Lithium depletion in solar-like stars: no planet connection

We have determined precise stellar parameters and lithium abundances in a sample of 117 stars with basic properties very similar to the Sun. This sample selection reduces biasing effects and systematic errors in the analysis. We estimate the ages of our sample stars mainly from isochrone fitting but also from measurements of rotation period and X-ray luminosity and test the connection between lithium abundance, age, and stellar parameters. We find strong evidence for increasing lithium depletion with age. Our sample includes 14 stars that are known to host planets and it does not support recent claims that planet-host stars have experienced more lithium depletion than stars without planets. We find the solar lithium abundance normal for a star of its age, mass, and metallicity. Furthermore, we analyze published data for 82 stars that were reported to support an enhanced lithium depletion in planet hosts. We show that those stars in fact follow an age trend very similar to that found with our sample and that the presence of giant planets is not related to low lithium abundances. Finally, we discuss the systematic biases that led to the incorrect conclusion of an enhanced lithium depletion in planet-host stars.

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