arXiv subjects

Bengt Edvardsson

Publications and source records attributed to Bengt Edvardsson.

At least 19 recordsLinked to original sources

arXiv2026

Stellar masses and ages in Gaia Data Release 4 from the Final Luminosity Age Mass Estimator algorithm

The masses and ages of stars are key quantities for understanding exoplanetary, stellar, and galactic evolution. In the context of Gaia, these parameters provide insights into the stellar populations, helping to trace the formation and history of the Galaxy. As part of the Gaia Data Processing and Analysis Consortium (DPAC), the Final Luminosity Age Mass Estimator (FLAME) pipeline processes Gaia data to derive stellar parameters comprising luminosities, radii, masses and ages. This paper discusses the methods and data used in FLAME for Gaia Data releases and the expected performances of FLAME for the 4th Gaia Data Release. FLAME comprises two main components: the first one, which is analytical, is used to estimate luminosity, radius, and radial velocity correction due to gravitational redshift by exploiting the atmospheric, astrometric, and photometric parameters produced within Gaia. The second is a model inference based on two main approaches: a classical minimization approach, and a Bayesian framework. It aims to derive mass, age, and evolutionary stage. The two step implementation offers flexibility in handling photometric properties that are prone to systematic errors. Tests with simulated data, the Sun, and well characterised samples of stars show that the methods in FLAME perform as expected, producing results in statistical agreement with the literature. We provide new stellar fundamental parameters for some high velocity stars, stars with very low mass companions, and a selection of stars in the Plato Field of View. In Gaia Data Release 4 approximately 500 million sources will have results from the pipeline. [abridged]

astro-ph.SR↗

arXiv2021

Models of Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. I. Bolometric Corrections Derived from New MARCS Synthetic Spectra and Their Implications for Observed Colour-Magnitude Diagrams

New, high-resolution MARCS synthetic spectra have been calculated for more than a dozen mixtures of the metals allowing, in turn, for variations in C:N:O, [CNO/Fe], and enhanced abundances of C, O, Mg, and Si. Bolometric Corrections (BCs) for many of the broad-band filters currently in use have been generated from these spectra. Due to improved treatments of molecules that involve atoms of C, N, and O, the BCs for UV and blue passbands, in particular, differ substantially from those derived from previous MARCS models. These differences, and the effects on BCs of varying the abundances of the metals, are shown in a number of instructive plots. Stellar evolutionary grids for -2.5 <= [Fe/H] <= -0.5 have also been computed for the different mixtures. Isochrones based on these tracks are intercompared on the theoretical H-R diagram and on a few of the colour-magnitude diagrams that can be constructed from HST Wide Field Camera 3 (WFC3) F336W, F438W, F606W, F814W, F110W, and F160W observations. For the first application of these models, isochrones have been fitted to WFC3 photometry of the globular cluster NGC 6496 from the HST UV Legacy Survey, with very encouraging results.

astro-ph.SR↗

arXiv2021

Models for Metal-Poor Stars with Different Initial Abundances of C, N, O, Mg, and Si. II. Application to the Colour-Magnitude Diagrams of the Globular Clusters 47 Tuc, NGC 6362, M5, M3, M55, and M92

Stellar models for -2.5 <= [Fe/H] <= -0.5 that have been computed for variations in the C:N:O abundance ratio (for two different values of [CNO/Fe]) are compared with HST Wide Field Camera 3 (WFC3) observations of the globular clusters (GCs) 47 Tuc, NGC 6362, M5, M5, M55, and M92. The bolometric corrections used to transpose the models to the observed planes are based on new MARCS synthetic spectra that incorporate improved treatments of molecules that involve atoms of C, N, and O. On the assumption of well-supported distance moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the other alpha elements, which are favoured by binary stars in GCs, generally reproduce the main features of observed colour-magnitude diagrams (CMDs) to within ~ 0.03 mag. In particular, they appear to match the spreads in the observed (M_F336W - M_F438W)_0 colours that are spanned by CN-weak and CN-strong stars along the lower giant branch quite well, but not the bluest giants, which are suspected to be N-poor ([N/Fe] <~ -0.5). Both the absolute (M_F438W - M_F606W)_0 colours and the variations in these colours at a given M_F606W magnitude on the giant branch are difficult to explain unless the reddest stars are C-rich ([C/Fe] >~ +0.5). Allowing for moderate He abundance variations (delta Y ~ 0.05) improves the fits to the observations.

astro-ph.SR↗

arXiv2020

Atomic data for the Gaia-ESO Survey

We describe the atomic and molecular data that were used for the abundance analyses of FGK-type stars carried out within the Gaia-ESO Survey. We present an unprecedented effort to create a homogeneous line list, which was used by several abundance analysis groups to calculate synthetic spectra and equivalent widths. The atomic data are accompanied by quality indicators and detailed references to the sources. The atomic and molecular data are made publicly available in electronic form. In general experimental transition probabilities were preferred but theoretical values were also used. Astrophysical gf-values were avoided due to the model-dependence of such a procedure. For elements whose lines are significantly affected by hyperfine structure or isotopic splitting a concerted effort has been made to collate the necessary data for the individual line components. We also performed a detailed investigation of available data for line broadening due to collisions with neutral hydrogen atoms. Synthetic spectra calculated for the Sun and Arcturus were used to assess the blending properties of the lines. Among a subset of over 1300 lines of 35 elements in the wavelength ranges from 475 nm to 685 nm and from 850 nm to 895 nm we identified about 200 lines of 24 species which have accurate gf-values and are free of blends in the spectra of the Sun and Arcturus. For the broadening due to collisions with neutral hydrogen we recommend data based on Anstee-Barklem-O'Mara theory, where available, and to avoid lines of neutral species otherwise. Theoretical broadening data by R.L. Kurucz should be used for Sc II, Ti II, and Y II lines. For ionised rare-earth species the Unsöld approximation with an enhancement factor of 1.5 for the line width can be used. Desirable improvements in atomic data were identified for a number of species, including Al I, S I, Cr II, Na I, Si I, Ca II, and Ni I.

astro-ph.IM↗

arXiv2020

APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South

The spectral analysis and data products in Data Release 16 (DR16; December 2019) from the high-resolution near-infrared APOGEE-2/SDSS-IV survey are described. Compared to the previous APOGEE data release (DR14; July 2017), APOGEE DR16 includes about 200000 new stellar spectra, of which 100000 are from a new southern APOGEE instrument mounted on the 2.5 m du Pont telescope at Las Campanas Observatory in Chile. DR16 includes all data taken up to August 2018, including data released in previous data releases. All of the data have been re-reduced and re-analyzed using the latest pipelines, resulting in a total of 473307 spectra of 437445 stars. Changes to the analysis methods for this release include, but are not limited to, the use of MARCS model atmospheres for calculation of the entire main grid of synthetic spectra used in the analysis, a new method for filling "holes" in the grids due to unconverged model atmospheres, and a new scheme for continuum normalization. Abundances of the neutron capture element Ce are included for the first time. A new scheme for estimating uncertainties of the derived quantities using stars with multiple observations has been applied, and calibrated values of surface gravities for dwarf stars are now supplied. Compared to DR14, the radial velocities derived for this release more closely match those in the Gaia DR2 data base, and a clear improvement in the spectral analysis of the coolest giants can be seen. The reduced spectra as well as the result of the analysis can be downloaded using links provided in the SDSS DR16 web page.

astro-ph.GA↗

arXiv2018

APOGEE Data Releases 13 and 14: Data and Analysis

Data and analysis methodology used for the SDSS/APOGEE Data Releases 13 and 14 are described, highlighting differences from the DR12 analysis presented in Holtzman (2015). Some improvement in the handling of telluric absorption and persistence is demonstrated. The derivation and calibration of stellar parameters, chemical abundances, and respective uncertainties are described, along with the ranges over which calibration was performed. Some known issues with the public data related to the calibration of the effective temperatures (DR13), surface gravity (DR13 and DR14), and C and N abundances for dwarfs (DR13 and DR14) are highlighted. We discuss how results from a data-driven technique, The Cannon (Casey 2016), are included in DR14, and compare those with results from the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP). We describe how using The Cannon in a mode that restricts the abundance analysis of each element to regions of the spectrum with known features from that element leads to Cannon abundances can lead to significantly different results for some elements than when all regions of the spectrum are used to derive abundances.

astro-ph.GA↗

arXiv2014

Isochrones for Old (> 5 Gyr) Stars and Stellar Populations. I. Models for $-2.4 \le$ [Fe/H] $\le +0.6$, $0.25 \le Y \le 0.33$, and $-0.4 \le$ [$α$\Fe] $\le +0.4$

Canonical grids of stellar evolutionary sequences have been computed for the helium mass-fraction abundances $Y = 0.25$, $0.29$, and $0.33$, and for iron abundances that vary from $-2.4$ to $+0.4$ (in 0.2 dex increments) when [$α$/Fe] $= +0.4$, or for the ranges $-2.0 \le$ [Fe/H] $\le +0.6$, $-1.8 \le$ [Fe/H] $\le +0.6$ when [$α$/Fe] $= 0.0$ and $-0.4$, respectively. The grids, which consist of tracks for masses from $0.12 {{\cal M}_\odot}$ to $1.1$-$1.5 {{\cal M}_\odot}$ (depending on the metallicity) are based on up-to-date physics, including the gravitational settling of helium (but not metals diffusion). Interpolation software is provided to generate isochrones for arbitrary ages between $\approx 5$ and $15$ Gyr and any values of $Y$, [$α$/Fe], and [Fe/H] within the aforementioned ranges. Comparisons of isochrones with published color-magnitude diagrams (CMDs) for the open clusters M67 ([Fe/H] $\approx 0.0$) and NGC 6791 ([Fe/H] $\approx 0.3$) and for four of the metal-poor globular clusters (47 Tuc, M3, M5, and M92) indicate that the models for the observed metallicities do a reasonably good job of reproducing the locations and slopes of the cluster main sequences and giant branches. The same conclusion is reached from a consideration of plots of nearby subdwarfs that have accurate Hipparcos parallaxes and metallicities in the range $-2.0 \le$ [Fe/H] $\le -1.0$ on various CMDs and on the ($\log T_{\rm eff},\,M_V$)-diagram. A relatively hot temperature scale similar to that derived in recent calibrations of the infrared flux method is favored by both the isochrones and the adopted color transformations, which are based on the latest MARCS model atmospheres.

astro-ph.SR↗

arXiv2010

A grid of MARCS model atmospheres for S stars

S-type stars are late-type giants whose atmosphere is enriched in carbon and s-process elements because of either extrinsic pollution by a binary companion or intrinsic nucleosynthesis and dredge-up on the thermally-pulsing AGB. A large grid of S-star model atmospheres has been computed covering the range 2700 < Teff < 4000 K with 0.5 < C/O < 0.99. ZrO and TiO band strength indices as well as VJHKL photometry are needed to disentangle Teff, C/O and [s/Fe]. A "best-model finding tool" was developed using a set of well-chosen indices and checked against photometry as well as low- and high-resolution spectroscopy. It is found that applying M-star model atmospheres (i.e., with a solar C/O ratio) to S stars can lead to errors on Teff up to 400K. We constrain the parameter space occupied by S stars of the vast sample of Henize stars in terms of Teff, [C/O] and [s/Fe].

astro-ph.SR↗

arXiv2010

Finding high-redshift dark stars with the James Webb Space Telescope

The first stars in the history of the Universe are likely to form in the dense central regions of 10^5-10^6 Msolar cold dark matter halos at z=10-50. The annihilation of dark matter particles in these environments may lead to the formation of so-called dark stars, which are predicted to be cooler, larger, more massive and potentially more long-lived than conventional population III stars. Here, we investigate the prospects of detecting high-redshift dark stars with the upcoming James Webb Space Telescope (JWST). We find that dark stars at z>6 are intrinsically too faint to be detected by JWST. However, by exploiting foreground galaxy clusters as gravitational telescopes, certain varieties of cool (Teff < 30000 K) dark stars should be within reach at redshifts up to z=10. If the lifetimes of dark stars are sufficiently long, many such objects may also congregate inside the first galaxies. We demonstrate that this could give rise to peculiar features in the integrated spectra of galaxies at high redshifts, provided that dark stars make up at least 1 percent of the total stellar mass in such objects.

astro-ph.CO↗

arXiv2009

Abundances in bulge stars from high-resolution, near-IR spectra I. The CNO elements observed during the science verification of CRIRES at VLT

The formation and evolution of the Milky Way bulge is not yet well understood and its classification is ambiguous. Constraints can, however, be obtained by studying the abundances of key elements in bulge stars. The aim of this study is to determine the chemical evolution of CNO, and a few other elements in stars in the Galactic bulge, and to discuss the sensitivities of the derived abundances from molecular lines. High-resolution, near-IR spectra in the H band were recorded using VLT/CRIRES. Due to the high and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred. The CNO abundances can all be determined simultaneously from the numerous molecular lines in the wavelength range observed. The three giant stars in Baade's window presented here are the first bulge stars observed with CRIRES. We have especially determined the CNO abundances, with uncertainties of less than 0.20 dex, from CO, CN, and OH lines. Since the systematic uncertainties in the derived CNO abundances due to uncertainties in the stellar fundamental parameters, notably Teff, are significant, a detailed discussion of the sensitivities of the derived abundances is included. We find good agreement between near-IR and optically determined O, Ti, Fe, and Si abundances. Two of our stars show a solar [C+N/Fe], suggesting that these giants have experienced the first dredge-up and that the oxygen abundance should reflect the original abundance of the giants. The two giants fit into the picture, in which there is no significant difference between the O abundance in bulge and thick-disk stars. Our determination of the S abundances is the first for bulge stars. The high [S/Fe] values for all the stars indicate a high star-formation rate in an early phase of the bulge evolution.

astro-ph.SR↗

arXiv2008

A grid of MARCS model atmospheres for late-type stars I. Methods and general properties

We have constructed a grid of about 10,000 spherically symmetric and plane-parallel models with the MARCS program, and make it available for public use. Parameter ranges are: Teff=2500 to 8000 K, log g =log(GM/R2)= -1 to 5 (cgs) with various masses and radii, [Me/H]=-5 to +1, with [Alpha/Fe] = 0.0 and 0.4 and different choices of C and N abundances to also represent stars of types R, S and N, and with microturbulence parameters from 1 to 5 km/s. We also list fluxes in approximately 108,000 wavelength points. Underlying assumptions in addition to 1D stratification include hydrostatic equilibrium, MLT convection and LTE. A number of general properties of the models are discussed, in relation to the effects of changing blanketing and sphericity. Models are compared with other available grids and excellent agreement is found with plane-parallel models of Castelli and Kurucz within the overlapping parameter range. Although there are departures from the spherically symmetric NextGen models, the agreement with more recent PHOENIX models is gratifying. The models of the grid show regularities, but some interesting departures from general patterns occur for the coolest models due to the molecular opacities. We have tested rules of thumb concerning effects of blanketing and sphericity and found them to often be astonishingly accurate. Some interesting new phenomena have been discovered, such as the intricate coupling between blanketing and sphericity, and the strong effects of carbon enhancement on metal-poor models. We give further details of models and comparisons with observations in subsequent papers.

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arXiv2007

On the Use of Blanketed Atmospheres as Boundary Conditions for Stellar Evolutionary Models

Stellar models have been computed for stars having [Fe/H] = 0.0 and -2.0 to determine the effects of using boundary conditions derived from the latest MARCS model atmospheres. The latter were fitted to the interior models at both the photosphere and at tau = 100, and at least for the 0.8-1.0 solar mass stars considered here, the resultant evolutionary tracks were found to be nearly independent of the chosen fitting point. Particular care was taken to treat the entire star as consistently as possible; i.e., both the interior and atmosphere codes assumed the same abundances and the same treatment of convection. Tracks were also computed using either the classical gray T(tau,T_eff) relation or that derived by Krishna Swamy (1966) to derive the boundary pressure. The latter predict warmer giant branches (by ~150 K) at solar abundances than those based on gray or MARCS atmospheres, which happens to be in good agreement with the inferred temperatures of giants in the open cluster M67 from the latest (V-K)-T_eff relations. Most of the calculations assumed Z=0.0125 (Asplund et al.), though a few models were computed for Z=0.0165 (Grevesse & Sauval) to determine the dependence of the tracks on Z_\odot. Grids of "scaled solar, differentially corrected" (SDC) atmospheres were also computed to try to improve upon theoretical MARCS models. When they were used as boundary conditions, the resultant tracks agreed very well with those based on a standard scaled-solar (e.g., Krishna Swamy) T(tau,T_eff) relation, independently of the assumed metal abundance. Fits of isochrones to the C-M diagram of the [Fe/H] = -2 globular cluster M68 were examined, as was the possibility that the mixing-length parameter varies with stellar parameters.

astro-ph↗

arXiv2007

A Constraint on Z_\odot from Fits of Isochrones to the Color-Magnitude Diagram of M67

The mass at which a transition is made between stars that have radiative or convective cores throughout the core H-burning phase is a fairly sensitive function of Z (particularly the CNO abundances). As a consequence, the ~4 Gyr, open cluster M67 provides a constraint on Z_\odot (and the solar heavy-element mixture) because (i) high-resolution spectroscopy indicates that this system has virtually the same metal abundances as the Sun, and (ii) its turnoff stars have masses just above the lower limit for sustained core convection on the main sequence. In this study, evolutionary tracks and isochrones using the latest MARCS model atmospheres as boundary conditions have been computed for 0.6-1.4 solar masses on the assumption of a metals mix (implying Z_\odot = 0.0125) based on the solar abundances derived by M. Asplund and collaborators using 3-D model atmospheres. These calculations do not predict a turnoff gap where one is observed in M67. No such difficulty is found if the analysis uses isochrones for Z_\odot = 0.0165, assuming the Grevesse & Sauval (1998) mix of heavy elements. Our findings, like the inferences from helioseismology, indicate a problem with the Asplund et al. abundances. However, it is possible that low-Z models with diffusive processes taken into account will be less problematic.

astro-ph↗

arXiv2005

Chemical abundances in 43 metal-poor stars

We have derived abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Ni and Ba for 43 metal-poor field stars in the solar neighbourhood with iron abundances [Fe/H] ranging from -0.4 to -3.0. Total abundance errors are estimated to about 0.1-0.2 dex for most elements, while relative abundances are most probably more accurate. We have made an extensive comparison with other similar studies and traced the reasons for the, in most cases, relatively small differences. Among the results we note the following: We find [O/Fe] from the O I IR- triplet lines to possibly increase beyond [Fe/H]=-1.0, though considerably less than obtained by others from abundances based on OH lines. We find correlations among the deviations for individual stars from the trends of individual alpha elements relative to Fe. This may support the idea that the formation of the Halo stars occurred in smaller systems with different star formation rates. We verify the finding by Gratton et al. (2003b) that stars that do not participate in the rotation of the galactic disk show a lower mean and larger spread in [alpha/Fe] than stars participating in the general rotation. The latter stars also seem to show some correlation between [alpha/Fe] and rotation speed. We finally advocate that a spectroscopic study of a larger sample of halo stars with well-defined selection criteria is very important, in addition to the very considerable efforts that various groups have already made. (Abridged)

astro-ph↗

arXiv2004

Subgiants as probes of galactic chemical evolution

Chemical abundances for 23 candidate subgiant stars have been derived with the aim at exploring their usefulness for studies of galactic chemical evolution. High-resolution spectra from ESO CAT-CES and NOT-SOFIN covered 16 different spectral regions in the visible part of the spectrum. Some 200 different atomic and molecular spectral lines have been used for abundance analysis of about 30 elemental species. The wings of strong, pressure-broadened metal lines were used for determination of stellar surface gravities, which have been compared with gravities derived from Hipparcos parallaxes and isochronic masses. Stellar space velocities have been derived from Hipparcos and Simbad data, and ages and masses were derived with recent isochrones. Only 12 of the stars turned out to be subgiants, i.e. on the "horizontal" part of the evolutionary track between the dwarf- and the giant stages. The abundances derived for the subgiants correspond closely to those of dwarf stars. With the possible exceptions of lithium and carbon we find that subgiant stars show no "chemical" traces of post-main-sequence evolution and that they are therefore very useful targets for studies of galactic chemical evolution.

astro-ph↗

arXiv2002

Alpha-process elements in the Galaxy: a possible GAIA contribution

The sensitivity of stellar spectra to alpha/Fe abundance changes is investigated with the aim to be detected photometricly and employed for scientific goals of the GAIA mission. A grid of plane parallel, line blanketed, flux constant, LTE model atmospheres with different [alpha/Fe] ratios was calculated. As a first step, the modelled stellar energy fluxes for solar-type stars and giants were computed and intercompared. The spectral sensitivity talpha/Fe abundance changes is noticeable and has to be taken into account when selecting photometric filters for GAIA. The Ca II H and K lines and Mg I b triplet are the most sensitive direct indicators of alpha/Fe abundance changes.

astro-ph↗

arXiv1997

The rise and fall of the NaMgAl stars

We have made new abundance determinations for a sample of NaMgAl stars. These stars, which are a subgroup of the nearby metal-rich field F and G disk dwarfs, were first identified by Edvardsson et al. (1993) on the basis of their apparent enrichment in Na, Mg and Al relative to other elements. The discovery of a planetary companion to the nearby solar type star 51 Peg (Mayor & Queloz 1995) combined with Edvardsson et al.'s earlier identification of 51 Peg as a NaMgAl star highlighted the group's potential importance. Our new analysis, which uses new spectra of higher resolution and better wavelength coverage than the analysis of Edvardsson et al., shows that the Na, Mg and Al abundances of the NaMgAl stars are indistinguishable from those of non-NaMgAl stars with otherwise similar properties. The group thus appears to be spurious. Our study, which includes 51 Peg, also provides the most complete set of abundances for this star available to date. The new Fe abundance, $[Fe/H] = +0.20 \pm 0.07$, of 51 Peg confirms earlier measurements of its metal richness. Abundances for 19 other elements, including C, N and O, reveal a fairly uniform enrichment similar to that of Fe and show no evidence of abnormality compared to other metal rich stars of similar spectral type.

astro-ph↗

arXiv1994

Carbon Abundances in F and G Dwarfs

We have determined carbon abundances or upper limits from the forbidden [C I] line at 8727.13 A for 85 F and early G type main sequence stars, with metallicities ranging between [Fe/H] = -1.0 and [Fe/H] = +0.25. The [C I] line has not been previously used for a study of this size. We find that the C/Fe abundance ratio is slowly decreasing with time and increasing metallicity in the disk, but with considerable observational scatter. A discussion of results in the literature supports this result. Our data do not reveal any gradients with galactocentric birth distance for our stars, but more accurate observations are desirable since possible gradients should be important to the question of the synthesis of C and Fe in the disk. (Accepted for publication in A&A. Postscript text and figures available via anonymous ftp to "chiron.astro.uu.se", in directory "pub/articles/atmos/P89")

astro-ph↗