arXiv subjects

M. Asplund

Publications and source records attributed to M. Asplund.

At least 55 records · Page 3Linked to original sources

arXiv2018

The GALAH survey: properties of the Galactic disk(s) in the solar neighbourhood

Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick disks near the solar neighbourhood. The data cover a small range of Galactocentric radius ($7.9 \leq R_\mathrm{GC} \leq 9.5$ kpc), but extend up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude $260 ^\circ \leq \ell \leq 280^\circ$). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined `thick' and `thin' disks of the Galaxy. The thin disk (low-$α$ population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/d$z=-0.18 \pm 0.01$ dex kpc$^{-1}$, which is broadly consistent with previous studies. In contrast, its vertical $α$-abundance profile is almost flat, with a gradient of d[$α$/M]/d$z$ = $0.008 \pm 0.002$ dex kpc$^{-1}$. The steep vertical metallicity gradient of the low-$α$ population is in agreement with models where radial migration has a major role in the evolution of the thin disk. The thick disk (high-$α$ population) has a weaker vertical metallicity gradient d[M/H]/d$z = -0.058 \pm 0.003$ dex kpc$^{-1}$. The $α$-abundance of the thick disk is nearly constant with height, d[$α$/M]/d$z$ = $0.007 \pm 0.002$ dex kpc$^{-1}$. The negative gradient in metallicity and the small gradient in [$α$/M] indicate that the high-$α$ population experienced a settling phase, but also formed prior to the onset of major SNIa enrichment. We explore the implications of the distinct $α$-enrichments and narrow [$α$/M] range of the sub-populations in the context of thick disk formation.

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arXiv2018

Accurate effective temperatures of the metal-poor benchmark stars HD 140283, HD 122563 and HD 103095 from CHARA interferometry

Large stellar surveys of the Milky Way require validation with reference to a set of "benchmark" stars whose fundamental properties are well-determined. For metal-poor benchmark stars, disagreement between spectroscopic and interferometric effective temperatures has called the reliability of the temperature scale into question. We present new interferometric measurements of three metal-poor benchmark stars, HD 140283, HD 122563, and HD 103095, from which we determine their effective temperatures. The angular sizes of all the stars were determined from observations with the PAVO beam combiner at visible wavelengths at the CHARA array, with additional observations of HD 103095 made with the VEGA instrument, also at the CHARA array. Together with photometrically derived bolometric fluxes, the angular diameters give a direct measurement of the effective temperature. For HD 140283 we find θ_LD = 0.324+/-0.005 mas, Teff = 5787+/-48 K; for HD 122563, θ_LD = 0.926+/-0.011 mas, Teff = 4636+/-37 K; and for HD 103095 θ_LD = 0.595+/-0.007 mas, Teff = 5140+/-49 K. Our temperatures for HD 140283 and HD 103095 are hotter than the previous interferometric measurements by 253 K and 322 K, respectively. We find good agreement between our temperatures and recent spectroscopic and photometric estimates. We conclude some previous interferometric measurements have been affected by systematic uncertainties larger than their quoted errors.

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arXiv2018

The Stagger-grid: A grid of 3D stellar atmosphere models V. Synthetic stellar spectra and broad-band photometry

Context. The surface structures and dynamics of cool stars are characterized by the presence of convective motions and turbulent flows which shape the emergent spectrum. Aims. We used realistic three-dimensional radiative hydrodynamical simulations from the Stagger-grid to calculate synthetic spectra with the radiative transfer code Optim3D for stars with different stellar parameters to predict photometric colors and convective velocity shifts. Methods. We calculated spectra from 1000 to 200 000 A with a constant resolving power of 20 000 and from 8470 and 8710 A (Gaia Radial Velocity Spectrometer - RVS - spectral range), with a constant resolving power of 300 000. Results. We used synthetic spectra to compute theoretical colors in the Johnson-Cousins UBV(RI), SDSS, 2MASS, Gaia, SkyMapper, Stromgren systems, and HST-WFC3. We showed that 1D versus 3D differences are limited to a small percent except for the narrow filters that span the optical and UV region of the spectrum. In addition, we derived the effect of the convective velocity fields on selected Fe I lines. We found the overall convective shift for 3D simulations with respect to the reference 1D hydrostatic models, revealing line shifts of between -0.235 and +0.361 km/s. We showed a net correlation of the convective shifts with the effective temperature: lower effective temperatures denote redshifts and higher effective temperatures denote blueshifts. We conclude that the extraction of accurate radial velocities from RVS spectra need an appropriate wavelength correction from convection shifts. Conclusions. The use of realistic 3D hydrodynamical stellar atmosphere simulations has a small but significant impact on the predicted photometry compared with classical 1D hydrostatic models for late-type stars. We make all the spectra publicly available for the community through the POLLUX database.

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arXiv2017

Discovery of a Metal-poor, Luminous Post-AGB Star that Failed the Third Dredge-up

Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is a A-type ($T_{\rm eff}$ = 8250 $\pm$ 250K) luminous (8200 $\pm$ 700 $\rm L_{\odot}$), metal-poor ($\textrm{[Fe/H]}$ = $- 1.18 \pm$ 0.10), low-mass (M$_{\rm initial}$ $\approx$ 1.5 $-$ 2.0 $\rm M_{\odot}$) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of [C/Fe] $<$ 0.50) nor enrichment of $s$-process elements. We derived an oxygen abundance of [O/Fe] = 0.29 $\pm$ 0.1. For Fe and O, we took into account the effects of non-local thermodynamic equilibrium (NLTE). We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB and be carbon-enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogues which are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any third dredge-up episodes.

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arXiv2017

The age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way

The four main findings about the age and abundance structure of the Milky Way bulge based on microlensed dwarf and subgiant stars are: (1) a wide metallicity distribution with distinct peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41; (2) a high fraction of intermediate-age to young stars where at [Fe/H]>0 more than 35 % are younger than 8 Gyr, (3) several episodes of significant star formation in the bulge 3, 6, 8, and 11 Gyr ago; (4) the `knee' in the alpha-element abundance trends of the sub-solar metallicity bulge appears to be located at a slightly higher [Fe/H] (about 0.05 to 0.1 dex) than in the local thick disk.

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arXiv2017

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. VI. Age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way

We present a detailed elemental abundance study of 90 F and G dwarf, turn-off and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing events, stellar ages and abundances for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. We find that the Galactic bulge has a wide metallicity distribution with significant peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41. We also find a high fraction of intermediate-age to young stars: at [Fe/H]>0 more than 35 % are younger than 8 Gyr. For [Fe/H]<-0.5 most stars are 10 Gyr or older. We have also identified several episodes when significant star formation in the bulge happened: 3, 6, 8, and 12 Gyr ago. We further find that the "knee" in the alpha-element abundance trends of the sub-solar metallicity bulge is located at about 0.1 dex higher [Fe/H] than in the local thick disk. The Galactic bulge has complex age and abundance properties that appear to be tightly connected to the main Galactic stellar populations. In particular, the peaks in the metallicity distribution, the star formation episodes, and the abundance trends, show similarities with the properties of the Galactic thin and thick disks. At the same time there are additional components not seen outside the bulge region, and that most likely can be associated with the Galactic bar. For instance, the star formation rate appears to have been slightly faster in the bulge than in the local thick disk, which most likely is an indication of the denser stellar environment closer to the Galactic centre. Our results strengthen the observational evidence that support the idea of a secular origin for the Galactic bulge, formed out of the other main Galactic stellar populations present in the central regions of our Galaxy.

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arXiv2017

Photometry and spectroscopy of multiple populations along the AGB of NGC2808 and NGC6121 (M4)

We present a photometric and spectroscopic study of multiple populations along the asymptotic-giant branch (AGB) of the intermediate-metallicity globular clusters (GCs) NGC2808 and NGC6121 (M4). Chemical abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Co, Ni, Zn, Y, and Ce in AGB stars from high-resolution FLAMES+UVES@VLT spectra are reported for both clusters. Our spectroscopic results have been combined with multi-wavelength photometry from the HST UV survey of Galactic GCs and ground-based photometry, plus proper motions derived by combining stellar positions from ground-based images and Gaia DR1. Our analysis reveals that the AGBs of both clusters host multiple populations with different chemical composition. In M4 we have identified two main populations of stars with different Na/O content, lying on distinct AGBs in the mF438W vs. C_F275W,F336W,F438W and the V vs.C_U,B,I pseudo-CMDs. In the more massive and complex GC NGC2808 three groups of stars with different chemical abundances occupy different locations on the so-called "chromosome map" photometric diagram. The spectroscopic+photometric comparison of stellar populations along the AGB and the red giants of this GC suggests that the AGB hosts stellar populations with a range in helium abundances spanning from primordial up to high contents of Y~0.32. On the other hand, from our dataset, there is no evidence for stars with extreme helium abundance (Y~0.38) on the AGB, suggesting that the most He-rich stars of NGC2808 do not reach this phase.

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arXiv2017

Non-LTE line formation of Fe in late-type stars IV: Modelling of the solar centre-to-limb variation in 3D

Our ability to model the shapes and strengths of iron lines in the solar spectrum is a critical test of the accuracy of the solar iron abundance, which sets the absolute zero-point of all stellar metallicities. We use an extensive 463-level Fe atom with new photoionisation cross-sections for FeI as well as quantum mechanical calculations of collisional excitation and charge transfer with neutral hydrogen; the latter effectively remove a free parameter that has hampered all previous line formation studies of Fe in non-local thermodynamic equilibrium (NLTE). For the first time, we use realistic 3D NLTE calculations of Fe for a quantitative comparison to solar observations. We confront our theoretical line profiles with observations taken at different viewing angles across the solar disk with the Swedish 1-m Solar Telescope. We find that 3D modelling well reproduces the observed centre-to-limb behaviour of spectral lines overall, but highlight aspects that may require further work, especially cross-sections for inelastic collisions with electrons. Our inferred solar iron abundance is log(eps(Fe))=7.48+-0.04.

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arXiv2017

Asymmetries on red giant branch surfaces from CHARA/MIRC optical interferometry

Context. Red giant branch (RGB) stars are very bright objects in galaxies and are often used as standard candles. Interferometry is the ideal tool to characterize the dynamics and morphology of their atmospheres. Aims. We aim at precisely characterising the surface dynamics of a sample of RGB stars. Methods. We obtained interferometric observations for three RGB stars with the MIRC instrument mounted at the CHARA interfer- ometer. We looked for asymmetries on the stellar surfaces using limb-darkening models. Results. We measured the apparent diameters of HD197989 (Epsilon Cyg) = 4.61+-0.02 mas, HD189276 (HR7633) = 2.95+-0.01 mas, and HD161096 (Beta Oph) = 4.43+-0.01 mas. We detected departures from the centrosymmetric case for all three stars with the tendency of a greater effect for lower logg of the sample. We explored the causes of this signal and conclude that a possible explanation to the interferometric signal is the convection-related and/or the magnetic-related surface activity. However, it is necessary to monitor these stars with new observations, possibly coupled with spectroscopy, in order to firmly establish the cause.

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arXiv2017

Sun-like stars unlike the Sun: Clues for chemical anomalies of cool stars

We present a summary of the splinter session "Sun-like stars unlike the Sun" that was held on 09 June 2016 as part of the Cool Stars 19 conference (Uppsala, Sweden). We discussed the main limitations (in the theory and observations) in the derivation of very precise stellar parameters and chemical abundances of Sun-like stars. We outlined and discussed the most important and most debated processes that can produce chemical peculiarities in solar-type stars. Finally, in an open discussion between all the participants we tried to identify new pathways and prospects towards future solutions of the currently open questions.

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arXiv2016

The Galah Survey: Classification and diagnostics with t-SNE reduction of spectral information

Galah is an ongoing high-resolution spectroscopic survey with the goal of disentangling the formation history of the Milky Way, using the fossil remnants of disrupted star formation sites which are now dispersed around the Galaxy. It is targeting a randomly selected, magnitude limited ($V \leq 14$) sample of stars, with the goal of observing one million objects. To date, 300,000 spectra have been obtained. Not all of them are correctly processed by parameter estimation pipelines and we need to know about them. We present a semi-automated classification scheme which identifies different types of peculiar spectral morphologies, in an effort to discover and flag potentially problematic spectra and thus help to preserve the integrity of the survey's results. To this end we employ a recently developed dimensionality reduction technique t-SNE (t-distributed Stochastic Neighbour Embedding), which enables us to represent the complex spectral morphology in a two-dimensional projection map while still preserving the properties of the local neighbourhoods of spectra. We find that the majority (178,483) of the 209,533 Galah spectra considered in this study represents normal single stars, whereas 31,050 peculiar and problematic spectra with very diverse spectral features pertaining to 28,579 stars are distributed into 10 classification categories: Hot stars, Cool metal-poor giants, Molecular absorption bands, Binary stars, H$α$/H$β$ emission, H$α$/H$β$ emission superimposed on absorption, H$α$/H$β$ P-Cygni, H$α$/H$β$ inverted P-Cygni, Lithium absorption, and Problematic. Classified spectra with supplementary information are presented in the catalogue, indicating candidates for follow-up observations and population studies of the short-lived phases of stellar evolution.

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arXiv2016

Measuring stellar granulation during planet transits

Stellar activity and convection-related surface structures might cause bias in planet detection and characterization that use these transits. Surface convection simulations help to quantify the granulation signal. We used realistic three-dimensional radiative hydrodynamical simulations from the Stagger grid and synthetic images computed with the radiative transfer code Optim3D to model the transits of three prototype planets: a hot Jupiter, a hot Neptune, and a terrestrial planet. We computed intensity maps from RHD simulations of the Sun and a K-dwarf star at different wavelength bands from optical to far-infrared. We modeled the transit using synthetic stellar-disk images and emulated the temporal variation of the granulation intensity. We identified two types of granulation noise that act simultaneously during the planet transit: (i) the intrinsic change in the granulation pattern with timescales smaller than the usual planet transit, and (ii) the fact that the transiting planet occults isolated regions of the photosphere that differ in local surface brightness. Our modeling approach shows that the granulation pattern has a non-negligible effect on the light curve depth during the transit, and, consequentially on the determination of the radius of the planet transiting. The granulation noise appears to be correlated among the different wavelength ranges either in the visible or in the infrared regions. The granulation has to be considered as an intrinsic uncertainty (as a result of stellar variability) on the precise measurements of exoplanet transits of planets. The full characterization of the granulation is essential for determining the degree of uncertainty on the planet parameters. In this context, the use of 3D RHD simulations is important to measure the convection-related fluctuations.

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arXiv2016

The solar silicon abundance based on 3D non-LTE calculations

We present three-dimensional (3D) non-local thermodynamic equilibrium (non-LTE) radiative transfer calculations for silicon in the solar photosphere, using an extensive model atom that includes recent, realistic neutral hydrogen collisional cross-sections. We find that photon losses in the SiI lines give rise to slightly negative non-LTE abundance corrections of the order -0.01 dex. We infer a 3D non-LTE based solar silicon abundance of 7.51 dex. With silicon commonly chosen to be the anchor between the photospheric and meteoritic abundances, we find that the meteoritic abundance scale remains unchanged compared with the Asplund et al. (2009) and Lodders et al. (2009) results.

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arXiv2016

Non-LTE line formation of Fe in late-type stars - III. 3D non-LTE analysis of metal-poor stars

As one of the most important elements in astronomy, iron abundance determinations need to be as accurate as possible. We investigate the accuracy of spectroscopic iron abundance analyses using archetypal metal-poor stars. We perform detailed 3D non-LTE radiative transfer calculations based on 3D hydrodynamic Stagger model atmospheres, and employ a new model atom that includes new quantum-mechanical neutral hydrogen collisional rate coefficients. With the exception of the red giant HD122563, we find that the 3D non-LTE models achieve Fe i/Fe ii excitation and ionization balance as well as not having any trends with equivalent width to within modelling uncertainties of 0.05 dex, all without having to invoke any microturbulent broadening; for HD122563 we predict that the current best parallax-based surface gravity is over-estimated by 0.5 dex. Using a 3D non-LTE analysis, we infer iron abundances from the 3D model atmospheres that are roughly 0.1 dex higher than corresponding abundances from 1D MARCS model atmospheres; these differences go in the same direction as the non-LTE effects themselves. We make available grids of departure coefficients, equivalent widths and abundance corrections, calculated on 1D MARCS model atmospheres and horizontally- and temporally-averaged 3D Stagger model atmospheres.

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arXiv2016

The chemical compositions of solar twins in the open cluster M67

Stars in open clusters are expected to share an identical abundance pattern. Establishing the level of chemical homogeneity in a given open cluster deserves further study as it is the basis of the concept of chemical tagging to unravel the history of the Milky Way. M67 is particularly interesting given its solar metallicity and age as well as being a dense cluster environment. We conducted a strictly line-by-line differential chemical abundance analysis of two solar twins in M67: M67-1194 and M67-1315. Stellar atmospheric parameters and elemental abundances were obtained with high precision using Keck/HIRES spectra. M67-1194 is essentially identical to the Sun in terms of its stellar parameters. M67-1315 is warmer than M67-1194 by ~ 150 K as well as slightly more metal-poor than M67-1194 by ~ 0.05 dex. M67-1194 is also found to have identical chemical composition to the Sun, confirming its solar twin nature. The abundance ratios [X/Fe] of M67-1315 are similar to the solar abundances for elements with atomic number Z <= 30, while most neutron-capture elements are enriched by ~ 0.05 dex, which might be attributed to enrichment from a mixture of AGB ejecta and r-process material. The distinct chemical abundances for the neutron-capture elements in M67-1315 and the lower metallicity of this star compared to M67-1194, indicate that the stars in M67 are likely not chemically homogeneous. This poses a challenge for the concept of chemical tagging since it is based on the assumption of stars forming in the same star-forming aggregate.

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arXiv2016

The nucleosynthetic history of elements in the Galactic disk: [X/Fe] - age relations from high-precision spectroscopy

Context: The chemical composition of stars is intimately linked to the Galaxy formation and evolution. Aims: We aim to trace the chemical evolution of the Galactic disk through the inspection of the [X/Fe]-age relations of 24 species from C to Eu. Methods: Using high-resolution and high-signal-to-noise UVES spectra of nine solar twins, we obtained precise estimates of stellar ages and chemical abundances. These determinations have been integrated with additional accurate age and abundance determinations from recent spectroscopic studies of solar twins existing in the literature, comprising superb abundances with 0.01~dex precision. Based on this data set, we outlined the [X/Fe]-age relations over a time interval of 10~Gyr. Results: We present the [X/Fe] - age relations for 24 elements (C, O, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Ba, La, Ce, Nd, and Eu). Each different class of elements showed distinct evolution with time that relies on the different characteristics, rates and timescales of the nucleosynthesis' sites from which they are produced. The $α$-elements are characterised by a [X/Fe] decrement as time goes on. Strikingly, an opposite behaviour is observed for Ca. The iron-peak elements show an early [X/Fe] increase followed by a decrease towards the youngest stars. The [X/Fe] for the n-capture elements decrease with age. We also found that both [Mg/Y] and [Al/Y] are precise stellar clocks, with [Al/Y] showing the steepest dependence with age. Conslusions: Knowledge of the [X/Fe]-age relations is a gold mine from which we can achieve a great understanding about the processes that governed the formation and evolution of the Milky Way. Through the reverse engineering of these relations we will be able to put strong constraints on the nature of the stellar formation history, the SNe rates, the stellar yields, and the variety of the SNe progenitors.

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arXiv2016

The Solar Twin Planet Search III. The [Y/Mg] clock: estimating stellar ages of solar-type stars

Solar twins are stars with similar stellar (surface) parameters to the Sun that can have a wide range of ages. This provide an opportunity to analyze the variation of their chemical abundances with age. Nissen (2015) recently suggested that the abundances of the s-process element Y and the $α$-element Mg could be used to estimate stellar ages. This paper aims to determine with high precision the Y, Mg, and Fe abundances for a sample of 88 solar twins that span a broad age range ($0.3-10.0$\,Gyr) and investigate their use for estimating ages. We obtained high-quality Magellan Inamori Kyocera Echelle (MIKE) spectra and determined Y and Mg abundances using equivalent widths and a line-by-line differential method within a 1D LTE framework. Stellar parameters and iron abundances were measured in Paper I of this series for all stars, but a few (three) required a small revision. The [Y/Mg] ratio shows a strong correlation with age. It has a slope of -0.041$\pm$0.001 dex/Gyr and a significance of 41 $σ$. This is in excellent agreement with the relation first proposed by Nissen (2015). We found some outliers that turned out to be binaries where mass transfer may have enhanced the yttrium abundance. Given a precise measurement of [Y/Mg] with typical error of 0.02 dex in solar twins, our formula can be used to determine a stellar age with $\sim$0.8 Gyr precision in the 0 to 10 Gyr range.

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arXiv2016

The Hyades open cluster is chemically inhomogeneous

We present a high-precision differential abundance analysis of 16 solar-type stars in the Hyades open cluster based on high resolution, high signal-to-noise ratio (S/N ~ 350 - 400) spectra obtained from the McDonald 2.7m telescope. We derived stellar parameters and differential chemical abundances for 19 elements (C, O, Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Ba) with uncertainties as low as ~ 0.01 - 0.02 dex. Our main results include: (1) there is no clear chemical signature of planet formation detected among the sample stars, i.e., no correlations in abundances versus condensation temperature; (2) the observed abundance dispersions are a factor of ~ 1.5 - 2 larger than the average measurement errors for most elements; (3) there are positive correlations, of high statistical significance, between the abundances of at least 90% of pairs of elements. We demonstrate that none of these findings can be explained by errors due to the stellar parameters. Our results reveal that the Hyades is chemically inhomogeneous at the 0.02 dex level. Possible explanations for the abundance variations include (1) inhomogeneous chemical evolution in the proto-cluster environment, (2) supernova ejection in the proto-cluster cloud, and (3) pollution of metal-poor gas before complete mixing of the proto-cluster cloud. Our results provide significant new constraints on the chemical composition of open clusters and a challenge to the current view of Galactic archeology.

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