arXiv subjects

M. Asplund

Publications and source records attributed to M. Asplund.

At least 145 records · Page 8Linked to original sources

arXiv2010

A possible signature of terrestrial planet formation in the chemical composition of solar analogs

Recent studies have shown that the elemental abundances in the Sun are anomalous when compared to most (about 85%) nearby solar twin stars. Compared to its twins, the Sun exhibits a deficiency of refractory elements (those with condensation temperatures Tc>900K) relative to volatiles (Tc<900K). This finding is speculated to be a signature of the planet formation that occurred more efficiently around the Sun compared with the majority of solar twins. Furthermore, within this scenario, it seems more likely that the abundance patterns found are specifically related to the formation of terrestrial planets. In this work we analyze abundance results from six large independent stellar abundance surveys to determine whether they confirm or reject this observational finding. We show that the elemental abundances derived for solar analogs in these six studies are consistent with the Tc trend suggested as a planet formation signature. The same conclusion is reached when those results are averaged heterogeneously. We also investigate the dependency of the abundances with first ionization potential (FIP), which correlates well with Tc. A trend with FIP would suggest a different origin for the abundance patterns found, but we show that the correlation with Tc is statistically more significant. We encourage similar investigations of metal-rich solar analogs and late F-type dwarf stars, for which the hypothesis of a planet formation signature in the elemental abundances makes very specific predictions. Finally, we examine a recent paper that claims that the abundance patterns of two stars hosting super-Earth like planets contradict the planet formation signature hypothesis. Instead, we find that the chemical compositions of these two stars are fully compatible with our hypothesis.

astro-ph.SR↗

arXiv2010

Radiative transfer with scattering for domain-decomposed 3D MHD simulations of cool stellar atmospheres

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

astro-ph.SR↗

arXiv2010

Observational signatures of lithium depletion in the metal-poor globular cluster NGC6397

The "stellar" solution to the cosmological lithium problem proposes that surface depletion of lithium in low-mass, metal-poor stars can reconcile the lower abundances found for Galactic halo stars with the primordial prediction. Globular clusters are ideal environments for studies of the surface evolution of lithium, with large number statistics possible to obtain for main sequence stars as well as giants. We discuss the Li abundances measured for >450 stars in the globular cluster NGC6397, focusing on the evidence for lithium depletion and especially highlighting how the inferred abundances and interpretations are affected by early cluster self-enrichment and systematic uncertainties in the effective temperature determination.

astro-ph.SR↗

arXiv2010

An absolutely calibrated effective temperature scale from the InfraRed Flux Method

Various effective temperature scales have been proposed over the years. Despite much work and the high internal precision usually achieved, systematic differences of order 100 K (or more) among various scales are still present. We present an investigation based on the Infrared Flux Method aimed at assessing the source of such discrepancies and pin down their origin. We break the impasse among different scales by using a large set of solar twins, stars which are spectroscopically and photometrically identical to the Sun, to set the absolute zero point of the effective temperature scale to within few degrees. Our newly calibrated, accurate and precise temperature scale applies to dwarfs and subgiants, from super-solar metallicities to the most metal-poor stars currently known. At solar metallicities our results validate spectroscopic effective temperature scales, whereas for [Fe/H]<-2.5 our temperatures are roughly 100 K hotter than those determined from model fits to the Balmer lines and 200 K hotter than those obtained from the excitation equilibrium of Fe lines. Empirical bolometric corrections and useful relations linking photometric indices to effective temperatures and angular diameters have been derived. Our results take full advantage of the high accuracy reached in absolute calibration in recent years and are further validated by interferometric angular diameters and space based spectrophotometry over a wide range of effective temperatures and metallicities.

astro-ph.SR↗

arXiv2010

Chemical similarities between Galactic bulge and local thick disk red giants: O, Na, Mg, Al, Si, Ca and Ti

... We confirm the well-established differences for [$α$/Fe] at a given metallicity between the local thin and thick disks. For all the elements investigated, we find no chemical distinction between the bulge and the local thick disk, in agreement with our previous study of C, N and O but in contrast to other groups relying on literature values for nearby disk dwarf stars. For -1.5 < [Fe/H] < -0.3 exactly the same trend is followed by both the bulge and thick disk stars, with a star-to-star scatter of only 0.03 dex. Furthermore, both populations share the location of the knee in the [alpha/Fe] vs [Fe/H] diagram. It still remains to be confirmed that the local thick disk extends to super-solar metallicities as is the case for the bulge. These are the most stringent constraints to date on the chemical similarity of these stellar populations. Our findings suggest that the bulge and local thick disk stars experienced similar formation timescales, star formation rates and initial mass functions, confirming thus the main outcomes of our previous homogeneous analysis of [O/Fe] from infrared spectra for nearly the same sample. The identical alpha-enhancements of thick disk and bulge stars may reflect a rapid chemical evolution taking place before the bulge and thick disk structures we see today were formed, or it may reflect Galactic orbital migration of inner disk/bulge stars resulting in stars in the solar neighborhood with thick-disk kinematics.

astro-ph.SR↗

arXiv2010

Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. II. Ages, metallicities, detailed elemental abundances, and connections to the Galactic thick disc

The Bulge is the least understood major stellar population of the Milky Way. Most of what we know about the formation and evolution of the Bulge comes from bright giant stars. The underlying assumption that giants represent all the stars, and accurately trace the chemical evolution of a stellar population, is under debate. In particular, recent observations of a few microlensed dwarf stars give a very different picture of the evolution of the Bulge from that given by the giant stars. [ABRIDGED] We perform a detailed elemental abundance analysis of dwarf stars in the Galactic bulge, based on high-resolution spectra that were obtained while the stars were optically magnified during gravitational microlensing events. [ABRIDGED] We present detailed elemental abundances and stellar ages for six new dwarf stars in the Galactic bulge. Combining these with previous events, here re-analysed with the same methods, we study a homogeneous sample of 15 stars, which constitute the largest sample to date of microlensed dwarf stars in the Galactic bulge. We find that the stars span the full range of metallicities from [Fe/H]=-0.72 to +0.54, and an average metallicity of <[Fe/H]>=-0.08+/-0.47, close to the average metallicity based on giant stars in the Bulge. Furthermore, the stars follow well-defined abundance trends, that for [Fe/H]<0 are very similar to those of the local Galactic thick disc. This suggests that the Bulge and the thick disc have had, at least partially, comparable chemical histories. At sub-solar metallicities we find the Bulge dwarf stars to have consistently old ages, while at super-solar metallicities we find a wide range of ages. Using the new age and abundance results from the microlensed dwarf stars we investigate possible formation scenarios for the Bulge.

astro-ph.GA↗

arXiv2009

Observational signatures for depletion in the Spite plateau: solving the cosmological Li discrepancy?

We present Li abundances for 73 stars in the metallicity range -3.5 < [Fe/H] < -1.0 using improved IRFM temperatures (Casagrande et al. 2010) with precise E(B-V) values obtained mostly from interstellar NaI D lines, and high-quality equivalent widths (errors ~ 3%). At all metallicities we uncover a fine-structure in the Li abundances of Spite plateau stars, which we trace to Li depletion that depends on both metallicity and mass. Models including atomic diffusion and turbulent mixing seem to reproduce the observed Li depletion assuming a primordial Li abundance ALi = 2.64 dex (MARCS models) or 2.72 (Kurucz overshooting models), in good agreement with current predictions (ALi = 2.72) from standard BBN. We are currently expanding our sample to have a better coverage of different evolutionary stages at the high and low metallicity ends, in order to verify our findings.

astro-ph.SR↗

arXiv2009

Does the anomalous solar chemical composition come from planet formation?

We derive the abundance of 19 elements in a sample of 64 stars with fundamental parameters very similar to solar, which minimizes the impact of systematic errors in our spectroscopic 1D-LTE differential analysis, using high-resolution (R=60,000), high signal-to-noise ratio (S/N=200) spectra. The estimated errors in the elemental abundances relative to solar are as small as 0.025 dex. The abundance ratios [X/Fe] as a function of [Fe/H] agree closely with previously established patterns of Galactic thin-disk chemical evolution. Interestingly, the majority of our stars show a significant correlation between [X/Fe] and condensation temperature (Tc). In the sample of 22 stars with parameters closest to solar, we find that, on average, low Tc elements are depleted with respect to high Tc elements in the solar twins relative to the Sun by about 0.08 dex (20%). An increasing trend is observed for the abundances as a function of Tc for 900<Tc<1800 K, while abundances of lower Tc elements appear to be roughly constant. We speculate that this is a signature of the planet formation that occurred around the Sun but not in the majority of solar twins. If this hypothesis is correct, stars with planetary systems like ours, although rare (frequency of 15%), may be identified through a very detailed inspection of the chemical compositions of their host stars.

astro-ph.SR↗

arXiv2009

The solar, exoplanet and cosmological lithium problems

We review three Li problems. First, the Li problem in the Sun, for which some previous studies have argued that it may be Li-poor compared to other Suns. Second, we discuss the Li problem in planet hosting stars, which are claimed to be Li-poor when compared to field stars. Third, we discuss the cosmological Li problem, i.e. the discrepancy between the Li abundance in metal-poor stars (Spite plateau stars) and the predictions from standard Big Bang Nucleosynthesis. In all three cases we find that the "problems" are naturally explained by non-standard mixing in stars.

astro-ph.SR↗

arXiv2009

Unprecedented accurate abundances: signatures of other Earths?

For more than 140 years the chemical composition of our Sun has been considered typical of solar-type stars. Our highly differential elemental abundance analysis of unprecedented accuracy (~0.01 dex) of the Sun relative to solar twins, shows that the Sun has a peculiar chemical composition with a ~20% depletion of refractory elements relative to the volatile elements in comparison with solar twins. The abundance differences correlate strongly with the condensation temperatures of the elements. A similar study of solar analogs from planet surveys shows that this peculiarity also holds in comparisons with solar analogs known to have close-in giant planets while the majority of solar analogs without detected giant planets show the solar abundance pattern. The peculiarities in the solar chemical composition can be explained as signatures of the formation of terrestrial planets like our own Earth.

astro-ph.EP↗

arXiv2009

Chemical abundances for 11 bulge stars from high-resolution, near-IR spectra

It is debated whether the Milky Way bulge has the characteristics of a classical bulge sooner than those of a pseudobulge. Detailed abundance studies of bulge stars is a key to investigate the origin, history, and classification of the bulge. The aim is to add to the discussion on the origin of the bulge and to study detailed abundances determined from near-IR spectra for bulge giants already investigated with optical spectra, the latter also providing the stellar parameters which are very significant for the results of the present study. Especially, the important CNO elements are better determined in the near-IR. High-resolution, near-infrared spectra in the H band are recorded using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances can all be determined from the numerous molecular lines in the wavelength range observed. Abundances of the alpha elements are also determined from the near-IR spectra. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up to metallicities of at least [Fe/H]=-0.3, after which they decline. This suggests that the Milky Way bulge experienced a rapid and early star-formation history like that of a classical bulge. However, a similarity between the bulge trend and the trend of the local thick disk seems present. Such a similarity could suggest that the bulge has a pseudobulge origin. Our [C/Fe] trend does not show any increase with [Fe/H] which could have been expected if W-R stars have contributed substantially to the C abundances. No "cosmic scatter" can be traced around our observed abundance trends; the scatter found is expected, given the observational uncertainties.

astro-ph.GA↗

arXiv2009

Abundance Analysis of the Halo Giant HD122563 with Three-Dimensional Model Stellar Atmospheres

We present a preliminary local thermodynamic equilibrium (LTE) abundance analysis of the template halo red giant HD122563 based on a realistic, three-dimensional (3D), time-dependent, hydrodynamical model atmosphere of the very metal-poor star. We compare the results of the 3D analysis with the abundances derived by means of a standard LTE analysis based on a classical, 1D, hydrostatic model atmosphere of the star. Due to the different upper photospheric temperature stratifications predicted by 1D and 3D models, we find large, negative, 3D-1D LTE abundance differences for low-excitation OH and Fe I lines. We also find trends with lower excitation potential in the derived Fe LTE abundances from Fe I lines, in both the 1D and 3D analyses. Such trends may be attributed to the neglected departures from LTE in the spectral line formation calculations.

astro-ph.SR↗

arXiv2009

Accounting for Convective Blue-Shifts in the Determination of Absolute Stellar Radial Velocities

For late-type non-active stars, gravitational redshifts and convective blueshifts are the main source of biases in the determination of radial velocities. If ignored, these effects can introduce systematic errors of the order of ~ 0.5 km/s. We demonstrate that three-dimensional hydrodynamical simulations of solar surface convection can be used to predict the convective blue-shifts of weak spectral lines in solar-like stars to ~ 0.070 km/s. Using accurate trigonometric parallaxes and stellar evolution models, the gravitational redshifts can be constrained with a similar uncertainty, leading to absolute radial velocities accurate to better than ~ 0.1 km/s.

astro-ph.SR↗

arXiv2009

Granulation across the HR diagram

We have obtained ultra-high quality spectra (R=180,000; S/N>300) with unprecedented wavelength coverage (4400 to 7400 A) for a number of stars covering most of the HR diagram in order to test the predictions of models of stellar surface convection. Line bisectors and core wavelength shifts are both measured and modeled, allowing us to validate and/or reveal the limitations of state-of-the-art hydrodynamic model atmospheres of different stellar parameters. We show the status of our project and preliminary results.

astro-ph.SR↗

arXiv2009

Elemental abundances in the Galactic bulge from microlensed dwarf stars

We present elemental abundances of 13 microlensed dwarf and subgiant stars in the Galactic bulge, which constitute the largest sample to date. We show that these stars span the full range of metallicity from Fe/H=-0.8 to +0.4, and that they follow well-defined abundance trends, coincident with those of the Galactic thick disc.

astro-ph.GA↗

arXiv2009

Signatures of intrinsic Li depletion and Li-Na anti-correlation in the metal-poor globular cluster NGC6397

To alleviate the discrepancy between the prediction of the primordial lithium abundance in the universe and the abundances observed in Pop II dwarfs and subgiant stars, it has been suggested that the stars observable today have undergone photospheric depletion of Li. To constrain the nature of such depletion, we conduct a homogeneous analysis of a very large sample of stars in the metal-poor globular cluster NGC6397, covering well all evolutionary phases from below the main sequence turn-off to high up the red giant branch. Non-LTE Li abundances or abundance upper limits are obtained for all stars, and for a size-able sub-set of the targets also Na abundances are obtained. The sodium abundances are used to distinguish stars formed out of pristine material from stars formed out of material affected by pollution from a previous generation of more massive stars. The dwarfs, turn-off, and early subgiant stars form a thin abundance plateau, disrupted in the middle of the subgiant branch by the lithium dilution caused by the first dredge-up. A second steep abundance drop is seen at the red giant branch bump. The turn-off stars are more lithium-poor, by up to 0.1 dex, than subgiants that have not yet undergone dredge-up. In addition, hotter dwarfs are slightly more lithium-poor than cooler dwarfs, which may be a signature of the so-called Li dip in the cluster, commonly seen among PopI stars. A considerably large spread in Na abundance confirms that NGC6397 has suffered from intra-cluster pollution in its infancy and a limited number of Na-enhanced and Li-deficient stars strongly contribute to form a significant anti-correlation between the abundances of Na and Li. The inferred Li abundances are compared to predictions from stellar structure models including atomic diffusion and ad-hoc turbulence below the convection zone.

astro-ph.SR↗

arXiv2009

Departures from LTE for neutral Li in late-type stars

We perform non-LTE calculations of lithium in late-type stars for a wide range of stellar parameters, including quantum mechanical cross-sections for collisions with neutral hydrogen and the negative hydrogen ion. Non-LTE abundance corrections for the lithium resonance line at 670.7nm and the subordinate line at 610.3nm, are calculated using 1D MARCS model atmospheres spanning a grid Teff=[4000,8000]K, log(g)=[1.0,5.0], and [Fe/H]=[0.0,-3.0], for lithium abundances in the range A(Li)=[-0.3,4.2]. The competing effects of ultraviolet over-ionization and photon losses in the resonance line govern the behaviour of the non-LTE effects with stellar parameters and lithium abundance. The size and sign of the non-LTE abundance corrections vary significantly over the grid for the 670.7nm line, but are typically positive and below 0.15dex for the 610.3nm, line. The new collisional data play a significant role in determining the abundance corrections.

astro-ph.SR↗

arXiv2009

Subaru high-resolution spectroscopy of Star G in the Tycho supernova remnant

It is widely believed that Type Ia supernovae (SN Ia) originate in binary systems where a white dwarf accretes material from a companion star until its mass approaches the Chandrasekhar mass and carbon is ignited in the white dwarf's core. This scenario predicts that the donor star should survive the supernova explosion, providing an opportunity to understand the progenitors of Type Ia supernovae.In this paper we argue that rotation is a generic signature expected of most non-giant donor stars that is easily measurable. \citep{2004Natur.431.1069R} examined stars in the center of the remnant of SN 1572 (Tycho's SN) and showed evidence that a subgiant star (Star G by their naming convention) near the remnant's centre was the system's donor star. We present high-resolution (R \simeq 40000) spectra taken with the High Dispersion Spectrograph on Subaru of this candidate donor star and measure the star's radial velocity as $79\pm 2$ \kms with respect to the LSR and put an upper limit on the star's rotation of 7.5 \kms. In addition, by comparing images that were taken in 1970 and 2004, we measure the proper motion of Star G to be $μ_l = -1.6 \pm 2.1$ \masyr and $μ_b = -2.7 \pm 1.6$ \masyr. We demonstrate that all of the measured properties of Star G presented in this paper are consistent with those of a star in the direction of Tycho's SN that is not associated with the supernova event. However, we discuss an unlikely, but still viable scenario for Star G to be the donor star, and suggest further observations that might be able to confirm or refute it.

astro-ph.SR↗