| 1. |
- Bergemann, M., et al.
(författare)
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The Gaia-ESO Survey : radial metallicity gradients and age-metallicity relation of stars in the Milky Way disk
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 565, s. A89-
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Tidskriftsartikel (refereegranskat)abstract
- We study the relationship between age, metallicity, and alpha-enhancement of FGK stars in the Galactic disk. The results are based upon the analysis of high-resolution UVES spectra from the Gaia-ESO large stellar survey. We explore the limitations of the observed dataset, i.e. the accuracy of stellar parameters and the selection effects that are caused by the photometric target preselection. We find that the colour and magnitude cuts in the survey suppress old metal-rich stars and young metal-poor stars. This suppression may be as high as 97% in some regions of the age-metallicity relationship. The dataset consists of 144 stars with a wide range of ages from 0.5 Gyr to 13.5 Gyr, Galactocentric distances from 6 kpc to 9.5 kpc, and vertical distances from the plane 0 < vertical bar Z vertical bar < 1.5 kpc. On this basis, we find that i) the observed age-metallicity relation is nearly flat in the range of ages between 0 Gyr and 8 Gyr; ii) at ages older than 9 Gyr, we see a decrease in [Fe/H] and a clear absence of metal-rich stars; this cannot be explained by the survey selection functions; iii) there is a significant scatter of [Fe/H] at any age; and iv) [Mg/Fe] increases with age, but the dispersion of [Mg/Fe] at ages > 9 Gyr is not as small as advocated by some other studies. In agreement with earlier work, we find that radial abundance gradients change as a function of vertical distance from the plane. The [Mg/Fe] gradient steepens and becomes negative. In addition, we show that the inner disk is not only more alpha-rich compared to the outer disk, but also older, as traced independently by the ages and Mg abundances of stars.
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| 2. |
- Duval, Florent, et al.
(författare)
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The Lyman alpha reference sample VI. Lyman alpha escape from the edge-on disk galaxy Mrk 1486
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
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Tidskriftsartikel (refereegranskat)abstract
- Context. Recent numerical simulations suggest that the strength of the Lyman alpha (Ly alpha) line of star-forming disk galaxies strongly depends on the inclination at which they are observed: from edge-on to face-on, we expect to see a change from a strongly attenuated Ly alpha line to a strong Ly alpha emission line.Aims. We aim to understand how a strong Ly alpha emission line is able to escape from the low-redshift highly inclined (edge-on) disk galaxy Mrk 1486 (z similar to 0.0338). To our knowledge, this work is the first observational study of Ly alpha transport inside an edge-on disk galaxy.Methods. Using a large set of HST imaging and spectroscopic data, we investigated the interstellar medium (ISM) structure and the dominant source of Ly alpha radiation inside Mrk 1486. Moreover, using a 3D Monte Carlo Ly alpha radiation transfer code, we studied the radiative transfer of Ly alpha and UV continuum photons inside a 3D geometry of neutral hydrogen (HI) and dust that models the ISM structure at the galaxy center. Our numerical simulations predicted the Ly alpha line profile that we then compared to the one observed in the HST/COS spectrum of Mrk 1486.Results. While a pronounced Ly alpha absorption line emerges from the disk of Mrk 1486, very extended Ly alpha structures are observed at large radii from the galaxy center: a large Ly alpha-halo and two very bright Ly alpha regions located slightly above and below the disk plane. The analysis of IFU H alpha spectroscopic data of Mrk 1486 indicates the presence of two bipolar outflowing halos of HI gas at the same location as these two bright Ly alpha regions. Comparing different diagnostic diagrams (such as [OIII](5007)/H beta versus [OI](6300)/H alpha) to photo-and shock-ionization models, we find that the Ly alpha production of Mrk 1486 is dominated by photoionization inside the galaxy disk. From this perspective, our numerical simulations succeed in reproducing the strength and shape of the observed Ly alpha emission line of Mrk 1486 by assuming a scenario in which the Ly alpha photons are produced inside the galaxy disk, travel along the outflowing halos, and scatter on cool HI materials toward the observer.Conclusions. Extended bipolar galactic winds are frequently observed from star-forming disk galaxies. Given the advantage Ly alpha photons take of such outflowing HI materials to easily escape from Mrk 1486, this mechanism may explain the origin of strong Ly alpha emission lines frequently observed from highly inclined galaxies at high-redshift. This therefore challenges the robustness of the expected viewing-angle effect on the Ly alpha properties of star-forming disk galaxies.
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| 3. |
- Gavel, Alvin, 1989-, et al.
(författare)
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Atomic diffusion and mixing in old stars VII : Abundances of Mg, Ti, and Fe in M 30
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 652, s. A75-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. We attempt to constrain the efficiency of additional transport or mixing processes that reduce the effect of atomic diffusion instellar atmospheres.Methods. We apply spectral synthesis methods to spectra observed with the GIRAFFE spectrograph on the VLT to estimate abundances of Mg, Ti, Fe, and Ba in stars in the metal-poor globular cluster M30. To the abundances we fit trends of abundances predicted by stellar evolution models assuming different efficiencies of additional transport or mixing processes. The fitting procedure attempts to take into account the effects of parameter-dependent systematic errors in the derived abundances.Results. We find that the parameter T0, which describes the efficiency of additional transport or mixing processes, can almost certainlybe constrained to the narrow range log10(T0/[K]) between 6.09 and 6.2. This corresponds to decreased abundances for stars at the main sequence turn-off point compared to the red giant branch by 0.2 dex for Mg, 0.1 dex for Fe, and 0.07 dex for Ti. We also find that while our estimates do have non-negligible systematic errors stemming from the continuum placement and the assumedmicroturbulence, our method can take them into account.Conclusions. Our results partly amend the results of an earlier paper in this article series, that tentatively used a value of log10(T0/[K])=6.0 when modelling the Spite plateau of lithium. To more easily distinguish physical effects from systematic errors, we recommend that studies of this kind focus on elements for which the expected surface abundances as functions of effective temperature have a distinct structure and cover a wide range.
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| 4. |
- Gavel, Alvin, et al.
(författare)
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The LUMBA UVES stellar parameter pipeline
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 629
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Tidskriftsartikel (refereegranskat)abstract
- Context. The Gaia-ESO Survey has taken high-quality spectra of a subset of 100 000 stars observed with the Gaia spacecraft. The goal for this subset is to derive chemical abundances for these stars that will complement the astrometric data collected by Gaia. Deriving the chemical abundances requires that the stellar parameters be determined. Aims. We present a pipeline for deriving stellar parameters from spectra observed with the FLAMES-UVES spectrograph in its standard fibre-fed mode centred on 580 nm, as used in the Gaia-ESO Survey. We quantify the performance of the pipeline in terms of systematic offsets and scatter. In doing so, we present a general method for benchmarking stellar parameter determination pipelines. Methods. Assuming a general model of the errors in stellar parameter pipelines, together with a sample of spectra of stars whose stellar parameters are known from fundamental measurements and relations, we use a Markov chain Monte Carlo method to quantitatively test the pipeline. Results. We find that the pipeline provides parameter estimates with systematic errors on effective temperature below 100 K, on surface gravity below 0.1 dex, and on metallicity below 0.05 dex for the main spectral types of star observed in the Gaia-ESO Survey and tested here. The performance on red giants is somewhat lower. Conclusions. The pipeline performs well enough to fulfil its intended purpose within the Gaia-ESO Survey. It is also general enough that it can be put to use on spectra from other surveys or other spectrographs similar to FLAMES-UVES.
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| 5. |
- Gruyters, Pieter, et al.
(författare)
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A VLT VIMOS IFU study of the ionisation nebula surrounding the supersoft X-ray source CAL 83
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 544, s. A86-
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Tidskriftsartikel (refereegranskat)abstract
- Context. CAL 83 is a prototype of the class of Super Soft X-ray Sources (SXS). It is a binary consisting of a low mass secondary that is transferring mass onto a white dwarf primary and is the only known SXS surrounded by an ionisation nebula, made up of the interstellar medium (ISM) ionised by the source itself. We study this nebula using integral field spectroscopy.Aims. The study of ionised material can inform us about the source that is responsible for the ionisation, in a way that is complementary to studying the source directly. Since CAL 83 is the only SXS known with an ionisation nebula, we have an opportunity to see if such studies are as useful for SXSs as they have been for other X-ray ionised nebulae. We can use these data to compare to models of how CAL83 should ionise its surroundings, based on what we know about the source emission spectrum and the physical conditions of the surrounding ISM.Methods. With the VIMOS integral field spectrograph we obtained spectra over a 25x25 '' field of view, encompassing one quarter of the nebula. Emission line maps - HI, HeII lambda 4686, [O III] lambda lambda 4959, 5007, [N II] lambda lambda 6548, 5683, and [S II] lambda lambda 6716, 6731 - are produced in order to study the morphology of the ionised gas. We include CAL 83 on diagrams of various diagnostic ion ratios to compare it to other X-ray ionised sources. Finally we computed some simple models of the ionised gas around CAL 83 and compare the predicted to the observed spectra.Results. CAL 83 appears to have a fairly standard ionisation nebula as far as the morphology goes: the edges where H is recombining are strong in the low stage ionisation lines and the central, clumpy regions are stronger in the higher stage ionisation lines. But the He II emission is unusual in being confined to one side of CAL 83 rather than being homogeneously distributed as with the other ions. We model the CAL 83 nebula with cloudy using model parameters for SXSs found in the literature. The He II emission does not fit in with model predictions; in fact none of the models is able to fit the observed spectrum very well.Conclusions. The spectral line images of the region surrounding CAL 83 are revealing and instructive. However, more work on modelling the spectrum of the ionised gas is necessary, and especially for the high-ionisation level emission from CAL83. In particular, we wish to know if the He II emission and the other nebular lines are powered by the same ionising source.
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| 6. |
- Gruyters, Pieter, et al.
(författare)
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Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752
- 2013
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 555, s. A31-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters off er useful laboratories to put observational constraints on this theory as one needs to compare abundances in unevolved and evolved stars, all drawn from the same stellar population. Aims. Atomic diffusion and additional mixing has been shown to be at work in the globular cluster NGC6397 at a metallicity of [Fe/H] similar to -2.1. We investigate possible abundance trends in Li, Mg, Ca, Ti, Sc, and Fe with evolutionary stage in another globular cluster NGC6752 at a metallicity of [Fe/H] similar to -1.6. This in order to better constrain stellar structure models including atomic diffusion and additional mixing. Methods. We performed a differential abundance analysis on VLT/FLAMES-UVES data of 16 stars in four groups between the turnoff point and the red giant branch. Continuum normalisation of the stellar spectra was performed in an automated way using DAOSPEC. Differential abundances relative to the sun were derived by fitting synthetic spectra to individual lines in the stellar spectrum. Results. We find weak systematic abundance trends with evolutionary phase for Fe, Sc, Ti, and Ca. The individual trends are weaker than the trends in NGC6397 and only significant at the 1-sigma level. However, the combined trend shows a significance on the 2-sigma level. The trends are best explained by stellar-structure models including atomic diffusion with more efficient additional mixing than needed in NGC6397. The model allows to correct for sub-primordial stellar lithium abundances of the stars on the Spite plateau. Conclusions. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the significance of the trends is weak, they all seem to indicate that atomic diffusion is operational along the evolutionary sequence of NGC6752. The trends are weaker than those observed in NGC6397, which is perhaps due to more efficient mixing. Using models of atomic diffusion including efficient additional mixing, we find a diffusion-corrected primordial lithium abundance of log epsilon(Li) = 2.58 +/- 0.10, in agreement with WMAP-calibrated Big-Bang nucleosynthesis predictions within the mutual 1-sigma uncertainties.
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| 7. |
- Gruyters, Pieter, 1986-, et al.
(författare)
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Atomic diffusion and mixing in old stars V : A deeper look into the globular cluster NGC6752
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 567, s. A72-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC6752 ([Fe/H] = −1.6). These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solartype, metal-poor stars.Aims. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC6752.Methods. We perform an abundance analysis by combining photometric and spectroscopic data of 194 stars located between the turnoff point and the base of the red giant branch. Stellar parameters are derived from ubvy Strömgren photometry. Using the quantitative-spectroscopy package SME, stellar surface abundances for light elements such as Li, Na, Mg, Al, and Si as well as heavier elements such as Ca, Ti, and Fe are derived in an automated way by fitting synthetic spectra to individual lines in the stellar spectra, obtained with the VLT/FLAMES-GIRAFFE spectrograph.Results. Based on ubvy Strömgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value.Conclusions. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC6752.
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| 8. |
- Gruyters, Pieter, 1986-, et al.
(författare)
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Atomic diffusion and mixing in old stars VI : Chemical abundance variations in M4
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6121 (M4) ([Fe/H] = −1.1). These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars.Aims. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up M4. Methods. We perform an abundance analysis by combining photometric and spectroscopic data of 194 stars located between the turnoff point and the base of the red giant branch. Stellar parameters are derived from uvby Strömgren photometry. Using the quantitative-spectroscopy package SME, stellar surface abundances for light elements such as Li, Na, Mg, Al, and Si as well as heavier elements such as Ca, Ti, and Fe are derived in an automated way by fitting synthetic spectra to individual lines in the stellar spectra, obtained with the VLT/FLAMES-GIRAFFE spectrograph.Results. Based on uvby Strömgren photometry, we are able to separate three stellar populations in M4 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of M4 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value.Conclusions. We find three stellar populations by combining photometric and spectroscopic data of 86 stars in the globular cluster M4. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of M4.
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| 9. |
- Gruyters, Pieter, et al.
(författare)
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Atomic diffusion and mixing in old stars VI. The lithium content of M30
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 589
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Tidskriftsartikel (refereegranskat)abstract
- Context. The prediction of the Planck-constrained primordial lithium abundance in the Universe is in discordance with the observed Li abundances in warm Population II dwarf and subgiant stars. Among the physically best motivated ideas, it has been suggested that this discrepancy can be alleviated if the stars observed today had undergone photospheric depletion of lithium.Aims. The cause of this depletion is investigated by accurately tracing the behaviour of the lithium abundances as a function of effective temperature. Globular clusters are ideal laboratories for such an abundance analysis as the relative stellar parameters of their stars can be precisely determined.Methods. We performed a homogeneous chemical abundance analysis of 144 stars in the metal-poor globular cluster M30, ranging from the cluster turnoff point to the tip of the red giant branch. Non-local thermal equilibrium (NLTE) abundances for Li, Ca, and Fe were derived where possible by fitting spectra obtained with VLT/FLAMES-GIRAFFE using the quantitative-spectroscopy package SME. Stellar parameters were derived by matching isochrones to the observed V vs. V I colour-magnitude diagram. Independent effective temperatures were obtained from automated profile fitting of the Balmer lines and by applying colour-T-eff calibrations to the broadband photometry.Results. Li abundances of the turno ff and early subgiant stars form a thin plateau that is broken off abruptly in the middle of the SGB as a result of the onset of Li dilution caused by the first dredge-up. Abundance trends with effective temperature for Fe and Ca are observed and compared to predictions from stellar structure models including atomic diffusion and ad hoc additional mixing below the surface convection zone. The comparison shows that the stars in M30 are affected by atomic diffusion and additional mixing, but we were unable to determine the efficiency of the additional mixing precisely. This is the fourth globular cluster (after NGC6397, NGC6752, and M4) in which atomic diffusion signatures are detected. After applying a conservative correction (T6.0 model) for atomic diffusion, we find an initial Li abundance of A(Li) = 2.48 +/- 0.10 for the globular cluster M30. We also detected a Li-rich SGB star with a Li abundance of A(Li) = 2.39. The finding makes Li-rich mass transfer a likely scenario for this star and rules out models in which its Li enhancement is created during the RGB bump phase.
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| 10. |
- Gruyters, Pieter, 1986-
(författare)
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Exploring the Chemical Evolution of Globular Clusters and their Stars : Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Through the cosmic matter cycle, the chemical evolution of the Milky Way is imprinted in the elemental abundance patterns of late-type stars (spectral types F to K). Due to their long lifetimes ( 1 Hubble time), these stars are of particular importance when it comes to studying the build-up of elements during the early times of our Galaxy. The chemical composition of the atmospheric layers of such stars is believed to resemble the gas from which they were formed. However, recent observations in globular clusters seem to contradict this assumption. The observations indicate that processes are at work that alter the surface compositions in these stars. The combined effect of processes responsible for an exchange of material between the stellar interior and atmosphere during the main sequence lifetime of the star, is referred to as atomic diffusion. Yet, the extent to which these processes alter surface abundances is still debated.By comparing abundances in unevolved and evolved stars all drawn from the same stellar population, any surface abundance anomalies can be traced. The anomalies, if found, can be compared to theoretical predictions from stellar structure models including atomic diffusion. Globular clusters provide stellar populations suitable to conduct such a comparison. In this thesis, the results of three independent analyses of two globular clusters, NGC 6752 and M4, at different metallicities are presented. The comparison between observations and models yields constraints on the models and finally a better understanding of the physical processes at work inside stars.
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| 11. |
- Gruyters, Pieter, et al.
(författare)
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First evidence of multiple populations along the AGB from Strömgren photometry
- 2017
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 603
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Tidskriftsartikel (refereegranskat)abstract
- Spectroscopic studies have demonstrated that nearly all Galactic globular clusters (GCs) harbour multiple stellar populations with different chemical compositions. Moreover, colour-magnitude diagrams based exclusively on Strömgrem photometry have allowed us to identify and characterise multiple populations along the RGB of a large number of clusters. In this paper we show for the first time that Strömgren photometry is also very efficient at identifying multiple populations along the AGB, and demonstrate that the AGB of M 3, M 92, NGC 362, NGC 1851, and NGC 6752 are not consistent with a single stellar population. We also provide a catalogue of RGB and AGB stars photometrically identified in these clusters for further spectroscopic follow-up studies. We combined photometry and elemental abundances from the literature for RGB and AGB stars in NGC 6752 where the presence of multiple populations along the AGB has been widely debated. We find that, while the MS, SGB, and RGB host three stellar populations with different helium and light element abundances, only two populations of AGB stars are present in the cluster. These results are consistent with standard evolutionary theory.
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| 12. |
- Guaita, Lucia, et al.
(författare)
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The Lyman alpha reference sample IV. Morphology at low and high redshift
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 576
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Tidskriftsartikel (refereegranskat)abstract
- Context. The transport of Ly alpha photons in galaxies is a complex process and the conditions under which Ly alpha photons manage to escape from certain galaxies is still under investigation. The Lyman alpha reference sample (LARS) is a sample of 14 local star-forming galaxies, designed to study Ly alpha in detail and relate it to rest-frame UV and optical emission. Aims. With the aim of identifying rest-frame UV and optical properties, which are typical of Ly alpha emitters (LAEs, galaxies with EW(Ly alpha) > 20 angstrom) at both low and high redshift, we investigated the morphological properties of the LARS galaxies, in particular the ones that exhibit intense Ly alpha radiation. Methods. We measured sizes and morphological parameters in the continuum, Ly alpha, and Ha images. We studied morphology by using the Gini coefficient vs. M20 and asymmetry vs. concentration diagrams. We then simulated LARS galaxies at z similar to 2 and 5.7, performing the same morphological measurements. We also investigated the detectability of LARS galaxies in current deep field observations. The subsample of LAEs within LARS (LARS-LAEs) was stacked to provide a comparison to stacking studies performed at high redshift. Results. LARS galaxies have continuum size, stellar mass, and rest-frame absolute magnitude typical of Lyman break analogues in the local Universe and also similar to 2 < z < 3 star-forming galaxies and massive LAEs. LARS optical morphology is consistent with the one of merging systems, and irregular or starburst galaxies. For the first time we quantify the morphology in Ly alpha images: even if a variety of intrinsic conditions of the interstellar medium can favour the escape of Ly alpha photons, LARS-LAEs appear small in the continuum, and their Ly alpha is compact. LARS galaxies tend to be more extended in Ly alpha than in the rest-frame UV. It means that Ly alpha photons escape by forming haloes around HII regions of LARS galaxies. Conclusions. The stack of LARS-LAE Ly alpha images is peaked in the centre, indicating that the conditions, which make a galaxy an LAE, tend to produce a concentrated surface brightness profile. On the other hand, the stack of all LARS galaxies is shallower and more extended. This can be caused by the variety of dust and HI amount and distribution, which produces a more complex, patchy, and extended profile, like the one observed for Lyman break galaxies that can contribute to the stack. We cannot identify a single morphological property that controls whether a galaxy emits a net positive Ly alpha flux. However, the LARS-LAEs have continuum properties consistent with merging systems.
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| 13. |
- Herenz, Edmund Christian, et al.
(författare)
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The Lyman alpha reference sample VII. Spatially resolved H alpha kinematics
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
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Tidskriftsartikel (refereegranskat)abstract
- We present integral field spectroscopic observations with the Potsdam Multi-Aperture Spectrophotometer of all 14 galaxies in the z similar to 0.1 Lyman Alpha Reference Sample (LARS). We produce 2D line-of-sight velocity maps and velocity dispersion maps from the Balmer alpha (H alpha) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies' intrinsic Ly alpha radiation field. We show our kinematic maps that are spatially registered onto the Hubble Space Telescope H alpha and Lyman alpha (Ly alpha) images. We can conjecture a causal connection between spatially resolved H alpha kinematics and Ly alpha photometry for individual galaxies, however, no general trend can be established for the whole sample. Furthermore, we compute the intrinsic velocity dispersion sigma(0), the shearing velocity v(shear), and the v(shear)/sigma(0) ratio from our kinematic maps. In general LARS galaxies are characterised by high intrinsic velocity dispersions (54 km s(-1) median) and low shearing velocities (65 km s(-1) median). The v(shear/sigma 0) values range from 0.5 to 3.2 with an average of 1.5. It is noteworthy that five galaxies of the sample are dispersion-dominated systems with v(shear)/sigma(0) < 1, and are thus kinematically similar to turbulent star-forming galaxies seen at high redshift. When linking our kinematical statistics to the global LARS Ly alpha properties, we find that dispersion-dominated systems show higher Ly alpha equivalent widths and higher Ly alpha escape fractions than systems with v(shear)/sigma(0) > 1. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Ly alpha radiation.
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| 14. |
- Hinkel, Natalie R., et al.
(författare)
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A Comparison Of Stellar Elemental Abundance Techniques And Measurements
- 2016
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 226:1
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Tidskriftsartikel (refereegranskat)abstract
- Stellar elemental abundances are important for understanding the fundamental properties of a star or stellar group, such as age and evolutionary history, as well as the composition of an orbiting planet. However, as abundance measurement techniques have progressed, there has been little standardization between individual methods and their comparisons. As a result, different stellar abundance procedures determine measurements that vary beyond the quoted error for the same elements within the same stars. The purpose of this paper is to better understand the systematic variations between methods and offer recommendations for producing more accurate results in the future. We invited a number of participants from around the world (Australia, Portugal, Sweden, Switzerland, and the United States) to calculate 10 element abundances (C, O, Na, Mg, Al, Si, Fe, Ni, Ba, and Eu) using the same stellar spectra for four stars (HD 361, HD 10700, HD 121504, and HD 202206). Each group produced measurements for each star using (1) their own autonomous techniques, (2) standardized stellar parameters, (3) a standardized line list, and (4) both standardized parameters and a line list. We present the resulting stellar parameters, absolute abundances, and a metric of data similarity that quantifies the homogeneity of the data. We conclude that standardization of some kind, particularly stellar parameters, improves the consistency between methods. However, because results did not converge as more free parameters were standardized, it is clear there are inherent issues within the techniques that need to be reconciled. Therefore, we encourage more conversation and transparency within the community such that stellar abundance determinations can be reproducible as well as accurate and precise.
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| 15. |
- Lind, Karin, et al.
(författare)
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The Gaia-ESO Survey : A globular cluster escapee in the Galactic halo
- 2015
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 575
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Tidskriftsartikel (refereegranskat)abstract
- A small fraction of the halo field is made up of stars that share the light element (Z <= 13) anomalies characteristic of second generation globular cluster (GC) stars. The ejected stars shed light on the formation of the Galactic halo by tracing the dynamical history of the clusters, which are believed to have once been more massive. Some of these ejected stars are expected to show strong Al enhancement at the expense of shortage of Mg, but until now no such star has been found. We search for outliers in the Mg and Al abundances of the few hundreds of halo field stars observed in the first eighteen months of the Gaia-ESO public spectroscopic survey. One halo star at the base of the red giant branch, here referred to as 22593757-4648029 is found to have [Mg/Fe] = -0.36 +/- 0.04 and [Al/Fe] = 0.99 +/- 0.08, which is compatible with the most extreme ratios detected in GCs so far. We compare the orbit of 22593757-4648029 to GCs of similar metallicity and find it unlikely that this star has been tidally stripped with low ejection velocity from any of the clusters. However, both chemical and kinematic arguments render it plausible that the star has been ejected at high velocity from the anomalous GC omega Centauri within the last few billion years. We cannot rule out other progenitor GCs, because some may have disrupted fully, and the abundance and orbital data are inadequate for many of those that are still intact.
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| 16. |
- Nordlander, Thomas, 1985-, et al.
(författare)
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Atomic diffusion and mixing in old stars : VIII. Chemical abundance variations in the globular cluster M4 (NGC 6121)
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 527:4, s. 12120-12139
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Tidskriftsartikel (refereegranskat)abstract
- Variations in chemical abundances with evolutionary phase have been identified among stars in globular and open clusters with a wide range of metallicities. In the metal-poor clusters, these variations compare well with predictions from stellar structure and evolution models considering the internal diffusive motions of atoms and ions, collectively known as atomic diffusion, when moderated by an additional mixing process with a fine-tuned efficiency. We present here an investigation of these effects in the Galactic globular cluster NGC 6121 (M4) ([Fe/H] = −1.13) through a detailed chemical abundance analysis of 86 stars using high-resolution ESO Very Large Telescope (VLT) Fibre Large Array Multi Element Spectrograph (FLAMES) spectroscopy. The stars range from the main-sequence turnoff point (TOP) to the red giant branch (RGB) just above the bump. We identify C-N-O and Mg-Al-Si abundance anticorrelations, and confirm the presence of a bimodal population differing by 1 dex in nitrogen abundance. The composition of the second-generation stars imply pollution from both massive (20–40 M) and asymptotic giant branch stars. We find evolutionary variations in chemical abundances between the TOP and RGB, which are robust to uncertainties in stellar parameters and modelling assumptions. The variations are weak, but match predictions well when employing efficient additional mixing. Without correcting for Galactic production of lithium, we derive an initial lithium abundance 2.63 ± 0.10, which is marginally lower than the predicted primordial big-bang nucleosynthesis value.
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| 17. |
- Nordlander, Thomas, 1985-, et al.
(författare)
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Atomic diffusion and mixing in old stars VII : Chemical abundance variations in M4 (NGC 6121)
- 2017
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Context. Variations in chemical abundances with evolutionary phase have been identified in previous papers of this series among stars in three globular clusters, M30 ([Fe/H] = −2.3), NGC 6397 ([Fe/H] = −2.1) and NGC 6752 ([Fe/H] = −1.6). These variations compare well with the predictions from stellar-structure models with atomic diffusion moderated by additional mixing of low and high efficiency at the respective ends of the metallicity scale.Aims. By extending these studies to higher metallicity, for a large number of elements, further empirical constraints can be provided to the nature of the additional mixing mechanism. We therefore investigate whether evolutionary abundance variations are present among stars in the globular cluster NGC 6121 (M4, [Fe/H] = −1.1).Methods. We perform a detailed chemical abundance analysis of 86 stars, ranging from the cluster turnoff point to the red giant branch just above the bump. We determine the abundances of 14 elements consistently by means of spectrum matching, using medium-resolution spectra obtained with VLT/FLAMES-GIRAFFE. Stellar parameters are obtained from UBVI broadband photometry, after correcting for differential reddening effects.Results. We observe the usual C-N-O anticorrelations and confirm the presence of a bimodal population characterised by their N content. We find systematic evolutionary variations in individual chemical abundances of weak statistical significance, but which are rather robust to uncertainties in stellar parameters and modelling assumptions, for magnesium, silicon, calcium, titanium and iron. These variations match predictions from stellar evolution models including atomic diffusion if efficient additional mixing is employed, in line with previous results. Using these models, we derive an initial lithium abundance for the cluster, 2.59 ± 0.10, which is fully compatible with those determined for M30, NGC 6397 and NGC 6752, falling slightly short of the predicted primordial BBN value.Conclusions. The observed abundance patterns of 14 elements investigated here suggest that the second generation stars in M4 were formed out of gas that was polluted by both massive (20−40 solar masses) stars and AGB stars. Element-specific abundance trends are identified in stars along the evolutionary sequence in M4. Although the significance of the individual trends is weak, they all seem to indicate that atomic diffusion is at work in M4. This is the fourth cluster in which atomic diffusion signatures are seen, giving more evidence for the assumption that atomic diffusion produces measurable surface-abundance changes in warm metal-poor stars and hence should be accounted for in stellar-evolution models and studies of Galactic chemical evolution.
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| 18. |
- Smiljanic, R., et al.
(författare)
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The Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
- 2014
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Ingår i: Astronomy & Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 570
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Tidskriftsartikel (refereegranskat)abstract
- Context. The ongoing Gaia-ESO Public Spectroscopic Survey is using FLAMES at the VLT to obtain high-quality medium-resolution Giraffe spectra for about 10(5) stars and high-resolution UVES spectra for about 5000 stars. With UVES, the Survey has already observed 1447 FGK-type stars. Aims. These UVES spectra are analyzed in parallel by several state-of-the-art methodologies. Our aim is to present how these analyses were implemented, to discuss their results, and to describe how a final recommended parameter scale is defined. We also discuss the precision (method-to-method dispersion) and accuracy (biases with respect to the reference values) of the final parameters. These results are part of the Gaia-ESO second internal release and will be part of its first public release of advanced data products. Methods. The final parameter scale is tied to the scale defined by the Gaia benchmark stars, a set of stars with fundamental atmospheric parameters. In addition, a set of open and globular clusters is used to evaluate the physical soundness of the results. Each of the implemented methodologies is judged against the benchmark stars to define weights in three different regions of the parameter space. The final recommended results are the weighted medians of those from the individual methods. Results. The recommended results successfully reproduce the atmospheric parameters of the benchmark stars and the expected T-eff-log g relation of the calibrating clusters. Atmospheric parameters and abundances have been determined for 1301 FGK-type stars observed with UVES. The median of the method-to-method dispersion of the atmospheric parameters is 55K for T-eff, 0.13dex for log g and 0.07 dex for [Fe/H]. Systematic biases are estimated to be between 50-100 K for T-eff, 0.10-0.25 dex for log g and 0.05-0.10 dex for [Fe/H]. Abundances for 24 elements were derived: C, N, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Mo, Ba, Nd, and Eu. The typical method-to-method dispersion of the abundances varies between 0.10 and 0.20 dex. Conclusions. The Gaia-ESO sample of high-resolution spectra of FGK-type stars will be among the largest of its kind analyzed in a homogeneous way. The extensive list of elemental abundances derived in these stars will enable significant advances in the areas of stellar evolution and Milky Way formation and evolution.
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| 19. |
- Östlin, Göran, et al.
(författare)
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THE Ly alpha REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 797:1
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Tidskriftsartikel (refereegranskat)abstract
- The Ly alpha Reference Sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Ly alpha line of neutral hydrogen. Ly alpha is the dominant spectral line in use for characterizing high-redshift (z) galaxies. This paper presents an overview of the survey, its selection function, and HST imaging observations. The sample was selected from the combined GALEX+Sloan Digital Sky Survey catalog at z = 0.028-0.19, in order to allow Ly alpha to be captured with combinations of long-pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) onboard HST. In addition, LARS utilizes H alpha and H beta narrowband and u, b, i broadband imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Ly alpha photons are produced (whether or not they escape), we demanded an H alpha equivalent width W(H alpha) >= 100 angstrom. The final sample of 14 galaxies covers far-UV (FUV, lambda similar to 1500 angstrom) luminosities that overlap with those of high-z Ly alpha emitters (LAEs) and Lyman break galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Ly alpha images, including our LARS eXtraction software (LaXs), which utilizes pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Ly alpha. We demonstrate that the use of SBC long-pass-filter combinations increase the signal-to-noise ratio by an order of magnitude compared to the nominal Ly alpha filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright and extended (indicative of resonance scattering) but strongly asymmetric Ly alpha emission. Spectroscopy from the Cosmic Origins Spectrograph on board HST centered on the brightest UV knot shows a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features) and Ly alpha emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Ly alpha line profile and confirms the presence of an outflow. From the integrated photometry we measure an Ly alpha luminosity of L-Ly alpha= 1.3x10(42) erg s(-1) an equivalent width W(Ly alpha) = 45 angstrom and an FUV absolute magnitude M-FUV = -19.2 (AB). Mrk 259 would hence be detectable in high-z Ly alpha and LBG surveys. The total Ly alpha escape fraction is 12%. This number is higher than the low-z average, but similar to that at z > 4, demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.
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