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Galactic archaeology with metal-poor stars
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The chemical fingerprints of old, metal-poor stars can be used to unravel the events of the newborn Universe and help us understand the properties of the first stars and star clusters. The study of nearby stars to infer properties in the distant past is often referred to as Galactic archaeology. However, the chemical composition of stars cannot be observed directly, but must be inferred by means of spectroscopic modelling. Traditionally, this modelling utilises one-dimensional (1D) stellar atmospheres in hydrostatic and local thermodynamic equilibrium (LTE). Today, we know that departures from LTE (known as NLTE), and differences between 1D model atmospheres and their hydrodynamical three-dimensional (3D) counterparts, become increasingly severe at lower metallicity. The development of NLTE modelling of spectral line formation in 3D atmospheres is still in its infancy, but constitutes a remarkable step forward that has been made possible by parallelised codes and supercomputers. The central theme of this thesis is the application of NLTE analyses to metal-poor stars, to help usher the field of Galactic archaeology forward with important consequences for the nature of the first stellar generations.

I present a theoretical NLTE study of aluminium, where I validate the analysis using a set of bright standard stars and provide calculated NLTE effects for a large parameter space. I perform 3D NLTE calculations for the solar spectrum to better constrain the zero-point of the cosmic abundance scale, and find excellent agreement with the meteoritic aluminium abundance.

I also present NLTE analyses of metal-poor stars in the globular clusters NGC 6397 and M4. While globular cluster stars were long expected to form from a chemically homogeneous medium, star-to-star abundance variations of light elements indicate multiple epochs of star formation. Massive first-generation stars polluted the interstellar medium from which later generations formed, and I use the observed abundance variations to deduce the properties of the polluting stars. Among the heavier elements, I uncover evolutionary abundance variations that match predictions of stellar evolution models with atomic diffusion. The results indicate that the chemical abundance ratios of unevolved metal-poor stars are affected by gravitational settling, with a bias of the order 25-50 %, increasing towards lower metallicity. This atmospheric depletion mechanism is a probable explanation to why the stellar abundances of lithium fall short of the predictions from standard Big Bang nucleosynthesis.

Finally, I apply a 3D NLTE abundance analysis to the red giant SMSS 0313-6708, which is the most iron-deficient star known. The chemical abundance pattern of this star indicates that it formed from gas affected only by Big Bang nucleosynthesis and a single faint supernova. Comparison of the inferred abundance pattern to theoretical predictions leads to constraints on the explosion mechanism and the mass of the metal-free progenitor star.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , 73 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1470
Keyword [en]
stars: abundances, radiative transfer, stars: Population II, stars: Population III
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:uu:diva-313595ISBN: 978-91-554-9803-0 (print)OAI: diva2:1067573
Public defence
2017-03-10, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Available from: 2017-02-16 Created: 2017-01-23 Last updated: 2017-02-27
List of papers
1. Non-LTE aluminium abundances in late-type stars
Open this publication in new window or tab >>Non-LTE aluminium abundances in late-type stars
2017 (English)In: Astronomy and AstrophysicsArticle in journal (Refereed) Submitted
Abstract [en]

Aims. Aluminium plays a key role in studies of the chemical enrichment of the Galaxy and of globular clusters. However, strong deviations from LTE (non-LTE) are known to significantly affect the inferred abundances in giant and metal-poor stars.

Methods. We present NLTE modeling of aluminium using recent and accurate atomic data, in particular utilizing new transition rates for collisions with hydrogen atoms, without the need for any astrophysically calibrated parameters. For the first time, we perform 3D NLTE modeling of aluminium lines in the solar spectrum. We also compute and make available extensive grids of abundance corrections for lines in the optical and near-infrared using one-dimensional model atmospheres, and apply grids of precomputed departure coefficients to direct line synthesis for a set of benchmark stars with accurately known stellar parameters.

Results. Our 3D NLTE modeling of the solar spectrum reproduces observed center-to-limb variations in the solar spectrum of the 7835 Å line as well as the mid-infrared photospheric emission line at 12.33 μm. We infer a 3D NLTE solar photospheric abundance of A(Al) = 6.43 ± 0.03, in perfect agreement with the meteoritic abundance. We find that abundance corrections vary rapidly with stellar parameters; for the 3961 Å resonance line, corrections are positive and may be as large as +1 dex, while corrections for subordinate lines generally have positive sign for warm stars but negative for cool stars. Our modeling reproduces the observed line profiles of benchmark K-giants, and we find abundance corrections as large as −0.4 dex for Arcturus. Our analyses of four metal-poor benchmark stars yield consistent abundances between the 3961 Å resonance line and lines in the UV, optical and near-infrared regions. Finally, we discuss implications for the galactic chemical evolution of aluminium. 

stars: abundances, stars: atmospheres, techniques: spectroscopic, line: formation
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-313592 (URN)
Available from: 2017-01-22 Created: 2017-01-22 Last updated: 2017-01-22
2. Atomic diffusion and mixing in old stars: III. Analysis of NGC 6397 stars under new constraints
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars: III. Analysis of NGC 6397 stars under new constraints
2012 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 753, no 1, 48- p.Article in journal (Refereed) Published
Abstract [en]

We have previously reported on chemical abundance trends with evolutionary state in the globular cluster NGC 6397 discovered in analyses of spectra taken with FLAMES at the Very Large Telescope. Here, we reinvestigate the FLAMES-UVES sample of 18 stars, ranging from just above the turnoff point to the red giant branch below the bump. Inspired by new calibrations of the infrared flux method, we adopt a set of hotter temperature scales. Chemical abundances are determined for six elements (Li, Mg, Ca, Ti, Cr, and Fe). Signatures of cluster-internal pollution are identified and corrected for in the analysis of Mg. On the modified temperature scales, evolutionary trends in the abundances of Mg and Fe are found to be significant at the 2 sigma and 3 sigma levels, respectively. The detailed evolution of abundances for all six elements agrees with theoretical isochrones, calculated with effects of atomic diffusion and a weak to moderately strong efficiency of turbulent mixing. The age of these models is compatible with the external determination from the white dwarf cooling sequence. We find that the abundance analysis cannot be reconciled with the strong turbulent-mixing efficiency inferred elsewhere for halo field stars. A weak mixing efficiency reproduces observations best, indicating a diffusion-corrected primordial lithium abundance of log epsilon(Li) = 2.57 +/- 0.10. At 1.2 sigma, this value agrees well with Wilkinson Microwave Anisotropy Probe calibrated big bang nucleosynthesis predictions.

diffusion, globular clusters: individual (NGC 6397), stars: abundances, stars: atmospheres, stars: evolution, stars: fundamental parameters, stars: Population II
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-177847 (URN)10.1088/0004-637X/753/1/48 (DOI)000305632500048 ()
Available from: 2012-07-20 Created: 2012-07-19 Last updated: 2017-12-07Bibliographically approved
3. Atomic diffusion and mixing in old stars VII: Chemical abundance variations in M4
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars VII: Chemical abundance variations in M4
2017 (English)In: Astronomy and AstrophysicsArticle in journal (Refereed) Submitted
Abstract [en]

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. 

stars: abundances, stars: atmospheres, stars: fundamental parameters, globular cluster and associations: M4, techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-313593 (URN)
Available from: 2017-01-22 Created: 2017-01-22 Last updated: 2017-01-29
4. 3D NLTE Analysis of the Most Iron-Deficient Star, SMSS0313-6708
Open this publication in new window or tab >>3D NLTE Analysis of the Most Iron-Deficient Star, SMSS0313-6708
Show others...
2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 597, A6Article in journal (Refereed) Published
Abstract [en]

Context. Models of star formation in the early universe require a detailed understanding of accretion, fragmentation and radiative feedback in metal-free molecular clouds. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low-mass (0.1-10 Msol), to massive (10-100 Msol), or even supermassive (100-1000 Msol). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova.

Aims. The photospheres of metal-poor stars are relatively transparent in the UV, which may lead to large three-dimensional (3D) effects as well as departures from local thermodynamical equilibrium (LTE), even for weak spectral lines. If 3D effects and departures from LTE (NLTE) are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 by means of the most realistic methods available, and compare the results to predicted supernova yields.

Methods. A 3D hydrodynamical Stagger model atmosphere and 3D NLTE radiative transfer were applied to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters.

Results. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] < -6.53 (3 sigma), than previous estimates based on <3D> NLTE (i.e., using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D-<3D> NLTE differences, all of which lead to a higher abundance determination.

Conclusions. We find that supernova yields for models in a wide range of progenitor masses reproduce the revised chemical composition. In addition to massive progenitors of 20-60 Msol exploding with low energies (1-2 B, where 1 B = 10^51 erg), we also find good fits for progenitors of 10 Msol, with very low explosion energies (<1 B). We cannot reconcile the new abundances with supernovae or hypernovae with explosion energies above 2.5 B, nor with pair-instability supernovae. 

radiative transfer, stars: abundances, Stars: Population III, techniques: spectroscopic, stars: individual: SMSS J031300.36, supernovae: general
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:uu:diva-313591 (URN)10.1051/0004-6361/201629202 (DOI)000392392900095 ()
Swedish National Space BoardAustralian Research Council, FL110100012Swedish Research Council, 2015-00415_3The Royal Swedish Academy of SciencesWenner-Gren FoundationsGöran Gustafsson Foundation for Research in Natural Sciences and MedicineKnut and Alice Wallenberg FoundationSwedish National Infrastructure for Computing (SNIC), p2013234
Available from: 2017-01-22 Created: 2017-01-22 Last updated: 2017-11-29Bibliographically approved

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