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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
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 91 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1168
Keyword [en]
stars: abundances – stars: atmospheres – stars: fundamental parameters – globular clusters: individual: NGC 6752 and M4 – techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
URN: urn:nbn:se:uu:diva-230182ISBN: 978-91-554-9008-9 (print)OAI: oai:DiVA.org:uu-230182DiVA: diva2:739780
Public defence
2014-10-03, Å2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-09-12 Created: 2014-08-20 Last updated: 2015-01-22
List of papers
1. Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars IV. Weak abundance trends in the globular cluster NGC6752
Show others...
2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 555, A31- p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
stars: abundances, stars: atmospheres, stars: fundamental parameters, globular clusters: individual: NGC 6752, techniques: spectroscopic
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-207105 (URN)10.1051/0004-6361/201220821 (DOI)000322008600031 ()
Available from: 2013-09-10 Created: 2013-09-09 Last updated: 2017-12-06Bibliographically approved
2. Atomic diffusion and mixing in old stars V: A deeper look into the globular cluster NGC6752
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars V: A deeper look into the globular cluster NGC6752
2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 567, A72- p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
stars: abundances – stars: atmospheres – stars: fundamental parameters – globular clusters: individual: NGC 6752 – techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-230180 (URN)10.1051/0004-6361/201423590 (DOI)000341185300086 ()
Available from: 2014-08-20 Created: 2014-08-20 Last updated: 2017-12-05Bibliographically approved
3. Atomic diffusion and mixing in old stars VI: Chemical abundance variations in M4
Open this publication in new window or tab >>Atomic diffusion and mixing in old stars VI: Chemical abundance variations in M4
(English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Other academic) Submitted
Abstract [en]

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.

Keyword
stars: abundances - stars: atmospheres - stars: fundamental parameters - globular cluster and associations: M4 - techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy with specialization in Astrophysics
Identifiers
urn:nbn:se:uu:diva-230181 (URN)
Note

To be submitted to A&A

Available from: 2014-08-21 Created: 2014-08-20 Last updated: 2017-12-05

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