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Atomic diffusion and mixing in old stars VI: Chemical abundance variations in M4
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
LUPM, Université Montpellier II, CNRS, CC072, Place E. Bataillon, 34095 Montpellier Cedex, France.
(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 [en]
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: urn:nbn:se:uu:diva-230181OAI: oai:DiVA.org:uu-230181DiVA: diva2:739571
Note

To be submitted to A&A

Available from: 2014-08-21 Created: 2014-08-20 Last updated: 2017-12-05
In thesis
1. Exploring the Chemical Evolution of Globular Clusters and their Stars: Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4
Open this publication in new window or tab >>Exploring the Chemical Evolution of Globular Clusters and their Stars: Observational Constraints on Atomic Diffusion and Cluster Pollution in NGC 6752 and M4
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
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:nbn:se:uu:diva-230182 (URN)978-91-554-9008-9 (ISBN)
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

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Gruyters, PieterNordlander, ThomasKorn, Andreas

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