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Solar Type Stars as Calibrators: A Photometric and Spectroscopic Study on the Atmospheric Properties of Late-type Stars
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. (Division of Astronomy)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Detailed knowledge of solar-type stars is essential in the understanding of the evolutionary past, presence and future of the Sun as well as the formation of its planetary system. Moreover, solar-type stars are of key significance for the study of the evolution of the Galaxy. The ages of solar-type stars map the full galactic evolution. Their surface layers are well mixed and just little affected by the interior nuclear processes. They may therefore be used as samples of the gas from which the stars were once formed.

Models of stellar atmospheres are used to derive fundamental stellar quantities such as chemical composition, effective temperature, surface gravity, age and rotation. It is therefore also important to investigate the progress and shortcomings of the atmospheric models and the reliability of calibrations based upon these. In this thesis we explore the potential of synthetic uvbyHβ colours for deriving atmospheric parameters. The theoretical colours are derived using high-resolution synthetic spectra based on 1D atmosphere models of late-type stars. Furthermore, possible applications of the established synthetic colours on globular stellar clusters are tested.

Observations of solar-type stars have demonstrated the existence of stars very similar to the Sun, so-called solar twins. A detailed chemical analysis of these stars, however, shows that most solar-twins are systematically richer, as compared with the Sun, in refractory elements such as Fe, Ni and Al, relative to volatile elements like C, N and O. This chemical abundance pattern has been suggested to be related to the formation of planets or the birth environment of the respective star. In this thesis we present a high-accuracy study on a solar-twin star in the old open cluster M67. We find that the star is very similar to the Sun when comparing their atmospheric parameters, effective temperature, surface gravity and metallicity. Remarkably enough, unlike most solar twins observed in the solar vicinity, the cluster twin shows the same refractory to volatile pattern as the Sun.The reason for this similarity is still unknown but further observations of the cluster will help to clarify the matter.

M dwarfs constitute a large fraction of the detectable baryonic matter. In spite of this, detailed knowledge on the numerous neighbouring low-mass stars is still not available. The presence of strong molecular features in the spectra, and incomplete line lists for the corresponding molecules have made metallicity determinations of M dwarfs difficult. Furthermore, the faint M dwarfs require long exposure times for a signal-to-noise ratio sufficient for detailed spectroscopic abundance analysis. In this thesis we present a high resolution spectroscopic study of early-type M dwarfs in the infrared. The lack of prominent molecular bands in parts of the infrared J-band (1100--1400 nm) allows a precise continuum placement. Furthermore, we verify the adequacy of using the model atmospheres for abundance determination by observing a set of binary systems with a solar-type primary and an M dwarf companion. We present a reliable zero-point for the metallicity scale of early-type M dwarfs and verify the reliability of spectroscopic abundance analyses in the infrared.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2011. , 55 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 855
Keyword [en]
stars: atmospheres, stars: late-type, techniques: photometric, stars: abundances, techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
URN: urn:nbn:se:uu:diva-158709ISBN: 978-91-554-8161-2 (print)OAI: diva2:442280
Public defence
2011-10-21, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:00 (English)
Available from: 2011-09-30 Created: 2011-09-13 Last updated: 2011-11-03Bibliographically approved
List of papers
1. Calibration of Stromgren uvby-H beta photometry for late-type stars - a model atmosphere approach
Open this publication in new window or tab >>Calibration of Stromgren uvby-H beta photometry for late-type stars - a model atmosphere approach
2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 498, no 2, 527-542 p.Article in journal (Refereed) Published
Abstract [en]

Context. The use of model atmospheres for deriving stellar fundamental parameters, such as T-eff, log g, and [Fe/H], will increase as we find and explore extreme stellar populations where empirical calibrations are not yet available. Moreover, calibrations for upcoming large satellite missions of new spectrophotometric indices, similar to the uvby-H beta system, will be needed. Aims. We aim to test the power of theoretical calibrations based on a new generation of MARCS models by comparisons with observational photomteric data. Methods. We calculated synthetic uvby-H beta colour indices from synthetic spectra. A sample of 367 field stars, as well as stars in globular clusters, is used for a direct comparison of the synthetic indices versus empirical data and for scrutinizing the possibilities of theoretical calibrations for temperature, metallicity, and gravity. Results. We show that the temperature sensitivity of the synthetic (b-y) colour is very close to its empirical counterpart, whereas the temperature scale based upon H beta shows a slight offset. The theoretical metallicity sensitivity of the m(1) index (and for G-type stars its combination with c(1)) is somewhat higher than the empirical one, based upon spectroscopic determinations. The gravity sensitivity of the synthetic c(1) index shows satisfactory behaviour when compared to obervations of F stars. For stars cooler than the sun, a deviation is significant in the c(1)-(b-y) diagram. The theoretical calibrations of (b-y), (v-y), and c(1) seem to work well for Pop II stars and lead to effective temperatures for globular cluster stars supporting recent claims that atomic diffusion occurs in stars near the turnoff point of NGC 6397. Conclusions. Synthetic colours of stellar atmospheres can indeed be used, in many cases, to derive reliable fundamental stellar parameters. The deviations seen when compared to observational data could be due to incomplete linelists but are possibly also due to the effects of assuming plane-parallell or spherical geometry and LTE.

stars: fundamental parameters, techniques: photometric, stars: atmospheres, stars: late-type
National Category
Physical Sciences
urn:nbn:se:uu:diva-148692 (URN)10.1051/0004-6361/200810786 (DOI)000265868600028 ()
Available from: 2011-03-10 Created: 2011-03-09 Last updated: 2017-12-11Bibliographically approved
2. M67-1194, an unusually Sun-like solar twin in M67
Open this publication in new window or tab >>M67-1194, an unusually Sun-like solar twin in M67
Show others...
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 528, A85- p.Article in journal (Refereed) Published
Abstract [en]

Context. The rich open cluster M67 is known to have a chemical composition close to solar, and an age around 4Gyr. It thus offers the opportunity to check our understanding of the physics and the evolution of solar-type stars in a cluster environment. Aims. We present the first spectroscopic study at high resolution, R approximate to 50 000, of the potentially best solar twin, M67-1194, identified among solar-like stars in M67. Methods. G dwarfs in M67 (d approximate to 900 pc) are relatively faint (V approximate to 15), which makes detailed spectroscopic studies time-consuming. Based on a pre-selection of solar-twin candidates performed at medium resolution by Pasquini et al. (2008, A&A, 489, 677), we explore the chemical-abundance similarities and differences between M67-1194 and the Sun, using VLT/FLAMES-UVES. Working with a solar twin in the framework of a differential analysis, we minimize systematic model errors in the abundance analysis compared to previous studies which utilized more evolved stars to determine the metallicity of M67. The differential approach yields precise and accurate chemical abundances for M67, which enhances the possibility to use this object in studies of the potential peculiarity, or normality, of the Sun. Results. We find M67-1194 to have stellar parameters indistinguishable from the solar values, with the exception of the overall metallicity which is slightly super-solar ([Fe/H] = 0.023 +/- 0.015). An age determination based on evolutionary tracks yields 4.2 +/- 1.6Gyr. Most surprisingly, we find the chemical abundance pattern to closely resemble the solar one, in contrast to most known solar twins in the solar neighbourhood. Conclusions. We confirm the solar-twin nature of M67-1194, the first solar twin known to belong to a stellar association. This fact allows us to put some constraints on the physical reasons for the seemingly systematic departure of M67-1194 and the Sun from most known solar twins regarding chemical composition. We find that radiative dust cleansing by nearby luminous stars may be the explanation for the peculiar composition of both the Sun and M67-1194, but alternative explanations are also possible. The chemical similarity between the Sun and M67-1194 also suggests that the Sun once formed in a cluster like M67.

stars: abundances, stars: fundamental parameters, stars: solar-type, Sun: abundances, open cluster and associations: individual: NGC 2682 (M67), techniques: spectroscopic
National Category
Physical Sciences
urn:nbn:se:uu:diva-151907 (URN)10.1051/0004-6361/201015138 (DOI)000288541600117 ()
Available from: 2011-04-19 Created: 2011-04-19 Last updated: 2017-12-11Bibliographically approved
3. M-dwarf metallicities: A high-resolution spectroscopic study in the near infrared
Open this publication in new window or tab >>M-dwarf metallicities: A high-resolution spectroscopic study in the near infrared
Show others...
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 542, A33- p.Article in journal (Refereed) Published
Abstract [en]

The relativley large spread in the derived metallicities ([Fe/H]) of M dwarfs shows that various approaches have not yet converged to consistency. The presence of strong molecular features, and incomplete line lists for the corresponding molecules have made metallicity determinations of M dwarfs diffcult. Furthermore, the faint M dwarfs require long exposure times for a signal-to-noise ratio suffcient for a detailed spectroscopic abundance analysis.

We present a high-resolution (R~50,000) spectroscopic study of a sample of eight single M dwarfs and three wide-binary systems observed in the infrared J-band.

The absence of large molecular contributions allow for a precise continuum placement. We derive metallicities based on the best fit synthetic spectra to the observed spectra. To verify the accuracy of the applied atmospheric models and test our synthetic spectrum approach, three binary systems with a K dwarf primary and an M dwarf companion were observed and analysed along with the single M dwarfs.

We obtain a good agreement between the metallicities derived for the primaries and secondaries of our test binaries and thereby confirm the reliability of our method of analysing M dwarfs. Our metallicities agree well with certain earlier determinations, and deviate from others.

We conclude that spectroscopic abundance analysis in the J band is a reliable method for establishing the metallicity scale for M dwarfs. We recommend its application to a larger sample covering lower as well as higher metallicities. Further prospectsof the method include abundance determinations for individual elements.

stars: atmospheres, stars: binaries visual, stars: low-mass, stars: abundances, techniques: spectroscopic
National Category
Astronomy, Astrophysics and Cosmology
Research subject
urn:nbn:se:uu:diva-158847 (URN)10.1051/0004-6361/201118101 (DOI)000305803300025 ()
Available from: 2011-09-18 Created: 2011-09-18 Last updated: 2017-12-08Bibliographically approved

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