Matthias Ammler  
Characterisation of young, nearby stars - The Ursa-Major group

Dissertation, 30.01.2007, A5, 188 Seiten, 8 Farbseiten

ISBN: 978-3-00-019738-3  VVPN 00001005

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The Ursa Major group is composed of the Ursa Major open star cluster in the Big Dipper constellation and of many co-moving stars spread over the whole sky. The stars of the Ursa Major group have a relatively young age of 200-600 million years.

Taking into account their proximity and their common motion, the youth makes these stars interesting targets for various astrophysical studies.

However the list of members is not complete. A few dozens of assured member face hundreds of additional candidates. The author presents a study which readdresses the spectroscopic properties of the Ursa Major and discusses their use for future decisions on the membership of candidates.


List of Figures

List of Tables



Remarks and Acknowledgements

1 Introduction

1.1 What is the Ursa Major group?

1.1.1 Co-moving stars in the Big Dipper constellation

1.1.2 Stellar motion and moving groups

1.1.3 Formation and evolution of open clusters and associations

1.1.4 The nature of the UMa group - cluster or association, or something else?

1.2 Why is the UMa group interesting?

1.2.1 A snapshot in stellar evolution

1.2.2 A laboratory in front of the door

1.2.3 The census of the solar neighbourhood

1.3 Constraining the UMa group - previous approaches

1.3.1 Spatial clustering

1.3.2 Kinematic criteria - derived from a "canonical" member list

1.3.3 Kinematic parameters - derived from kinematic clustering

1.3.4 Stellar parameters and abundances

1.3.5 The age of the UMa group - photometric criteria

1.3.6 Spectroscopic indicators for age and activity

1.3.7 Combining kinematic, spectroscopic, and photometric criteria

1.4 A new homogeneous spectroscopic study

1.4.1 Defining the sample

1.4.2 How to obtain precise stellar parameters?

2 Observations, reduction and calibration

2.1 Required data

2.2 Instruments

2.3 Observations

2.4 Reduction and calibrations

3 Deriving the stellar parameters - Methods

3.1 Differential analysis

3.2 Model atmospheres and synthetic line formation

3.2.1 Radiative transfer

3.2.2 Radiative transfer in solar-like stars - defining the geometry

3.2.3 Thermodynamic equilibrium

3.2.4 Local thermodynamic equilibrium (LTE)

3.2.5 Contributions to absorption and emission

3.2.6 Line profiles

3.2.7 Hydrostatic equilibrium

3.2.8 Convection

3.2.9 Three-dimensional hydrodynamics versus one-dimensional hydrostatics

3.3 The stellar parameters

3.3.1 Effective temperature

3.3.2 Surface gravity

3.3.3 Abundances and microturbulence

3.3.4 Instrumental profile, rotation, and macroturbulence

3.3.5 Estimating the stellar mass

4 Results and implications for the UMa group

4.1 How accurate are the resulting stellar parameters?

4.1.1 The Moon spectra and the solar parameters

4.1.2 Consistency with Fuhrmann (2004)

4.1.3 Comparison of spectroscopic distance with Hipparcos distance

4.1.4 Notes on individual stars

4.1.5 Comparison of single star parameters with previous determinations

4.2 The properties of the UMa group

4.2.1 Kiel diagram

4.2.2 Abundances of iron and magnesium

4.2.3 Rotation

4.2.4 Equivalent width of the Li I λ 6707.8Å absorption line

4.2.5 Filling-in of the H α line core

4.3 Conclusions on the age

4.4 Concluding remarks on membership criteria

5 Summary and outlook


A. The kinematic membership criteria of Eggen (1958, 1995)

A.1. Preliminaries

A.2. Moving cluster method

A.3. Peculiar velocity criterion

A.4. Radial velocity criterion

A.5. Adaption by Montes et al. (2001a)

B. Details on the used spectra

C. Line data

D. Solution of the model atmosphere problem with MAFAGS

D.1. Flux conservation

D.2. Solution of the model atmosphere

E. Individual spectra near H α and Li I 6707.8Å

F. Residuals of LTE fits to the observed H α profile


Persönliche Danksagung

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