[Implementation of Gaia Photometric System in Astro Science]

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ABSTRACT

The European Gaia astrometry mission is due for launch in 2011. Gaia will rely on the proven principles of the ESA Hipparcos mission to create an all-sky survey of about one billion stars throughout our Galaxy and beyond, by observing all objects down to 20 mag. Through its massive measurement of stellar distances, motions and multicolour photometry, it will provide fundamental data necessary for unravelling the structure, formation and evolution of the Galaxy. This paper presents the design and performance of the broad- and medium-band set of photometric filters adopted as the baseline for Gaia. The 19 selected passbands (extending from the UV to the far-red), the criteria and the methodology on which this choice has been based are discussed in detail. We analyse the photometric capabilities for characterizing the luminosity, temperature, gravity and chemical composition of stars. We also discuss the automatic determination of these physical parameters for the large number of observations involved, for objects located throughout the entire Hertzsprung-Russell diagram. Finally, the capability of the photometric system (PS) to deal with the main Gaia science case is outlined.

TABLE OF CONTENTS

ABSTRACTII

CHAPTER 1: INTRODUCTION1

CHAPTER 2: INSTRUMENT DESCRIPTION5

CHAPTER 3: THE G AND GS PASSBANDS12

CHAPTER 4: DESIGNING THE PHOTOMETRIC SYSTEM16

CHAPTER 5: THE G AIA PHOTOMETRIC SYSTEM29

The C1B broad passbands29

Astrophysical diagnostics33

Chromaticity evaluation35

The C1M medium passbands39

Astrophysical diagnostics42

CHAPTER 6: PHOTOMETRIC PRECISION47

Simulated Photometry47

Aperture photometry and associated errors48

CHAPTER 7: PHOTOMETRY PERFORMANCES55

CHAPTER 7: PHOTOMETRY PERFORMANCES55

CHAPTER 8: SCIENCE IMPLICATIONS69

CHAPTER 9: CONCLUSIONS76

REFERENCES80

APPENDIX A89

CHAPTER 1: INTRODUCTION



Gaia has been approved as a cornerstone mission in the ESA scientific programme. The main goal is to provide data to study the formation and subsequent dynamical, chemical and star formation evolution of the Milky Way galaxy (Perryman et al. 2008: 45; Mignard 2009: 105). Gaia will achieve this by providing an all-sky astrometric and photometric survey complete to 20 mag in unfiltered light. During the mission, on-board object detection will be employed and more than 1 billion stars will be observed (as well as non-stellar objects to similar completeness limits). The full-mission (5-yr) mean-sky parallax accuracies are expected to be around 7 microarcsec (7 µas) at V= 10, 12-25 µas at V= 15 and 100-300 µas at V= 20 (depending on spectral type). Multi-epoch, multicolour photometry covering the optical wavelength range will reach the same completeness limit. Radial velocities will be obtained for 100-150 million stars brighter than V? 17-18 mag with accuracies of around 1-15 km s-1, depending on the apparent magnitude and spectral type of the stars and the sky density (for details see Katz et al. 2010: 165; Wilkinson et al. 2009: 277).

The photometric measurements provide the basic diagnostics for classifying all objects as stars, quasars, Solar system objects, or otherwise and for parametrizing them according to their nature. Stellar classification and parametrization across the entire Hertzsprung-Russell (HR) diagram is required as well as the identification of peculiar objects. This demands observation in a wide wavelength range, extending from the UV to the far-red. The photometric data must determine:

(i) effective temperatures and reddening ...