[Implementation of Gaia Photometric System in Astro Science]

by

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION3

CHAPTER 2: INSTRUMENT DESCRIPTION7

CHAPTER 3: THE G AND GS PASSBANDS14

CHAPTER 4: DESIGNING THE PHOTOMETRIC SYSTEM18

REFERENCES33

APPENDIX A43

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 at least for O-B-A stars (needed both as tracers of Galactic spiral arms and as reddening probes);

(ii) at least effective temperatures and abundances for F-G-K-M giants and dwarfs;

(iii) luminosities (gravities) for stars having large relative parallax errors;

(iv) indications of unresolved multiplicity and peculiarity; and

(v) a map of the interstellar extinction in the Galaxy.

All of this has to be done with an accuracy sufficient for stellar age determination in order to allow for a quantitative description of the chemical and dynamical evolution of the Galaxy over all galactocentric distances. Separate determination of Fe- and a-element abundances is essential for mapping Galactic chemical evolution and understanding the formation of the Galaxy.

Photometry is also crucial to identify and characterize the set of ~500 000 quasars that the mission will detect. Apart from being astrophysically interesting in their own right, quasars are key objects for defining the fixed, non-rotating Gaia Celestial Reference Frame, the optical equivalent of the International Celestial Reference Frame (Mignard 2009: 108). On the other hand, Gaia will identify about 900 quasars with multiple images produced by macrolensing. Because this number is sensitive to cosmological parameters, the Gaia observations will be able to constrain the latter.

Due to diffraction and the optical aberrations of the instrument, the position of the centre of the stellar images ...