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Photometry with Gaia

Photometry with Gaia

Photometry with Gaia

The core science case for Gaia requires measurement of luminosity, effective temperature, mass, age, and composition for the stellar populations in our own Galaxy, the Milky Way and in its nearest galaxy neighbours. These quantities can be derived from the spectral energy distribution of the stars, through multi-band photometry. Obtaining this information is an essential part of the Gaia payload.

A broad-band magnitude can be obtained from the primary Gaia mission data. This combined with the parallax and correction for interstellar absorption give an estimate of the absolute magnitude for a very large fraction of the 1 billion objects which will be observed by Gaia. However, multi-band photometry must be separately obtained for each object to derive astrophysical measurements.

Multi-colour photometry measures the energy output of a star in different wavebands. The distribution of this energy is related to physical parameters such as the effective (surface) temperature. Photometric measurements can also be used to determine: the spectral type of a star; the luminosity class of a star; photospheric chemical abundances; surface gravity; interstellar reddening etc.

A photometric system defines standard wavebands and sets of standard stars. Many systems exist e.g. (Johnson, Geneva, Strvmgren, Sloan, Vilnius) but none satisfy all the Gaia requirements. The Gaia photometric system must be able to classify stars across the entire Hertzsprung-Russell diagram, as well as to identify peculiar objects. Thus it is necessary to observe a large spectral domain, extending from the UV to the far-red. It must be able, for example, to determine temperatures and reddening for OBAF and G stars (needed as tracers of Galactic spiral arms and as reddening probes) and effective temperatures and abundances for late-type giants and dwarfs. Additionally, determination of the abundance of Fe and alpha-elements will be essential for mapping Galactic chemical evolution.

A Gaia photometric system has been derived following study of the scientific and technical requirements of the Gaia payload. The current baseline system is for five to seven broad bands which will be in the main astrometric instrument focal plane and 11 medium band filter observations in the spectrometric instrument focal plane.

Last Update: 1 September 2019
9-May-2021 22:33 UT

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