Gaia Treatment of Astrometric Binaries with a Variable Component: VIM, VIMA, VIMO

Details Gaia Treatment of Astrometric Binaries with a Variable Component: VIM, VIMA, VIMO Gaia Treatment of Astrometric Binaries with a Variable Component: VIM, VIMA, VIMO

Aug 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007iaus..240..267h&link_type=abstract

Binary Stars as Critical Tools and Tests in Contemporary Astrophysics, International Astronomical Union. Symposium no. 240, he

Astronomy and Astrophysics

Astrophysics

Scientific paper

Gaia is an ESA project that will conduct a census of one thousand million stars in our Galaxy. Each target star will receive about 100 1-D high-precision astrometric measurements over a five-year period. The magnitude of the stars will be accurately estimated in the same time. Among the huge quantity of stars observed by Gaia, some will be unresolved binaries hosting a photometric variable; since the astrometric abscissa and the total luminosity of these stars will be recorded simultaneously, three distinct models will be considered to describe these observations: (a) the VIM model (VIM=Variability Induced Movers), is dedicated to binaries with components having fixed relative positions; (b) the VIMA model introduces acceleration terms in order to describe a short part of orbit; (c) the VIMO model is used to derive the orbital parameters of the binaries with orbital period up to about 10 years. The VIM model was already used in the Hipparcos data reduction. Each astrometric measurement is fitted with a linear function of the parameters of the stars, and their derivation is therefore straightforward. The VIMA model is more complicated. It includes a parameter (called "h" hereafter), which is a function of the mass ratio of the system and of the magnitude of the non-variable component. When "h" is fixed, the model remains linear. The VIMO model is still more complicated, since, additionally to "h", it is necessary to fix P, e, T0 and w to obtain a linear system. A method of computation was tried with simulated data, and its results look fairly good. In conclusion, the unresolved binaries with a variable component will receive accurate astrometric parameters in the final Gaia catalogue, and other properties of these systems will still be derived with the software that we are preparing.

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