Evidence for a defect of young and old stars in Milky Way inner in-plane disc

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  • 25 March 2004
Abstract
We give independent proofs on the defect of stars in the in-plane central disc with respect to the predictions of a pure exponential disc of the density distribution. We use three different methods: 1) the inversion of red clump giant distribution in near-infrared colour-magnitude diagrams in order to obtain the star density along the line of sight; 2) the determination of the density distribution of 1612 MHz sources by means of the distance determination of OH/IR sources from their kinematical information; 3) an analysis of near and mid-infrared star counts and comparison with models. All the tests give the same result: a defect of stars in the inner disc with respect to an exponential disc (either with constant scaleheight or extrapolated from the outer disc), but only in near plane regions. This defect might be interpreted as a flare in the vertical distribution. The in-plane density is almost independent of R and not an exponential law. Further away from the plane, however, the density increases towards the centre due to the increase of the scaleheight. Tests also show that this result cannot be due to the extinction. This affects both the young and the old populations, so it is probably a rather stable feature of the disc, and might be due to the existence of an in-plane bar sweeping the near-plane stars.

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