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

We give independent proof of the deficit of stars in the in-plane central disc (2<R<4 kpc, \b\less than or similar to 3degrees) with respect to the predictions of a pure exponential density distribution. We use three different methods: 1) the inversion of the red clump giant distribution in near-infrared colour-magnitude diagrams 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 deficit 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 (\b\less than or similar to3degrees). This deficit might be interpreted as a flare in the vertical distribution. The in-plane density is almost independent of R and not an exponential law of the type rho proportional to exp(-R/h). 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 extinction. This deficit 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. An approximate expression of the disc density within 2<R<8 kpc is: rho(R,z) proportional to e(-(R/1970 pc + 3740 pc/R)) e(-\z\/hz(R)), with h(z)(R)approximate to285 [1 + 0.21 kpc(-1) (R-R-circle dot)+0.056 kpc(-2) (R-R-circle dot)(2)]pc.