We give independent proof of the deficit of stars in the in-plane central disc ( 2 < R < 4 kpc , |b| \la 3 ^ { \circ } ) 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| \la 3 ^ { \circ } ) .
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 \propto \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 can not 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 ) \propto e ^ { - \left ( \frac { R } { 1970 { pc } } + \frac { 3740 { pc } } { R } % \right ) } e ^ { \frac { - |z| } { h _ { z } ( R ) } } , with h _ { z } ( R ) \approx 285 [ 1 + 0.21 { kpc } ^ { -1 } ( R - R _ { \odot } ) +0.056 { kpc } ^ { -2 % } ( R - R _ { \odot } ) ^ { 2 } ] { pc } .