Old stellar Galactic disc in near-plane regions according to 2MASS: Scales, cut-off, flare and warp

We have pursued two dierent methods to analyze the old stellar population near the Galactic plane, using data from the 2MASS survey. The first method is based on the isolation of the red clump giant population in the color-magnitude diagrams and the inversion of its star counts to obtain directly the density distribution along the line of sight. The second method fits the parameters of a disc model to the star counts in 820 regions. Results from both independent methods are consistent with each other. The qualitative conclusions are that the disc is well fitted by an exponential distribution in both the galactocentric distance and height. There is not an abrupt cut-o in the stellar disc (at least within R 6 kpc) that: the scale- height in the solar circle is hz(R)= 3:6 10 2 R, the scale-length of the surface density is hR= 0:42 R and the scale-length of the space density in the plane (i.e. including the eect of the flare) is H= 0:25 R. The variation of the scale-height due to the flare follows roughly a law hz(R) hz(R )e xp R R (12 0:6R(kpc)) kpc (for R< 15 kpc; R= 7:9 kpc). The warp moves the mean position of the disc to a height zw= 1:2 10 3 R(kpc) 5:25 sin(+ 5) pc (for R< 13 kpc; R= 7:9 kpc).