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Astron. Astrophys. 320, 428-439 (1997)

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5. Conclusions

The vertical structure of the galactic disc results from the combined effect of the star formation history and the secular heating of the disc. Combining these two factors in a dynamical consistent model, within the range of possibilities left open by local contraints, we get the following conclusions.

The shape of the vertical density law strongly deviates from the classical exponential even though this approximation may hold over restricted distance ranges. At low distances (say below 500 pc), a single exponential is inadequate under any reasonable scenario.

Concerning the IMF, the synthetic approach used here where the predicted solar neighbourhood luminosity function comes out strait from the combination of SFR, IMF, via reliable evolutionary tracks strongly reduces the dependence of the result on arbitrary mass- [FORMULA] relations. As a result the bimodal IMF derived by Scalo (1986) in association with a decreasing SFR turns out to be most likely an artefact due to the crude mass- [FORMULA] approximation.
Within the possibilities left open by realistic vertical density profiles, and by compatibility with the observed luminosity function, the range of possible star formation scenarii (IMF+SFR) is significantly reduced yet not pointing to a unique solution. These include a decreasing SFR with an IMF index [FORMULA] 1.5, a constant SFR in combination with an IMF index of [FORMULA] 2, or an increasing SFR with an IMF index [FORMULA] 2.5. The age- [FORMULA] relation (i.e typical timescale of dynamical heating) is also an important residual source of uncertainty. In Paper II we shall show that faint star counts at the galactic poles can help removing these ambiguities.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998