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We constructed hydrodynamical model atmospheres for mid M-type main-, as wellas pre-main-sequence (PMS) objects. Despite the complex chemistry encounteredin these cool atmospheres a reasonably accurate representation of the radiativetransfer is possible, even in the context of time-dependent andthree-dimensional models. The models provide detailed information about themorphology of M-type granulation and statistical properties of the convectivesurface flows. In particular, we determined the efficiency of the convectiveenergy transport, and the efficiency of mixing by convective overshoot. Theconvective transport efficiency was expressed in terms of an equivalentmixing-length parameter alpha in the formulation of mixing-length theory (MLT)given by Mihalas (1978). Alpha amounts to values around 2 for matching theentropy of the deep, adiabatically stratified regions of the convectiveenvelope, and lies between 2.5 and 3.0 for matching the thermal structure ofthe deep photosphere. For current spectral analysis of PMS objects this impliesthat MLT models based on alpha=2.0 overestimate the effective temperature by100 K and surface gravities by 0.25 dex. The average thermal structure of theformally convectively stable layers is little affected by convective overshootand wave heating, i.e., stays close to radiative equilibrium conditions. Ourmodels suggest that the rate of mixing by convective overshoot declinesexponentially with geometrical distance to the Schwarzschild stabilityboundary. It increases at given effective temperature with decreasinggravitational acceleration.Comment: 17 pages, 23 figures, accepted for publication in A&

Energy transport, overshoot, and mixing in the atmospheres of M-type main- and pre-main-sequence objects