Convection in M-type dwarfs: predictions from 3D hydrodynamical model atmospheres J. Klevas1, A. Kučinskas1, H.-G. Ludwig2 1 Astronomical Observatory of Vilnius University, Saulėtekio al. 3, Vilnius 10257, Lithuania (email: jonas.klevas@tfai.vu.lt) 2 Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany Although convection plays an important both in the atmospheres and interiors of M-type dwarfs, its impact on the atmospheric structures and observable properties of these stars is still relatively poorly understood. In particular, standard 1D model atmospheres that are used in the analysis of M-type dwarfs rely on the calibration of convection by using mixing length parameter, MLT. As a rule, 1D model atmospheres of M- type dwarfs are computed using a fixed MLT value while in reality it may vary considerably in stellar parameter space (Magic 2015, Sonoi 2019). With the aid of 3D hydrodynamical model atmospheres, we investigate how the mixing length parameter, MLT, changes with effective temperature, gravity and metallicity typical to those observed in Galactic M-type dwarfs. We also provide a grid of MLT values that could be used with the 1D classical model atmospheres. To study convective properties of M-type dwarfs we used a new grid of 3D hydrodynamic model atmospheres (Klevas et al., in prep.) computed with CO5BOLD code (Freytag et al. 2012). The models were calculated using opacities that were grouped into 14 opacity bins based on continuum formation depth. All model had the same resolution of 140x140x160 which roughly corresponded to 1000 x 1000 x 400 km, and covered atmospheric layers in the range of -6 < log Ross <4. To investigate how convection works in M-type dwarfs, for comparison we used 1D hydrostatic models computed with LHD code using the same equation of state and opacities as the 3D hydrodynamical CO5BOLD models. This allows direct differential comparison with CO5BOLD models, thus leaving 1D hydrostatic nature and parametrization of convection as the only source of differences from the 3D hydrodynamic model atmospheres. The LHD model atmospheres were computed using the mixing length theory (Mihalas 1978), for a set of different values of MLT . Fig. 1. The mixing length parameter, MLT, obtained as the best fit of 1D model atmosphere entropy profile to that of the corresponding mean 3D model. Models in each column have identical Teff and log g values but differ in their metallicities ([M/H]) and best-fitted MLT. Grey dots are M-dwarfs observed by Kepler and TESS space missions. We present new results aimed at calibrating MLT parameter for use with the 1D hydrostatic model atmospheres of M-type dwarfs. In the range of atmospheric parameters typical to M- type dwarfs, the obtained MLT estimates vary in the range of MLT = 1.6 to 2.2 depending on the model effective temperature, gravity and metallicity, with [M/H] having the largest impact on MLT. The effect of Teff and log g are smaller and lead to changes of D MLT <0.3 and < 0.2 over the intervals of DTeff =1000K and Dlog g = 0.5, respectively. While the 1D MLT models may be used to represent optically thick layers in M-type dwarfs reasonably well, there significant deviations in the entropy profiles predicted by the 1D MLT and averaged 3D hydrodynamical model atmospheres in the outer atmospheric layers. This happens because the overshooting of convective flow into layers beyond the optical surface and subsequent adiabatic cooling are not accounted for in the 1D models. These differences can not be neglected, especially in more metal-poor stars where cooling of these layers is more efficient (Fig. 3). Extra caution is advised when layers outside of overshooting zone are of interest (log Ross < -2) because of this cooling. As shown in Fig. 1, MLT does vary significantly along the atmospheric parameter space. In particular, it is very sensitive to metallicity where the difference in MLT in model