1995A&A...295..736B

We have computed a new grid of model photospheres for M dwarfs which covers the parameter ranges 4000 K≤T_eff≤2600K, logg=4.5, 5.0 and abundances of (1.0, 2.0, 0.1, 0.01) times the solar value. Although these models retain the standard assumptions of `classical models' they are computed (for the first time) with the accurate and realistic opacity sampling technique. This has been applied to an extensive set of atomic and molecular line opacites computed with laboratory values of transition strengths, which will permit a critical comparison to observations. We outline the computation of the models and describe the sensitivity of the computed structures to the input parameters T_eff, logg and abundance as well as the mixing length parameter l/h. Interesting effects connected with molecular thermodynamics are found in the coolest models. Comparisons to other model photospheres for M dwarfs demonstrate the importance of having sufficient opacity sources, good opacity technique and correct solar abundances for the resulting model structure. Despite the much improved treatment of opacity, a comparison to observations reveals problems which are highlighted in the coolest models by the strength and sensitivity of H₂O absorption bands. An analysis of this problem indicates that it probably due to the models being too cool in the upper photospheric layers. Two likely causes are identified as the neglect of chromospheric heating and the simple treatment of convection employed. More sophisticated models will be required to investigate this problem further.