OPACITY SAMPLING MODEL PHOTOSPHERES FOR M-DWARFS .1. COMPUTATIONS, SENSITIVITIES AND COMPARISONS

We have computed a new grid of model photospheres for M dwarfs which c overs the parameter ranges 4000 K less-than-or-equal-to T(eff) less-th an-or-equal-to 2600 K, log g = 4.5, 5.0 and abundances of (1.0, 2.0, 0 .1, 0.01) times the solar value. Although these models retain the stan dard assumptions of 'classical models' they are computed (for the firs t time) with the accurate and realistic opacity sampling technique. Th is has been applied to an extensive set of atomic and molecular line o pacites computed with laboratory values of transition strengths, which will permit a critical comparison to observations. We outline the com putation of the models and describe the sensitivity of the computed st ructures to the input parameters T(eff), log g and abundance as well a s 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 pr oblems which are highlighted in the coolest models by the strength and sensitivity of H2O absorption bands. An analysis of this problem indi cates that it probably due to the models being too cool in the upper p hotospheric 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 probl em further.