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Chromospheric line blanketing and the Hydrogen spectrum in M dwarfs C. I. Short & J. G. Doyle We present non-LTE calculations of the H I spectrum in a grid of chromospheric models that represents a dM0 star in which the activity level ranges from quiescent to very active. We investigate three different treatments of the background opacity: 1) continuous opacity only, 2) blanketing due to lines that form in the photosphere below Tmin, and 3) blanketing by lines that form throughout the entire outer atmosphere. We show that the predicted EW of Ly alpha in all models is reduced by as much as a factor of ~4, and that of H alpha in very active (dMe) stars is enhanced by a factor of about two by the inclusion of background line opacity. A consistent treatment of line blanketing that includes the effect of the chromospheric and transition region temperature structure in the calculation of background line opacity is necessary for the accurate calculation of Ly alpha, and in some cases H alpha, in these stars. The H alpha line in less active models, and the Pa beta line in all models, is negligibly affected by the treatment of background opacity. We also show that, in addition to the expected suppression of emergent flux in the visible by line blanketing, the broad-band continuum flux in regions where lambda < 2000 A is increased by as much as a factor of three in some models by the inclusion of line blanketing. This reduces the equivalent width of the Lyman series by a factor of four and is due to the veil of background lines going into emission in the UV, and to the stronger coupling of the background source function to the Planck function in the presence of blanketing by thermal lines. We confirm the results of earlier calculations that suggest the dominance of the continuum emission in the radiative cooling of the chromosphere. Therefore, any proposed heating mechanisms must supply at least an order of magnitude more non-radiative heating than would be required on the basis of an analysis in which only emission line cooling is considered. We also include a preliminary assessment of non-LTE effects in the background opacity on the emergent UV continua.