OBSERVATION AND MODELING OF MAIN-SEQUENCE STAR CHROMOSPHERES .5. ULTRAVIOLET EXCESS EMISSION IN ACTIVE M-DWARFS

The variation in the continuum intensity (50-5000nm) of late-type M dw arfs is investigated via two grids of model atmospheres with different temperature minima. It is shown that the (E)UV intensity is mostly de pendent on the transition region pressure, although the temperature mi nimum also plays an important role. We also observe a significant freq uency redistribution of the photospheric flux when changing the minimu m temperature, and a black-body type of emission from the lower chromo sphere. We examine the formation of the continuum and point out that, although some differences appear for very low or very high activity le vels, in general the global picture is much alike the Sun. We show tha t the UV continua are very good diagnostics of cool dwarf atmospheres, from the temperature minimum to the transition region. Our calculatio ns give a good overview of the domains where physical parameters and s pectral signatures are most likely observed. We compute the UBV(RIJKL) (J) broad band fluxes for our models and conclude that the chromospher ic contribution should be detectable in the U band and possibly also i n the B band. We compare our calculations to recent high resolution ob servations for selected stars in a narrow spectral range ((R-I)(K)=0.8 75+/-0.05); we show that H alpha line profiles behave as expected, wit h a tight correlation between the line width and equivalent width. H a lpha emission line stars show an excess in U-B color but not in B-V. T hey are also more luminous than their less active absorption line coun terparts, which indicates that active dwarfs have not yet reached the main sequence and are intermediate between T Tauri stars and main sequ ence stars. The anomalously large proportion of active stars towards l ate spectral types is attributed to the very slow contraction phase fo r low mass stars. We calculate the fluxes in the Extreme Ultraviolet E xplorer Sn/SiO band (500-740 Angstrom) and show that they are compatib le with observed upper limits. We further compare our results for the upper activity range to pre-main sequence models and observations. The y strongly support the case for a chromospheric contribution to H alph a and the blue/UV excess for those objects (T Tauris, naked T Tauris, YSOs). An important conclusion is that at high pressures, correspondin g to active dMe stellar atmospheres, the chromosphere becomes a very e fficient radiator at continuum wavelengths. From log(M)similar to-5 (c olumn mass), radiative losses in the continuum rise exponentially and faster than in Hydrogen spectral lines because of their larger optical depths. As a consequence, HI spectral lines have a small or negligibl e contribution to the total HI (lines and continua) and white light ra diative budget. We show that the hydrogen series dominate the radiativ e cooling in spectral lines (from 40% to 90%) for H alpha emission lin e stars. The cooling in the (E)UV continuum overwhelms the total radia tive budget and is much larger than that in outstanding chromospheric and transition region lines (e.g. Ca II, Mg II, H I Lyman and Balmer). We propose this as a possible contribution for the apparent saturatio n observed in some spectral lines, therefore questioning the suggestio n of saturation in magnetic activity levels.