1997A&A...320..525P

We analyze ROSAT X-ray spectra of a sample of young late type stars by fitting them with a continuous emission measure model with power-law temperature dependence up to a maximum temperature. This model gives successful fits to all spectra of our sample. While most of the spectra can also fitted equally successful with the frequently used two temperature model, we argue and provide evidence that the continuous temperature distribution model probably describes the coronal temperature structure in a more meaningful way. From our fitting results we find a very good correlation between the X-ray surface flux and the maximum coronal temperature with the functional form F_X∝T_max^(2.2±0.2) . This correlation extends over several orders of magnitude and is valid for the very young T Tauri stars as well as for the much older young main sequence stars in our sample. Since the X-ray surface flux is a good measure of the stellar activity and shows a clear dependence on the stellar rotational velocity, the correlation means that the coronal temperature is related to the X-ray activity and ultimately determined by stellar rotation. Finally, we interpret our results with a simple analytic loop model and find that the coronae of the active young stars are probably dominated by loops with generally higher pressure as compared to typical solar active region loops.