SIMBAD references

We have investigated some aspects of the fitting of stellar X-ray spectra by means of hydrostatic models of coronal loops. We have performed an extensive set of simulations in which we have synthesized and fitted loop model spectra, considering the instrument responses of the Einstein Observatory Imaging Proportional Counter, of the ROSAT Position Sensitive Proportional Counter, and of the ASCA Solid-state Imaging Spectrometer, and we have derived theoretical confidence regions for the determination of the input loop parameters. We have found that our ability to constrain the loop model parameters depends on the distribution of the differential emission measure. More specifically, we have found that, for loops significantly below the pressure scale height, the spectral fitting allows to constrain well the plasma peak temperature and hence the product of the base pressure with the loop semi-length, but not each of the two separately, due to scale invariance of the equations describing the hydrostatic loop model of uniform pressure. For input loops with lengths of the order of the pressure scale height or larger, the spectral fitting yields closed confidence regions in the parameter space, allowing to constrain all the loop parameters, with different levels of discrimination depending on photon counting statistics and instrument energy response. Loops much longer than the star's radius and with relatively high base pressure behave like the short loops, because the pressure decrease along the loop is moderated by the local increase of the pressure scale height.