LOOP MODELING OF CORONAL X-RAY-EMISSION FROM AR LACERTAE

We fitted a hydrostatic loop model (including the effects of increasin g cross section and of gravitation) to the pulse height spectra of the RS CVn binary AR Lac, which were obtained in June 1990 by the PSPC de tector on board the ROSAT spacecraft. This observation of the quiescen t emission comprises 18 spectra between binary phases -0.9 to 0.6. As resulting loop parameters we find a maximum temperature of approximate ly 28 10(6) K, a length of approximately 30 10(10) cm, an expansion fa ctor of approximately 2, and a binary filling factor of less than or s imilar to 10%, without showing a significant variation with orbital ph ase. However, a temperature decrease prior to phase 0.5 (secondary ecl ipse) appears to be present. The magnetic fields required to confine t hese extended low-pressure loops are only about two times larger than the solar magnetic fields, but the energy input per loop seems to exce ed the solar value by about 2-3 orders of magnitude. The loop paramete rs are consistent with the active region model of AR Lac deduced for t he same observation by Ottmann et al. (1992). Especially, the extended loops should be associated with the K star, and the combined spectrum should either be dominated by the K star spectrum or be a blend of th e spectra of the G and K star. The loop depth below chromosphere is in agreement with the convection zone depth, predicted from computations about stellar evolution after the main sequence.