Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...273..546o&link_type=abstract
Astronomy and Astrophysics, Vol.273, NO. 2/JUNII, P. 546, 1993
Astronomy and Astrophysics
Astrophysics
16
Late-Type Stars, Rs Cvn Binaries, X-Rays, Coronae
Scientific paper
We fitted a hydrostatic loop model (including the effects of increasing 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 detector on board the ROSAT spacecraft. This observation of the quiescent emission comprises 18 spectra between binary phases -0.9 to 0.6. As resulting loop parameters we find a maximum temperature of ˜28 106 K, a length of ˜30 1010 cm, an expansion factor of ˜2, and a binary filling factor of ≲10%, without showing a significant variation with orbital phase. However, a temperature decrease prior to phase 0.5 (secondary eclipse) appears to be present. The magnetic fields required to confine these extended low-pressure loops are only about two times larger than the solar magnetic fields, but the energy input per loop seems to exceed the solar value by about 2-3 orders of magnitude. The loop parameters are consistent with the active region model of AR Lac deduced for the 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 the 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.
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