IUE observations of RS CVn-like binary sigma Corona Borealis.

Abstract

High and low resolution IUE observations of the triple system sigma CrB are presented. The spectra show the presencee of chromospheric (O I, Si II, Mg II) and transition region lines (He II, C II, C IV) in emission. The surface fluxes of these lines are orders of magnitude higher than those observed in the sun but are comparable to their values in RS CVn binaries like UX Ari. A comparison of continuum and emission line fluxes through the small and large apertures suggests that the observed continuum and chromospheric and transition region lines are from the primary star HD 146361 which is a spectroscopic binary; the secondary, HD 146362 which is 5.3 arcsec away from the primary may be contributing a small fraction to the O I emission only. This confirms that HD 146361 is indeed an RS CVn binary and the source of X-rays from the triple system. The profiles of the h and k lines of Mg II suggest almost equal contribution from the two components of the spectroscopic binary HD 146361. This hypothesis is supported by the profiles of absorption lines. Interstellar Fe II absorptions give a value of 0.05 per cu cm for the mean interstellar hydrogen density towards sigma CrB. A hydrostatic model for the chromosphere, transition region and corona of HD 146361 has been developed and its implications have been discussed.