EUVE Investigation of Three Short-Period Binary Stars

EUVE satellite spectroscopic observations (SW, MW and LW bands covering 80 – 160, 170 – 350 and 450 Å with resolutions 0.5, 1 and 2 Å) and photometric observations (Deep Sky Survey, broad-band 70 – 140 Å) have been obtained for two contact, 44i Boo B and VW Cep, and one detached, ER Vul, close binary stars. All three systems have orbital periods shorter than one day and thus are expected to show “saturated” levels of chromospheric, transition-region and lower-corona emissions. The spectroscopic data were of sufficient quality for an attempt at an emission-measure determination only for 44i Boo B. This determination, based entirely on iron lines, and utilizing Singular Value Decomposition formalism developed by Schmitt et al. (1996) indicates lack of any dominating temperature regime with the emission measure rising from log T ≃ 6 to log T ≃ 7.2. However, strong dependence of the resulting emission-measure curve on the inclusion of individual lines formed at high temperatures casts some doubts about the quality of the solution. A comparison of the line strengths have been made for selected strongest chromospheric, transition-region and lower-coronal emission lines; it included the data for the single, rapidly-rotating star AB Dor which is the only such star with the rotation period shorter than one day which has been observed with the EUVE. If the single-epoch observations are representative, the results indicate that AB Dor is under-active relative to the three binary stars which show similar levels of activity. Subject headings: binaries: close — binaries: eclipsing