ITAL]Extreme[/ITAL] [ITAL]Ultraviolet[/ITAL] [ITAL]Explorer[/ITAL] Investigation of Three Short-Period Binary Stars

Abstract

Extreme Ultraviolet Explorer spectroscopic observations (SW, MW, and LW bands, covering 80–160, 170–350, and 300–450 Å with resolutions 0.5, 1, and 2 Å, respectively) and photometric observations (Deep Sky Survey [DSS], broadband 70–140 Å) have been obtained for two contact binaries, 44i Boo B and VW Cep, and one close but detached binary, ER Vul. All three systems have orbital periods shorter than 1 day and thus are expected to show "saturated" levels of chromospheric, transition region, and lower corona emissions. Only moderate flaring activity was seen in the DSS light curves of all three stars. The phased DSS light curve of 44i Boo B shows one minimum at the eclipse of the hotter, smaller component, while the phased light curve of ER Vul has a slightly higher level at the first maximum, following the deeper eclipse. An unsuccessful attempt has been made to determine the temperature dependence of the differential emission measure (DEM) of the coronal plasma for 44i Boo B on the basis of iron ion emission lines. It utilized the singular value decomposition formalism developed by Schmitt et al. to handle the inverse problem of the spectrum-to-DEM mapping. Implementation of this formalism suffered from the low number of available spectral features and possibly also inconsistencies in the line strengths, which become amplified in the mathematical inversion process. However, experiments with the direct reconstruction of the spectra, which were generated with the DEM functions available from the literature, favor the distribution of Dupree et al. in preference to the two-temperature model of McGale et al. A comparison of the relative line fluxes fline/fbol for selected strongest chromospheric, transition region, and low-corona emission lines in 44i Boo B, VW Cep, ER Vul, and the single, rapidly rotating star AB Dor shows that the currently available data are consistent with the period-independent ("saturated") levels of activity for features forming at log T ≤ 5 in all four stars, while features formed at log T ≃ 6.8–7.1 are definitely the weakest in AB Dor.