A New Effect in Eclipsing Binaries.
Daniel M. Popper, Merle F. Walker · The Astronomical Journal · 1963
In an earlier note (Publ. Astron. Soc. Paci4tc 73, 333,1961)) an ultraviolet excess of approximately 0.8 mag. (B- V=+1.5, U-B=+0.8) was reported for the 38-day eclipsing binary, KU Cyg, at primary mini- mum. The spectral types of the components are KS III and F4p. An image-tube spectrogram has now been obtained at minimum with the 120-ii~ch reflector. lt shows a strong continuum dominating the spectrum in the ultraviolet. From N3500, the shortward limit of the exposure, to N4000 the spectral energy distribution is close to that of BD +2804211, used as spectrophotometric standard. The color temperature of + 2804211 is greater than 50 0000 (J. B. Oke, unpublished). Allowance for the effects of interstellar reddening and the low- temperature continuum of the K star would increase the color temperature of the added ultraviolet to well above that of +2804211. The appearance of the continuum in KU Cyg is quite different from that observed in "ultraviolet excess" stars in young clusters in Orion and Monoceros, and does not result to an appreciable extent from Balmer emlsslon. The observations do not serve to locate the place of origin of the emission in the binary system except to show that it is not confined to regions close to the photosphere of the hotter star. KU Cyg shows the "RW Tauri" effect during eclipse, evidence of an emitting ring around the hotter component. RZ Oph, another system showing the RW Tauri effect and with spectral types similar to those of KU Cyg, shows an ultraviolet excess at primary eclipse of about 0.5 mag. (B V +1.8, U-B +1.4). Thus excess ultraviolet radiation may be a general feature of similar systems. TT Hya, a system showing the RW Tauri effect but with considerably earlier spectral types than KU Cyg (A3,G8), appears not to have an appreciable ultraviolet excess during total eclipse. This investigation was supported in part (Popper) by a contract with the Office of Naval Research.