Dr. David Boyd
British Astronomical Association, UK
The enigmatic eclipsing Be binary EE Cephei
Abstract:
EE Cephei is an eclipsing binary with a 5.6 year orbital period comprising a B5 giant primary and an enigmatic invisible dark secondary described as an opaque dusty disk surrounding a low luminosity object. Since its discovery as a variable in 1952, and prior to the eclipse in 2025, 12 primary eclipses have been observed varying in depth from 0.5 to 2 magnitudes.
Attempts to model the different photometric profiles of these eclipses in terms of the varying thickness, inclination and precession of the secondary disk have so far failed to accurately reproduce the behaviour of all previous eclipses. The limited spectroscopic coverage of previous eclipses revealed Balmer line emission indicating that the hot component is a rapidly rotating Be star with a gaseous decretion disk and transient shell-like absorption.
We will report on a coordinated international campaign by amateur astronomers to obtain comprehensive spectroscopic coverage of the eclipse of EE Cephei in November 2025 with low and high resolution spectra. By calibrating our spectra in absolute flux and using multi-Gaussian fits, we measured the emission and absorption components of Balmer, Fe II and Na I spectral lines and monitored how these varied through the eclipse. This, together with analysis of the spectral energy distribution, revealed new information about the nature of the secondary and, combined with the use of TESS photometry, about the primary Be star.