The interacting early-type binary BD+40 degrees 4220 (V729 CYG): Modeling the colliding winds region

We present an analysis of an extensive set of spectroscopic observations of the mysterious early-type binary ED + 40 degrees 4220. A new orbital solut ion is derived from the radial velocities of the absorption lines. We confi rm that the secondary star is highly overluminous for its mass. The absorpt ion lines of both components display phase-locked profile variations, with some of the secondary's lines going into emission between phi = 0.20 and ph i = 0.55. A detailed investigation of the profile variability of the We II lambda 4686 emission line reveals that the pattern of variability of this l ine is very stable. We show that Dart of the He II lambda 4686 emission is produced in the wind interaction region between the stars. Most of the emis sion lines in the visual spectrum of ED +40 degrees 4220 display variations that are reminiscent of those observed on the He II lambda 4686 line, poin ting toward a similar origin. We present numerical simulations of emission- line profiles in a colliding winds binary, showing that an important part o f the variability observed in ED +40 degrees 4220 can be explained by a col liding winds phenomenon. The properties of the wind interaction region can be accounted for if we assume that the secondary star is an evolved object, most probably some kind of Ofpe/WN9 transition star, with a mass loss rate of similar to 5.5 x 10(-6) M. yr(-1). We finally discuss the fundamental p arameters of the binary, concluding that mass transfer must have played a c rucial role in the evolution of this system.