The Wind‐Wind Collision Region of the Wolf‐Rayet Binary V444 Cygni: How Much Optical Line Emission Does It Produce?

Abstract

We model the emission-line profile variations that are expected to be produced by physical and wind eclipses in the Wolf-Rayet (W-R + O) binary system V444 Cyg. A comparison of the theoretical profiles with the He II 4686 A line observed in V444 Cyg allows us to isolate the effects that are likely to be due to the wind-wind collision region in this particular line. We estimate that the wind-wind collision region contributes no more than ~12% of the equivalent width of the emission line, with smaller values during elongations, when part of the shock cone is being eclipsed by the O star. The upper limit implies a maximum contribution from the wind-wind collision region of ~1 × 1035 ergs s-1 to the total luminosity of He II 4686 A line. Using the analytical solution of Canto et al., we find that the bulk of this emission arises along the shock cone walls where the flow velocity is ~800 km s-1, at a distance of ~8 R☉ from the O star's surface, and at θ = 65°-75° from the line joining the centers of the two stars, with origin in the O star. The derived surface density of this region is σ = 0.22 g cm-2, which, together with the He II 4686 A luminosity, indicates that the thickness of the shock lies in the range 2-10 × 1010 cm and the total density is 1-6 × 1012 cm-3.