The Formation of a Helium White Dwarf in Close Binary System with Diffusion

We study the evolution of a system composed by a 1.4 M⊙ neutron star and a normal, solar composition star of 2 M⊙ in orbit with a period of 1 day. Calculations were performed employing the binary hydro code presented in Benvenuto & De Vito (2003) that handle the mass transfer rate in a fully implicit way. Now we included the main standard physical ingredients together with diffusion processes and a proper outer boundary condition. We have assumed fully non conservative mass transfer episodes. In order to study the interplay of mass loss episodes and diffusion we considered evolutionary sequences with and without diffusion in which all Roche lobe overflows (RLOFs) produce mass transfer. Another two sequences in which thermonuclearlydriven RLOFs are not allowed to drive mass transfer have been computed with and without diffusion. To our notice, this study represents the first binary evolution calculations in which diffusion is considered. The system produces a helium white dwarf of ∼ 0.21 M⊙ in an orbit with a period of ∼ 4.3 days for the four cases. We find that mass transfer episodes induced by hydrogen thermonuclear flashes drive a tiny amount of mass transfer. As diffusion produces stronger flashes, the amount of hydrogen rich matter transferred is slightly higher than in models without diffusion. We find that diffusion is the main agent in determining the evolutionary timescale of low mass white dwarfs even in presence of mass transfer episodes.