On the evolution of close triple stars that produce Type Ia supernovae

Current observational estimates suggest that similar to 30% of all binary s tars are in triple systems. In similar to 70% of these, the enclosed binary is close enough that the primary in the binary can evolve to fill its Roch e lobe. In similar to 10%-20%, the third, more distant component can evolve to fill its Roche lobe, leading to configurations inaccessible to isolated binary stars. Triple stars are unstable if the ratio of the orbital period of the enclosed binary to the period of the third component exceeds a crit ical value. Hence, an increase in the orbital period of the binary due to c onservative mass transfer between components or to wind mass loss from the binary can destabilize an initially stable triple system, causing it to dec ompose into a rapidly moving single star and an evolved binary recoiling in the opposite direction with a velocity large compared with velocities typi cal of primordial binaries. To highlight the different possibilities inhere nt in triple-star evolution, we discuss qualitatively several possible scen arios whereby triple stars with component masses in the range 1-10 M. can e volve into Type Ia supernovae, which we assume to be explosions of merging carbon-oxygen or oxygen-neon white dwarfs of total mass larger than 1.4 M.. Before quantitative predictions of the likelihood of these scenarios can b e made, it is necessary to determine the initial distribution of young trip le stars over their masses and orbital separations and to calculate the rea ction of the enclosed binary to matter transferred to it by the third compo nent when it fills its Roche lobe or supports a strong wind.