The Evolution of 3 He , 4 He and D in

In this work we present the predictions of a modified version of the “two-infall model” (Chiappini et al. 1997) for the evolution of He, He and D in the Galaxy, as well as their distributions along the Galactic disk. In particular, we show that when allowing for extra-mixing process in low mass stars (M< 2.5 M⊙), as predicted by Charbonnel and do Nascimento (1998), a long standing problem in chemical evolution is solved, namely: the overproduction of He by the chemical evolution models as compared to the observed values in the sun and in the interstellar medium. Moreover, we show that chemical evolution models can constrain the primordial value of the deuterium abundance and that a value of (D/H)p < 3 × 10 −5 is suggested by the present model. Finally, adopting the primordial He abundance from Izotov & Thuan (1998) we obtain a value for ∆Y/∆Z ≃ 2 and a better agreement with the solar He abundance.