The Evolution of 3He, 4He and D in the Galaxy
  In this work we present the predictions of a modified version of the ``two-infall model'' (Chiappini et al. 1997) for the evolution of 3He, 4He and D in the solar vicinity, as well as their distributions along the Galactic disk. In particular, we show that when allowing for extra-mixing process in low mass stars, as predicted by Charbonnel and do Nascimento (1998), a long standing problem in chemical evolution is solved, namely: the overproduction of 3He 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 primordial D/H < 3 x 10(-5) is suggested by the present model. Finally, adopting the primordial 4He abundance suggested by Viegas et al. (1999), we obtain a value for dY/dZ around 2 and a better agreement with the solar He abundance. 
Cristina Chiappini (1) and Francesca Matteucci (2) ((1) Columbia   University, USA (2) Universita di Trieste, Italy)