Y versus 10⁴ O/H in the solar vicinity (left panels) and in the inner disc (R_G = 6 kpc; right panels), as predicted by models with different nucleosynthesis prescriptions. Top panels: Model 1 [short-dashed (red) lines] versus Model 2 [solid (black) lines], showing the effect of changing the mass cut in models of SNII explosions. Bottom panels: Model 6 [dot-dashed (black) lines] versus Model 7 [solid (black) lines], with and without rotation in high-mass stars, respectively. Left panels: the filled circles on each theoretical track indicate the Y and 10⁴ O/H values predicted at the time of the birth of the Sun, 4.5 Gyr ago. These are compared with the bulk (initial) abundances of He and O in the Sun (Sun symbols, from Grevesse & Sauval 1998 – higher O abundance – and Asplund et al. 2009 – lower O abundance). The effects of diffusion are taken into account in defining the protosolar abundances displayed in the figure (see the discussion in Asplund et al. 2009). Right panels: the final Y and 10⁴ O/H values attained by the models are highlighted (filled circles) and compared to the abundances of He and O measured for M 17, an H ii region located at 2.1 kpc distance from the Sun (filled triangles; see text for discussion).