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tokens: 2015 ). For illustration, in Fig. 3 , we plot the radial profiles of various quantities for two models: inside-out scenario ( Case 1) and for constant short time-scale gas infall rate t f = 2.25 Gyr using the boundary τ b = 100 Myr. As we see, the distributions do not differ significantly for the two shown examples. Notice that, although near the corotation the reduced SFR for high-mass stars , their sum does not tend to zero at the corotation. 3 Our computed profiles for μ g , μ s and are close to observed ones derived by other authors (cf. our Fig. 3 with fig. 1 in Yin et al. 2009 or fig. 4 in Kubryk et al. 2015 ). Figure 3. Radial distributions of various quantities at present time. Solid lines: model with constant t f = 2.25 Gyr. Dashed lines: inside-out scenario. The boundary τ b = 100 Myr. The bend in μ g ( r ) at 4 kpc, seen for inside-out scenario (dashed lines), is due to change in t f approximation for r ≤ 4 kpc. The feature in at this distance is due to (see Sections 3.1 and 3.2). The upward jump in μ g , at r ≈ 12 kpc for the model with constant t f (solid lines) is associated with the outer Lindblad resonance r L , which is located at the same distance (see Section 3.3). The jump maybe easily explained by the cut-off at the same distance. 3.4 Oxygen synthesis in the Galactic disc In general, our ‘chemical’ equation, which describes the radial evolution of oxygen abundance along the Galactic disc in ISG, is close to the classical ones proposed by Tinsley ( 1980 ), Lacey Fall ( 1985 ), WS , PC , Matteucci ( 2003 ) and others with one correction that we work with composite SFR (see equation 8 ): where μ O is the mass density of oxygen in the disc, Z O = μ O /μ g , is the enrichment rate of ISG by newly synthesized oxygen: correspondingly P O is the mean mass of newly synthesized oxygen ejected per CC SN event. The lower limit in the integral, entering equation ( 18 ), we adopt to be equal to 10 M ⊙ since exploding CC SNe do not produce oxygen if the initial masses of their progenitors are less than 10 M ⊙ (e.g. Wanajo et al. 2009 ). The meaning of various terms in equation ( 17 ) is as follows. The second term on the right-hand side of the equation describes the increase of oxygen density due to intergalactic gas infall on to the disc. The third term describes the rate of oxygen return into ISG due to wind from low-mass stars and their pass to remnant; the fourth, fifth and sixth terms describe the depletion of oxygen in ISG due to capture it by the forming objects. The last term in equation ( 17 ) is the radial diffusion of oxygen that we associate with turbulence in ISG. The inclusion effect of diffusion was provoked by the following: if we do not take into account the diffusion, in our model we would have deep gap in oxygen radial distribution near the corotation because here ψ H → 0. To smooth away this sharp gap in the distribution, we must include the effect of diffusion. The combined effect of corotation resonance and