We propose a statistical method for decomposition of contributions to iron production from various sources: Type II supernovae and the subpopulations of Type Ia supernovae, prompt (their progenitors are short-lived stars with ages lower than ∼100 Myr) and tardy (their progenitors are long-lived stars with ages >100 Myr). To do that, we develop a theory of oxygen and iron synthesis that takes into account the influence of the spiral arms on the amount of the above elements synthesized by both Type II supernovae and prompt Type Ia supernovae. In the framework of the theory, we processed statistically the new, more precise, observational data on Cepheid abundances, which, as is well known, demonstrate non-trivial radial distributions of oxygen and iron in the Galactic disc with bends in the gradients. In our opinion, such fine structure in the distribution of elements along the Galactic disc enables one to decompose the amount of iron unambiguously into three components produced by the above three sources. In addition, by means of our statistical methods we solve this task without any preliminary suppositions about the ratios between the proportions of iron synthesized by the above sources.
The total mass supplied to the Galactic disc during its life by all types of supernovae is ∼(4.0 ± 0.4) × 107 M⊙, while the mass of iron occurring in the present interstellar medium (ISM) is ∼(1.20 ± 0.05) × 107 M⊙, i.e. about two thirds of iron is contained in stars and stellar remnants.
The relative proportion of iron synthesized by tardy type Ia supernovae for the lifetime of the Galaxy is ∼35 per cent (in the present ISM this portion is ∼50 per cent). Correspondingly, the total proportion of iron supplied to the disc by Type II supernovae and prompt Type Ia supernovae is ∼65 per cent (in the present ISM this proportion is ∼50 per cent). The above result depends slightly on the adopted mass of oxygen and iron synthesized during one supernova explosion and the shape (bimodal or smooth) of the so-called delay-time distribution function.
The proportions of iron mass distributed between short-lived supernovae are usually as follows: depending on the ejected masses of oxygen or iron during one Type II supernova event, the relative proportion of iron supplied to the Galactic disc for its age varies in the range 12–32 per cent (in the present ISM 9–25 per cent); the proportion supplied by prompt Type Ia supernovae to the Galactic disc is 33–53 per cent (in the ISM 26–42 per cent).
Our method also confirms that the bend in the observed slope of the oxygen radial distribution and the minimum in [O/Fe] at ∼7 kpc form in the vicinity of the location of the corotation resonance.