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Multi-zone chemical evolution models (CEMs), differing in the nucleosynthesisprescriptions (yields) and prescriptions of star formation, have been computedfor the Milky Way. All models fit the observed O/H and Fe/H gradients well andreproduce the main characteristics of the gas distribution, but they are alsodesigned to do so. For the C/H gradient the results are inconclusive withregards to yields and star formation. The C/Fe and O/Fe vs. Fe/H, as well asC/O vs. O/H trends predicted by the models for the solar neighbourhood zonewere compared with stellar abundances from the literature. For O/Fe vs. Fe/Hall models fit the data, but for C/O vs. O/H, only models with increased carbonyields for zero-metallicity stars or an evolving initial mass function providegood fits. Furthermore, a steep star formation threshold in the disc can beruled out since it predicts a steep fall-off in all abundance gradients beyonda certain galactocentric distance (~ 13 kpc) and cannot explain the possibleflattening of the C/H and Fe/H gradients in the outer disc seen inobservations. Since in the best-fit models the enrichment scenario is such thatcarbon is primarily produced in low-mass stars, it is suggested that in everyenvironment where the peak of star formation happened a few Gyr back in time,winds of carbon-stars are responsible for most of the carbon enrichment.However, a significant contribution by zero-metallicity stars, especially atvery early stages, and by winds of high-mass stars, which are increasing instrength with metallicity, cannot be ruled out by the CEMs presented here. Inthe solar neighbourhood, as much as 80%, or as little as 40% of the carbon mayhave been injected to the interstellar medium by low- and intermediate-massstars. The stellar origin of carbon remains an open question, althoughproduction in low- and intermediate-mass stars appears to be the simplestexplanation of observed carbon abundance trends.

The origin of carbon: Low-mass stars and an evolving, initially top-heavy IMF?