The origin and chemical evolution of carbon in the Galactic thin and thick discs

In order to trace the origin and evolution of carbon in the Galactic disc, we have determined carbon abundances in 51 nearby F and G dwarf stars. The sample is divided into two kinematically distinct subsamples with 35 and 16 stars that are representative of the Galactic thin and thick discs, respectively. The analysis is based on spectral synthesis of the forbidden C i line at 872.7 nm using spectra of very high resolution (R≈ 220 000) and high signal-to-noise ratio (S/N ≳ 300) that were obtained with the Coudé Echelle Spectrograph (CES) spectrograph by the European Southern Observatory (ESO) 3.6-m telescope at La Silla in Chile. We find that C/Fe versus Fe/H trends for the thin and thick discs are totally merged and flat for subsolar metallicities. The thin disc that extends to higher metallicities than the thick disc shows a shallow decline in C/Fe from Fe/H≈ 0 and up to Fe/H≈+0.4. The C/O versus O/H trends are well separated between the two discs (due to differences in the oxygen abundances) and bear a great resemblance to the Fe/O versus O/H trends. Our interpretation of our abundance trends is that the sources that are responsible for the carbon enrichment in the Galactic thin and thick discs have operated on a time-scale very similar to those that are responsible for the Fe and Y enrichment i.e. SN Ia and asymptotic giant branch (AGB) stars, respectively. We further note that there exist other observational data in the literature that favour massive stars as the main sources for carbon. In order to match our carbon trends, we believe that the carbon yields from massive stars then must be very dependent on metallicity for the C, Fe and Y trends to be so finely tuned in the two disc populations. Such metallicity-dependent yields are no longer supported by the new stellar models in the recent literature. For the Galaxy, we hence conclude that the carbon enrichment at metallicities typical of the disc is mainly due to low- and intermediate-mass stars, while massive stars are still the main carbon contributor at low metallicities (halo and metal-poor thick disc).