Carbon, isotopic ratio $^{12}$C/$^{13}$C and nitrogen in solar twins:
  constraints for the chemical evolution of the local disc
  Abundances of light elements in dwarf stars of different ages are important
constraints for stellar yields, Galactic chemical evolution and exoplanet
chemical composition studies. We have measured C and N abundances and
$^{12}$C/$^{13}$C ratios for a sample of 63 solar twins spanning a wide range
in age, based on spectral synthesis of a comprehensive list of CH\,A-X and
CN\,B-X features using HARPS spectra. The analysis of 55 thin disc solar twins
confirms the dependences of [C/Fe] and [N/Fe] on [Fe/H]. [N/Fe] is investigated
as a function of [Fe/H] and age for the first time for these stars. Our derived
correlation [C/Fe]-age agrees with works for solar-type stars and solar twins,
but the [N/Fe]-age correlation does not. The relations [C,N/Fe]-[Fe/H] and
[C,N/Fe]-age for the solar twins lay under-solar. $^{12}$C/$^{13}$C is found
correlated with [Fe/H] and seems to have decreased along the evolution of the
local thin disc. Predictions from chemical evolution models for the solar
vicinity corroborate the relations [C,N/Fe]-[Fe/H], $^{12}$C/$^{13}$C-age and
[N/O]-[O/H], but do not for the $^{12}$C/$^{13}$C-[Fe/H] and [C/O]-[O/H]
relations. The N/O ratio in the Sun is placed at the high end of the
homogeneous distribution of solar twins, which suggests uniformity in the N-O
budget for the formation of icy planetesimals, watery super-earths and giant
planets. C and N had different nucleosynthetic origins along the thin disc
evolution, as shown by the relations of [C/N], [C/O] and [N/O] against [O/H]
and age. [C/N] and [C/O] are particularly observed increasing in time for solar
twins younger than the Sun.
