  We have measured the gamma-ray line production cross sections in
proton-induced nuclear reactions on various target nuclei abundant in
astrophysical sites over the incident energy range of Ep = 30 - 200 MeV. We
carried out experimental campaigns in joint collaboration at the K = 200
cyclotron of iThemba LABS using a high-energy resolution, high-efficiency
detection array composed of 8 Compton-suppressed clover detectors comprising 32
HP-Ge crystals for recording the gamma-ray spectra. In the current paper, we
focus on de-excitation lines produced in proton irradiations of natC and Mylar
targets. In particular, on the prominent 4.439 and 6.129 MeV lines of $^{12}$C
and $^{16}$O which are among the strongest lines emitted in solar flares and in
interactions of low-energy cosmic rays with the gas and dust of the inner
galaxy. We report new gamma-ray production experimental cross section data for
ten nuclear lines that we compare to previous low-energy data sets from the
literature, to the predictions of the TALYS code of modern nuclear reactions
and to a semi-empirical compilation. In first approach, performing calculations
with default input parameters of TALYS we observed substantial deviations
between the predicted cross sections and experimental data. Then, using
modified optical model potential and nuclear level deformation parameters as
input data we generated theoretical excitation functions for the above two main
lines fully consistent with experimental data. In contrast, the experimental
data sets for the other eight analyzed lines from the two proton-irradiated
targets exhibit significant deviations with the predicted cross section values.
We also report line-shape experimental data for the line complex observed at
$E_g$ = 4.44 MeV in irradiations of the two targets. Finally, we emphasize the
astrophysical implications of our results.
