Carbon, Nitrogen, and Oxygen Abundances in the Chemically Peculiar Stars of the Upper Main Sequence.

Abundances of the three light elements, carbon, nitrogen, and oxygen (CNO), are obtained from low-noise, high-resolution reticon spectra of a variety of chemically peculiar (CP) stars with surface temperatures in the range 7000 to 18000K. Our survey includes 13 Am and related stars, 9 HgMn stars, 15 magnetic Ap stars (8 SrCrEu-type and 7 Si-type), 1 He-weak star, and 5 suitable standard composition stars. A quantitative abundance analysis was carried out using neutral lines of CNO and once-ionized lines of C at mostly red and near-infrared wavelengths and line-blanketed model atmospheres. Local thermodynamic equilibrium was assumed. This analysis provides the first comprehensive and uniform set of CNO abundances in the traditional classes of CP stars. Relative to the standard stars, each class has the following average abundances (in log abundance units) for CNO, respectively: Am stars (-0.24, -0.24, -0.40), HgMn stars (+0.10, -0.67, -0.30), SrCrEu stars (-0.68, -0.60, -1.29), and Si stars (-0.12, -0.22, -0.49). The C and O abundances show a roughly montonic increase with surface temperature in both the non-magnetic (Am and HgMn) stars and the magnetic (SrCrEu and Si) stars. At a given surface temperature, both elements are usually more deficient in the magnetic stars. These results are successfully accounted for by the radiative diffusion theory. The presence of turbulence (a diffusion inhibitor) is particularly evident in the less deficient Am stars, which also have higher than average microturbulent velocities. The N abundance in the non-magnetic stars is most deficient in the stars above 10000K (i.e., the HgMn stars), which is in conflict with our predictions for an overabundance at these temperatures based on models where diffusion is operating alone. In the magnetic stars, only stars with temperatures between 8000 and 10000K show large N deficiencies. We tentatively propose that a modest stellar wind coupled with upward diffusion can explain the observed large N deficiencies in the HgMn stars, and, perhaps, in the SrCrEu stars. We also find that the largest N depletions are associated with weak or absent magnetic fields, which provides an argument against an alternative explanation of CP star abundance anomalies, the magnetic accretion theory.