Based on the spectrum fitting method applied to C i 5380 , N i 7486 , and O i 6156–8 lines , we determined the abundances of C , N , and O for 100 mostly A-type main-sequence stars ( late B through early F at 11000 K \gtsim T _ { eff } \gtsim 7000 K ) comprising normal stars as well as non-magnetic chemically peculiar ( CP ) stars in the projected rotational velocity range of 0 km s ^ { -1 } \ltsim v _ { e } \sin i \ltsim 100 km s ^ { -1 } , where our aim was to investigate the abundance anomalies of these elements in terms of mutual correlation , dependence upon stellar parameters , and difference between normal and CP stars .
We found that CNO are generally underabundant ( relative to the standard star Procyon ) typically by several tenths dex to \sim 1 dex for almost all stars ( regardless of CP or normal ) , though those classified as peculiar ( Am or HgMn ) tend to show larger underabundance , especially for C in late Am stars and for N in HgMn stars of late B-type , for which deficiency amounts even up to \sim 2 dex .
While the behaviors of these three elements are qualitatively similar to each other , the quantitative extent of peculiarity ( or the vulnerability to the physical process causing anomaly ) tends to follow the inequality relation of C > N > O .
Regarding the considerable star-to-star dispersion observed at any T _ { eff } , the most important cause is presumably the difference in rotational velocity .
These observational facts appear to be more or less favorably compared with the recent theoretical calculations based on the model of atomic diffusion and envelope mixing .