With an aim to investigate how the surface abundances of intermediate-mass stars off the main sequence ( evolving toward the red-giant stage ) are affected by the evolution-induced envelope mixing , we spectroscopically determined the abundances of Li , C , N , O , and Na for selected 62 late A through G subgiants , giants , and supergiants , which are often called “ Hertzsprung-gap stars , ” by applying the synthetic spectrum-fitting technique to Li i 6708 , C i 5380 , N i 7460 , O i 6156–8 , and Na i 6161 lines .
A substantially large star-to-star dispersion ( \gtrsim 2 dex ) was confirmed for the Li abundances , indicating that this vulnerable element can either suffer significant depletion before the red-giant stage or almost retain the primordial composition .
Regarding C , N , O , and Na possibly altered by dredge-up of nuclear-processed products , their abundances turned out to show considerable scatter .
This suggests that these abundance results are likely to suffer appreciable uncertainties , the reason for which is not clear but might be due to some kind of inadequate modeling for the atmospheric structure .
Yet , paying attention to the fact that the relative abundance ratios between C , N , and O should be more reliable ( because systematic errors may be canceled as lines of similar properties are used for these species ) , we could confirm a positive correlation between [ O/C ] ( ranging from \sim 0 to \sim + 0.5 dex ) and [ N/C ] ( showing a larger spread from \sim 0 to \sim + 1 dex ) , which is reasonably consistent with the theoretical prediction .
This observational detection of C deficiency as well as N enrichment in our program stars manifestly indicates that the dredge-up of H-burning product can take place before entering the red-giant stage , with its extent differing from star to star .