Blue stragglers and the OBN phenomenon.

Abstract

Observational data for the hottest stars in five OB associations are reported to show that blue stragglers exhibit nitrogen overabundances. The coude spectra were taken in the ranges 3500-5400 A and 3800-4900 A at ESO and Observatoire de Haute Provence, respectively. The survey covered the objects HD 34656, HD 42088, HD 96715, HD 151804, HD 152233, HD 192639, HD 193524 and HD 254755, the hottest members of the Aur OB1, Gem OB1, Car OB2, Sco OB2 and Cyg OB1 associations. Five of the blue stragglers in the associations displayed nitrogen abundances. It is suggested that the massive blue stragglers experience rotation-induced extra-mixing that brings nitrogen enhancement to the surface and keeps the stars close to the main sequence. The rotationally-induced effects could also explain the bimodal rotational velocity distribution of main sequence O stars, whose lifetimes are extended by the extra mixing.