Blue stragglers and the OBN phenomenon
Observational data for the hottest stars in five OB associations arereported to show that blue stragglers exhibit nitrogen overabundances.The coude spectra were taken in the ranges 3500-5400 A and 3800-4900 Aat ESO and Observatoire de Haute Provence, respectively. The surveycovered 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 bluestragglers in the associations displayed nitrogen abundances. It issuggested that the massive blue stragglers experience rotation-inducedextra-mixing that brings nitrogen enhancement to the surface and keepsthe stars close to the main sequence. The rotationally-induced effectscould also explain the bimodal rotational velocity distribution of mainsequence O stars, whose lifetimes are extended by the extra mixing.
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