2009A&A...503..985C

We present a study of the extreme O-type supergiant He3-759 using new high-resolution FEROS data, revealing that it is a near spectroscopic twin of HD 151804 (O8Iaf). We investigate the extinction towards He3-759 using a variety of methods, revealing A_V∼4.7^m. If we assume He3-759 has an identical absolute K-band magnitude to HD151804 we find that it lies in the Sagittarius-Carina spiral arm at a distance of ∼6.5kpc. We derive the physical and wind properties for He3-759, revealing T_*=30.5kK, logL/L_☉=5.9 and {dot}(M)=10^–5.17M_☉/yr for a clumped wind whose terminal velocity is estimated at 1000km/s. The atmosphere of He3-759 is enriched in helium (X_He=49%) and nitrogen (X_N=0.3%). A reanalysis of HD151804 and HD152408 (WN9ha) reveals similar parameters except that the WN9ha star possesses a stronger wind and reduced surface hydrogen content. HD151804 and HD152408 lie within the Sco OB1 association, with initial masses of ∼60M_☉ and ages ∼2.7Myr, consistent with NGC6231 cluster members using standard Geneva isochrones. Improved agreement with observed surface abundances are obtained for similar initial masses with more recent Geneva group predictions from which higher ages of ∼3.75Myr are obtained. No young, massive star cluster is known to be associated with He3-759.