Quantitative spectroscopy of Wolf—Rayet stars in HD 97950 and R136a — the cores of giant H ii regions

Abstract

We present quantitative analyses of Wolf–Rayet stars in the cores of two giant H ii regions — HD 97950 in NGC 3603 and R136a in 30 Doradus — based on archive Hubble Space Telescope (HST) spectroscopy. We confirm previous WN6h+abs classifications for components A1, B and C in HD 97950, while classifications for R136a1–3 are revised from O3 If*/WN6 to WN5h. From detailed non-local thermodynamic equilibrium analyses, we find that all Wolf–Rayet stars exhibit products of CNO-processed material at their surface since they are rich in both helium (H/He ≈ 3–6, by number) and nitrogen (N/He ≈ 0.002–0.006). Their luminosities, log (L/L⊙) = 6.0–6.3, are amongst the highest known for Wolf–Rayet stars. Consequently they are very massive stars (Minit ≥ 100 M⊙) at a relatively low age (∼ 2 Myr), reminiscent of the late WN stars in the Carina Nebula. We obtain a revised distance modulus of 15.03 mag (= 10.1 kpc) to NGC 3603 based on available photometry, an updated Mv calibration for early O stars and a reddening of E (B − V) = 1.23 mag towards its core. From a census of the massive stellar content of the two central clusters we conclude that their global properties are comparable. We evaluate the contribution made by Wolf–Rayet stars to the total Lyman continuum ionizing flux and kinetic energy released into the ISM. We discuss how simple calibrations can be used to estimate stellar luminosities, ionizing fluxes and mass-loss rates of luminous OB stars. Wolf–Rayet stars provide ∼ 20 per cent of the total ionizing flux (∼ 1.3 × 1051 Ly photon s−1) within 0.5 pc of their cores, and ∼ 60 per cent of the total kinetic energy injected into the ISM (5–6 × 1038 erg s−1), despite representing only 10 per cent of the massive stellar population. For the larger R136 cluster in 30 Doradus (r ≤ 10 pc), 117 massive stars provide a total ionizing flux of 4 × 1051 Ly photon s−1 and release a total kinetic energy of 1.6 × 1039 erg s−1 into the ISM, the latter being dominated by nine WR (43 per cent) and six O3 If*/WN (29 per cent) stars.