We introduce a Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph (STIS) stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution far-ultraviolet STIS spectroscopy of R136 using 17 contiguous 52 arcsec × 0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90 per cent of the 57 sources brighter than m F555W = 16.0 mag within a radius of 0.5 parsec of R136a1, plus 8 additional nearby sources including R136b (O4 If/WN8). We measure wind velocities for 52 early-type stars from C IVλλ1548-51, including 16 O2-3 stars. For the first time, we spectroscopically classify all Weigelt and Baier members of R136a, which comprise three WN5 stars (a1-a3), two O supergiants (a5-a6) and three early O dwarfs (a4, a7, a8). A complete Hertzsprung-Russell diagram for the most massive O stars in R136 is provided, from which we obtain a cluster age of 1.5 +0.3 −0.7 Myr. In addition, we discuss the integrated ultraviolet spectrum of R136, and highlight the central role played by the most luminous stars in producing the prominent He II λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100 M . The presence of strong He II λ1640 emission in the integrated light of very young star clusters (e.g. A1 in NGC 3125) favours an initial mass function extending well beyond a conventional upper limit of 100 M . We include montages of ultraviolet spectroscopy for Large Magellanic Cloud O stars in the appendix. Future studies in this series will focus on optical STIS medium resolution observations.

Abstract. We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based (2.3 m MSSSO, NTT) spectroscopy to determine the physical parameters of six WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellar parameters are revised significantly relative to Gräfener et al. (1998) based on improved observations and more sophisticated model atmosphere codes, which account for line blanketing and clumping. We find that stellar luminosities are revised upwards by up to 0.4 dex, with surface abundances spanning a lower range of 0.1 ≤ C/He ≤ 0.35 (20-45% carbon by mass) and O/He ≤ 0.06 (≤10% oxygen by mass). Relative to Galactic WC5-8 stars at known distance, and analysed in a similar manner, LMC WC4 stars possess systematically higher stellar luminosities, ∼0.2 dex lower wind densities, yet a similar range of surface chemistries. We illustrate how the classification C  λ5696 line is extremely sensitive to wind density, such that this is the principal difference between the subtype distribution of LMC and Galactic early-type WC stars. Temperature differences do play a role, but carbon abundance does not affect WC spectral types. We illustrate the effect of varying temperature and mass-loss rate on the WC spectral type for HD 32257 (WC4, LMC) and HD 156385 (WC7, Galaxy) which possess similar abundances and luminosities. Using the latest evolutionary models, pre-supernova stellar masses in the range 11-19 M are anticipated for LMC WC4 stars, with 7-14 M for Galactic WC stars with known distances. These values are consistent with pre-cursors of bright typeIc supernovae such as SN 1998bw (alias GRB 980425) for which a minimum total mass of C and O of 14 M has been independently derived.

We present a quantitative classification scheme for carbon and oxygen sequence Wolf-Rayet stars. Our scheme uses new high-quality optical AAT and INT observations of 20 stars for which we provide narrow-band photometry and estimates of interstellar reddenings. In increasing order of excitation, our spectral classes range from WC11 to WC4 for WolfRayet stars with a dominant carbon line visual spectrum, and subsequently from WO4 to WO1 for those with predominantly oxygen lines. We refine existing WC and WO schemes to incorporate stars with higher and lower excitation spectral features. Both massive stars and central stars of planetary nebulae (CSPNe) can be classified with the unified system. We have found no criterion that cleanly separates spectra of the two types of star, including elemental abundances (C/O or C/He). However, CSPNe show a wider range of line strength and width than massive stars in the same ionization subclass. Systematically lower FWHM(C iv X5808) values are observed from WO-type CSPNe than from massive WO stars.For WC4-11 stars, our primary diagnostic is the equivalent width or line flux ratio C Iv XX5801-12/C iii X5696. We extend the use of this as the principal criterion throughout the WC sequence, with few reclassifications necessary relative to Smith, Shara & Moffat. For WO stars, C iii is absent and our new criteria, using primarily oxygen lines, take over smoothly. We define subclasses W04-1, using 0 vi XX3811-34/O v X5590 as our primary diagnostic. The continuation in spectral sequence from WC to WO is used to indicate that the sequence is a result primarily of excitation effects, rather than significant abundance differences.Our scheme allows us to confirm that massive stars and CSPNe are differently distributed over the subclasses. Around 3/5 of massive WC stars lie within the range WC5-8, while -1/5 of CSPNe are found within these spectral types. Stars within both the highest (WO 1) and lowest (WC10-11) excitation spectral classes are unique to CSPNe. A WC classification for the hot R CrB star V348 Sgr is excluded (previously [WC12]) since both C iii X5696 and C iv X5808 are absent in its optical spectrum. Additional criteria allow us to distinguish between WC-type, 'weak emission line' CSPNe, and 0 stars, allowing us to reclassify the central star of IRAS 21282+5050 (previously [WC11]) as an 0 star.

Stars stripped of their hydrogen-rich envelope through interaction with a binary companion are generally not considered when accounting for ionizing radiation from stellar populations, despite the expectation that stripped stars emit hard ionizing radiation, form frequently and live 10 − 100 times longer than single massive stars. We compute the first grid of evolutionary and spectral models specially made for stars stripped in binaries for a range of progenitor masses (2-20 M ) and metallicities ranging from solar to values representative for pop II stars. For stripped stars with masses in the range 0.3-7 M , we find consistently high effective temperatures (20 000-100 000 K, increasing with mass), small radii (0.2-1 R ) and high bolometric luminosities, comparable to that of their progenitor before stripping. The spectra show a continuous sequence that naturally bridge subdwarf-type stars at the low mass end and Wolf-Rayet like spectra at the high mass end. For intermediate masses we find hybrid spectral classes showing a mixture of absorption and emission lines. These appear for stars with mass loss rates of 10 −8 − 10 −6 M yr −1 , which have semi-transparent atmospheres. At low metallicity, substantial hydrogen-rich layers are left at the surface and we predict spectra that resemble O-type stars instead. We obtain spectra undistinguishable from subdwarfs for stripped stars with masses up to 1.7 M , which questions whether the widely adopted canonical value of 0.47 M is uniformly valid. Only a handful of stripped stars of intermediate mass have currently been identified observationally. Increasing this sample will provide necessary tests for the physics of interaction, internal mixing and stellar winds. We use our model spectra to investigate the feasibility to detect stripped stars next to an optically bright companion and recommend systematic searches for their UV excess and possible emission lines, most notably HeII λ4686 in the optical and HeII λ1640 in the UV. Our models are publicly available for further investigations or inclusion in spectral synthesis simulations.

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.