The R136 star cluster dissected withHubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of He ii λ1640 in young star clusters

Abstract

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.

The Tarantula Nebula as a template for extragalactic star forming regions from VLT/MUSE and HST/STIS

“…Establishing the stellar content of R136 has generally required high spatial spectroscopy from HST at UV and optical wavelengths or a ground-based 8m telescope with Adaptive Optics. Massey & Hunter (1998) have used HST/FOS to observe stars in the R136 region (few parsec), while Crowther et al (2016) used HST/STIS to map the core of R136, obtaining complete UV/optical spectroscopy of the central parsec. Spectroscopic analysis of the brightest 52 O stars reveals a cluster age of 1.5 +0.3 −0.7 Myr, confirming this as one of the youngest massive clusters in the Local Group.…”

The very massive star content of the nuclear star clusters in NGC 5253

“…4, we compare the cluster #5 UV spectrum with G140L STIS spectra of massive stars in the 1.5 Myr old LMC cluster R136 (Crowther et al 2016, ID 12465). The stars plotted are R136a3 (WN5h; M = 180 ± 30 M ), R136a5 (O2If/WN5; M = 100 ± 30 M ) and R136 H36 (O2If; M = 70 ± 10 M ), with current mass values taken from Crowther et al (2016). These stars have been chosen because their spectral types bracket the spectral characteristics of the cluster #5 spectrum.…”

“…O v λ1371 is detected in cluster #5 as a blueshifted absorption component with a velocity of −850 km s −1 (Table 1), indicating a wind-affected O v absorption. The terminal velocity measured from the C iv P Cygni absorption component is −2900 km s −1 , where the edge velocity has been corrected for turbulent motions in the outflow, following Crowther et al (2016). All four spectra have weak to absent Si iv λ1393, 1402, with interstellar absorption providing the dominant contribution.…”

“…5 the observed UV spectrum of cluster #5 with the integrated STIS spectrum of R136a from Crowther et al (2016). To make this comparison, we simply scaled the R136a fluxes to the distance of NGC 5253 and tweaked the fluxes by multiplying the scaled R136a spectrum by 0.92 to match the central part of the cluster # 5 spectrum.…”

“…The four stars they find have initial masses between 165 and 320 M , based on model fitting. More recently, Crowther et al (2016) have presented a stellar census of the content of R136 from HST/STIS UV spectroscopy. They derive a cluster age of 1.5 ± 0.5 Myr and find that the He ii λ1640 emission flux in the R136 cluster originates exclusively from stars with masses above 100 M .…”