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

Abstract

We introduce a &lt;it&gt;Hubble Space Telescope&lt;/it&gt; (&lt;it&gt;HST&lt;/it&gt;)/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 &lt;it&gt;m&lt;/it&gt;&lt;inf&gt;F555W&lt;/inf&gt; = 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 &lt;scp&gt;iv&lt;/scp&gt;λλ1548–51, 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&lt;f&gt;$^{+0.3}_{-0.7}$&lt;/f&gt; 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 &lt;scp&gt;ii&lt;/scp&gt; λ1640 emission line. This emission is totally dominated by very massive stars with initial masses above ∼100 M&lt;inf&gt;&amp;odot;&lt;/inf&gt;. The presence of strong He &lt;scp&gt;ii&lt;/scp&gt; 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&lt;inf&gt;&amp;odot;&lt;/inf&gt;. 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.