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

By P. Crowther, S. Caballero-Nieves, K. Bostroem, J. M. Apellániz, F. Schneider, N. Walborn, C. Angus, I. Brott, Alceste Z. Bonanos, A. D. Koter and 10 more. 2016

150 citations recorded by the source corpus.

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 x 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 IV lambda lambda 1548-51, 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(-0.7)(+0.3) 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 lambda 1640 emission line. This emission is totally dominated by very massive stars with initial masses above similar to 100M(circle dot). The presence of strong He II lambda 1640 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 100M(circle dot). 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/3debe9d567570649107543328c81224cd3b63cfa

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In Astronomy and Astrophysics

1. Naveen A. Reddy
2. C. Steidel
3. M. Pettini
4. A. Shapley
5. G. Rudie
6. D. Erb
7. R. Trainor
8. Michela Mapelli

In Instrumentation

1. Naveen A. Reddy
2. C. Steidel
3. M. Pettini
4. A. Shapley
5. D. Erb
6. G. Rudie
7. R. Trainor
8. A. Strom

Citations in the corpus

• RECONCILING THE STELLAR AND NEBULAR SPECTRA OF HIGH-REDSHIFT GALAXIES (2016, background): “Massive star binaries naturally produce a harder ionizing-UV spectrum with a higher duty cycle than single star models, which can produce He II-ionizing photons only during brief intervals when very high mass Wolf-Rayet stars are present (see, e.g., Shirazi & Brinchmann 2012; Crowther et al. 2016).”
• Very massive stars, pair-instability supernovae and intermediate-mass black holes with the sevn code (2017, background): “VMSs have been observed in extreme star forming regions (e.g. Crowther et al. 2016) and might be the product of runaway collisions (i.e. multiple collisions in very dense stellar systems, Bonnell et al. 1998; Portegies Zwart et al. 1999).”
• The Observer's Guide to the Gamma-Ray Burst-Supernova Connection (2016, background): “, [487, 488]) which contains many of the most massive and luminous stars known, including R136a1 (M ∼ 315M⊙, L ∼ 8.”
• Dawes Review 6: The Impact of Companions on Stellar Evolution (2016, background): “This said, a recent investigation of the R136a cluster by Crowther et al. (2016) favours a scenario where not all very massive star are merger products, leaving open the question of how to form such stellar monsters.”