The R136 star cluster dissected with Hubble Space Telescope/STIS. I. Far-ultraviolet spectroscopic census and the origin of HeII 1640 in young star clusters (1603.04994v1)

Abstract: We introduce a HST/STIS stellar census of R136a, the central ionizing star cluster of 30 Doradus. We present low resolution far-ultraviolet STIS/MAMA spectroscopy of R136 using 17 contiguous 52x0.2 arcsec slits which together provide complete coverage of the central 0.85 parsec (3.4 arcsec). We provide spectral types of 90% 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 CIV 1548-51, including 16 O2-3 stars. For the first time we spectroscopically classify all Weigelt & 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 HeII 1640 emission line. This emission is totally dominated by very massive stars with initial masses above ~100 Msun. The presence of strong HeII 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 100 Msun. We include montages of ultraviolet spectroscopy for LMC O stars in the Appendix. Future studies in this series will focus on optical STIS/CCD medium resolution observations.

• The paper uses Hubble Space Telescope/STIS far-ultraviolet spectroscopy to classify 90% of 57 bright sources within 0.5 parsecs of the R136 cluster center, identifying WN5, O supergiant, and O dwarf stars.
• Wind velocity measurements for 52 early-type stars in R136 show lower average velocities for O2-4 dwarfs compared to LMC counterparts, potentially indicating a cluster effect.
• The study suggests very massive stars drive the cluster's integrated UV spectrum, with HeII 1640 emission serving as a diagnostic for identifying similar massive star clusters in distant galaxies.

Insights into R136 Star Cluster with Hubble Space Telescope/STIS

The paper by Crowther et al. provides an in-depth spectroscopic analysis of the R136 star cluster using data from the Hubble Space Telescope's Space Telescope Imaging Spectrograph (HST/STIS). Located in the 30 Doradus region of the Large Magellanic Cloud (LMC), R136 is pivotal in understanding the formation and evolution of the most massive stars. This paper presents critical insights into the ultraviolet (UV) properties of the cluster, focusing on spectral classifications, stellar winds, and implications on the initial mass function (IMF).

Key Findings

1. UV Spectroscopy and Stellar Classification: Utilizing 17 contiguous slits of the STIS, the paper provides spectral classifications for 90% of 57 sources brighter than $m_{\rm F555W}$ = 16.0 mag within 0.5 parsecs of the cluster center. The authors identify three WN5 stars (a1-a3), two O supergiants (a5-a6), and three early O dwarfs (a4, a7-a8). Such classifications are invaluable for constructing the Hertzsprung-Russell (HR) diagram and understanding stellar evolution in dense clusters.
2. Wind Velocities: The paper measures wind velocities for 52 early-type stars using C\,{\sc iv} $\lambda\lambda$1548--51 data. Statistical analysis reveals lower average wind velocities for O2--4 dwarfs in R136 compared to counterparts in the LMC, suggesting a potential cluster effect or evolutionary state impacting stellar winds.
3. Cluster Age and Mass Estimates: From the HR diagram, the cluster's age is inferred to be 1.5$^{+0.3}_{-0.7}$ million years. This suggests a very young cluster dominated by massive stars with initial masses exceeding 100 $M_{\odot}$. Importantly, these estimates leverage Bayesian models to refine mass and age predictions, crucial for dynamic evolutionary modeling.
4. Integrated Ultraviolet Spectrum: The paper emphasizes the role of very massive stars (VMS) in the cluster's integrated UV spectrum, notably the He\,{\sc ii} $\lambda$1640 emission line, which indicates the presence of stars exceeding 100 $M_{\odot}$. This aspect challenges conventional IMF upper limits and supports a more extended mass function for young clusters.
5. Implications for Stellar Formation and Evolution: The detailed observations suggest that the R136 cluster hosts a population of VMS that potentially results from dynamic interactions and mergers. The presence of strong He\,{\sc ii} emission in integrated spectra could serve as a diagnostic for identifying similar massive star clusters in distant galaxies.

Implications and Future Directions

The results of this paper advance our understanding of massive star formation and the initial mass function in extreme environments. Practically, the identification of VMS beyond the traditional 100 $M_{\odot}$ threshold necessitates a reconsideration of stellar evolution models, especially in resolving the binary fraction and potential for stellar mergers in dense clusters.

Future studies will benefit from optical datasets, particularly for refining spectral types and assessing binarity and rotational velocities, which are not extensively covered in the present UV-focused analysis. Moreover, tackling the dynamic evolution of R136, along with consistent comparisons across different wavebands, should illuminate the life cycles of massive stars and their feedback effects on surrounding interstellar environments.

The integration of additional data modalities, such as new HST Fine Guidance Sensor observations, will likely resolve further the nuances of massive star formation models and validate the implications drawn on IMF extensions in young star-forming regions. This will offer a broader context for understanding massive stellar populations in both local and distant galactic contexts.