The R136 star cluster dissected with Hubble Space Telescope/STIS. II. Physical properties of the most massive stars in R136 (2009.05136v1)

Abstract: We present an optical analysis of 55 members of R136, the central cluster in the Tarantula Nebula of the Large Magellanic Cloud. Our sample was observed with STIS aboard the Hubble Space Telescope, is complete down to about 40\,$M_{\odot}$, and includes 7 very massive stars with masses over 100\,$M_{\odot}$. We performed a spectroscopic analysis to derive their physical properties. Using evolutionary models we find that the initial mass function (IMF) of massive stars in R136 is suggestive of being top-heavy with a power-law exponent $\gamma \approx 2 \pm 0.3$, but steeper exponents cannot be excluded. The age of R136 lies between 1 and 2\,Myr with a median age of around 1.6\,Myr. Stars more luminous than $\log L/L_{\odot} = 6.3$ are helium enriched and their evolution is dominated by mass loss, but rotational mixing or some other form of mixing could be still required to explain the helium composition at the surface. Stars more massive than 40\,$M_{\odot}$ have larger spectroscopic than evolutionary masses. The slope of the wind-luminosity relation assuming unclumped stellar winds is $2.41\pm0.13$ which is steeper than usually obtained ($\sim 1.8$). The ionising ($\log Q_0\,[{\rm ph/s}] = 51.4$) and mechanical ($\log L_{\rm SW}\,[{\rm erg/s}] = 39.1$) output of R136 is dominated by the most massive stars ($>100\,M_{\odot}$). R136 contributes around a quarter of the ionising flux and around a fifth of the mechanical feedback to the overall budget of the Tarantula Nebula. For a census of massive stars of the Tarantula Nebula region we combined our results with the VLT-FLAMES Tarantula Survey plus other spectroscopic studies. We observe a lack of evolved Wolf-Rayet stars and luminous blue and red supergiants.

• The paper identifies a top-heavy initial mass function in R136 with a power-law exponent of about 2±0.3, challenging traditional models.
• Using HST/STIS spectroscopy, the study measures physical properties for 55 stars, including seven exceeding 100 M⊙ and one reaching ~251 M⊙.
• Stellar wind diagnostics reveal a steep wind-luminosity relationship (2.41±0.13), emphasizing significant mass loss that impacts stellar evolution.

Overview of the R136 Star Cluster's Most Massive Stars

The paper "The R136 star cluster dissected with Hubble Space Telescope/STIS. II. Physical properties of the most massive stars in R136" by J. M. Bestenlehner et al. undertakes a comprehensive analysis of the R136 cluster, focusing on its most massive stars. Using the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST), the paper provides valuable insights into the physical properties of these luminous objects located in the Tarantula Nebula of the Large Magellanic Cloud (LMC). This work builds upon previous studies and enhances our understanding of stellar evolution for massive stars.

Methodology and Findings

The paper analyzed 55 stars within the R136 cluster, achieving a completeness down to approximately 40 solar masses ($M_{\odot}$). Among these, seven stars exceed 100 $M_{\odot}$, with the most massive identified as having an initial mass of $251^{+48}_{-35} M_{\odot}$. Spectroscopic tools were employed to deduce various stellar properties, including the distribution of helium enrichment at the stellar surfaces and loss rates driven by stellar winds.

A key finding of the paper is the indication that the initial mass function (IMF) in R136 might be slightly top-heavy, with a power-law exponent $\gamma \approx 2 \pm 0.3$. This deviates from the traditional Salpeter IMF ($\gamma = 2.35$), suggesting that higher mass stars might be more prevalent in this region than previously thought. The analysis places the age of R136 between 1 and 2 million years, with a median age of around 1.6 Myr. Stars more luminous than $\log L/L_{\odot} = 6.3$ exhibit surface helium enrichment due to substantial mass loss.

The paper uncovers that stars with masses above 40 $M_{\odot}$ tend to have greater spectroscopic masses compared to their evolutionary masses, indicating a shift in our understanding of massive star evolution. Furthermore, the wind-luminosity relationship in these stars revealed a steeper slope ($2.41 \pm 0.13$) than the traditionally observed value of approximately $1.8$, and is steeper than the theoretical predictions for LMC conditions.

Theoretical and Practical Implications

This work has significant theoretical implications, particularly in the modeling of massive stellar evolution and the calibration of theoretical models against observed data. The findings challenge the proposed upper mass limit of 150 $M_{\odot}$ and support the presence of very massive stars—potential progenitors of rare astrophysical events like gamma-ray bursts and pair-instability supernovae.

The paper also impacts the understanding of galaxy evolution, as these massive stars greatly influence the ionization and mechanical feedback within their host galaxies. R136 contributes significantly to the ionizing radiation and mechanical energy of the Tarantula Nebula, a fact crucial for understanding star formation rates and processes in this region.

Future Prospects

Future explorations into wind parameters, possibly integrating ultraviolet diagnostics, will provide a more refined understanding of the mass-loss processes in these massive stars. Further studies, particularly focusing on the unresolved discrepancies between spectroscopic and evolutionary masses, are essential. Incorporating these stars' evolution into broader models of galactic evolution could enhance our comprehension of star cluster dynamics and feedback mechanisms.

In conclusion, this paper advances our knowledge of the physical characteristics of massive stars within R136, while inviting additional scrutiny and development of stellar evolutionary models. The interplay of mass loss and internal mixing mechanisms, coupled with large spectroscopic datasets, continues to hold promise for new insights into the lifecycle of the universe's heaviest stars.