[Paper Review] The Wolf-Rayet stars in the Large Magellanic Cloud: A comprehensive analysis of the WN class

R. Hainich, U. Rühling+7 more|Jan 21, 2014

Stellar, planetary, and galactic studies131 references83 citations

TL;DR

This study presents a comprehensive spectroscopic analysis of nearly all known WN-type Wolf-Rayet stars in the Large Magellanic Cloud (LMC) using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to UV and optical data, it reveals that 88% of single WN stars have luminosities between log(L/L☉) = 5.3–5.8 and low hydrogen content, indicating they evolved through the red supergiant phase, while a small fraction (12%) are extremely luminous with significant hydrogen, likely originating from very high initial masses.

ABSTRACT

Aims: Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods: For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results: We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10^6 Lsun and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/Lsun) = 5.3...5.8. Conclusions: While the few extremely luminous stars (log (L/Lsun) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/Lsun) = 5.3...5.8, these stars originate from initial masses between 20 and 40 Msun. This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.

Motivation & Objective

• To conduct a comprehensive spectroscopic analysis of nearly all known WN-type Wolf-Rayet stars in the Large Magellanic Cloud (LMC).
• To determine fundamental stellar and wind parameters using advanced model atmosphere techniques.
• To investigate the evolutionary pathways of WN stars in the LMC and compare them with predictions from stellar evolution models.
• To assess the impact of metallicity on the observed properties of WN stars, particularly in comparison to the Milky Way.
• To test whether current stellar evolution tracks, including rotational mixing, can reproduce the observed luminosity and mass distributions.

Proposed method

• Employed the Potsdam Wolf-Rayet (PoWR) model atmosphere code to analyze wind-dominated emission-line spectra.
• Fitted synthetic spectra to observed spectral energy distributions (SEDs) and flux-calibrated UV and optical spectra.
• Used iron-line blanketing and microclumping in the wind models to improve accuracy of derived parameters.
• Applied distance modulus (DM = 18.5 mag) and low reddening (E(B-V) < 0.25 mag) to derive absolute luminosities.
• Analyzed photometry from visual narrowband, 2MASS, and IRAC instruments for stars with limited spectral data.
• Compared results with theoretical models, including those with rotational mixing, to assess consistency with evolutionary tracks.

Experimental results

Research questions

• RQ1 What are the fundamental stellar and wind parameters of the nearly complete sample of WN stars in the LMC?
• RQ2 How do the luminosity and hydrogen content of WN stars in the LMC compare to predictions from current stellar evolution models?
• RQ3 To what extent does the observed distribution of WN stars in the LMC reflect metallicity-dependent evolution, particularly in comparison to the Milky Way?
• RQ4 Why do observed stellar radii and effective temperatures deviate from model predictions, and what physical mechanisms might explain this?
• RQ5 Can current stellar evolution models, including rotational mixing, reproduce the observed luminosity range of WN stars in the LMC?

Key findings

• The analysis covers 107 WN stars in the LMC, providing a nearly complete sample of the class.
• 88% of single WN stars have log(L/L☉) between 5.3 and 5.8 and exhibit little to no hydrogen, indicating they evolved through the red supergiant phase.
• 12% of the sample are more luminous than 10⁶ L☉ and contain significant hydrogen, suggesting they may originate from very high initial masses (>40 M☉).
• Stellar radii are generally larger and effective temperatures lower than predicted by current models, likely due to subphotospheric inflation.
• Even with rotational mixing, current stellar evolution tracks fail to fully reproduce the observed luminosity and initial mass ranges.
• The metallicity dependence of evolution is not clearly observed in the LMC, as the initial mass range (20–40 M☉) for the bulk of WN stars is similar to that in the Milky Way despite lower metallicity (Z ≈ 0.4 Z☉).