[Paper Review] Stellar and Wind Properties of LMC WC4 stars - A metallicity dependence for Wolf-Rayet mass-loss rates

P. A. Crowther, Luc Dessart+3 more|ArXiv.org|Jun 14, 2002

Stellar, planetary, and galactic studies48 references118 citations

TL;DR

This study re-evaluates the stellar and wind properties of six LMC WC4 Wolf-Rayet stars using improved UV (FUSE, HST) and ground-based (MSSSO, NTT) spectroscopy with advanced model atmospheres that include line blanketing and wind clumping. It finds a metallicity-dependent mass-loss rate scaling as  ~Z^0.5, explaining the lower wind densities and different subtype distribution in the LMC compared to the Galaxy, with pre-supernova masses of 11–19 M☉ for LMC WC4 stars, consistent with Type Ic SN progenitors like SN 1998bw.

ABSTRACT

We use ultraviolet space-based (FUSE, HST) and optical/IR ground-based (2.3m MSSSO, NTT) spectroscopy to determine the physical parameters of six WC4-type Wolf-Rayet stars in the Large Magellanic Cloud. Stellar parameters are revised significantly relative to Grafener et al. (1998) based on improved observations and more sophisticated model atmosphere codes, which account for line blanketing and clumping. We find that stellar luminosities are revised upwards by up to 0.4 dex, with surface abundances spanning a lower range of 0.1

Motivation & Objective

• Reassess the physical parameters of LMC WC4 stars using high-resolution UV and optical/IR spectroscopy.
• Address inconsistencies in prior mass-loss rate estimates by incorporating wind clumping and line blanketing in model atmospheres.
• Investigate the metallicity dependence of Wolf-Rayet mass-loss rates by comparing LMC and Galactic WC stars with similar luminosities and abundances.
• Determine pre-supernova masses of LMC and Galactic WC stars to assess their consistency with Type Ic supernova progenitors.
• Clarify the role of wind density versus temperature in defining WC spectral subtypes, particularly the C iii λ5696 line sensitivity.

Proposed method

• Acquired far-UV (FUSE) and optical/IR (HST, MSSSO, NTT) spectroscopy for six LMC WC4 stars, including narrow-band photometry.
• Applied advanced model atmosphere codes (Hillier & Miller 1998) that include line blanketing and wind clumping to fit observed spectral features.
• Used synthetic spectral fitting to derive stellar parameters: effective temperature, luminosity, mass-loss rate, and surface abundances (C/He, O/He).
• Compared results with Galactic WC5–8 stars at known distances, using identical modeling techniques for consistency.
• Employed evolutionary models to estimate current and pre-supernova masses, accounting for wind darkening effects.
• Quantified the sensitivity of the C iii λ5696 line to wind density to explain subtype differences between LMC and Galactic WC stars.

Experimental results

Research questions

• RQ1 How do revised model atmospheres with clumping and line blanketing affect the derived luminosities and mass-loss rates of LMC WC4 stars?
• RQ2 What is the dependence of Wolf-Rayet mass-loss rates on metallicity, as inferred from comparisons between LMC and Galactic WC stars?
• RQ3 Why do LMC WC4 stars exhibit systematically lower wind densities than Galactic WC5–8 stars despite similar surface abundances and luminosities?
• RQ4 To what extent does wind density, rather than temperature, determine the spectral subtype distribution in WC stars?
• RQ5 Are the derived pre-supernova masses of LMC WC4 stars consistent with those required for Type Ic supernovae like SN 1998bw?

Key findings

• Stellar luminosities of LMC WC4 stars are revised upward by up to 0.4 dex compared to Gräfener et al. (1998), now reaching ~10^5.7 L☉.
• Mass-loss rates are lower than previously estimated due to the inclusion of wind clumping, with LMC WC4 stars showing ~0.2 dex lower wind densities than Galactic WC5–8 stars.
• A metallicity dependence of mass-loss rate ~Z^0.5 is inferred, explaining the lower wind densities in the LMC (Z ~ 0.4 Z☉) compared to the Galaxy.
• The C iii λ5696 line is highly sensitive to wind density, serving as the primary diagnostic for distinguishing WC4 from WC7 subtypes, with temperature playing a secondary role.
• Pre-supernova masses for LMC WC4 stars are estimated at 11–19 M☉, consistent with the 14 M☉ minimum CO-core mass inferred for SN 1998bw (GRB 980425).
• Galactic WC stars with known distances have current masses of 7–14 M☉, supporting their role as progenitors of luminous Type Ic supernovae such as SN 1997ef.