[Paper Review] The gamma Velorum binary system. II. WR stellar parameters and the photon loss mechanism

Orsola De Marco, W. Schmütz+5 more|arXiv (Cornell University)|Apr 6, 2000

Stellar, planetary, and galactic studies4 citations

TL;DR

This paper revises the stellar parameters of the WC8 Wolf-Rayet star WR11 in the γ Velorum binary system using non-LTE, clumped, line-blanketed models (cmfgen), finding a hotter effective temperature (57 kK), higher luminosity (log(L/L⊙) = 5.00), and lower mass-loss rate (log(Ṁ/M⊙/yr) = –5.0) than previous studies. The key contribution is demonstrating that line-line interactions—particularly between He ii Lyα and nearby metal lines—significantly reduce wind ionization, implying that photon loss mechanisms must be included in models to accurately determine wind structure and stellar parameters.

ABSTRACT

In this paper we derive stellar parameters for the Wolf-Rayet star in the gamma Velorum binary system (WR11), from a detailed non-LTE model of its optical and infrared spectra. Compared to the study of Schaerer et al., the parameters of the WC8 star are revised to a hotter effective temperature (Teff~57kK),a higher luminosity (log(L/Lsun = 5.00), and a lower mass loss rate (log(Mdot / Msun/yr) = -5.0 - using a 10% clumping filling factor). These changes lead to a significant decrease in wind efficiency number, from 144 to 7, so that the driving mechanism of the wind of this WR star may be simply radiation pressure on lines. The derived spectroscopic luminosity is found to be 40% lower than that derived by De Marco & Schmutz through the mass-luminosity relationship for WR stars (log(L/Lsun = 5.2). The paper furthermore presents a comparison of the independently-developed modelling programs, CMFGEN and ISA-Wind . Overall, there seems to be very reasonable agreement between the derived parameters for WR11, except for the carbon content, which is 2 times higher for CMFGEN (C/He=0.15 vs. 0.06, by number).The comparison also confirms a disparity in the predicted flux at lambda

Motivation & Objective

• To determine accurate stellar parameters (T_eff, L, Ṁ) for the WC8 Wolf-Rayet star WR11 in the γ Velorum binary system using detailed non-LTE modeling.
• To test the validity of the photon loss mechanism proposed by Schmutz (1997), where line-line interactions between strong resonance lines and weaker metal lines reduce effective ionization.
• To compare results from the cmfgen and isa-wind modeling codes to assess consistency and identify discrepancies, particularly in extreme UV fluxes and metal abundances.
• To evaluate the impact of line blanketing, especially near critical resonance lines like He ii λ303.78, on wind ionization and derived stellar parameters.
• To assess the importance of macro-turbulence and line clustering in determining the strength of photon loss effects in Wolf-Rayet atmospheres.

Proposed method

• Employed the non-LTE stellar atmosphere code cmfgen to model the optical and infrared spectra of WR11, including clumping (10% filling factor) and extensive line blanketing.
• Used hydrodynamic wind equations and orbital parameters from Schmutz et al. (1997) to derive the O star mass and infer the WR star mass (9.0 ± 1.0 M⊙).
• Applied the photon loss approximation from Schmutz (1997) within cmfgen to assess how line-line interactions between He ii Lyα and metal lines (e.g. O iii, C iii) affect ionization structure.
• Compared results from cmfgen with those from the independently developed isa-wind code, using the same input parameters and including line blanketing via Monte Carlo methods.
• Quantified the effect of varying turbulent velocity (50 vs. 5 km s⁻¹) near 303 Å to isolate the role of line-line interactions from continuum blanketing.
• Evaluated the impact of missing lines in the cmfgen model on the photon loss mechanism, acknowledging potential limitations in completeness.

Experimental results

Research questions

• RQ1 How do revised non-LTE, clumped, line-blanketed models alter the derived stellar parameters (T_eff, L, Ṁ) of the WC8 star WR11 compared to previous studies?
• RQ2 To what extent does the line-line interaction between He ii Lyα and nearby metal lines (e.g. O iii, C iii) reduce wind ionization, and how does this affect the derived effective temperature and luminosity?
• RQ3 How do the results from the cmfgen and isa-wind modeling codes compare, particularly in the extreme UV (λ < 400 Å), and what explains the discrepancy in flux hardness?
• RQ4 What is the quantitative impact of including or excluding the photon loss mechanism on the derived wind ionization structure and stellar parameters?
• RQ5 How do macro-turbulence and the completeness of the line list influence the strength of the photon loss effect in WR star models?

Key findings

• The effective temperature of WR11 is revised to 57 kK, significantly hotter than previous estimates, due to improved treatment of line blanketing and line-line interactions.
• The bolometric luminosity is found to be log(L/L⊙) = 5.00, 0.2 dex lower than the value derived from the mass-luminosity relation (log(L/L⊙) = 5.2), indicating a discrepancy in indirect luminosity estimates.
• The mass-loss rate is revised downward to log(Ṁ/M⊙/yr) = –5.0, leading to a dramatic reduction in wind efficiency from 144 to 7, suggesting radiation pressure on lines alone can drive the wind.
• The photon loss mechanism—where line-line interactions between He ii Lyα and metal lines reduce effective ionization—was confirmed to lower the wind ionization by ~10,000 K, equivalent to a 20% increase in luminosity.
• The cmfgen and isa-wind codes show good agreement overall, except for the carbon abundance (C/He = 0.15 vs. 0.06 by number) and extreme UV fluxes, where isa-wind predicts a harder spectrum.
• The study demonstrates that even weak lines near strong resonance lines (e.g. He ii λ303.78) are critical for accurate ionization structure, and that mean blanketing factors may miss key physical effects.