Hydrodynamic atmosphere models for WNL stars

In a recent work we investigated the properties of the luminous, H-rich WNL

stars with our hydrodynamic PoWR models (Grafener & Hamann 2007). The most important conclusion from that work is that WR-type mass loss is primarily trig-geredbyhigh L/Mratios or, equivalently, Eddington factors Te = xeL*/(4ncGM*)

approaching unity. Note that high L/M ratios are expected for very-massive stars and for He-burning objects, giving a natural explanation for the occurrence of the WR phenomenon.

In Figure 33.2 we show the results from grid computations for WNL stars with a fixed luminosity of 106 3L0 and stellar temperatures T* in the range 30-60 kK. For the stellar masses, values of 67M0 and 55M0 are adopted, corresponding to Eddington factors of Te = 0.55 and 0.67. Notably, the mass loss strongly depends on re and T*. The obtained synthetic spectra nicely reflect the observed sequence of weak-lined WNL subtypes, starting with WN 6 at 55 kK and extending to WN

9 at 31 kK. From a more-detailed investigation of the WN 7 component in WR 22, an eclipsing WR + O binary system in Car OB1, we infer a stellar mass of 78M0 (re = 0.67), in agreement with Rauw etal. (1996), who obtained (72 ± 3)M0 from

140 120 100 90 80 70 60 50 40 35 30

- A

A

1 1 WNL

- ■

WNE-w

- •

O

WNE-s

6.5

— •

6WD128

8^A148 \ 107 16aa66 a108 \ a.87 a156 \ 4(AA123 \ a124 a116 \ \ \

#14 ^10 17128^84 D63

120ML

40Mm

\ZAMS

Figure 33.1. Recent spectral analyses of Galactic WR stars with line-blanketed models, according to Hamann et al. (2006) and Barniske et al. (2006): symbols in light gray denote H-rich WR stars, whereas H-free objects are indicated in dark gray, and WC stars are indicated in black; w, weak lines; and s, strong lines. For objects with large symbols distance estimates are available (van der Hucht 2001), whereas objects with small symbols were calibrated by their spectral subtype. Evolutionary tracks for non-rotating massive stars (Meynet & Maeder 2003) are shown for comparison.

the binary orbit. Such high stellar masses imply that the weak-lined WNL stars are still in the phase of central H-burning, suggesting an evolutionary sequence of the form O ^ WNL ^ LBV ^ WN ^ WC for very massive stars.

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