WNL stars at various metallicities

In Figure 33.3 we present results from a grid of models for luminous WNL stars at various metallicities (Gräfener & Hamann 2007). The models were computed for

4.80

4.70

4.60

Figure 33.2. Wind models for Galactic WNL stars: mass-loss rates for various stellar temperatures T and Eddington factors Te are shown. The corresponding spectral subtypes are indicated, together with WR numbers of specific Galactic objects according to van der Hucht (2001), in brackets, showing the good agreement with the synthetic line spectra. Note that the models were computed for a fixed stellar luminosity of 106 3L0. The WN 7 model (WR 22) is slightly offset from the standard grid models because it is calculated with an enhanced hydrogen abundance (see the text).

4.80

4.70

4.60

4.50

Figure 33.2. Wind models for Galactic WNL stars: mass-loss rates for various stellar temperatures T and Eddington factors Te are shown. The corresponding spectral subtypes are indicated, together with WR numbers of specific Galactic objects according to van der Hucht (2001), in brackets, showing the good agreement with the synthetic line spectra. Note that the models were computed for a fixed stellar luminosity of 106 3L0. The WN 7 model (WR 22) is slightly offset from the standard grid models because it is calculated with an enhanced hydrogen abundance (see the text).

a fixed luminosity of 106 3 L0, a stellar temperature of T = 45 kK, and a hydrogen surface mass fraction of XH = 0.4. In addition to the metal abundances, we varied the Eddington factor Te (or equivalently the stellar mass). Note that we scaled all metals with Z, assuming a CNO-processed (i.e. N-enriched) WN surface composition. The wind-driving in our models is chiefly due to radiation pressure on Fe-group line opacities. In accordance with Vink & Koter (2005) we thus find a strong dependence on Z. However, as in our previous computations, the proximity to the Eddington limit plays an equally important role. We find that optically thick winds with high WR-type mass-loss rates are formed over the whole range of metallicities, from (1/1,000) Z0 to 3 Z0, if the stars are close enough to the Eddington limit. Only the limiting value of Te at which the WR-type winds start to form changes. For solar Z, values of Te = 0.5-0.6 lead to the formation of weak-lined WNL stars. As we have seen for the case of WR 22, this corresponds to very-massive, slightly over-luminous stars in a late phase of H-burning. Note that it is indeed observed that the most massive stars in very young galactic clusters are in the WNL phase (e.g. Figer et al. (2002)) and Najarro et al. (2004) for the Arches cluster; Drissen (1999) and Crowther & Dessart (1998) for NGC 3603).

For higher values of Z, the limit for the formation of WR-type winds shifts towards even lower values of Te. For 3Z0, we find that stars with Te ~ 0.4 already have typical WNL mass-loss rates (see Figure 33.3). This corresponds to objects with mass 120M0 on the ZAMS. We thus expect that metal-rich stars with very

2,000

500 0

Figure 33.3. Mass loss for WNL stars over a broad range of metallicities (Z): mass-loss rates (top) and terminal wind velocities (bottom) obtained from our hydrodynamic models are plotted against the Eddington factor Te. The solid curves indicate model series for WNL stars, where Te is varied for a given value of Z. The dashed gray lines indicate models with 3Z0 and XH = 0.7, corresponding to very-massive, metal-rich stars on the ZAMS.

high masses start their lives in the WNL phase, i.e. the occurrence of WNL stars in young massive clusters is strongly favored for high metallicities.

References

Barniske, A., Hamann, W.-R., & Grafener, G. (2006), in Stellar Evolution at Low Metallicity: Mass-Loss, Explosions, Cosmology, ed. H. Lamers, N. Langer, & T. Nugis (San Francisco, CA, Astronomical Society of the Pacific), p. 243 Crowther, P. A. & Dessart, L. (1998), MNRAS 296, 622

Drissen, L. (1999), in Wolf-Rayet Phenomena in Massive Stars and Starburst Galaxies, eds. K. A. van der Hucht, G. Koenigsberger, & P. R. J. Eenens (San Francisco, CA, Astronomical Society of the Pacific), p. 403 Figer, D. F., Najarro, F., Gilmore, D. et al. (2002), ApJ 581, 258 Grafener, G., & Hamann, W.-R. (2005), A&A 432, 633

(2006), in Stellar Evolution at Low Metallicity: Mass-Loss, Explosions, Cosmology, ed. H. Lamers, N. Langer, & T. Nugis (San Francisco, CA, Astronomical Society of the Pacific), p. 171

2,000

500 0

Figure 33.3. Mass loss for WNL stars over a broad range of metallicities (Z): mass-loss rates (top) and terminal wind velocities (bottom) obtained from our hydrodynamic models are plotted against the Eddington factor Te. The solid curves indicate model series for WNL stars, where Te is varied for a given value of Z. The dashed gray lines indicate models with 3Z0 and XH = 0.7, corresponding to very-massive, metal-rich stars on the ZAMS.

0.6 0.7 0.8 Eddington Factor re

0.6 0.7 0.8 Eddington Factor re

Gräfener, G., Koesterke, L., & Hamann, W.-R. (2002), A&A 387, 244 Hamann, W.-R., & Gräfener, G. (2003), A&A 410, 993 Hamann, W.-R., Grafener, G., & Liermann, A. (2006), A&A 457, 1015 Hamann, W.-R., & Koesterke, L. (1998), A&A 335, 1003 Koesterke, L., Hamann, W.-R., & Grafener, G. (2002), A&A 384, 562 Meynet, G., & Maeder, A. (2003), A&A 404, 975

Najarro, F., Figer, D. F., Hillier, D. J., & Kudritzki, R. P. (2004), ApJ 611, L105 Rauw, G., Vreux, J.-M., Gosset, E. et al. (1996), A&A 306, 771 van der Hucht, K. A. (2001), New Astronomy Review 45, 135 Vink, J. S. & de Koter, A. (2005), A&A 442, 587

Was this article helpful?

0 0

Post a comment