Why late WC stars at high metallicity

The ubiquitous detection of late WC stars within metal-rich environments (and their absence at low metallicity) requires explanation. The observed trend for WC subtypes in the LMC versus the Milky Way was initially believed to originate from a difference in carbon abundances (Smith & Maeder 1991), yet quantitative analysis reveals similar carbon abundances (Koesterke & Hamann 1995; Crowther et al.

2002). Alternatively, late WC stars might evolve preferentially from relatively low-mass OB stars that enter the WR phase owing to stronger stellar winds at earlier evolutionary phases. This scenario is not supported by cluster studies (e.g. Massey et al. 2000; Crowther et al. 2006).

The most compelling evidence suggests that late WC stars are favoured in the case of high wind densities (Crowther et al. 2002). Consequently, the presence of late WC stars within metal-rich galaxies favours metallicity-dependent winds for Wolf-Rayet stars. The impact of a metallicity dependence for WR winds upon spectral types is as follows. At high metallicity, recombination from high to low ions (early to late subtypes) is very effective in very dense winds, whilst the opposite is true for low-metallicity, low-density winds. Stellar temperatures further complicate this picture, such that the spectral type of a WR star results from a subtle combination of ionization and wind density, in contrast with the case for normal stars.

Theoretically, Nugis & Lamers (2002) argued that the iron opacity peak was the origin of the wind driving in WR stars, which Grafener & Hamann (2005) supported via a hydrodynamic model for an early-type WC star in which lines of Fe ix-xvii deep in the atmosphere provided the necessary radiative driving. Vink & de Koter (2005) applied a Monte Carlo approach to investigate the metallicity dependence for cool WN and WC stars, revealing M a Za, where a = 0.86 for WN stars and a = 0.66 for WC stars for 0.1 < Z < Z0. The weaker WC dependence originates from an increasing Fe content and constant C and O content at high metallicity. Empirical results for the Solar neighbourhood, LMC and SMC are broadly consistent with theoretical predictions, although detailed studies of individual WR stars within galaxies with a broader range in metallicity would provide stronger constraints.

A metallicity dependence of WR winds affects evolutionary-model calculations as follows. Recent evolutionary models of Meynet & Maeder (2005) allow for rotational mixing, but not a metallicity dependence of WR winds. Improved agreement with respect to earlier models is achieved, but the ratio of WC versus WN stars for continuous star formation does does not reproduce that observed at high metallicity, as illustrated in Figure 29.6. In contrast, recent (non-rotating) evolutionary models by Eldridge & Vink (2006) in which the Vink & de Koter (2005) WR metallicity dependence has been implemented provide a much better match to observations.

With regard to the inferred WR populations at high metallicity, a note of caution is necessary. At high metallicity, WR optical recombination lines will (i) increase in equivalent width, since their strength scales with the square of the density; and (ii) increase in line flux, since the lower wind strength will reduce the line blanketing, resulting in an increased extreme-UV continuum strength at the expense of the UV and optical (Crowther & Hadfield 2006). Indeed, the equivalent widths of optical

0

1 1 IC10

i i i i i i i i i i

■ M83

Solar Neighbourhood ^

:

/<-■ M31 -

/ ■ M33(inner)

■ M33(middle) -

M33(outer)

NGC300(inner)

..•'LMC "

- £ SMC'

Figure 29.6. Ratios of subtype distribution of WC to WN stars for nearby galaxies, updated from Massey & Johnson (1998) to include M83 (Hadfield et al. 2005). Evolutionary predictions from Meynet & Maeder (2005) (solid line) and Eldridge & Vink (2006) (dotted line) are included.

emission lines of WN stars in the Milky Way and LMC are well known to be higher than those of their SMC counterparts (Conti et al. 1989).

To date, the standard approach for the determination of unresolved WR populations in external galaxies has been to assume metallicity-independent WR line fluxes - obtained for Milky Way and LMC stars (Schaerer & Vacca 1998) - regardless of whether the host galaxy is metal-rich (Mrk 309) (Schaerer et al. 2000) or metal-poor (I Zw 18) (Izotov et al. 1997). Ideally, one would wish to use WR template stars appropriate to the metallicity of the galaxy under consideration. Unfortunately, this is feasible only for the LMC, SMC and Solar neighbourhood, since it is challenging to isolate individual WR stars from ground-based observations of more distant galaxies, which span a larger spread in metallicity.

Enhanced WR line fluxes are also predicted for WR atmospheric models at high metallicity if one follows the metallicity dependence from Vink & de Koter (2005), such that WR populations inferred from Schaerer & Vacca (1998) at high metallicity may overestimate actual populations.

Wavelength (A) Wavelength (A)

Figure 29.7. Predicted Lyman continuum ionizing fluxes for model reference WN #11 (T = 100 kK, L = 105 48L0) from Smith et al. (2002), illustrating harder ionizing fluxes at lower metallicity, notably below X = 228 A due to weaker stellar winds.

Wavelength (A) Wavelength (A)

Figure 29.7. Predicted Lyman continuum ionizing fluxes for model reference WN #11 (T = 100 kK, L = 105 48L0) from Smith et al. (2002), illustrating harder ionizing fluxes at lower metallicity, notably below X = 228 A due to weaker stellar winds.

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