4 Trends of WR populations with metallicity

Figure 6: Observed WR-bump intensities (left panel) and equivalent widths (right panel) as a function of metallicity from the compilation of Schaerer (1999, crosses), the samples of Guseva et al. (2000, small filled circles), Castellanos et al. (2002a, small triangles), SGIT00 (large open squares and circles), BK02 (large filled squares) and the present data (large filled triangles). Typical error bars for the Guseva et al. sample are shown. Maximum predicted intensities and $W({\rm WR})$ from the SV98 synthesis models are shown for various star formation histories: instantaneous bursts (solid), burst durations $\Delta t = 2$ Myr (dotted) and 4 Myr (long dashed). See discussion in text.

4.1 Behaviour of the "WR bump''

Figure 6 shows the WR bump intensities and equivalent widths as a function of metallicity for our metal-rich H  II regions (large filled triangles) and the 11 WR regions in spiral galaxies recently reported by Bresolin & Kennicutt (2002, large filled squares), together with data compiled by Schaerer (1999, small crosses) and Schaerer et al. (2000).

Our new measurements at high O/H are found to fill in the range from the previously observed maximum intensities/equivalent widths down to lower values. Physically the maxima of $I({\rm WR})/I({\rm H}\beta)$ and $W({\rm WR})$ are expected to reflect the maximum WR/O star ratio achieved in bursts. No lower limit is expected; if present in a given sample, such a lower limit presumably reflects the detection limit of the WR features.

The increase of the upper envelope of $I({\rm WR})/I({\rm H}\beta)$ with metallicity has been known since the work of Arnault et al. (1989) and has been reviewed by Schaerer (1999). With few exceptions, max( $W({\rm WR})$) also seems to show an increase with O/H as shown here for the first time. The increase of max( $I({\rm WR})/I({\rm H}\beta)$) is naturally interpreted as due to the increase of stellar wind mass loss with metallicity leading to lower minimum mass limit for the formation of WR stars, $M_{\rm WR}$, thereby favouring the presence of WR stars at high metallicity (cf. Maeder et al. 1981; Arnault et al. 1989; Maeder 1991). Other effects, e.g. a lowering of the ${\rm H}\beta$ flux due to a) increasing amounts of dust absorbing ionising radiation or b) lower average stellar temperatures at high O/H due to modified stellar evolution, could also play a role (cf. Schaerer 1999), but are likely secondary.

The maxima of the predicted WR bump intensities and equivalent widths computed with the code of SV98  with a "standard'' Salpeter IMF for instantaneous bursts (solid line), and extended bursts of duration $\Delta t = 2$ Myr (dotted), and 4 Myr (long dashed) are overplotted in Fig. 6. As already shown earlier (cf. Schaerer 1996, 1999; Mas-Hesse & Kunth 1999; Guseva et al. 2000) the range of observations at subsolar metallicities ( $12 + \log({\rm O/H})\la 8.6$) is fairly well reproduced by the models, when accounting for the various uncertainties (e.g. missing ${\rm H}\beta$ flux in slit observations, some objects with small numbers of WR stars, some poor spectra; cf. discussion in Guseva et al.). The new sample of metal-rich objects plotted here shows WR bump strengths smaller than the maxima predicted by the "standard'' models. The possible reasons for this behaviour are discussed in Sect. 5 where detailed model comparisons are undertaken.

Figure 7: Estimated number ratio of WC/WNL stars versus metallicity. Data derived from C  IV $\lambda$5808/WR bump observations from the sample of Guseva et al. (2000) and SGIT00 are shown by stars; the assumed mean spectral type of the WC stars is WC4 for $12 + \log({\rm O/H})< 8.4$ and WC7 for higher metallicities. Results from our VLT data are shown for different assumptions: (1) from C  IV $\lambda$5808/WR bump assuming a WC7 spectral type (filled squares), or WC4 (crosses). (2) from C  III $\lambda$5696/WR bump assuming WC7 spectral type (filled triangles), or WC8 (open cirles). Maximum predicted WC/WN ratios from the SV98 models are shown for instantaneous bursts (solid), and burst durations $\Delta t = 2$ Myr (dotted), and 4 Myr (long dashed). The observed trend of WC/WN with metallicity in Local Group galaxies, thought to represent an average for constantly star forming regions, from Massey & Johnson (1998) is shown by the dash-dotted line.

4.2 WC/WN ratio

We have estimated the relative number ratio of WC and WN stars, shown in Fig. 7, in several ways. First the number of WN stars, $N({\rm WNL})$ assuming late WN subtypes dominate, is derived from the luminosity of the blue WR bump, as described above. The number of WC stars, $N({\rm WC})$, is estimated from the C  IV $\lambda$5808 or C  III $\lambda$5696 luminosity where measured, again assuming that WN stars do not contribute to these lines. As the observed average luminosity of WC stars in these lines varies strongly with subtype (see SV98), the estimated $N({\rm WC})$ depends on the assumption of the dominant WC subtype. As the observations (see above, Guseva et al. 2000; Schaerer et al. 1999a) indicate that early types ($\sim$WC4) dominate at low metallicity, while WC7-8 dominate at high $12 + \log({\rm O/H})$, we assume these mean WC subtypes for the sample of Guseva et al. (2000). For our high metallicity sample, the estimated $N({\rm WC})/N({\rm WNL})$ ratios is estimated adopting different assumptions on the WC subtype and using C  IV $\lambda$5808 or C  III $\lambda$5696 (see Fig. 7).

The resulting estimates show a fairly clear trend of an increasing upper envelope for $N({\rm WC})/N({\rm WNL})$ with metallicity. Furthermore, and in contrast with the limited sample of Guseva et al. (2000), we now find at the high metallicity end a number of objects with $N({\rm WC})/N({\rm WNL}) \ga$ 0.5-1. and a WC/WN number ratio larger than the observed trend in Local Group galaxies by Massey & Johnson (1998), indicated by the dash-dotted line in Fig. 7. Indeed, while the regions observed by these authors are thought to correspond to averages large enough to represent the equilibrium $N({\rm WC})/N({\rm WNL})$ value at constant star formation, larger (and obviously also smaller) values should be found in regions with fairly short bursts.

A more quantitative interpretation of the observed WC to WN ratio appears difficult for the following reasons. First the uncertainties in the estimated $N({\rm WC})/N({\rm WNL})$ are quite large (cf. above); second, detailed evolutionary synthesis model predictions of $N({\rm WC})/N({\rm WNL})$ depend quite strongly on the adopted interpolation techniques (cf. SV98, comparison between results from SV98 models and Starburst99 (Leitherer et al. 1999), also Massey 2002); third, other comparisons with synthesis models reveal potential difficulties (cf. below). In any case the SV98 models predict the maximum WC/WN number ratios indicated in Fig. 7 by the solid line for instantaneous bursts, and burst durations of $\Delta t = 2$ Myr (dotted) and 4 Myr (dashed) respectively.