The statistics of massive O stars and Wolf Rayet stars

We measured the WR bump in the residual spectra described above. To measure the blue WR bump and the red WR bump at 5808 A, we employed a method similar to that described by Guseva et al. (2000). In six galaxies (25% of the sample) we could clearly detect the blue WR bump. However, we measured the WR bumps in all other galaxies, thus establishing upper limits for the numbers of observed WR stars. The numbers of O stars were calculated using the Hp luminosity and the calibration proposed by Leitherer & Heckman (1995).

Figure 30.2. The observed WR/O-star ratio versus metallicity. Multiple extractions are shown for each galaxy, when available. Wolf-Rayet galaxies are represented as squares and all others (upper limits) are plotted as triangles. The lines correspond to STARBURST99 models with two different star-formation scenarios: continuous and instantaneous. Three different mass functions are plotted for each scenario: a = 2.35, Mupp = 100M0 (solid lines); a = 3.3, Mupp = 100M0 (dashed lines); and a = 2.35, Mupp = 30M0 (dotted lines). Most of our galaxies present a combination of the two star-forming scenarios.

Figure 30.2. The observed WR/O-star ratio versus metallicity. Multiple extractions are shown for each galaxy, when available. Wolf-Rayet galaxies are represented as squares and all others (upper limits) are plotted as triangles. The lines correspond to STARBURST99 models with two different star-formation scenarios: continuous and instantaneous. Three different mass functions are plotted for each scenario: a = 2.35, Mupp = 100M0 (solid lines); a = 3.3, Mupp = 100M0 (dashed lines); and a = 2.35, Mupp = 30M0 (dotted lines). Most of our galaxies present a combination of the two star-forming scenarios.

We for the first time categorize galaxies ESO 485-G003, NGC 6090, and NGC 2798 as WR galaxies, on the basis of the presence of the WR bump and the calculated numbers of WR stars.

In Figure 30.2 we compare the WR/O-star ratio with the metallicity of the host galaxy and with the STARBURST99 models. In this figure, all galaxies for which WR bumps have been detected are plotted as squares; all others are the upper limits, plotted as triangles. A combination of star-formation regimes (continuous and instantaneous) explains the observed WR/O-star ratios.

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