Reduced Wolf-Rayet line luminosities at low metallicity

Abstract

New NTT/EMMI spectrophotometry of single WN2-5 stars in the Small and Large Magellanic Clouds are presented, from whichHe ii λ4686 line luminosities have been derived, and compared with observations of other Magellanic Cloud Wolf-Rayet stars. SMC WN3-4 stars possess line luminosities which are a factor of 4 times lower than LMC counterparts, incorporating several binary SMC WN3-4 stars from the literature. Similar results are found for WN5-6 stars, despite reduced statistics, incorporating observations of single LMC WN5-9 stars from the literature. C iv λ5808 line luminosities of carbon sequence WR stars in the SMC and IC 1613 (both WO subtypes) from the recent literature are a factor of 3 lower than LMC WC stars from Mt Stromlo/DBS spectrophotometry, although similar results are also obtained for the sole LMC WO star. We demonstrate how reduced line luminosities at low metallicity follow naturally if WR winds are metallicity-dependent, as recent empirical and theoretical results suggest. We apply mass loss-metallicity scalings to atmospheric non-LTE models of Milky Way and LMC WR stars to predict the wind signatures of WR stars in the metal-poor star forming WR galaxy I Zw 18. WN He ii λ4686 line luminosities are 7-20 times lower than in metal-rich counterparts of identical bolometric luminosity, whilst WC C iv λ5808 line luminosities are 3-6 times lower. Significant He + Lyman continuum fluxes are predicted for metal-poor early-type WR stars. Consequently, our results suggest a larger population of WR stars in I Zw 18 than is presently assumed, particularly for WN stars, potentially posing a severe challenge to evolutionary models at very low metallicity. Finally, reduced wind strengths from WR stars at low metallicities impacts upon the immediate circumstellar environment of long duration GRB afterglows, particularly since the host galaxies of high-redshift GRBs tend to be metal-poor.

Gemini GMOS spectroscopy of HeII nebulae in M 33

“…Alternatively, could MC8 be an Of supergiant, since they possess much weaker He II λ4686 emission than WN stars plus brighter absolute magnitudes of M V = −7 mag. We are able to discriminate between these two possibilies since Of supergiants possess narrow λ4686 emission, with FWHM ∼ 2-7 Å (Crowther et al 2002b) versus FWHM ∼= 25-60 Å for early-type WN stars (Crowther & Hadfield 2006). From our MC8 Gemini GMOS spectrum, He II λ4686 emission is broad (FWHM ∼ 30 Å), allowing us to conclude that MC8 is a probable cluster hosting an early-type WN star.…”

“…An extinction of E(B−V) = 0.30 mag is required to reproduce the spectral energy distribution of the GMOS dataset (Fig. 10), from which we obtain a λ4686 line luminosity of 4.5 × 10 35 erg s −1 for the WN star, versus an average of 8.4 ± 7.1 × 10 35 erg s −1 for LMC early-type WN stars (Crowther & Hadfield 2006).…”

Massive star formation in Wolf-Rayet galaxies

“…The numbers of WNL and WCE stars were computed assuming metallicity-dependent WR luminosities (Crowther & Hadfield 2006). We detected the blue WR bump (the broad He ii λ4686 emission line) in all objects with high N/O ratio: UM 420, NGC 5253 A,B, HCG 31 AC, IRAS 08208+2816, III Zw 107 and ESO 566-8, indicating that these bursts host an important WNL star population.…”

Narrow He II emission in star-forming galaxies at low metallicity

“…As a result of their dependence on the iron-group opacities, the winds of classical WR stars are expected to be weaker at low Z (Vink & de Koter 2005), resulting in significantly weaker emission lines (Crowther & Hadfield 2006). However, the situation is slightly different for WC stars because their surfaces are substantially self-enriched with the products of He-burning, namely C, O, and most likely Ne and Mg.…”