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arXiv · 2609.22918

Distinct Population of Wolf-Rayet Stars in the Low-Metallicity Galaxy NGC 6822. Quantitative analysis of optical and UV spectra of the complete sample

R. Trigg·L. M. Oskinova·W. -R. Hamann·H. Todt·D. Pauli·T. Shenar·A. A. C. Sander·M. Chatzis·A. Mang·R. R. Lefever·V. Ramachandran·D. Massa
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Abstract

Context: Wolf-Rayet (WR) stars are particularly rare in low-metallicity galaxies and remain poorly understood. Detailed studies of the presumably complete population of WR stars in the nearby Small Magellanic Cloud (SMC) galaxy led to perplexing results. Aims: The nearby galaxy NGC 6822 has a similar metallicity to the SMC, and hence provides an excellent laboratory for a comparative study. We aim to derive the stellar and wind parameters of each known WR star in NGC 6822 by means of quantitative spectroscopy. The results are compared with the WR population of the SMC. Methods: New optical and UV spectroscopic observations have been obtained with the Very Large Telescope (VLT) and Hubble Space Telescope (HST). We employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code for our analysis, consistently fitting synthetic spectra to the observed spectral energy distribution, and optical and UV spectra. Results: We spectroscopically confirm all four stars in our sample belong to the nitrogen sequence with early subtypes (WN3-WN6). All stars show indications of binarity. Only one star in our sample has a high fraction of hydrogen in its atmosphere, as typical for the WN population in the SMC, and two other stars in our sample are hydrogen-free. Finally, the spectrum and parameters of the fourth object closely resemble those of the qWR-type strongly magnetic merger-product star in the HD 45166 binary. Conclusions: The stellar properties of our sample are diverse. We have identified the first H-free WN-type stars at metallicities $\sim$ 1/7Z$_\odot$. According to evolutionary calculations, the WR stars in NGC 6822 stem from progenitors with initial masses below 40 M$_\odot$. We report a discovery of a low luminosity stripped qWR star in NGC 6822, and conclude that the WR star population in NGC 6822 and the SMC are distinct despite similar metallicities of their host galaxies.

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R. Trigg, L. M. Oskinova, W. -R. Hamann, H. Todt, D. Pauli, T. Shenar, A. A. C. Sander, M. Chatzis, A. Mang, R. R. Lefever, V. Ramachandran, D. Massa. 2026-09-19. Distinct Population of Wolf-Rayet Stars in the Low-Metallicity Galaxy NGC 6822. Quantitative analysis of optical and UV spectra of the complete sample. https://arxiv.org/abs/2609.22918

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