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A. Pellerin (Space Telescope Science Institute), L. Drissen (Université Laval)
NGC\,595 is a well-known {H~{\sc{ii}}} region within the nearby spiral galaxy M\,33 for which individual stars can be resolved. The massive stellar content of NGC\,595 have been studied with the spectral synthesis technique below 1200\,Å\ using archival data from the FUSE telescope. The FUSE data show well-developed P\,Cygni profiles, a typical UV signature of massive stars with strong winds, for the stellar lines of {OVI~{\sc{vi}}}~\lambda\lambda1031.9, 1037.6, {S~{\sc{iv}}}~\lambda\lambda1062.7, 1073.0, 1073.5, {P~{\sc{v}}}~\lambda\lambda1118.0, 1128.0, and {C~{\sc{iii}}}~\lambda1175.6. However, the spectral synthesis model LavalSB was unable to reproduce these line profiles because the high mass end of the mass function in NGC\,595 is not well represented by an analytical IMF, as used by the model, due to the relatively small number of massive stars.
With the failure of the synthesis model to reproduce the observations, we then concentrate our efforts on individual stars. We co-added the FUSE spectra of individual hot stars to find that O7\,I stars dominate the stellar line profiles of NGC\,595. These stars usually dominate the line profiles for a population of 2.5-4.0\,Myr old. Intriguingly, we obtained that 30% of the UV flux comes from Wolf-Rayet (WR) stars, which is a large contribution compared to standard populations. More surprisingly, we found that the WR spectral types needed to reproduce the line profiles of NGC\,595 do not correspond to the classification obtained for individual WR in the visible range. Within the FUSE archives, we also found that Galactic WR stars of a same spectral type, with no known companion, show very different stellar line profiles. These far-UV line variations brings interrogations about the precision of the spectral classification of WR stars in the visible range. If not associated with binary systems, these variations may be related to evolutionary stages of WR stars.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.