The Wolf-Rayet stars in the Large Magellanic Cloud

Abstract

Context. Massive stars, although being important building blocks of galaxies, are still not fully understood. This especially holds true for Wolf-Rayet (WR) stars with their strong mass loss, whose spectral analysis requires adequate model atmospheres. Aims. Following our comprehensive studies of the WR stars in the Milky Way, we now present spectroscopic analyses of almost all known WN stars in the LMC. Methods. For the quantitative analysis of the wind-dominated emission-line spectra, we employ the Potsdam Wolf-Rayet (PoWR) model atmosphere code. By fitting synthetic spectra to the observed spectral energy distribution and the available spectra (ultraviolet and optical), we obtain the physical properties of 107 stars. Results. We present the fundamental stellar and wind parameters for an almost complete sample of WN stars in the LMC. Among those stars that are putatively single, two different groups can be clearly distinguished. While 12% of our sample are more luminous than 10 6 L and contain a significant amount of hydrogen, 88% of the WN stars, with little or no hydrogen, populate the luminosity range between log (L/L ) = 5.3 ... 5.8. Conclusions. While the few extremely luminous stars (log (L/L ) > 6), if indeed single stars, descended directly from the main sequence at very high initial masses, the bulk of WN stars have gone through the red-supergiant phase. According to their luminosities in the range of log (L/L ) = 5.3 ... 5.8, these stars originate from initial masses between 20 and 40 M . This mass range is similar to the one found in the Galaxy, i.e. the expected metallicity dependence of the evolution is not seen. Current stellar evolution tracks, even when accounting for rotationally induced mixing, still partly fail to reproduce the observed ranges of luminosities and initial masses. Moreover, stellar radii are generally larger and effective temperatures correspondingly lower than predicted from stellar evolution models, probably due to subphotospheric inflation.

Towards a better understanding of the evolution of Wolf–Rayet stars and Type Ib/Ic supernova progenitors

“…1 is likely due to the uncertainty of the distance measurement. Hainich et al (2014) found M ∝ L 1.18 for single WNE stars in the LMC, for which the distance uncertainty is much smaller than in the case of Galactic WR stars. Therefore, it seems to be real that WNE stars have a steeper luminosity dependence of mass-loss rates compared to the case of WC/WO stars.…”

“…This relation gives 51 per cent higher mass-loss rates than those of LMC WNE stars (i.e., log M = −11.5 + 1.18 log(L/L ⊙ ); Hainich et al 2014). Assuming that LMC metallicity is half the solar value as in TSK, we get the metallicity dependence of M ∝ Z 0.6 init .…”

“…For this purpose, we may use the relation of M ∝ L 1.18 that is found with single WNE stars in the LMC (Hainich et al 2014). Given that the scatter in the distance estimate is minimized with the LMC sample, we may consider this relation more reliable than that obtained with the Galactic sample.…”

“…This discrepancy is because they did not correct the Galactic WNE mass-loss rates for D = 10, as mentioned above. Hainich et al (2014) used D = 10 in their analysis of LMC WNE stars and a correction of (4/10) 0.5 to Galactic WNE mass loss rates by Hamann et al (2006) is needed to derive the metallicity dependence in a consistent way.…”

“…In particular, the mass-loss rate scales with the clumping factor D as M ∝ D −1/2 . Several studies prefer D = 10 (e.g., Sander et al 2012;Hainich et al 2014), but it is not clear yet if the complex features resulting from hydrodynamic instabilities in the wind material can be well described by this single free parameter. This allows us to have some freedom in our choice of f WR .…”