The Galactic WN stars

Abstract

Context. Very massive stars pass through the Wolf-Rayet (WR) stage before they finally explode. Details of their evolution have not yet been safely established, and their physics are not well understood. Their spectral analysis requires adequate model atmospheres, which have been developed step by step during the past decades and account in their recent version for line blanketing by the millions of lines from iron and iron-group elements. However, only very few WN stars have been re-analyzed by means of line-blanketed models yet. Aims. The quantitative spectral analysis of a large sample of Galactic WN stars with the most advanced generation of model atmospheres should provide an empirical basis for various studies about the origin, evolution, and physics of the Wolf-Rayet stars and their powerful winds. Methods. We analyze a large sample of Galactic WN stars by means of the Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line blanketing and clumping. The results are compared with a synthetic population, generated from the Geneva tracks for massive star evolution. Results. We obtain a homogeneous set of stellar and atmospheric parameters for the Galactic WN stars, partly revising earlier results. Conclusions. Comparing the results of our spectral analyses of the Galactic WN stars with the predictions of the Geneva evolutionary calculations, we conclude that there is rough qualitative agreement. However, the quantitative discrepancies are still severe, and there is no preference for the tracks that account for the effects of rotation. It seems that the evolution of massive stars is still not satisfactorily understood.

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

“…1 presents the WR mass-loss rates given by NL and TSK at solar metallicity, compared to the empirical values of Galactic hydrogen-free WR stars 1 . Note that the Potsdam group inferred mass-loss rates with clumping factors of D = 4 for Galactic WNE stars and with D = 10 for WC stars, respectively (Hamann et al 2006;Sander et al 2012). Given that both NL and TSK prescriptions are based on the data compatible with D = 10 rather than with D = 4, here we corrected the WNE mass-loss rates of the Potsdam group by a factor of (4/10) 0.5 for this comparison.…”

“…However, consideration of wind clumping (e.g., Moffat & Robert 1994;Lépine & Moffat 1999) in later empirical estimates of WR mass-loss rates resulted in much lower values (e.g., Hamann & Koesterke 1998;Hamann et al 2006;Crowther 2007;Sander et al 2012). For example, currently the most popular prescription by Nugis & Lamers (2000, hereafter, NL) gives WR massloss rates almost 10 times lower than those used in the above-quoted theoretical studies.…”

“…Recent studies by the Potsdam group presents a homogeneous set of WR star properties for both WN and WC types, which gives an excellent observational constraint on WR star properties (Hamann et al 2006;Sander et al 2012;Hainich et al 2014). As discussed below, the observed population of WN/WC stars cannot be well explained by the models using the NL prescription either.…”

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

“…Hamann et al 2006;Crowther et al 2010;Hainich et al 2014;Bestenlehner et al 2014). The HRD positions of these objects suggest that they are very massive stars (VMS) with initial masses in excess of ∼100 M , and reaching up to 300 M (cf.…”

“…Comprehensive studies of the local population of WR stars of the nitrogen sequence (WN stars) have been performed for the Galaxy by Hamann et al (2006) The dividing luminosity at roughly 10 5.9 L suggests that the classical WN stars are core He-burning objects in a post-red supergiant (post-RSG) phase. This picture is further supported by the work of Sander et al (2012), who found that the WR stars of the carbon sequence (WC stars), which are showing the products of He-burning at their surface, form a natural succession of the sequence of classical WN stars in the HR diagram (HRD).…”