A&A 457, 1015–1031 (2006) Astronomy DOI: 10.1051/0004-6361:20065052 & c ESO 2006 Astrophysics The Galactic WN stars Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation W.-R. Hamann, G. Gräfener, and A. Liermann Lehrstuhl Astrophysik der Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany e-mail: [email protected] Received 20 February 2006 / Accepted 6 June 2006 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 atmo- spheres 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 evolution- ary 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. Key words. stars: mass-loss – stars: winds, outflows – stars: Wolf-Rayet – stars: atmospheres – stars: early-type – stars: evolution 1. Introduction of the stellar parameters. For many WN stars, a substantial re- vision of previous results can be expected. However, only very Very massive stars pass through the Wolf-Rayet (WR) stage be- γ few WN stars have been re-analyzed so far by means of line- fore they finally explode as supernovae or, possibly, -ray bursts. blanketed models (Herald et al. 2001; Marchenko et al. 2004). The WR stars are important sources of ionizing photons, mo- In the present paper we now re-analyze all Galactic WN stars mentum, and chemical elements. However, the evolution of mas- for which observed spectra are available to us. sive stars has not yet been safely established, and their physics is not well understood. The best way to analyze a larger sample of stars systemat- The empirical knowledge about WR stars is hampered by ically is first to establish “model grids”, i.e. sets of model at- difficulties with their spectral analysis. Adequate model atmo- mospheres and synthetic spectra that cover the relevant range of spheres, which account for the non-LTE physics and their super- parameters. For WN stars we have already prepared such grids of sonic expansion, have been developed step by step during the iron-line blanketed models (Hamann & Gräfener 2004), which past decades. In a previous paper we presented a comprehen- will be used in the present paper. sive analysis of the Galactic WN stars from their helium, hy- drogen, and nitrogen spectra (Hamann & Koesterke 1998, here- The main objective of analyzing the Galactic WN stars is after quoted as Paper I). The major deficiency of this generation to understand the evolution of massive stars. The empirical re- of model atmospheres was its neglect of line blanketing by the sults from our previous comprehensive analyses were in conflict millions of lines from iron and iron-group elements. Hillier & with the evolutionary calculations. However, with the applica- Miller (1998) were the first to include this important effect in tion of the more advanced models, we will revise the empirical their code cmfgen. The line-blanketed version of the Potsdam stellar parameters. Moreover, the evolutionary models have also Wolf-Rayet (PoWR) model atmosphere code became available been significantly improved recently, since the effects of stellar with Gräfener et al. (2002). rotation are now taken into account (Meynet & Maeder 2003). The improved models provide a much better fit to the ob- Hence, the question of whether stellar evolution theory still con- served spectra, and hence lead to more reliable determination flicts with the empirical stellar parameters should be reassessed. For this purpose, we use these new evolutionary tracks for gen- Figures 12 to 74 are only available in electronic form at erating synthetic stellar populations and compare them with the http://www.edpsciences.org results from our analyses of the Galactic WN sample. Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20065052 1016 W.-R. Hamann et al.: The Galactic WN stars The paper is organized as follows. In Sect. 2 we briefly char- 227 stars in total. Among them, 127 stars2 belong to the acterize the applied model atmospheres. The program stars and WN sequence considered here, while the others belong to the observational data are introduced in Sect. 3. The spectral ana- WC sequence or are of intermediate spectral type (WN/WC). lyses and their results are given in Sect. 4, together with a long Many of the WN stars in the catalog are highly reddened and list of detailed comments on individual WN stars. Our empirical therefore invisible or at least very faint in visual light. Our study results are compared with evolutionary models in Sect. 5, espe- started with 74 WN stars for which we had sufficient spectral cially by means of a population synthesis. observations at our disposal. As we restrict the present analyses to single-star spectra, well-established binaries with composed spectra (such as WR 139 alias V444 Cyg) have been omitted 2. The models from the beginning. But so far our sample still comprised many stars that have The Potsdam Wolf-Rayet (PoWR) model atmospheres (see been suspected of binarity, on more or less sound basis. We con- Hamann & Gräfener 2004, and references therein, for more de- sidered all these cases in detail, checking the literature and in- tails) are based on the “standard” assumptions (spherically sym- specting our spectral fits for indications of binarity (cf. Hamann metric, stationary mass-loss). The velocity field is pre-specified & Gräfener 2006). For 11 objects we finally conclude that they in the standard way. For the supersonic part we adopt the usual are indeed binaries, where the companion of the WN stars con- β-law, with the terminal velocity ∞ being a free parameter. The tributes more than 15% of the total light in the visual. Those exponent β is set to unity throughout this work. In the subsonic stars were excluded from the present single-star analyses. They region, the velocity field is defined such that a hydrostatic den- are marked in Table 2 as “composite spectrum”, the reasoning sity stratification is approached. given below in the “Comments on individual stars” (Sect. 4.3). The “stellar radius” R∗, which is the inner boundary of our Among the remaining 63 stars (see Table 2), which we are model atmosphere, corresponds by definition to a Rosseland op- going to analyze in the present paper, there are still four stars tical depth of 20. The “stellar temperature” T∗ is defined by the that are most likely close binaries, but the light contamina- luminosity L and the stellar radius R∗ via the Stefan-Boltzmann tion from the non-WN companion can be neglected. These law; i.e. T∗ denotes the effective temperature referring to the ra- stars are marked by the superscript b at their WR number in dius R∗. Table 2, Col. (1), indicating their possible origin from close- Wind inhomogeneities (“clumping”) are now accounted for binary evolution. Note that six stars of our present sample in a first-order approximation, assuming that optically thin (WR 28, WR 63, WR 71, WR 85, WR 94, WR 107) have not clumps fill a volume fraction fV while the interclump space is been analyzed before. void. Thus the matter density in the clumps is higher by a fac- The observational material is mostly the same as used in = −1 tor D fV , compared to an un-clumped model with the same our previous papers and as was published in our atlas of WN parameters. D = 4 is assumed throughout this paper. The Doppler spectra (see Hamann et al. 1995 for more details). These spectra velocity D, which reflects random motions on small scales (“mi- were taken at the “Deutsch-Spanisches Astronomisches Zentrum croturbulence”), is set to 100 km s−1. (DSAZ)”, Calar Alto, Spain, and at the European Southern The major improvement of the models compared to those Observatory (ESO), La Silla, Chile. For four stars (WR 6, applied in Paper I is the inclusion of line blanketing by iron and WR 22, WR 24, WR 78) we can employ high-resolution optical other iron-group elements. About 105 energy levels and 107 line spectra from the ESO archive obtained with UVES. All optical transitions between those levels are taken into account in the ap- spectra are not flux-calibrated and therefore were normalized to proximation of the “superlevel” approach. the continuum “by eye” before being compared with normalized The PoWR code was used to establish two grids of WN-type synthetic spectra.