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Astron. Astrophys. 320, 500-524 (1997)

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2. Observations

We have sought to obtain observations of every bona-fide late WN star in the LMC. For the purposes of this paper, we define a WNL star as being of spectral type WN6 or later and containing hydrogen. Throughout this paper, we have adopted the new three-dimensional classification scheme of L.F. Smith et al. (1996) which classifies WN stars on the basis of their helium line strengths and widths, and the presence of hydrogen (denoted by 'h'). Our primary source of LMC WN stars is the catalogue of Breysacher (1981, 1986). Since we can only reliably derive parameters for single stars, we have excluded those stars which are believed to be multiple (Brey 82 (R136), Brey 65 (HDE 269828), Brey 73 (AB10) and Brey 76a (Mk37Wa)). Our own unpublished spectra suggest that Brey 57 (WN7h) and Brey 79 (WN6(h)) suffer significant contamination from unknown companions, and these stars have also been excluded. From our sample, Brey 26 and Brey 90 show radial velocity variability (Moffat 1989), suggestive of unseen companions, while Parker (1993) found Brey 81 to be multiple, although its companion is also not seen spectroscopically. Some stars in the Breysacher catalogue have been recently re-classified to earlier WN spectral types (Brey 56 to WN5h?+OB; Brey 92 to WN4-6h?pec; L.F. Smith et al. 1996) or to Of spectral type (Brey 86 to O6 Iaf) and thus these have also been omitted. We provide what we consider to be the complete list of LMC WNL stars in Table 1, and give their various catalogue names. From a known LMC population of 19 WN6-8 stars (including AB18), we are left with 13 single stars suitable for a quantitative analysis, of which we have observed 12 stars - only Brey 58 (WN5-6?) is missing. Following the recent literature, we preferentially utilise Breysacher (1981) catalogue names and spectral classifications from L.F. Smith et al. (1996), Melnick (1985) or Breysacher (1981, 1986).


[TABLE]

Table 1. A complete list of LMC WN6-8 and Ofpe/WN9 stars (excluding O3 If/WN6 stars). We preferentially use Breysacher (1981, Brey), Henize (1956, S) and Bohannan & Epps (1974, BE) catalogue numbers, as indicated with an underline. Other catalogue references are Feast et al. (1960, R), Westerlund & Smith (1964, WS), Sanduleak (1969, Sk), Fehrenbach et al. (1976, FD), Azzopardi & Breysacher (1985, AB) and Melnick (1985, Mk). WR spectral classifications are taken from L.F. Smith et al. (1996), Melnick (1985) and herein


In Paper I we presented arguments for the re-classification of spectrally composite Ofpe/WN9 stars to Wolf-Rayet stars, at least for some members of this class. We have therefore included in Table 1 the ten known LMC Ofpe/WN9 stars from Bohannan & Walborn (1989). We have observed the remaining five Ofpe/WN9 stars (excluding the LBV R127, Stahl et al. 1983) from Paper I to verify their spectroscopic and evolutionary status. Since no standard nomenclature exists for each star, we follow the recent tradition of using Henize (1956) and Bohannan & Epps (1974) catalogue numbers, in preference to the Feast et al. (1960), Sanduleak (1969) and HD catalogues (except for R99). Finally, for completeness, we note that we have omitted from this study the O3 If/WN6 stars introduced by Walborn (1986) which show a normal O3 If spectrum plus relatively broad, strong He II [FORMULA] 4686 emission.

2.1. Optical observations

We have obtained new optical spectra of our seventeen programme LMC stars at the 3.9m Anglo-Australian Telescope (AAT) between 1991 December-1994 December and at the 1.9m telescope at the Mount Stromlo Observatory (MSO) in 1995 December. The journal of observations is given in the Appendix (Table 4).

AAT optical observations were principally obtained with the RGO spectrograph, 25cm camera, Tektronix CCD (1024 [FORMULA] 1024, 24µm pixels), 1200V grating (1200B also for 1992 November) and a slit width of [FORMULA]. The measured spectral resolution in the extracted spectra is 1.6-1.8Å, using the FWHMs of the Cu-Ar arc spectra taken either before or after every stellar exposure. Each object was observed at three spectrally overlapping settings covering in total 3670-6005Å. The CCD data were bias-corrected, flat-fielded and then optimally extracted using the PAMELA (Horne 1986) routines within FIGARO (Meyerdierks 1993). Photometric standards were observed each night to flux calibrate the stellar spectra. Once calibrated, the spectra were rectified using low order polynomials and measured within DIPSO (Howarth et al. 1995).

The Mount Stromlo observations were taken with the coudé spectrograph, 32 in. camera and Tektronix CCD (2048 [FORMULA] 2048, 24µm pixels). We used the 300 and 600 l mm-1 gratings, both blazed at 5000Å, resulting in dispersions of 0.96Å pixel-1 and 0.49Å pixel-1 and spectral resolutions of 2.0Å and 1.0Å, respectively, as measured from the widths of Cu-Ar arc lines. This dataset extended our wavelength coverage further into the red for our brightest objects, enabled higher spectral resolution observations of He I [FORMULA] 5876 and H [FORMULA] for programme Of/WN stars, and allowed observations of two previously unobserved stars (Brey 75, 81) to be made (see Table 4). These data were reduced in an identical manner to our AAT-RGO observations.

Further AAT high resolution observations of S119 and R99 were collected with the UCL echelle spectrograph (UCLES), 31.6 lines mm-1 grating, and blue Thomson CCD (1024 [FORMULA] 1024, 19µm pixels). A slit width of [FORMULA] yielded a spectral resolution in the extracted spectra of [FORMULA] 0.10 Å, as measured from the widths of Th-Ar arc lines. Two overlapping settings were required for complete spectral coverage between [FORMULA] 4070-5100. The CCD frames were first bias-subtracted, and the echelle orders were optimally extracted using the software package ECHOMOP (Mills & Webb 1994). Subsequent analysis was again performed using the DIPSO package.

In addition to the new observations detailed above, we have made use of the atlas of low resolution flux calibrated WR spectra of Torres-Dodgen & Massey (1988), of which eight are common to the programme stars.

2.2. Ultraviolet observations

We have obtained short wavelength (SWP) high resolution (HIRES) IUE observations of Brey 13, 24, and 26 at the ESA tracking station at Vilspa, Madrid during 1992-1993 (see Appendix: Table 5) while archive observations of two further programme stars (Brey 89, and 90) were obtained from the World Data Centre at the Rutherford Appleton Laboratory and reduced using IUEDR (Giddings et al. 1995).

All programme stars have previously been observed by IUE at low resolution (LORES) between 1978-1993, with the exception of AB18, Brey 75 and Brey 81. These observations were generally accessed via the Uniform Low-Dispersion Archive (ULDA; Talavera 1988) and provided useful interstellar reddening constraints.

We have also obtained Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observations of R99, BE294, S119, and S61 from the HST data archive (see Appendix: Table 5), additionally including the WN9h star Sk- [FORMULA]  249c (=HDE 269927c) from Paper I These observations were carried out in 1993 by Dr C. Leitherer (see Table 5) using both the G130H ([FORMULA] 1153-1606), and G190H ([FORMULA] 1573-2330) gratings.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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