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Astron. Astrophys. 350, 1007-1017 (1999)

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2. Observations of WR124 and its associated nebula, M1-67

We discuss the observed properties of WR124 and M1-67 in this section, and provide a summary in Table 1. Unlike most massive WR nebulae, which are difficult to identify, M1-67 was so striking that it was included in the Bertola (1964) Planetary Nebula catalogue (PK 50+3o 1) with WR124 as its central star. Cohen & Barlow (1975) instead proposed a massive WN origin, which was supported by Solf & Carsenty (1982) and Esteban et al. (1991), who found nebular abundances to be consistent with processed stellar ejecta, indicative of a massive central star. Confirmation of a non-PN origin was made by Crawford & Barlow (1991) who estimated a distance of 4-5 kpc from interstellar Na I D2 observations. (In order for M1-67 to be a PN, its distance should be [FORMULA]460 pc according to van der Hucht et al. 1985).


Table 1. Summary of basic stellar and nebular properties of WR124/M1-67.

2.1. WR124

WR124 (alias 209 BAC, He 2-427) is a Galactic, heavily reddened WN8h star that is well known as the ionizing star of M1-67, with a high heliocentric recession velocity of [FORMULA]200 km s-1 (Merrill 1936). Crowther et al. (1995b) presented optical spectroscopy of WR124, including comparisons with other late-type WN (WNL) stars.

For the current analysis, spectrophotometry obtained in 1991 September at the 2.5m Isaac Newton Telescope (INT), covering [FORMULA]4400-7300 (resolution 2-3 Å) are used, taken from Crowther et al. (1995b). Observations obtained at the Deutsch-Spanisches Astronomisches Zentrum (DSAZ) 2.2m observatory in June 1991 provide an additional blue dataset covering [FORMULA]3400-4400 ([FORMULA]2 Å resolution), details about which can be obtained from Hamann et al. (1995).

New near-IR spectroscopy of WR124 was obtained in 1998 July at the 3.8m U.K. Infrared Telescope (UKIRT) with the cooled grating spectrograph CGS4, the long (300mm) camera, a 256[FORMULA]256 InSb array and a 40 l/mm grating. First order observations provided data covering 1.66-2.30 µm (R[FORMULA]800) and 2.98-3.62 µm (R[FORMULA]1300) with a slit width of 0.6" (1 pixel). This data set was bias-corrected, flat-fielded, extracted and sky-subtracted using CGS4DR (Daly & Beard 1992). Subsequent reductions and analysis were carried out using FIGARO (Meyerdierks 1993) and DIPSO (Howarth et al. 1995). In order to remove atmospheric features, the observations were divided by a standard star (whose spectral features were artificially removed) observed at around the same time and similar air mass. In regions of low atmospheric transmission at UKIRT the reliability of line shape and strength must be treated with caution (e.g. Paschen [FORMULA]).

The reddening to M1-67 is extremely high, with previous estimates of [FORMULA] in the range 0.90 (Esteban et al. 1991) to 1.50 (Solf & Carsenty 1982). Analysis of the stellar spectrum by Crowther et al. (1995b) indicated [FORMULA]=1.18 mag, which implied a distance of [FORMULA]6kpc, assuming a typical [FORMULA]=-6.0 mag for WN8 stars. Here we attempt an improved stellar reddening determination by including International Ultraviolet Explorer (IUE) newly extracted spectra (INES, Rodriguez-Pascual et al. 1998) of low resolution (LORES) datasets. The quality of LORES INES datasets are preferred to NEWSIPS (Schartel & Skillen 1998). Four large aperture IUE datasets of WR124 were obtained between 1978-1986, including 3 long wavelength (LWR) datasets, totalling 270 minutes exposure time, plus 1 short wavelength (SWP) dataset of 212 minutes.

Pre-empting the results from the next section, we compare spectrophotometry with reddened, line-blanketed model atmosphere distributions obtained with the CMFGEN code (see Sect. 3.1) in Fig. 1, indicating [FORMULA][FORMULA]1.3 mag ([FORMULA]=4.0), 0.1-0.2 mag higher than previous results based solely on optical stellar datasets. A comparison with alternative ISA -wind (see Sect. 3.2) model fluxes indicates an equivalent reddening.

[FIGURE] Fig. 1. Comparison between the spectrophotometry of WR124 (open circles: IUE; solid line: INT) and theoretical CMFGEN models, reddened by [FORMULA]=1.0, 1.15, 1.3 and 1.45 mag (dotted lines) according to Cardelli et al. (1989). For clarity, comparisons are offset vertically by 0, +2, +4 and +6 units.

Our measurement is in reasonable agreement with the nebular study of M1-67 by Chu & Treffers (1981) who obtained [FORMULA]=1.23 from a comparison of its 1.4 GHz and H[FORMULA] flux. In contrast, Esteban et al. (1991) obtained an significantly lower value of [FORMULA]=0.90 mag from nebular Balmer line strengths.

Since the reddening towards WR124 is so high, we prefer to select the distance from the K-band absolute magnitude, [FORMULA], where the interstellar reddening is much lower. Using WN8-9 stars at known distance in our Galaxy or the Large Magellanic Cloud (LMC) as calibrators (see Morris et al. 1999 and references therein) we obtain [FORMULA]=-6.2[FORMULA]0.3 mag. Using the observed K-band magnitude of WR124 from van der Hucht et al. (1985) plus [FORMULA] = 0.12 [FORMULA] = 0.49 (Cardelli et al. 1989) we find a distance modulus of 13.44 (i.e. 4.9 kpc), in accord with Crawford & Barlow (1991). Consequently, we adopt a distance of 5 kpc to WR124, i.e. [FORMULA]=-6.25 mag. Note that the resulting V-band absolute magnitude of [FORMULA]=-6.1 mag is in excellent agreement with LMC WN8-9 stars (Crowther & Smith 1997).

2.2. M1-67

M1-67, discovered by Minkowski (1946), has a clumpy, ejecta-type, morphology (Chu 1981), with an angular diameter of 90-120 arcsec (Solf & Carsenty 1982; Grosdidier et al. 1998). High spatial resolution spectroscopy of M1-67 was obtained by Esteban et al. (1991), revealing nebular conditions of [FORMULA][FORMULA]103 and [FORMULA][FORMULA]6,200 K. Relative to normal H II regions, oxygen is deficient, with nitrogen enhanced, such that N/O ([FORMULA]3). Their results suggest that O has been processed into N mainly through the ON cycle, and confirm that the M1-67 nebula is composed of stellar ejectra. Esteban et al. estimated an upper limit of 0.2 for the ratio between the ISM mass swept-up by M1-67 and the nebular mass, suggesting that relatively little mixing with the local interstellar gas has occurred.

Fabry-Perot interferometry (Chu & Treffers 1981) and long-slit spectroscopy (Sirianni et al. 1998) indicate a complicated velocity field, with two different motions: a spherical, hollow shell, expanding at 46 km s-1, plus a `bipolar' outflow with a larger velocity of 88 km s-1. Sirianni et al. (1998) interpreted this structure as the result of two subsequent outbursts. Grosdidier et al. (1998) used the very high spatial quality of the Wide Field and Planetary Camera 2 (WFPC2) aboard HST to reveal filamentary, chaotic substructures, with a number of `bullets' in the nebula, which seem to concentrate within two conical regions in agreement with the previous results of Sirianni et al. (1998). Globally, Grosdidier et al. (1998) found that the de-projected radial density distribution of M1-67 is well described by a power law, [FORMULA], with a cutoff at [FORMULA]50".

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© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999