Astron. Astrophys. 344, 143-150 (1999)

2. The sample and the measurements

The sample contains all known nebulae associated with galactic LBVs and LBV candidates (Nota & Clampin 1997, Hutsemékers 1994, 1997). All these nebulae are recognized to be dusty stellar ejecta. Only two LBVs, HD168607 and HD160529, are not surrounded by nebulae although imaged in similar conditions. To this sample we add the four WR ring nebulae known to be mainly constituted of matter ejected by the star: M1-67, RCW58, NGC6888, and S308 (Smith 1995). Other WR ring nebulae are essentially constituted of interstellar gas and are not considered here. The only LBV nebula for which no preferential axis can be defined is that around P Cygni. Interestingly, it has also several peculiarities which distinguish it from other LBV nebulae (Nota et al. 1995, Hutsemékers 1997). Finally, the total sample amounts to twelve galactic nebulae. All objects are at low galactic latitudes.

While there are as many LBVs in the LMC as in the Galaxy, the consequence of the larger distance to the LMC is that only two nebulae (those around R127 and S119) are resolved with enough detail to reveal their morphology and the association with the central star (e.g. Nota & Clampin 1997). Among WR ring nebula resolved in the LMC, only that one surrounding Br13 is presently confirmed as an ejecta (Garnett & Chu 1994). In addition, we also consider the famous three-ring nebula associated with SN1987A (Burrows et al. 1995). Although its exact nature is still unclear and not strictly related to LBV or WR ring nebulae, it is nevertheless constituted of material ejected by the massive supernova progenitor (Panagia et al. 1996). Excited by the supernova ultraviolet flash, this nebula may therefore represent an advanced, short-lived, stage of nebulae ejected by massive stars.

The adopted nebular position angles refer to the projected morphological long axes of the nebulae whatever their type, i.e. true bipolar or elliptical. As suggested by Nota et al. (1995), both morphologies may result from a unique shaping mechanism as in the case of planetary nebulae. Bipolar nebulae (e.g. HD168625) often have a "waist" i.e. a brighter ring ¹ perpendicular to the long (bipolar) axis. For elliptical nebulae, this waist manifests itself as emission enhancement along the minor axis (e.g. AG Car). However, there may be some ambiguity about the exact morphological type. Indeed, some elliptical nebulae could in fact be bipolar objects seen along the bipolar axis, or even the brighter waists of bipolar nebulae with very faint and undetected lobes. In the latter case the measured position angle will be perpendicular to the true long axis, while in the former one, morphological details will affect the measurements. Ideally, one should have used only fully developed bipolar nebulae to detect at best a possible alignment effect. But, in our case, this reduces the sample too drastically for it to remain statistically useful. We therefore consider all the nebulae of the sample at the risk of masking a possible deviation from a random orientation of axes.

The nebular morphological long axes are determined from narrow-band visible images (mostly H). For the two highly reddened objects G25.5+0.2 and G79.29+0.46, optical images are not available and radio maps are used. In addition to values explicitly provided in the literature, the nebular position angles are measured on published images, as well as on several H+[NII ] images that we obtained ourselves with the ESO 3.6m telescope equipped with EFOSC1 in its coronographic mode (cf. Hutsemékers et al. 1994 for technical information). The measurements for individual objects are detailed in Appendix A. The uncertainties of position angles are estimated from the dispersion of measurements obtained after several trials and combined with measurements given in the literature if any. They are typically around 5^o. It should be noted that even in situations where the ellipticity of the nebula is small, or when the overall nebular morphology is dubious, one may define a preferential axis with a reasonable accuracy. This is the case of the nebulae S308 and G79.29+0.46 which are nearly circular, and for WRA751 and M1-67 which cannot be unambiguously classified as bipolar or elliptical, but which are clearly elongated. Possible errors due to a mis-interpretation of the observed nebular morphological features can however not be excluded. It is therefore important to emphasize that possible errors on position angles or morphological types, systematic or not, can only act to reduce the deviation from a uniform distribution and certainly not to produce coherent orientations. Also, possible biases in the data (lack of independence) are minimized. Indeed, the position angles are measured in the equatorial coordinate system before being transformed into position angles in the galactic coordinate system in which the alignments are finally searched for.

The resulting position angles are given in Tables 1 and 2, together with the object distance d, the galactic longitude and galactic latitude , or the right ascension and declination . and respectively refer to the position angles expressed in the equatorial and galactic coordinate systems. In agreement with the usual convention, the equatorial position angle (= P.A. in Appendix A) refers to the direction of the nebular axis projected onto the plane of the sky, and measured from the north ( = 0^o) towards the east ( = 90^o). The galactic position angle similarly refers to the direction of the projected nebular axis but now measured from the galactic north ( = 0^o) towards the galactic east ( = 90^o). Both position angles are defined from to 180^o. They are related by

where and are the galactic coordinates of the equatorial north pole. represents the uncertainty of the position angle estimated in Appendix A. For the galactic objects, is the acute angle between the nebular long axis projected onto the plane of the sky and the galactic plane (all objects roughly lie in the galactic plane). For LMC objects, represents the difference between the nebular position angle and a local average of the magnetic field direction (see Sect. 3.3).

Table 1. The sample of galactic nebulae

Table 2. The sample of LMC nebulae

© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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