2. Observational data and parameters
Table 1 lists the PNe with H-deficient central stars and the observational data we have used in the present study. Column (1) lists the PN G numbers from the Strasbourg-ESO catalogue (Acker et al. 1992). Usual names of the objects are given in Column (2). Column (3) shows the spectral classes of the central stars. The angular diameters of the nebulae (in arcsec) can be found in Column (4). The nebular densities (in cm-3) are given in Column (5). The observed fluxes (in erg cm-1 s-1) and the radio fluxes at 6 cm (in mJy) are listed in Columns (6) and (7), respectively. Column (8) gives the line ratio. The central star B and V magnitudes are given in Columns (9) and (10), respectively.
Table 1. Observational data for planetary nebulae with H-deficient central stars.
The [WC] classification primarily comes from Tylenda et al. (1993) and Acker et al. (1999) with preferences given to the latter paper as it was based on a newer and better quality observational material. The classification of M 1-60 (019.7-04.5) is according to Acker et al. (1996) while that of NGC 2452 (243.3-01.0) and NGC 2867 (278.1-05.9) is from Koesterke & Hamann (1997).
Crowther et al. (1998) have recently classified [WC] PNNi with a somewhat different classification scheme as in the above studies. However, for the sake of uniformity of our data set we have decided not to use the results from this paper. They are given only for a limited sample (20 objects) and the classification criteria of Crowther at al. cannot be applied to other objects without having access to direct spectroscopic data.
The angular diameters are primarily taken from the Strasbourg-ESO catalogue. In the case of small nebulae (angular diameter ) we adopt preferentially the dimensions from VLA measurements. For He 2-113 (321.0+03.9) and He 2-1333 (332.9-09.9) the diameters are from de Marco et al. (1997).
The nebular densities, , have been derived from the [S II ] line ratio and come from Stanghellini & Kaler (1989), Acker et al. (1989, 1991), Kingsburgh & Barlow (1994), Cuisinier (1994), Philips (1998) and from other sources in individual cases.
The fluxes are from the Strasbourg-ESO catalogue. The radio fluxes are taken from the same compilation and references as in Stasinska et al. (1992). The values of the ratio come from Tylenda et al. (1994). The central star magnitudes are from the Strasbourg-ESO catalogue with the exception of NGC 6765 (062.4+09.5) for which the V magnitude is from Napiwotzki & Schönberner (1995).
The interstellar extinction has been determined from the ratio of the radio to fluxes if good quality radio flux measurements are available. Otherwise the extinction has been derived from the ratio. In this case the resulting logarithmic extinction at has then been divided by 1.17 in order to correct for a systematic difference between the radio and optical extinctions as discussed in Stasinska et al. (1992).
It is well known that the observationally derived distances to the Galactic PNe are not reliable in most cases. This is the principal source of uncertainties while comparing theoretical models to the observations on the HR diagram or other diagrams involving distances. Therefore in this study instead of analysing luminosities, absolute magnitudes, nebular dimensions or expansion ages we investigate parameters which do not involve distances.
Thus we use derived from forbidden line ratios as an indicator of the nebular age. Another parameter which we use in our investigations is the surface brightness which is defined as
where is the nebular flux corrected for extinction and is the nebular angular radius. Similarly as , also measures the nebular age but it is derived from other independent observational measurements. The third parameter we are exploring is defined as
where is the stellar flux (corrected for extinction) in the V band. has been proposed in Górny et. al. (1997a) and is closely related to the parameter f introduced in Tylenda & Stasinska (1989). It combines information on the evolutionary advancement of the central star with the expansion stage of the nebula. The value of decreases by many orders of magnitude during the PN phase (much more than and ) and therefore it is a very useful parameter for distinguishing young, newly formed objects from evolved ones.
The values of and calculated for our sample of H-deficient PNNi are given in Table 1 in Columns (11) and (12), respectively. In both cases the units are erg cm-2 s-1 sr- 1.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 2000