2 The observed sample

PNe are dynamical systems existence of which requires the synchronization of the evolution between the central star and the nebula. During the lifetime of a PN, the central star is interacting both radiatively and dynamically with the nebula (Kwok 1994). An evolved (old) PN is usually defined by a high dynamical age of the nebula ($t_{\rm dyn}=R/V\gt 10^4$ yr, where R and V are respectively the radius and expansion velocity of the nebula). Since $t_{\rm dyn}$ is proportional to distance and reliable distances are often difficult to determine, old PNe can also be identified as objects with low surface brightness ($I=F/\pi \theta^2$, where F is the total flux from the nebula and $\theta$ is the angular radius). Since both F and $\theta$ have the same dependence on distances ($\propto r^{-2}$), the surface brightness criterion is therefore distance independent. Since circumstellar dust also disperses as the nebula evolves, evolved PNe often show less extinction and less infrared excess (Zhang & Kwok 1993). These distance-independent parameters are also often used to distinguish young and old PNe. A quantitative measure of the infrared excess is the infrared excess index (IRE), defined as

$$\begin{eqnarray} {\rm IRE} &=& \frac{L_{\rm IR}}{L({\rm Ly}_\alpha)} \\ &=& A... ...\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1})}{F_{\rm 5GHz}({\rm mJy})}\end{eqnarray}$$ (1) (2)

where $F_{\rm IR}$ is the total amount of flux emitted in the infrared and the coefficient A has the value of 1.0 and 1.5 at high and low densities respectively (e.g. Pottasch 1984; Zijlstra's thesis 1989). Young PNe usually have IRE values larger than 1, whereas old PNe have IRE values much less than 1. Although central stars of evolved PNe are expected to have high temperatures and low luminosity, these parameters are not necessarily related to the nebular age. Thus, a central star of high mass will evolve very fast ($t \propto M_*^{-10}$), and will be on the cooling track even though the nebula is relatively young.

The PNe presented in this study are all selected from the Abell's (1966) catalogue. The only observations that have been reported were made with wide bandpass filters (Manchado et al. 1996). Since H$\alpha$ and [N II] respond differently to nebular conditions, narrow-band observations separating these two emission lines will provide useful information on the nebular structure. The only object which was previously imaged through narrowband interference filters is A 13 (Rosado & Moreno 1991), but these observations were made with 103 aG films coupled with an image-tube. In this paper, we report new observations through narrow-band filters and using a fast and high quality CCD detector. The coordinates and observed fluxes (when available in literature) are given in Table 1. Most of the line flux entries are taken from Cahn et al. (1992) and the Strasbourg-ESO Catalogue (Acker et al. 1992) as well. Strikingly, from this Table 1 and except for A 13, the observed PNe have the H$\alpha$/H$\beta$ intensity ratios rather smaller than the theoretical value of 2.87 in the recombination case B (e.g. Pottasch 1984), in contradiction with the optical extinction values quoted in Table 2 (Col. 5, and taken from Cahn et al. 1992). For instance, considering A 28 and A 30 which apparently show no extinction ($c_{\alpha}=0.0$), one should have obtained the predicted ratio (287) instead of 163 and 200 respectively. These cases need to be checked by further accurate spectrophotometric measurements. Furthermore, while most of the sample (except A 28 and A 30) have 5 GHz radio fluxes measured, only two (A 30 and A 36) are bright enough in the far infrared to have IRAS measurements. In the case of A 30, most of the infrared emission comes from the inner part and not from the shell. If we except A 13 characterized by a rather strong extinction ($c_{\rm H\beta}{\rm (radio)}=1.47$), all objects would satisfy the above criteria as being old PNe.

  

Table 1: List of observed PNe

(a) The H$\alpha$ and H$\beta$ intensities are from Kaler (1983). (b) From Kaler et al. (1990). Values quoted in Acker et al. (1992). Note the surprisingly "small'' values (except for A 13) of the Balmer decrement compared with the recombination case B prediction of 2.87!