4 Morphology of the nebulae

4.1 A 13. PN G 204.02-08.52

Figure 2 shows the H$\alpha$ image of A 13. It shows an elliptical (or "skull-like'' $105\hbox{$^{\prime\prime}$}\times 135\hbox{$^{\prime\prime}$}$) ring structure similar to that of NGC 6720 (Hua 1997; Guerrero et al. 1997). Our angular resolution (28.65 arcsec mm^-1) is much higher than that (49 arcsec mm^-1) of Rosado & Moreno (1991), allowing us to better outline the inner "hole'' of this new ring-nebula. The high ratio of the surface brightness between the ring and the "hole'' suggests that this is not a projected spherical shell, but rather a bipolar nebula viewed pole-on, the line of sight being slightly off-axis, since the ionizing star is not exactly centred. The bright ring-structure is probably the high density equatorial waist of the bipolar nebula. As in NGC 6720, the nitrogen (inner) structure is slightly ($155\hbox{$^{\prime\prime}$}\times114\hbox{$^{\prime\prime}$}$) larger than the hydrogen distribution. The outer [N II] emission is definitely much more extended than that in H$\alpha$, along with diffuse extensions over $\sim 12\hbox{$^\prime$}$ (Fig. 3), which displays a PA almost perpendicular to that of the bright ring. This result is quite in very good agreement with Rosado & Moreno [N II] image. Due to their use of a larger bandpass filter including H$\alpha$ and [N II] lines, Manchado et al. (1996) were not able to outline these above features. Our observations instead enable us to discriminate specific contribution from each ion: e.g. Fig. 4 shows the [N II]/H$\alpha$ intensity ratio obtained by superimposing the two monochromatic images. This ratio varies from 0.4 to 2.5 across the nebula (cross-section in Fig. 6), reflecting the changing ionization/excitation conditions of the nebula. Nitrogen is much more enhanced along the S-W part of the elliptical ring. [O III] is rather faint (probably absent) since it is hardly detected against the sky background, even after 1 hour exposure. This result confirms the [O III] image of Manchado et al. The structure outlined in the intensity ratio map suggests that the apparent morphology of A 13 results from the projection on the plane of sky of a "wool-ball''-shape nebula, alike Sh 1-89 (see Hua et al. 1998), the bright elliptical waist being the equatorial area, with the fuzzy surrounding features.

4.2 A 21. PN G 205.14 +14.25

The filamentary appearance of A 21 has led to this object having been considered as a supernova remnant, H II region, Wolf-Rayet nebula, as well as PN (Arkhipova & Lozinskaia 1978). A 21 has a PG 1159 central star and is classified by Napiwotzki & Schönberner (1995) as spectral type hgO(C) in the scheme of Méndez et al. (1986) and DOZ in the white dwarf classification scheme of McCook & Sion (1987). Since PG 1159 stars are believed to be transition objects between central stars of PNe and white dwarfs, there is little doubt that this object is an old PN. Spectroscopic observations of A 21 by Kwitter et al. (1983) show high N/O and He/H ratios, which is consistent of its being a Type I PN. The actual dimensions of this PN are much larger than our field of view ($11\hbox{$^\prime$}7$). According to Manchado et al. (1996, images given on p. 122), these could be even twice as large.

The H$\alpha$ image, shown in Fig. 8, displays a network of filamentary structures in the S-E direction with fainter extensions completing a nearly spherical shell in the N-W. These filamentary structures are even more intense in the [N II] image (Fig. 9). The sizes of the nebula as measured from these two images are approximately the same, with a diameter of $\sim 550\hbox{$^{\prime\prime}$}$ The [N II]/H$\alpha$ intensity ratio (Fig. 10) strongly varies (0.6-1.2) from point to point with its highest value in the N-W area. As a matter of fact, the two components are not distributed evenly throughout the nebula, reflecting differences in physical conditions, since the two emission lines depend upon different mechanisms. The complete ring (with the symmetry axis in the NW-SE direction) can clearly be seen, with a morphology bearing some resemblance to the double-ring structures seen in some PNe (e.g. MyCn 18, Sahai et al. 1998) as the result of the interacting winds process.

4.3 A 24. PN G 217.18+14.76

The H$\alpha$ (Fig. 13) and [N II] images (Fig. 14) of A 24 show two strong elongated E-W lobes with a fragment of (W-side) arc which could be part of a helical structure. The H$\alpha$ diffuse underlying emission has an irregular contour, along with an absorbing patch below the E lobe. In contrast, the [N II] image, noticeably larger, displays a double shell with remarkable radial structures outwards. These jet-like structures are similar to those observed in the [N II] image of NGC 6543 (HST archives, program 5403, P. Harrington, P.I.). The patchy lane in the north shows up even more sharply. The two bright lobes could be the "waist'' of a bipolar PN, with the diffuse underlying emission being the projection of two bubbles on top and below the waist. This structure is clearly evident in the [N II]/H$\alpha$ intensity ratio image (Fig. 15). The jet-like radial structures, not apparent in wide bandpass images of Manchado et al. (1996, p. 125), are clearly seen in the NE and SW directions in (Fig. 14).

4.4 A 28. PN G 158.81+37.19

A 28 has a nearly circular shape of size $320\hbox{$^{\prime\prime}$}$ in the H$\alpha$ image (Fig. 18) with the (rather faint) surface brightness distribution consistent of its being a sphere. In addition, a fragment of outer envelope (or shell) can be seen beyond the limb-brightened rim at the SW side extending to $\sim 360\hbox{$^{\prime\prime}$}$. The [N II] image (Fig. 19) is obviously smaller. However, nitrogen would suggest a more conspicuous bilobal structure (alike A 33 below but with much fainter brightness) than in the H$\alpha$ image owing to a sort of void around the central star, which is not the case in H$\alpha$. Figure 20 shows the [O III] image of A 28. [O III] is only weakly detected in this PN (undetected in Manchado et al. 1996, p. 109).

4.5 A 30. PN G 208.56+33.29

A 30 consists of a normal outer envelope and a hydrogen deficient core (Jacoby & Ford 1983). We have imaged A 30 in H$\alpha$, [N II], and [O III] and found the PN to have very different structures in the three images. The almost perfectly circular shell is most prominent in the H$\alpha$ image (Fig. 22). We note the existence of four fragments of enhanced intensity along the outer edge of the shell. The same four enhancements along the rim can also be seen in the [O III] image (Fig. 24). The surface brightness distribution of the circular shell is consistent of it being a sphere. A series of "spider'' features (first detected by Jacoby (1979) but shown in better details here) appear to whirl around the central knots. The inner core ($15^{\prime\prime} \times 18^{\prime\prime}$ of A 30 has been imaged by the HST in [O III] and found to have spoke-like structures (Borkowski et al. 1995). The "spider'' features lie outside of the spokes found in the HST image. The high velocity ($\sim 200$ km s^-1) outflows detected by Meaburn & López (1997 could arise from these "spider'' features. Most interestingly, the shell is not detected in the [N II] image, suggesting that the shell may be nitrogen poor. Figure 25 shows the inner regions of A 30 in H$\alpha$, [NII] and [O III]. Two bright knots (in the NW and SE directions) close to the central star can be seen in all three images. Another knot (in the SW direction) is progressively stronger from H$\alpha$, [N II] to [O III].

4.6 A 33. PN G 238.03+34.87

The signal-to-noise ratio of the present observations is much higher than in a previous report (Hua & Nguyên-Trong 1983). The central star is classified as O(H) (low gravity and hydrogen rich) by Méndez (1992). By fitting the absorption line profiles by NLTE models, Méndez et al. (1985) derive an effective temperature of $100 \pm 30\ 10^3$ K and a surface gravity of $\log~g=6.0 \pm 0.5$. This is another example of perfectly spherical structure PN with the H$\alpha$ (Fig. 27) and [O III] images (Fig. 28) showing similar structures. The intensity distribution is consistent with a 3-D sphere, not a ring as in A 13. The inner region is rather non-uniform, giving an "owl''-shape. [N II] was not detected.

4.7 A 36. PN G 318.47+41.50

The present observations are of much better quality than in a previous report based on data obtained with a RCA image tube coupled with baked IIaO plates (Hua & Nguyên-Trong 1983). A 36 has a bright (m_V=11.5) central star and faint nebulosity. The central star is classified as O(H) by Méndez (1992). By fitting the absorption line profiles by NLTE models, Méndez et al. (1981) derive an effective temperature of $65~\pm~10 \ 10^3$ K and a surface gravity of $\log~g=5.2~\pm~0.5$. Comparison with the Schönberner H-burning tracks yields a central star mass of 0.55 $M_\odot$. The helical shape with a central elliptical ring is seen in both H$\alpha$ and [O III] images, [N II] was not detected. Its morphology resembles that of NGC 6543 (Balick & Preston 1987), showing two pairs of bipolar lobes. Features such as "rims'' and "caps'' can be seen in A 36 as in NGC 6543, using the terminology of Balick et al. (1994). The "caps'' probably represent the region where the collimated outflows interact with the remnants of the circumstellar envelope of its asymptotic giant branch progenitor. The ROSAT X-ray spectrum of A 36 shows a two-component structure (Leahy et al. 1996). The high-energy (corresponding to a temperature of $7 \ 10^6$ K) component could be the result of this interaction. It would be interesting to take an image with an even larger field of view to see if the halo is present. Since A 36 is $\sim 15$ times larger than NGC 6543 in angular size, it must be much closer than NGC 6543 if the two objects are intrinsically similar. Given its galactic latitude of $41^\circ$, A 36 is probably no more than a couple hundred parsecs away.