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Astron. Astrophys. 351, 1036-1040 (1999)

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3. Results and discussion

3.1. Morphology and detailed description

The morphology of our object is extremely unique. Our H[FORMULA] and [N II] narrow-band image (Fig. 3) shows a variety of structures not clearly apparent in the POSS I image (Hartl & Weinberger 1987). The deep H[FORMULA] + [N II] image presented in the IAC catalogue (Manchado et al. 1996) shows most of these features but it is not as deep as our image. They classified the object as a bipolar PN. Clearly from the morphological point of view, the object is not bipolar. This is mainly because the central star (CS) is roughly in the geometrical center of the sphere and outside the main emission nebula.

One can see from the narrow band image (Fig. 3) a number of "disturbed" morphologies, characterized by (1) A bright emission region (94" x 82") with two bright blobs. This part itself resembles a bipolar PN. (2) The central star seems to be a high gravity star farther south. (3) To the north, there is a faint and extended diffuse emission. (4) Many filamentary structures. Two of these filamentary structures appear to protrude from the main nebula north and northeast. In the east, several filaments can be described as explosion-like events from the eastern part of the main nebula. Among these several filaments there are two of them separated from the other emission, faint, sharp, and straight filaments. The emission of filament 1 (see Fig. 1, right) is clearly detected in the H[FORMULA], H[FORMULA], and [N II] lines. (5) No emission is seen south of the CS.

[FIGURE] Fig. 1. The two contour maps are presented to clarify the bright (left) and faint (right) structures of PN G218.9-10.7. North is up and east is left. The straight line displays the slit position of the spectrum of Fig. 2. The central star is marked by arrow 4, and arrows 1, 2 and 3 mark three filaments which are detectable in the spectrum of the object.

[FIGURE] Fig. 2. Low-dispersion spectra of PN G218.9-10.7, calibrated flux (10-14erg cm- 2 s-1 Å-1) [FORMULA] wavelength in Å. Left is the visible spectrum in the blue band, while right is the red band.

[FIGURE] Fig. 3. H[FORMULA] + [N II] narrow band image of PN G218.9-10.7 1. North is left, and east is down. Note the one-sidedness, with no detectable emission on the other side of the CS (see the black arrow), the sharp separated filaments to the east, the two jet-like features north and northwest of the main emission, and the smoothly filamented structure of the faint northern emission. The different colours represent different intensities. The highest intensity is at the two blobs in the central part of the main nebula.

Why there is no emission detectable to the south?, no clear sharp edge for the PN as is normally seen?, a faint emission to the north?, and many filaments, some completely separated, with no clear orientation to the nebula, some even seeming to end in stars?. All of these questions prompted us to ask for the real nature of the object.

In Fig. 1 (left), the possible central star is located in the lower middle, south of the central emission. There is no other star that might be considered as a CS candidate. The two brightest blobs of the PN have about equal distance from the CS. They may be interpreted as part of a knotty ringlike structure around the CS. The inner (southern) part of the lowest isophotes is oval but centered approximately on a faint star southeast of the CS. This star is not seen on POSS I. The outer parts of these isophotes look very irregular but would be consistent with a near-half circle around the CS. Fig. 1 (left) also shows the 2 brightest filaments, north-east and north of the brightest PN emission. Both are quite narrow on the direct image (Fig. 3). Both point to stars. The north-east one does not end at the star but passes it with decreased intensity. The north one seems to end at the star. A third filament pointing on and ending abruptly at a star appears in Fig. 1 (right) lower left. We mention this because one would not expect to find such a chance of positional coincidences so often. Is one of the stars physically connected with the object?

Fig. 1 (right) shows details of the low-intensity emission. More filaments are obvious but only Fig. 3 reveals that nearly all of the faint emissions are filamentary structured. The faint emission shows little symmetry towards the CS. There is just an indication of very faint emission around the CS but this might as well be an extension of the faint emission south of the brightest part, which reaches approximately to the CS. In the east and north, the faint emission reaches at least 5 times the distance from the CS to the brightest blobs of the PN. The faint emission in the north can be alternatively described by a rectangle.

3.2. The nature of the object

The filamentary structure is particularly obvious in the cases of those PNe which interact with the interstellar medium (ISM), e.g., DHW5, MWP1, IW2, S176 and S188 which have one-sided filamentary structure (Tweedy & Kwitter, 1996). Tweedy et al. (1995) claim that the ISM magnetic field enhanced the pressure and shape the filaments to acquire parallel morphology. NGC 6894 which interacts with magnetized ISM (see Soker & Zucker 1997) is a good example for comparison with our object. It has two straight and parallel stripes on one side (like the two separate eastern filaments, Fig. 1 (right)) which are parallel to the galactic plane. NGC 6894 is at a distance of 0.07 kpc below the galactic plane, where the magnetic field is supposed to be parallel. In our case, the eastern two filaments are inclined to the galactic plane by 18o (roughly parallel to the galactic plane). In order to check the direction of the magnetic field in the vicinity of our object one needs to know its height (Z) from the galactic plane. There is no published distance except the rough estimated value 1.48 kpc by Hartl & Weinberger (1987). From this value one can derive [FORMULA] kpc, which should be within the galactic disc. Therefore, the eastern two straight filaments can be explained possibly as the effect of the interstellar magnetic field.

The faint and diffuse emission to the north is comparable to S68 which have a faint emission to the north and east of the PN (Tweedy & Kwitter, 1996).

The absence of emission opposite to the brightest part is comparable to A35 (Jacoby, 1981) and S188 (Manchado et al. 1996). The deep H[FORMULA] images of A35 and S188 show, however, extended faint emission opposite to the brightest parts (Tweedy & Kwitter, 1996). These authors gave, for S188, a simple explanation for the faint material and filamentary structure. The faint emission is just the northwest side of the nebula that has diffused away. This may imply a highly inhomogeneous ISM in the vicinity of S188. This idea of inhomogenity in the ISM could be accepted as an explanation for at least some features shown in our object.

The PN He-2-428 shows similarity with our object in the sense, that the main body of the nebula is sickle-like and that there is some filamentary emission beyond the main body. There are, however, two significant differences. First, in the case of He 2-428 there is the CS located in the main body of the nebula at a location where you would expect it; this is not at all our case. Second, the filamentary emission in the case of He 2-428 is located in the inner side of the sickle-shaped main body of the nebula, where one would expect, but the situation is reversed in our case. Also, there are no significant straight filaments to be seen in He 2-428.

Recently Soker (1997) listed our object as a PN interacting with ISM, where the PN direction is south relative to the ISM. He also mentioned that it is hard to tell from the image whether the ISM really influences the PN morphology or not.

A comparison with other PNe mentioned above suggests some interpretations (these are, however, very preliminary and would need further observations for confirmation): (1) The dark southern part is due to extinction by a cold dark cloud rather than due to missing emission. This interpretation would explain why the number density of stars is less here than in the north (POSS I plate). As well as the presence of weak radio emission (VLA NVSS 1.4 Ghz) 1 from the central region of the main nebula and extending to the north and to the south. (2) The faint northern emission comes from radiation that has penetrated the optically thin PN and has ionized the ISM. Consequently, part of the object could be an HII region surrounding the PN. (3) The eastern filaments stem from an interaction of PN material with the ISM, where magnetic fields may play a role (see, Heiles 1987, Soker & Zucker 1997, and Soker & Dgani 1997).

3.3. PN or H II region?

The object (Fig. 3) neither look like a PN, nor like an H II region. In the log(H[FORMULA]/[S II]) vs log(H[FORMULA]/[N II]) diagram of Garcia-Lario et al. (1991), the object falls into the overlap region of PNe and H II regions. The low [O III] emission as well as the low electron density are in favour of an H II region. On the other hand, we find only one reasonably blue star in the vicinity of the object (as checked from POSS I), namely the possible central star south of the object. Mendez (1991) classified this star as a high gravity star but still has some doubt concerning the CS classification. He suggested that a higher-quality spectrum would be needed to settle the question (private communication).

3.4. Spectrum and plasma properties

In Table 1, we compare the observed flux ratios relative to H[FORMULA] with those mentioned in the Strasbourg-ESO Catalogue of galactic PNe. He I [FORMULA] 5876 Å has been observed, while it is not mentioned in the catalogue. The ratio of the two [S II] lines is almost the same in both cases. Our values are generally higher than the ones published in the catalogue except the [O III] [FORMULA] 5007 Å. This may be mainly due to different orientation of the slit across this extended object as well as in the catalogue the slit width was [FORMULA] while we used [FORMULA] slit.


[TABLE]

Table 1. Comparison of the flux ratios to H[FORMULA] between the Strasbourg-ESO catalogue of galactic PNe and our values for the object


In Fig. 2, low dispersion spectra are shown for the blue (Fig. 2, right) and red (Fig. 2, left) spectral part of the object. The spectrum exhibits Balmer lines of hydrogen and lines of ionized oxygen, nitrogen, sulfur, neon, and helium. The emission lines are listed in Table 2. The [O III] emission is smaller than the typical observed emission in the advanced PNe. This confirms the very faint [O III] narrow band image given by Manchado et al. (1996). The [N II]/H[FORMULA] ratio in the bright part of the nebula is about 0.9 but it is only about 0.4 in the two filaments 1 and 2 east of the main part of the nebula. The emission of the two filaments is below the detected limit except in H[FORMULA], H[FORMULA], and [N II]. The line fluxes were corrected for interstellar extinction using the extinction law from Savage & Mathis (1979). The extinction coefficient was calculated using the Balmer ratio H[FORMULA]/H[FORMULA] under case B, with the value 0.6. It compares well with the value 0.6 derived by Tylenda et al. (1992).


[TABLE]

Table 2. Identified emission lines and their fluxes relative to H[FORMULA]. The estimated errors in these units are between 2 and 3. The line of H[FORMULA] 3965 is merged with He I 3970.07 Å and [Ne III] 3967.46 Å.


After correction for extinction, the excitation class of the object (EC = 1) was calculated according to Dopita & Meatheringham (1990). The absence of Helium emission line He II 4686 Å in addition to the presence of neutral atoms [O I] 6300, 6364 Å, and [N I] 5198 Å confirm that the object is in very low excited class. The electron temperature (Te [FORMULA] 9000 K) and electron density (Ne [FORMULA] 225 cm-3) are determined from the [N II] [FORMULA] and [S II] [FORMULA] line ratios, respectively. The estimation of the electron temperature depends on the intensity value of [N II] 5755 Å which is uncertain. The ratio of the [S II] doublet yields a quite low value of electron density (Ne [FORMULA] 250 cm-3), clearly showing that the object is in an advanced stage of evolution.

Taking into consideration that the maximum estimated error of line fluxes is 3[FORMULA] of the H[FORMULA] line flux, then the maximum line intensity of HeI 5876 is 13[FORMULA] of the H[FORMULA]. By using the values of HeI line flux and the upper limit of electron temperature (13500 K), we estimate a maximum He abundance of 0.102. Since the He aboundance is less than 0.125, we could roughly state that our object is consistent with type II PNe according to Peimbert & Torres- Peimbert (1983).

To sum up, the object is unique in its morphology where many peculiar structures are noticed. We have given a preliminary interpretation for its complex morphology. The electron temperature, electron density, and excitation class are determined. For a future work, we recommend deep images with different narrow-band filters and high resolution spectra with different slit positions, to give us more clear picture on the physics and the nature of the object.

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© European Southern Observatory (ESO) 1999

Online publication: November 16, 1999
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