6 Discussion

Several new emission line structures, as described above, have been discovered to the north-east of the supernova remnant CTB 80. Even though their average absolute fluxes span a wide range of values, their sulfur line emission is strong relative to H$\alpha$, as would be expected for emission from a supernova remnant.

6.1 Area I

The two filamentary arcs to the north of LBN 156 are very faint but clearly visible, especially in the [O II] image (Fig. 3). The angular extent of these filaments covers roughly more than a quarter of a circle's circumference. In addition, the faint filamentary emission detected at position Ia (Fig. 4) seems to be related to the long arcs and is probably projected at the specific location. The [O III] image also shows weak emission in areas I and Ia, while the absolute amount of flux in this line is comparable to the H$\beta$ flux suggesting the presence of complete recombination zones behind the shock front (Raymond et al. 1988). The strong [O II] emission relative to the weaker [O III] emission indicates a shock velocity in the range of 90-120 km s^-1 and/or a partially neutral preshock medium (Cox & Rayond 1985).

Deep long-slit spectra taken at area I provide us with more information about the physical properties at the specified location. The total sulfur line flux amounts to 90% of the H$\alpha$ flux identifying it as emission from shock heated gas. The [O III] flux is only $\sim$2 times higher than the H$\beta$ flux, while the latter is a factor of $\sim$10 weaker than the H$\alpha$ flux. This implies a significant attenuation of the optical emission due to interstellar absorption. Adopting the interstellar reddening curve of Whitford (1958) as presented by Kaler (1976), we find a logarithmic interstellar extinction, $c=1.6\pm0.3$ towards area I. The computerized model of Hakkila et al. (1997) on the visual interstellar extinction allows us to obtain a very rough estimate on the distance to area I. Use of this code shows that distances greater than 2 kpc are compatible with the measured extinction of 1.6 and the following results will be scaled to this distance.

The observed angular radius of 42$^\prime$ is equivalent to 24.2  $D_{\rm 2\,kpc}$ pc, while a typical projected FWHM of the filaments in the north-west is 0.15  $D_{\rm 2\,kpc}$ pc. (Where $D_{\rm 2\,kpc}$ is the distance to the filaments in units of 2 kpc.) The observed sulfur line ratio approaches the low density limit but given the statistical uncertainties we estimate that the actual electron density is less than $\sim$160 cm^-3 at a 2$\sigma$ confidence.

An estimate of the preshock cloud density can be made through the use of the equation given by Fesen & Kirshner (1980) which relates the electron density, the preshock cloud density and the shock velocity. Given the above upper limit on the electron density and the range of shock velocities, we expect preshock cloud densities less than $\sim$4 cm^-3. For this density of 4 cm^-3, we find that the energy of the explosion E should lie in the range of 0.8- $1.6 \times 10^{51}$  $D^{3}_{\rm 2\,kpc}$ erg (Hailey & Craig 1994). Since the derived preshock cloud density is only an upper limit, the energy E should be less than $1.6 \times 10^{51}$  $D^{3}_{\rm 2\,kpc}$ erg.

The radio emission found, though weak and certainly in need of confirmation, appears to support our suggestion that observed optical emission originates from a SNR. There is little evidence for fine-scale structure. Indeed, the 92 cm maps show no evidence for emission features as narrow as the optical filaments, as is seen in some SNRs.

The nondetection of X-ray emission is not too surprising, as soft X-rays can be readily absorbed by the ISM, and the emission might be intrinsically weak. As for the "hole'' in the infrared background, while it might be fortuitous, similar features have been noted in association with other SNRs. For example, Braun & Strom (1986) find a cavity associated with the Cygnus Loop.

6.2 Area II

In the southern areas of our field we have discovered a long, relatively thin structure which is characterized by very strong oxygen emission both in the low and medium ionization images (Figs. 2 and 3, Table 4). The sulfur to H$\alpha$ line ratio is 0.6, a value which is not far away from the limiting value of 0.4-0.5 used for distinguishing between H II regions and shock heated gas (Fesen et al. 1985). In favor of the latter case is the fact that the spectra taken to the south and north of the [O III] filament, i.e. into the diffuse emission, are characterized by a [S II]/H$\alpha$ ratio of $\sim$0.4.

In the case where the observed emission originates indeed from shock heated gas, we estimate a shock velocity around 100 km s^-1 and an electron density close to the low density limit. Note that a spectrum from a planar shock propagating at $\sim$110 km s^-1 with equilibrium ionization matches, acceptably well, the observations (Hartigan et al. 1987). The authors have also constructed shock models with complete preionization. The model that most closely approximates the observations corresponds to a shock velocity of $\sim$80 km s^-1, and probably less, even though the calculated sulfur line flux is less than that observed. The presence of radio emission along the [O III] filament may suggest their physical association but it is only the determination of its non-thermal nature that would firmly establish this proposition.

6.3 Area III

The new peculiar source of emission exhibits the highest surface brightness among the rest of the new structures in the field, with very strong sulfur line emission (Table 4). Spectra extracted from different apertures along the slit indicate a [S II]/H$\alpha$ ratio of 0.8-1.1, characteristic of emission from shock heated gas. The difference seen in the low and medium ionization images is also evident in the results from the long-slit spectra reported in Table 4, where substantial variations of the [O III] flux are observed. In addition, the calibrated images suggest that the [O II] flux is much stronger than the [O III] flux in area IIIs than in area IIIn. These observational constraints cannot be met by the equilibrium ionization models of Hartigan et al. (1987). Better agreement is achieved by the complete preionization models.

The 60 km s^-1 model predicts an [O III] flux comparable to the H$\beta$ flux, while the [O II] flux is several times stronger than the [O III] flux. This is similar to what we observe in area IIIs. However, the spectrum from area IIIn exhibits an [O III] flux which is $\sim$6 times stronger than the H$\beta$ flux, while at the same location the [O II] flux is only 2 times stronger than the [O III] flux. The 80 km s^-1 complete preionization model of Hartigan et al. (1987) shows similar characteristics. We note here that the model calculations were performed for a preshock density of 100 cm^-3, while our long-slit spectra and the estimates of the shock velocity suggest that the preshock cloud density should not exceed a few nuclei per cm^-3. Calculations focusing on the specific problem under study would probably be more favorable since we are in the range of velocities that dramatically affect the [O III] flux.

Figure 4: The radio emission from Area I at 4850 MHz is shown in grey scale. The contours represent the 11 cm emission detected in the Effelsberg survey (Reich et al. 1990) and the contour levels are set at 8, 16 and 32 mJy beam-1.

The strong oxygen emission from feature III reminds one of the oxygen-rich filaments which have been observed in a small number of other remnants. Although usually associated with young objects (Van den Bergh 1988), one of the protrusions seen in the $\sim$$10\,000$ yr old Vela supernova remnant (Aschenbach et al. 1995; referred to as D) is also oxygen rich. Indeed the wedge shape of III (Fig. 3), and its apparent location near the rim of the candidate remnant, are also features which it shares with D.

6.4 Area IV

The fourth new structure detected in this part of the sky has a semi-circular shape and is a weak source of forbidden line radiation. The morphology of this source is mainly diffuse and patchy but yet, the flux emitted in the sulfur lines is stronger than that emitted in the H$\alpha$ line. Spectra extracted from different apertures along the slit provide evidence for emission from shock heated gas. The interstellar extinction is not accurately determined due to the low confidence of the H$\beta$ flux. In favor of a supernova remnant association of the optical emission from this area is the radio emission seen in Fig. 4, where its morphology seems to match that of the optical. However, it is clear that the non-thermal nature of the radio emission must be established before one can accept such an association.