The modern self-consistent photoionization model of planetary nebula luminescence is described. All of the processes which play an important role in the ionization and thermal equilibrium of the nebular gas are taken into consideration. The diffuse ionizing radiation is taken into account completely. The construction of the model is carried out for the radial distribution of gas density in the nebular envelope which is consistent with isophotal map of the nebula. The application of the model is illustrated on the example of the planetary nebulae BD+30°3639 and NGC 7293. For each nebula, the intensities of the emission lines of ten basic chemical elements in the UV, optical and IR spectral ranges are calculated and matched with observational data. Both the chemical composition of the nebular gas and the continuum of the central star at the wavelengths ≤912Å are determined during the process of model calculation. It is shown that the continuum of the central star at ≤912Å does not correspond to the blackbody spectrum but agrees with the spectrum of the corresponding non-LTE model atmosphere. The radial distributions of electron density, electron temperature and other parameters in the nebular envelopes are found. The optical thickness of the nebulae in the Lyman continuum is derived.

For planetary nebulae, empirical abundances can be obtained from the observed emission-lines as long as the electron density, the electron temperature, and the ionization corrections factor are determined. However, due to temperature fluctuations in the emitting gas, the evaluation of the temperature from the observational data is strongly dependent on the method used. The temperature fluctuation is usually characterized by the mean square temperature fluctuation, t 2 (Peimbert and Costero, 1969 — PC).

We show that for the study of emission from weakly ionized low temperature hydrogen plasmas, the processes A + + A → A + + A + ħω and A + + A → A + 2 + ħω (where A denotes a neutral atom in ground state and, A + and A + 2 atomic and molecular ions) must be treaded as a source of continual electro–magnetic radiation from low temperature plasma. Both reaction channels are treated separately and the corresponding total and separate spectral intensities are determined for hydrogen plasma at T < 10000K. The obtained results have been also compared with the corresponding spectral intensities for electron–ion bremsstrahlung and electron–ion photorecombination.

Deep, medium resolution, long-slit spectrophotometric data have been taken for a number of high-excitation planetary nebulae, covering a wavelength range from 3100 Å to 7200 Å with some selected regions observed at higher resolution. For about half the objects, the whole optical region has been observed, from 3100 Å to 11600 Å. Accurate flux calibration is achieved over this whole wavelength range. These data allow a detailed quantitative study of the Bowen fluorescence mechanism and the charge transfer reaction in planetary nebulae, the primary goal of this program. In this paper measurements of O2+ Bowen fluorescence and charge transfer lines are presented. We show that LS coupling fails for the O2+ permitted transitions studied in this program and intermediate coupling may be a better assumption. Efficiencies of the Bowen fluorescence mechanism are derived for 15 objects, and a wide range of possible values is apparent. There is a remarkable linear positive correlation between the Bowen efficiency and the fractional abundance of oxygen in the ionization stage of O2+ and the fractional abundance of helium in the form of He+. Evidence that the Bowen efficiency is anticorrelated with the electron temperature, as first noted by Likkel and Aller, is established. The Bowen efficiency drops substantially when the nebular expansion velocity 2Vexp (O2+) > 55 km/sec. For lower expansion velocities there is no detectable correlation between these two quantities. There are no observable differences in Bowen efficiency among objects of different morphological type nor between objects excited by stars of different spectral types as suggested by Likkel and Aller.

New and published line intensities are used to test the role of the pumping of the Bowen Lines of O III by absorption of the O1 and O3 lines and population by Charge Exchange. Although general agreement is obtained, noticeable deviations from theory exist.

The dielectronic recombination (DR) is of importance at ‘low’ (nebular) temperature. This process leads to cooling the electron gas in nebulae. The cooling rate by recombination of ion X+n is

The Hamilton Echelle Spectrograph at the coudé focus of the Shane 3 m telescope at Lick Observatory permits us to obtain high resolution spectra of bright planetary nebulae over the spectral range from 3500á to 10300á. Not only is it possible to separate pairs such as λ 5198, 5200 [NI], 4860 HeII and 4861 HI, but one may secure the profiles of Doppler broadened lines. It appears to be possible to separate the HI and HeI contributions of the 3889 line in Hu 1–2. The nebulae most suitable for observation are those of high surface brightness. NGC 7027 was the first nebula intensively studied with this equipment (Keyes et al. 1990); subsequently we have observed NGC 2440, 6537, 6543, 6567, 6572, 6741, 7009, 7662, IC 351, 418, 2149, 2165, 4634, 4997, 5217, Hb 12, and Hu 1–2. In NGC 7009 we measured line fluxes at the ends of major and minor axes. Small compact objects were centered on the slit; other were observed in the bright ring. These planetary nebulae cover a range in excitation level, chemical composition, and evolutionary status.

We present new results from a program of emission-line imaging spectrophotometry of planetary nebulae using the Ohio State University Imaging Fabry-Perot Spectrograph (IFPS). High-quality emission-line maps of the important diagnostic lines [NII]λλ5755,6583, [SII]λλ6717,6731, [OI]λ6300, [OIII]λ5007, Hα, and Hβ have been obtained. Maps of the ionization structure ([S II]/Hα, [N II]/Hα, [O III]/Hβ, and [O I]/[O III]), temperature in the N+ region, density in the S+ region, and Balmer decrement across the nebula are presented. These show considerable variation in ionization state, temperature and density. This detailed information will provide powerful constraints on photoionization models for the Ring Nebula.

The sample of planetaries with the strong [NII] emission lines are analysed. All available parameters are discussed. The enrichments of all elements and the tendency to rather low surface brightness of ne bulae in Hβ flux are remarked. The deficiency of hydrogen in envelopes is proposed as the possible reason of observational spectra.

During a program of observations of planetary nebulae (PN) made at CTIO in 1982 using the 1.5 m telescope equipped with a Cassegrain spectrograph and a Vidicon detector, and repeated in 1991 using the same configuration with a CCD detector, we found four PN for which Hα < [N II] λ6583: He2-109, He2-145, He2-152, and He2-163. The analysis of the data was made using the computing facilities of Cerro Tololo Computing Center at La Serena, and of the Centro de Procesamiento Digital de Imágenes of the Departamento de Astronomía de la Universidad de Chile.

We present first results from the ROSAT All Sky Survey on X-ray emission of planetary nebulae (PNe). For the first time extended X-ray emission from PNe was detected. This is the case for NGC 6543, NGC 6853, A 12, NGC 4361 (and LoTr 5). X-ray emission compatible with a point source was detected from BD+30°3639, however, the spectral distribution of the X-ray photons is leading to temperatures beyond 2 106 K. Thus in all cases, with the possible exception of LoTr 5, the central star of the PNe can be excluded as the main source of the observed X-ray emission. X-ray images and ROSAT spectra for all detected PNe are presented. The best observed PN in X-ray emission is NGC 6543. Due to the close vicinity to the north ecliptic pole, this object was regularly observed, every 90 minutes during the whole half year of the ROSAT All Sky Survey, resulting in 41 ksec of integration time. In addition NGC 6543 was observed in a 50 ksec pointed observation to the north ecliptic pole, taken in June 1990 during the calibration phase (Kreysing et al. 1992). A comparison of the semi-ring-like distribution of the X-ray emission of NGC 6543 with optical CCD-images shows, that most of the X-ray emission seems to originate from the boundary region between the nebula and the halo. Neither the central star nor the hot wind from the central star wind is the main source of the X-ray emission, as proposed by the interacting stellar wind model (Kwok 1982). An alternative model employing a possible coronal heating mechanism has been discussed by Kreysing (1992); accoustic waves, travelling outward from the nebula, encounter a sudden density decline at the boundary to the halo. As a consequence the waves degenerate into shock waves, dissipating their energy in a thin region of only some 1015 cm into the ambient medium.

IUE spectra of the Ring Nebula (M57=NGC 6720) were taken 16–18 May 1991 using the large aperture (10 × 20 arc sec oval) at low dispersion. SWP and LWP spectra of seven locations were acquired at 10 arc sec intervals extending along a PA = 124° line and passing through the central star of the nebula. This direction was such that the long axis of the large aperture was aligned and overlapped, thus enabling the line profiles to be “spliced” together. This provided continuous spatial variation curves at approximately 1 arc sec spatial resolution over the entire diameter (80 arc sec) of the main body of the Ring Nebula.

Sh 2–71 appears to represent a diffuse, ellipsoidal nebulosity extending over a range Δα × Δδ = 1.7 = 3 arcmin2. Early spectroscopy by Glushkov et al (1975), and Chopinet and Lortet-Zuckerman (1976) suggested the presence of a high excitation central star, and Kaler (1983) has more recently determined Zanstra temperatures Tz (HeII) > 7.7 · 104 K. The observed central star clearly constitutes a much cooler companion, and luminosity variations in this source have been attributed to binary eclipse.

A photoionization model is presented for M4–18. The model includes the presence of dust mixed with gas. The model shows sulphur and nitrogen are underabundant (relative to solar) and partially succeeds in explaining the observed IR excess.

Imaging and long–slit spectroscopy of the nebula surrounding the WC11 star IRAS 17514–1555 are presented that allow, for the first time, the nebula to be studied without contamination from the underlying WC11 star. These data show that the nebula is photoionized and does not appear to have an obvious scattering component. The nebula consists of two shells: a low density outer region (diameter 18 arcsec) where ne ∼ 100cm−3 and an unresolved inner nebula (diameter < 1.5 arcsec) where ne > 104cm−3. The abundances in the nebula are similar to those found in Type I PN (He and N enriched). Simple models are constructed which demonstrate that the nebula is probably in equilibrium with the central star (T∗ ∼ 30,000 K). The nebular and stellar properties are compared to those of other late–type WC stars and the evolutionary status of IRAS 17514–1555 discussed.

The planetary nebula Hubble 5 shows a very striking bi-symmetry; a 180° rotation around its center would bring the object into fair coincidence. Similar to this nebula is NGC 6537 and, to a lesser degree, NGC 6302. Jet outflows are reported from this “post-main sequence” nebula over a range of 260 km s−1 (Phillips and Mampaso, 1988). Otherwise not much has appeared on this object in the literature.

Fg 1 (He 2–66) is a southern planetary nebula that presents an elliptical shape. Deep CCD imaging and long-slit spectroscopy have been obtained at Las Campanas Observatory of this object. The images were obtained in the light of Hα+[NII], [SII] λλ6716,6731, HeII λ4686 and in the broad-band R gunn filter. The spectra were obtained oriented N-S, P.A. 90°, and P.A. 85°. The instrumental combination yields a spectral resolution of ∼ 2 Å FWHM, covering a spectral range ≃ λλ 6290–6805 Å.

We present monochromatic false-color images of three planetary nebulae obtained by means of a CCD camera at the f/13.5 focus of the 0.91m reflector of the Cerro Telolo Interamerican Observatory in August, 1985. The resultant image scale is 0.363 arcsec square per pixel. The objects, IC 1297, NGC 7009, and M2-9 were imaged through narrow band filters in the V band, [O III] λ5007, and Hα λ6563, respectively. The V band filter corresponds to the standeard Johnson V magnitude. The λ5007 filter has a FWHM of 14 Å. The Hα filter has a FWHM of 73 Å, thus including contributions from the [N II] λλ6548, 6584 lines. IC 1297 shows a double shell structure with a pronounced bright rim at the extreme edge of the faint outer envelope, suggesting that it is density bounded. NGC 7009 (the “Saturn” nebula), in addition to the well-known double shell and extended W-W ansae, also shows a faint large, circular outer halo which strongly suggests that NGC 7009 is a member of the rare class of triple-shell nebulae. The bipolar object M2-9 (the “Butterfly” nebula) shows a bright, non-stellar central core from which the wings extend in a nearly N-S direction. Condensations in the wings are seen particularly well for the [O III] λ5007 images. The CCD images were obtained by B. Schaefer at CTIO and were processed at the Goddard Image Processing Facility.

Using Hα and Hβ images obtained at the CFHT, we derived the spatially varying reddening within NGC 6537 and NGC 7027. We then corrected the observed Hβ picture for dust extinction to obtain pictures more in accord with radio maps. The left panels below show NGC 6537; the right panels, NGC 7027. The top panels show the observed Hβ picture, and the bottom panels, the Hβ picture corrected for spatially varying extinction. Note that the bright optical knot in NGC 7027 corresponds to a hole in the extinction.

Images with narrow band filters centred at the most significant emission lines, such as Hβ, Hα, [SII] 6717 and [SII] 6731, were taken to explore the physical conditions of NGC 6302. Observations were secured on May 1991 with the 2.0m telescope of the Observatorio Astronómico Nacional at San Pedro Mártir, B.C., Mexico, the f/7.5 secondary, and a 384 × 576 CCD chip. The most significant results are:

1. 1. Excitation mechanism. The image of the Hα/[SII] lines ratio reveals that photoionization and shocks are equally important throughout most of the object. Photoionization is dominant in the central region and a few isolated patches. Elongated structures pointing towards the central region can be identified with a shock impinging the wall of a bipolar cavity.

2. 2. Extinction. The image of the Hα/Hβ lines ratio shows that extinction changes over all distance scales. C(Hα) is nearly equal to 1.7 in the central region. It falls off very rapidly away from it, and at a distance of 10 arcsec it is approximately equal to 1.3. In the mean, Hα/Hβ is 1.25 times larger in the western lobe than in the eastern.

3. 3. Excitation. Different lines ratios, such as [SII] 6724/[SIII] 9069, [OIII] 5007/[OI] 6300 and Hel 5876/Hβ, indicate that excitation is larger in the eastern lobe. For instance, Hel 5876/Hβ ≃ 0.15 in the eastern lobe and ≃ 0.13 in the western region.

4. 4. Electron density was calculated from the [SII] 6717 and [SII] 6731 images. The density structure is very complex. Within the central region the highest density is found at both sides of the dark lane. No clear correlation exists between density and shock excited regions. Wave-like patterns can be recognized in the density image. These can be attributed to instabilities, sporadic ejections events and/or multiple shocks.