The ultraviolet emission lines that were me asured in the spectrum obtained from our rocket observation of NGC 7662 have been compared with those predicted by a set of models. The models allow a determination of the carbon abundance which is found to be solar and the C/0 ratio which is equal to unity. In order to obtain a good fit, dielectronic recombination and charge exchange between neutral hydrogen and C IV were included, both of which increase the concentration of C III. The observed continuum consists of light from the central star and from the nebula, which contributed primarily via the two photon process in the H° and the Balmer continuum.

The detailed spectral study in the region λλ6,000–11,000 of eight planetary nebulae: NGC 6210, 6572, 6891, 7662, IC 2149, 4593, 4997 and BD+30° 3639 was carried out at the Crimean Station of Sternberg Astronomical Institute, using the 125-cm parabolic reflector with the contact image tube.

The new results have been obtained for 15 planetary nebulae, using the new spectrograms obtained at the Abastumany Observatory in 1971. The planetaries belong to the large Galactic centre group, studied by the authors earlier.

We obtained spectra of twelve faint planetary nebulae at the Cassegrain focus of the 193 cm telescope at Haute Provence Observatory (dispersion: 230 Åmm−1 spectral range 8000–11000 Å, receiver: ITT image tube with SI photocathode).

We have observed BD +30 3639, IC 418, NGC 6572, NGC 6543, NGC 3918, NGC 2440, NGC 3242, A30, Ml-78, and NGC 6210 in two broad passbands with effective wavelengths of 35μand 75μ. All were detected at 35μ, and all but IC 4997 at 75μ. Their spectral energy distributions are similar in that most have peak flux densities at wavelengths ranging from 30μto 45μ. The two nebulae with markedly different energy distributions are IC 4997, which has a hot spectrum peaking near 20μ, and Ml-78, whose flux density rises rapidly from 35μto 75μ, characteristic of compact H II regions.

NGC 7027 was observed on two nights, 1976 May 18–19 and 20–21, using the 36″ telescope of the KAO. A medium resolution, two channel, Helium-cooled grating spectrometer was used to observe the bands, 16-23μ (Δλ = 0.5) and 20-38μ (Δλ = 1.2). The beam size was 30″. A nearly flat smooth continuum over the entire band was observed with a peak intensity of 7 ×10−16 W/cm2μ between 20 and 25μ.

Recent observations of the 18.7 ym fine-structure line of S++ in NGC 7027 and BD+30°3639 (Greenberg, Dyal and Geballe, 1977 Ap.J.(Letters), 213, L74) allow the first determination of an ionic column density in ionized nebulae. The line ratios 18.7 μm/A9532 and λ6312/λ9532, besides yielding both electron density and temperature in the S++ region, have been used to indicate that the fine-structure levels of S++ are collisionally saturated. In this case the 18.7 ym surface brightness directly measures the column density of S++ ions with little dependence upon nebular structure, the major uncertainty being the experimental error. This research has been partially supported by NASA Grants NGR 05-003-511 and NGL 05-003-272.

A new spectrometer has been built for use in the 10μm atmospheric window which is substantially more sensitive than previous instruments for observations of 10]im emission and absorption features narrower than about 0.25 cm−1. Measurements have been made of fine structure emission lines in planetary nebulae as well as in galactic and extragalactic H II regions. The spectrometer consists of a liquid Helium cooled grating with a resolution of 1–4 cm−1 and a liquid Nitrogen cooled Fabry-Perot spectrometer which is scanned across the grating bandpass, resulting in a resolution of 0.1 − 0.2 cm−1. The system NEP is about , resulting in a minimum detectable line flux of about 10−19 w/cm2, with a 6″ aperture on the Lick 3m telescope.

The H110α radio recombination line has been observed toward the planetary nebulae NGC 7027, IC 418, and NGC 6543 in order to ascertain the physical characteristics of the bulk nebular gas. The observations of NGC 7027 confirm the earlier findings of Chaisson and Malkan (Ap.J., 210, 108, 1976) and Churchwell, Terzian and Walmsley (A&A, 48, 331, 1976) who reported evidence for a substantial increase in linewidth with principal quantum number. Attributed to electron-ion impact broadening (Stark Effect), the observations imply an electron density Ne ≃ 50,000/cm3. The LTE-derived electron-ion temperature Te ≃ 18,000°K agrees reasonably well with most radio-line analyses, as well as with previous analyses of the radio continuum, of forbidden optical line ratios, and of optical recombination lines and their associated continuum. IC418's HllOa line is also wider than radio lines observed at higher frequencies, suggesting a Stark Effect consistent with Ne < 20,000/cm3; NGC 6543 shows no appreciable line broadening, providing an upper limit to the density Ne < 10,000/cm3. The LTE-derived Te values for IC 418 and NGC 6543 are approximately 14,000 and 7000°K, reasonably consistent with those found by other techniques. On the basis of this and other recent studies, I suggest that the bulkemission in the Hnα recombination lines observed to date, 77 < n < 111, can be explained by a simple model of optically thin planetary nebular gas largely homogeneous in temperature and in density, and only slightly removed from LTE.

The forbidden lines in planetary nebulae are due to transitions between energy levels belonging to configurations ns2npq with q = 1 to 5, and with n = 2 ([O II], [O III], …) or n = 3 ([S II], [S III], …). For electron impact excitation of these levels, the expansion

Excitation, ionization and recombination processes are discussed elsewhere in this volume by Seaton (1977). I will restrict my attention to those processes involving heavy particles that may modify the ionization structure of planetary nebulae and that may affect the molecular composition of condensations and of the transition regions between the ionized nebulae and the surrounding interstellar gas.

Our knowledge of the ionization structure of gaseous nebulae may be dated from the classic paper of Strömgren (1939), who showed that the transition between ionized and neutral hydrogen is sharp. In fact the very sharpness of the transition region has at last become a difficulty. Another milestone was the paper of Hummer and Seaton (1964) which clarified the structure of nebulae containing helium as well as hydrogen. Then, just a decade ago, the first models of planetary nebulae were constructed which detailed the changes in the temperature and in the ionization of the heavy elements as the stellar radiation was progressively diminished by absorption in the nebular gas (Goodson, 1967 Harrington, 1968; Williams, 1968; Flower, 1969a). These first models were of a general nature, but soon models were constructed to match specific nebulae (Flower, 1968, 1969b; Harrington, 1969; Kirkpatrick, 1970).

We have calculated electron impact excitation cross sections between most of the low metastable levels of the 3d6 configuration in Fe III and of the 3d3 configuration in Fe VI have also been computed. The statistical equilibrium equations have been solved for a range of electron temperatures and electron densities, and the resulting level populations used to calculate emissivities for the forbidden lines of Fe III and Fe VI. Comparisons with observed line intensities and abundance determinations in planetary nebulae are in progress. (Paper submitted to The Astrophysical Journal.)

The transfer equation for the interlocked Hell Lya/OIII 2p2P2 − 2p 3d P2° lines is solved for spherical symmetry and for comparison in plane parallel approximation in the framework of the discrete space theory of radiative transfer. We used Harrington's ionization model (1969) without velocity fields and applied the partial redistribution function RIIA for both lines.

From existing models of planetary nebulae an important discrepancy comes out with the low ionization potential ions, as N+ and 0+, whose calculated line intensities are smaller than the observed ones. In this work the effect of charge transfer reactions between single, double, and triple ionized oxygen and nitrogen with neutral hydrogen is taken into account in a model of NGC 7662. The ionization structure of oxygen and nitrogen becomes strongly modified. The computed fractional abundances of 0+ and N+ turn out substantially larger than in previous models, thus helping to reconcile the computed line emissions from low ionization potential ions with the observed ones. (Paper will appear in Astronomy and Astrophysics.)

It is demonstrated that ionization structure of NGC 7027 cannot be explained by static models based on the standard photoionization theory unless some complementary physical process is included. The main discrepancies are successfully eliminated by including charge transfer reactions with hydrogen for the following ions: C+, C+3, N+2, O+2, Ne+2, S+ and S+3, the rate coefficients being probably of the order of or larger than 10−9cm+3s−1 except for C+ and Ne+2; in addition the charge exchange rate for N+ is probably ∼200 times smaller than the value proposed by Steigman et al. (1971).

Isophotal contour maps pröduced from monochromatic electronographic exposures of the low excitation planetary nebula IC 418 are interpreted in terms of a three-dimensional non-spherical gas distribution.

The linear response of the electronographic process provides a simple method for producing relative intensity ratios of emission lines across a complete nebula. Using narrow-band filters to isolate those lines which are sensitive to electron temperature and density changes, it is possible to map the gross variation of these quantities across the object.

A method of determination of the electron temperature in the He++ regions in planetary nebulae from observed intensities of [NeV] λ3426 and [NeIV] λ4714–25 relative to HeII λ4686 is presented. It has been used for 22 planetary nebulae for which the observational data have been taken from Kaler's Catalogue (Ap. J. Suppl., 31, 517). The electron temperatures in the He++ regions range from 15,000°K to 30,000°K for most of the studied planetaries and are usually significantly higher than the temperatures determined from [01II] lines which refer to the He++ regions. A correlation between the electron temperatures and effective temperatures of central stars has been found; the planetaries with hotter nuclei have slightly higher electron temperatures in the He++ regions. Finally, intensities of ultraviolet lines of [NeV] λ2975 and [NeIV] λ2424 have been predicted for the studied nebulae.

In addition to numerous emission lines arising from the ionization of chemical elements, well-exposed nebular spectrograms reveal a Balmer continuum in emission hitherto attributed to the second level radiative recombination of the hydrogen atoms and to the two-photon contribution. The very intense lines with regard to the adjacent continuous spectrum have been easily measured by many workers for identification, Balmer decrements, line ratios … whereas few data are available involving continuum measurements. As well as the mean slope of the continuum, the Balmer discontinuity is very sensitive to the physical conditions prevailing and the presence of dust in the nebulae.