A number of objects which may be proto-planetary nebulae are being studied. They were found through a radio search of early-type emission-line stars, and the radio emission is believed to originate in extensive circumstellar ionised gas. The objects have an infrared spectrum which indicates the presence of dust, and a continuum radio spectrum characteristic of prolonged mass outflow from the central star. Of the five objects discussed here, Vy2-2, Hbl2, HD167362, Hl-36 and V1016 Cygni, a distance is available for only the last: for that object the calculated mass of the ionized gas is 0.02 M0 (see Ahem et al, 1977). If this is typical of the five, then they may represent the early stages of the low-mass planetary nebulae discussed by Wood and Cahn (1977). References: (1) Ahern, F.A., Fitzgerald, P.M., Marsh, K.A., and Purton, C.R. 1977, Astron, & Astrophys., in press. (2) Wood, P.R., and Cahn, J.H. 1977, Astrophys. J. 211, 499.

We present a model of the emission object V1016 Cygni consistent with the current optical, infrared and radio observations. V1016 Cygni is 4.2 ± 1.0 kpc from the sun. It was originally an M star which ejected ∼0.017 M0 at 35 kms"1 over a period of ∼600 years producing a neutral nebula with an r"2 density distribution. This process ceased in 1961.3 ±3.0 when the nebula detached from a remnant hot core (T> 80,000K, R< 1.5 Rg). By 1964 0.1 ± 0.3 the nebula was ionized by the UV radiation from the core, producing the observed emission line spectrum. The infrared energy excess comes from two dust components of M000°K and ∼250°K, which absorb UV photons from the star. The dust is concentrated into clumps which 1) provide high density areas on their inner side, allowing strong [OHI] X4363 to remain; 2) shield the regions on their outer side permitting low excitation lines to be present; 3) explain the observed emission line structure. A stellar wind of 105 km/s helps to keep the centre of the ejected nebula hollow. We suggest V1016 Cygni is an example of a low mass planetary nebula in the formation stage.

From the list of stellar emission-line objects which exhibit radio spectra characteristic of uniform radial mass outflow, we have selected those objects which might be considered possible embryonic planetary nebulae. Our criterion was to choose those mass outflow objects with exciting stars of the types'characteristic of central stars of non-stellar (i.e., normal) planetary nebulae. These objects are Hb 12, Hen 1044, HD 167362, and possible M2-9, Vy 2-2, and Hl-23.

Radio recombination lines of hydrogen (H167ot) have been observed from 45 HII regions using the Arecibo telescope. Nine of these sources also show the C167a line, and twenty show H2103 lines. Some of the carbon lines, particularly those from W48 and G62.2 + 0.5, show a significant negative displacement from the H167a velocities. This can be explained if the lines originate in the ionization front or in neutral material swept up by the ionization front. The displacement is 3.8 km/sec for W48, and 5.7 km/sec for G62.2 + 0.5. The source G37.7 + 0.1 ex-hibited two H167a lines, one 25 km/sec wide at 87.3 km/sec and one 16 km/sec wide at 48 km/sec, velocities with respect to the local standard of rest. This may be a case of two HII regions in the same line of sight. Parameters for the HII regions, such as electron temperatures and densities, are derived. (Paper to appear in the Astronomical Journal.)

Observations of the integrated continuum flux density of several stellar planetary nebulae have been made at 90 GHz. These are reported, with some remarks on their significance for objects with positive radio-frequency spectral index, and to display graphically their relation to published microwave and near-infrared data. The 90-GHz data have been obtained with the 11-m dish of the National Radio Astronomy Observatory* on Kitt Peak, under the Lockheed Independent Research Program. Three of the objects have been included among a dozen stellar planetary nebulae observed with the Fabry-Perrot interferometer at the Cerro Tololo Inter-American Observatoryt so as to provide kinematical information with the microwave spectral data, since they are related in the theories of the positive spectral index that appeared in 1975. The Fabry-Perot observations are reported in Astrophys. J. 1977 September 15. No other journal article is yet in press.

M2-9 is not a bona fide planetary, nor simply a star: it most probably is a planetary nebula in the formative stages. High densities, the presence of [01], Fell and [Fell] emission lines, and prominent infrared continua are characteristic of such objects. There is a striking progression of their spectra from B[e] stars such as HD45677 to M2-9 itself; Merrill's class of objects intermediate between Be stars and planetary nebulae is represented e.g. by MWC 645, HD 51585, V 1016 Cygni, HM Sagittae, and MIVC 349. Remarkable similarities exist between the spectra of the latter objects and that of M2-9, especially in the recently covered region between 8000 and 11000 Å which is rich in emission lines of the Paschen series, of Hel, 01, [SIII], Fell and [Fell] (Andrillat and Swings, 1977, in preparation).

The requirements of the interstellar medium for ionization, heating and “stirring” are reviewed. The role of planetary nebulae is compared with that of supernova remnants and of OB-stars.

The chemical evolution of galaxies is controlled by star formation, stellar mass loss, gas flows, and the compositions of matter involved in these processes. Planetary nebulae (PN) thus affect chemical evolution directly in representing mass ejection at the end of the nuclear burning lives of many stars, and in enriching the interstellar medium in elements that are overabundant in the nebulae. Supernovae often eclipse PN in discussions of these effects, so it is noteworthy that planetary precursor stars not only provide much of the present stellar mass loss in the solar neighborhood, but also show as clear evidence as supernova remnants for the ejection of elements newly synthesized in the stars themselves. Furthermore, PN are tracers of the past star formation history in a galaxy, since their rate of occurrence depends on the birthrate of progenitor stars during the whole lifetime of the system.

The present-day birth rate of planetary nebulae, 5·10−12 PN/pc3yr according to Cahn and Wyatt (1976), seems somewhat high compared to white dwarfs, for which a redetermination, including the statistics of Sion and Liebert (1977), yields 2·10−12 WD/pc3yr to within a factor of two. However, an increase of the distance scale for PN by a factor of 1.3 compared to Seaton (1968) - necessary in order to shift the extremum of PN radial velocities to the distance of the galactic center (9 kpc), and to increase the luminosities of the central stars from the position given by Pottasch et al. (1977) to a minimum value compatible with evolutionary constraints (Weidemann, 1977a) - brings birth rates of PN and WD into almost complete agreements.

Six high-excitation PN are found to have gas-phase iron abundances less than solar by M.2 dex, and carbon abundances larger than solar by M3.8 dex. The objects have solar abundances of oxygen, and this supports the idea that the missing iron has condensed into grains. Efficient condensation of iron, and of all carbon not in CO molecules, is expected before the shell becomes ionized. The presence of some gas-phase iron therefore requires partial destruction of the grains.

108–76°1 was discovered by Boeshaar and Bond (1977, Ap. J., 213, 421) to be a member of the galactic halo component. Herein are presente the results of a computer model analysis of these observations and those of Hawley and Miller (this volume). Models producing the 23% of helium in the doubly-ionized state required by the Hell X4686 line strength utilized the 95,000°K, non-gray, spherical atmosphere model of Cassi-nelli (1971, Ap. J., 165, 265) for the central star. Even 100,000°K plane-parallel atmospheres gave less than half the needed He++. The results for model 1 (homogeneous sphere) and model 2 (Gaussian shell) show good agreement with observed fluxes, even for [OH] X3727. The abundance values

We have studied the central regions of the planetary nebulae A 30 and A 78 by UBVRI photometry, optical spectroscopy, and near-infrared photometry. The spectra contain high-excitation emission lines and strongly resemble those of Wolt-Rayet stars of the carbon sequence. We infer stellar temperatures > 50,000°K. The observed 3.5-ym flux of each nebula exceeds reasonable extrapolations of both the stellar flux and any possible free-free emission. The colour temperature of this excess between 2.28 and 3.5 /im is ∼ 1000°K. For each nebula, the aperture dependence of the excess emission suggests an extended (y10 arc-sec radius) region centred on the nucleus. Thermal radiation from a distribution of dust that is concentrated near the nuclei seems the most plausible explanation for the excess, but no theory of dust formation or heating seems totally adequate at present. (Paper will appear in the Monthly Notices of the Royal Astronomical Society.)

Some of the results in the spectroscopic observation of HBV475 are presented.

The data available on permitted and forbidden line intensities of the planetary nebula NGC 7027 have been analyzed in terms of non-constant density models. The significance of the derived density distribution is discussed. Some implications to the determination of the chemical abundances are also discussed.

As I am sure you realize I cannot really give you conclusions, I can only give you my own impressions of this conference; George Field will give you his impressions, and you must draw your own conclusions from all that you have heard here.

Dr. Osterbrock has given a comprehensive summary of the Symposium. I will discuss a few topics which seemed particularly interesting to me as a non-specialist: dust, the fate of pre-planetary ejecta, and inhomogeneities. Let me begin by describing a simple model of the planetary nebula (PN) phenomenon, which is suggested by the discussions we have heard at this Symposium, and which provides the framework for my other comments.

Woltjer: Dr. Field raised the issue of the discrepancy between some estimates of the amount of graphite. I wonder if any of the authors have a comment on this point?