I outline recent advances in numerical simulations of gaseous nebulae. These fall into three major areas; the Opacity Project and its extensions, the role of grains within the ionized gas, and the effects of mechanical heat on the nebula. These advances, together with improvements in stellar atmosphere calculations, should lead to a new generation of more realistic simulations of conditions in planetary nebulae and predictions of their emitted spectra.

The shapes of planetary nebulae (PNs) provide paleontological clues about the origin and evolution of the gas expelled in the late phases of stellar evolution. The morphological classes of planetaries and various structural components of the nebulae are interpreted as hydrodynamic interactions of episodes of relatively brief, axisymmetric wind driven mass-loss events. Theoretical studies of the past five years are compared with extant data to show that astrophysical hydro models are achieving a very high level of success as explanations for the shapes of most PNs.

The most successful models are those for which the star is assumed to expel much or most of its mass in an equatorial wasteband. In stark contrast are dense ansae, dense and low-ionization knots of fast-moving gas, for which the mass distribution is decidedly polar. The origins of both the equatorial wastebands and polar knots remain decidedly enigmatic.

Planetary nebulae (PNe) and ring nebulae around massive stars are not just superficially similar in morphologies. For massive stars that evolve through red supergiant phase, the final fast wind would sweep up the slow red supergiant wind and form a bubble of stellar material, reminiscing the two-wind formation of a PN. Sometimes it can be really difficult to determine whether the central star of a ring nebula is a PN nucleus or a Pop I massive star. Parallel studies of PNe and ring nebulae around massive stars can greatly benefit each other.

This paper summarizes recent developments in the study of planetary nebulae using observations of molecular lines and the 21 cm line of H I. The observations reveal that many planetary nebulae are surrounded by envelopes of neutral gas, whose mass often exceeds that of the ionized nebulae. They also provide valuable information on the physical and chemical properties of the envelopes, their structure, and kinematics. The neutral envelopes firmly link the formation of planetary nebulae with the mass loss by AGB stars, and can play an important role in the subsequent evolution of the nebulae.

FUV photons (<13.6eV) from the central star create a region of warm (≈1000K) atomic and molecular gas around Planetary Nebulae (PN). This paper reviews theoretical and observational characteristics of such regions, commonly called photodissociation regions or PDRs. PDRs around PN differ in some aspects from those in other galactic objects and this is briefly discussed with an emphasis on time dependent effects. It is concluded that, in evolved PN (texp>103 yr), molecules will only survive inside dense clumps (>106 cm−3). H2 emission from such dense gas will show a thermal spectrum in the low v states. Finally, the physical conditions in the PDR associated with NGC 7027 are compared to those in other galactic and extragalactic PDRs

Developments in the field since the review of Roche (1989) are discussed. Section 1 describes the results from detailed models of the dust emission from planetary nebulae and summarises the derived dust-to-gas mass ratios. The properties of the remarkable gas-poor planetary nebula IRAS 18333–2357, located in the globular cluster M22, are discussed in Section 2. The use of thermal infrared imaging to map the distribution of different dust components is reviewed in Section 3, and two recently applied optical probes of nebular dust properties are described: emission line spectropolarimetry, and spectroscopy of the Extended Red Emission (ERE). The final section describes recent results from infrared spectroscopy. The spectra of carbon-rich post-AGB objects have provided valuable new insights into the nature and evolution of the materials responsible for the UIR bands, while unusual oxide grains and crystalline ice emission have been detected around oxygen-rich post-AGB objects. The spectra of a small number of objects show evidence for the simultaneous presence of both oxygen-rich and carbon-rich material.

The distances are determined for 142 Galactic planetary nebulae, using two methods. Method A is based on the stellar mass and surface gravity, while Method B makes use of the stellar luminosity. These properties of the central star are inferred from the modeling of the distance-independent parameters (Zhang and Kwok 1992). The results from the two methods used in this paper are consistent with each other. A distance of 10.8 kpc is found for K648 using Method A of this work. This is in excellent agreement with the distance of 10 kpc of its hosting globular cluster M15. The distances obtained in this work are in good agreement with the distances of Mendez et al. (1992), based on the recently developed non-LTE model atmospheres including spherical extension and stellar winds. The agreement of our distances with the individually determined distances of Gathier et al. (1986a, 1986b), using the extinction-distance method and the HI 21 cm absorption method, is worse than that found between ours and Mendez et al.'s. The distances obtained by Gathier et al. are likely often to be underestimated. The various statistical distance scales, except for Cudworth's (1974) distance scale, show no agreement with the distances determined in this work. CYZ was supported by the NASA grant NAG 2–67.

Determination of the positions of central stars of planetary nebulae in the HR-diagram requires the knowledge of nebular distances. For almost all nebulae, these can only be given in terms of statistical scales. These scales have in common that they assume all nebulae to have the same structure (e.g. constant density) and that a unique ionized mass-radius relation exists. If the mass-radius relation is given by Mion = M 0 · (R/R 0)η, the distance d(pc) of planetary nebulae can be expressed as a function the de-reddened Hβ-flux (erg cm−2s−1) and the angular radius θ(arcsec): M0 and R0 are in solar masses and pc (Te = 10000 K, He/H = 0.1). The parameter η characterizes the distance scale: e.g. Shklovsky (1956) η = 0, Maciel L. Pottasch (1980) η = 1, Pottasch (1984) η = 3/2, Daub (1982) η = 5/3, and Kwok (1985) η = 9/4.

Parallaxes are presented for nine Planetary Nebulae central stars.

The VLA observations presented by Pottasch et al. (1988, A&A 205, 248) and Ratag et al. (1990, A&A 233, 181) demonstrate that IRE (Pottasch et al. 1984, A&A 138, 10) correlates very well with F(6 cm) for the Galactic center PN, especially when IRE is large The correlation (see figure) can be approximated by:

We have compared the extinction of planetary nebulae (PN) as derived from the Balmer decrement and from the ratio of radio to Hβ fluxes.

Photoelectric UBV photometry of stars was carried out in the angular vicinity (radius = 15′) of 3 planetary nebulae. An extinction distance relation was constructed for the line of sight towards each planetary nebula. This relation was verified by published data on nearby stars and star clusters. The distances derived are 850 ± 300 pc for PK 128 −04 1 (S 188, Simeis 22), 400 ± 200 pc for PK 149 −09 1 (HW 4), and (less reliable) 800 pc ± 400 pc for PK 224 +01 1 (We 1–6). For the central star of PK 224 +01 1 our observations gave V = 15.76 ± 0.03 mag, (B–V) = −0.08 ± 0.03 mag and (U–B) = −0.87 ± 0.03 mag.

Shklovsky estimated distances to planetary nebulae (PNs) based on an assumed constant ionized mass and the relationship between flux and radius under the assumption of a constant density, fully ionized shell. He found that a mass of ∼0.2 M⊙ yielded the best results. Estimates of the ionized masses of PNs with independently determined distances also rarely exceed a few tenths of a solar mass. This is surprising since many PNs are thought to derive from high mass progenitors (up to 8 M⊙). Recent optical work (Plait and Soker, 1990) and our own computer simulations show that this simple mass estimation method may severely underestimate the total ionized mass. This is because of a halo of lower density ionized material which often contributes only a small fraction of the PN luminosity even though it may contain many times the mass of the dense inner shell. The precipitous drop in surface brightness (both optical and radio) beyond the inner part of the ionized shell also lead to underestimates of the PN's actual ionized radius. Since the evolution of PNs is driven by the expansion of the nebular shell coupled with the evolution of the nucleus (PNN), we ran several simulations using a simple momentum conserving two-wind model as well as employing density profiles derived by more sophisticated energy conserving models, with a wide range of wind parameters and using two different models of PNN evolution. From our simulations (assuming a 4 M⊙ progenitor) we derive an apparent “Shklovsky Mass” – defined as the ionized mass that would be derived from the observationally determined fluxes and radii of our model PNs. While the total ionized masses and Strömgren radii of the model PNs varied widely depending on the PNN and wind parameters, the derived Shklovsky mass consistently remained below one solar mass. This result is almost independent of the total ionized mass or the mass of the progenitor envelope and is fairly insensitive to the wind parameters chosen as input to the models. The observed spread of masses (based on an error analysis of the work of Gathier, 1987) is similar to the mass dispersion in our models for PNs of moderate age. This may explain why the Shklovsky distance method has been found to agree well with kinematic distances (Schneider and Terzian, 1983) even though the fundamental assumptions may be inappropriate for the nebula as a whole.

If the zero age main sequence is expressed in the (V–I) versus (B–V)–(V–I) plane the reddening lines are found to lie at a great enough angle to allow reasonably accurate spectral type classification for stars later than ∼ F5. Earlier spectral types can also be identified but with lower accuracy. Comparison with the Q method of UBV photometry and with spectra of some of the program stars shows that the BVIc technique produces reliable results. As late–type stars constitute the most numerous spectral types and are plentiful in all galactic plane directions BVIc reddening distances can be derived close to the desired direction (although to smaller distances than techniques that utilize early type stars). The applicability of the technique is further enhanced by the use of CCDs which generally have a spectral response well suited for BVIc imaging observations. Using the new technique the distance to the PN NGC2440 was found to be (3100 ± 320)pc.

Ground-based and satellite spectrophotometric observations of emission-line intensities over the spectral range 1400–7200 Å have been made in 5 or more positions in a total of 9 planetary nebulae.

We have undertaken a detailed photometric study of the open cluster NGC 2453 in Puppis with a view to determining accurately its distance modulus and to investigate its possible association with the PN NGC 2452. We have observed the core region of the cluster in V and I photometric bands using a Thomson-CSF TH 7882 CCD chip with the format 384×576 pixels, at the f/13 cassegrain focus of the 102-cm telescope at Vainu Bappu Observatory, Kavalur, India. Each image frame covers a field of 2′.3 × 3′.4 of the sky. Bias frames and flat field exposures on the twilight sky were obtained as per standard practice.

In order to study the physical properties of nebulae and determine their elemental abundances, it is important to observe lines from many different ionic species. Such studies have been enhanced in recent years with the measurement of lines in the far-infrared (FIR). The [N III]57μm line provides a way to assess the N++ abundance – which is not readily done from any other spectral region. Recent detection of the [N II]122 and 205μm lines provides a new way to assess both the electron density in the N+ region and the total N abundance in an object. When there are few observations to warrant a detailed modeling approach, it may be necessary to use another approach which has been referred to as a semi-empirical method (hereafter SEM) (e.g. Aller 1984). We delineate a SEM scheme for doing this and apply it to observations for the H II region G333.6–0.2.

We present a study on the filling factors and ionized masses of four sets of galactic and extra–galactic planetary nebulae (PNe) at known distances. The calculation of filling factors and ionized masses has been pursued as to get a deeper insight on the evolution of this class of objects. We used a galactic set of PNe, another set of nebulae that are averagely located near the galactic center, and two sets of nebulae in the Magellanic Clouds. As input data, we used the electron densities derived from the forbidden line intensity ratios, the Hβ nebular fluxes, the distances of galactic PNe derived from extinction, and the distances of galactic center PNe and of extra–galactic PNe derived from galaxy (or galactic region) memberships. All these quantities, plus the input angular radii, have been selected among the most recent measurements available in the literature. We obtained several interesting results. (1) The calculated filling factors are on average much smaller than what is usually assumed, independently for each set. (2) The ionized masses are all in good agreement with the theoretical predictions, with the possible exception of the Galactic Bulge PNe. (3) Both filling factors and ionized masses cover a wide range of values as it is shown in the cumulative histograms below (filled circles=local PNe; open circles=galactic center PNe; filled squares=LMC PNe; open squares=SMC PNe).

Mean electron densities are presented for over 100 planetary nebulae (PN). Distances to the majority of these PN are then derived, based on calibrations from observations of Magellanic Cloud PN. Absolute radii and filling factors have also been determined. A trend is seen in that for larger radii, smaller filling factors are found, however we show that such a trend results from uncertainties in the observed angular diameters.

In Liu and Danziger(1992b) we present Te derived from the Balmer discontinuities for 14 objects by long slit spectrophotometry. It is found that on the average the Te derived in this way are systematically lower than those derived the forbidden line ratios, indicating the presence of large temperature fluctuations(Peimbert 1967, 1971). The spectra obtained have been analyzed point-by-point along the slit for NGC 2392 and NGC 3242, for which t 2 = 0.13 and 0.058, respectively, as measured on the summed spectra integrated along the slits.