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We report results of an HST "snapshot" survey aimed at finding resolved binary companions of the central stars of Galactic planetary nebulae (PNe). Using WF/PC and WFPC2, we searched the fields of 113 PNe for stars whose close proximity to the central star suggests a physical association. We find 10 binary nuclei that are very likely to be physically associated, and another six that are possible binary associations. By correcting for interstellar extinction and placing the central stars' companions on the main sequence, we derive distances to the objects, and thereby significantly increase the number of PNe with reliable distances. Comparison of our derived distances with those obtained from various statistical methods shows that all of the latter have systematically overestimated the distances, by factors ranging up to a factor of two or more. We show that this error is most likely due to the fact that the properties of our PNe with binary nuclei are systematically different from those of PNe used heretofore to calibrate statistical methods. Specifically, our PNe tend to have lower surface brightnesses at the same physical radius than the traditional calibration objects. This difference may arise from a selection effect: the PNe in our survey are typically nearby, old nebulae, whereas most of the objects that calibrate statistical techniques are low-latitude, high-surface-brightness, and more distant nebulae. As a result, the statistical methods that seem to work well with samples of distant PNe, e.g., those in the Galactic bulge or external galaxies, may not be applicable to the more diverse population of local PNe.

We have produced simulated images of 110 planetary nebulae using the ellipsoidal shell model. This process has allowed us to remove the projection effects from the morphological classification of planetary nebulae and has provided quantitative measures of the intrinsic asymmetries of the nebulae. It is shown that the morphology of most planetary nebulae can be reproduced with pole-to-equator density ratios of 0.1-1. Many planetary nebulae also show a modest departure from axial symmetry. Contrary to previous findings by Khromov & Kohoutek, the sky orientation of planetary nebulae in this sample is consistent with a purely random distribution. Extremely bipolar nebulae (e.g., those of butterfly shape) point to a steep density profile in the AGB envelope and are more likely to be type I (high helium and/or nitrogen abundance) nebulae. We found evidence that these nebulae are likely to have more massive progenitors and are at a more advanced stage of dynamical evolution.

Sub-arcsecond resolution V and I images have been obtained for two proto—planetary nebulae. Both are found to show a definite bipolar morphology. A circumstellar disk is clearly seen in the V-I color image, suggesting that the bipolar lobes are due to starlight scattered into the polar openings. This indicates that bipolar morphologies develop early in the evolution of planetary nebulae, even before the onset of photoionization.

From numerical solutions of two colliding electromagnetic solitary waves, it is shown that their shapes are modified, and a trailing wake of plasma oscillations is produced by the collision. They therefore cannot be classified as solitons. For sufficiently large relative velocities, however, it is shown that the modifications to the vector and scalar potentials, as well as to the wake due to the collision, are small.

The properties of one-dimensional, weakly nonlinear electromagnetic solitary waves in a plasma are investigated. The solution of the resulting eigenvalue problem shows that the solitary waves have amplitudes which are allowed discrete values only and their vector and scalar potentials are proportional to [omega][sub [ital p]]/[omega][sub 0] and ([omega][sub [ital p]]/[omega][sub 0])[sup 2], respectively, where [omega][sub [ital p]] and [omega][sub 0] are the plasma and electromagnetic wave frequencies, respectively. Their widths are comparable to the plasma wavelength [lambda][sub [ital p]]=2[pi][ital c]/[omega][sub [ital p]] (where [ital c] is the velocity of light), except for the lowest-order solitary wave, whose width is large compared with [lambda][sub [ital p]], which is a true wakeless solitary wave only in the limit of vanishing amplitude. Simple analytical solutions are derived for higher-order solitary waves, whose vector-potential envelope is highly oscillatory, and are shown to consist, in the group-velocity frame, of two trapped, oppositely traveling waves.

The effects of a cold ion beam on an ion‐acoustic solitary wave whose potential is large enough to reflect the ion beam are studied in the regime in which the solitary‐wave response to the beam is adiabatic. It is shown that the solitary wave can be amplified by a large factor, and that it is accelerated until its velocity equals the beam velocity.

The properties of solitary waves in a plasma with an ion beam are investigated. The two conditions for the existence of solitary‐wave solutions are determined and presented graphically in parameter space. Electron inertia is shown to restrict the regime of the fast and slow beam solitary waves to beam velocities that are less than the electron thermal velocity.

IRAS Pointed Observations are used to study the pre-main sequence (PMS) objects associated with the reflection nebula Cederblad 110 in the Chamaeleon I star forming cloud. Nine PMS stars are identified, and near-infrared observations are presented and combined with IRAS data to investigate their evolutionary status. The nine objects represent various low-mass PMS stages from deeply embedded protostar candidates to naked T Tauri stars approaching the ZAMS, indicating that star formation has been going on for millions of years in the immediate vicinity of Cederblad 110

The properties of arbitrary‐amplitude solitary waves in a plasma consisting of drifting ions and nondrifting electrons are investigated. The conditions for the existence of solitary‐wave solutions are determined. It is shown that electron inertia has the significant effect of restricting the regime of solitary waves to ion drift velocities that are less than the electron thermal speed. Because of this restriction relativistic effects are small.

For a weakly nonuniform medium, a solitary wave solution of the variable‐coefficient nonlinear Schrodinger equation is obtained through second order in the expansion parameter. From a generalization of the second‐order result, the form of the solution to all orders is derived from which the conditions for an exact, distortionless solitary wave in a nonuniform medium are obtained. By use of a transformation, these results are shown to apply a weakly dissipative medium.

The soliton modifications resulting from the addition of a small third derivative term, due to higher‐order dispersion, to the nonlinear Schrodinger equation are investigated. Based on direct perturbation theory, it is shown that through first order, the soliton phase and velocity are modified, but the shape, amplitude, and width are unchanged. The radiation stimulated by the third derivative term, which is not predicted by the perturbation theory, is also derived. The expression obtained confirms the result of Wai (Ph.D. thesis, University of Maryland, 1988), which was obtained by a different method. The rate of change of the soliton amplitude due to this radiation, which emerges in front of the soliton, is calculated. The nonlinear term that drives the radiation is shown to grow to a value that is much larger than the unperturbed value.

Some properties of small‐amplitude ion‐acoustic solitons in a plasma consisting of nondrifting electrons and drifting ions are investigated. Electron inertia effects are shown to be considerably more important than relativistic effects. It is also shown that ion‐acoustic soliton solutions exist only if the ion drift velocity is less than the electron thermal velocity.

Confrontations of the dredge-up theory with observed patterns of chemical abundances of planetary nebulae (PNs) have been carried out by many authors (see, e.g., Kaler & Jacoby 1990, 1991; Stasinska & Tylenda 1990). Although these studies suggest that the observational abundance ratios of PNs can qualitatively be explained by the current dredge-up theory, scatters around the theoretical predictions in their diagrams are always large. This has led Ratag (1991) to conclude that there is no correlation at all between the nebular abundances and the core mass of CSPNs (see also Pottasch 1993).

The variety of different morphologies of planetary nebulae (PNs) have always fascinated researchers in the field. Recent progress in CCD imaging techniques has resulted in a large number of homogeneous optical images of more than a few hundreds PNs (Balick 1987, Schwarz, Corradi and Melnick 1992). In addition, two major radio surveys using the Very Large Array (VLA) by Aaquist and Kwok (1990, 1991) and by Zijstra, Pottasch and Bignell (1989) have revealed detailed morphologies in the radio, including many compact star-like PNs. This treasure of data sets contains useful information about morphologies, density distributions, formation and evolution of PNs, yet to be fully explored.

The influence of polarization on the two-dimensional structure of heterogeneous dumbbells is studied by Monte Carlo simulation. The dumbbell consists of two fused hard spheres whose centers are embedded with point dipoles and , respectively. The axis of the dumbbell can rotate freely in the x-y plane but and are always along the x-axis which is the polarization direction. When the dipolar strength ?a is much larger than ?b, the dumbbells are perpendicular to the polarization direction and form zigzag chains; when ?a and ?b are comparable, the dumbbells will form end-to-end chains if and are parallel, or side-by-side chains if and are antiparallel. All the chains are along the polarization direction and the detailed information about them is given through extracting the radial and angular distributions from the simulation results. Finally, we calculate the energies of four kinds of ideal chains to explain structural transition. Our simulation results can explain the very recent experiment by Nagao D. et al. (Langmuir, 28 (2012) 6546) which studied the assembly of dumbbells with inducible dipoles, where magnitude of the dipoles was frequency-dependent and by changing frequency, different assemblies could be obtained.

We have selected a sample of planetary nebulae, for which the core masses are determined using distance-independent parameters (Zhang and Kwok 1992). The chemical abundances of He, N, O, and C are taken from the literature for them. Relationships of the ratios of He/H, N/O, and C/O with various stellar parameters of planetary nebulae (PN), such as the core mass, the mass of the core plus the ionized nebular gas, the stellar age and temperature, are examined. It is found that the N/O increases with increasing mass, while the C/O first increases and then decreases with the core mass. No strong correlation seems to exist between the He/H and the core mass. A correlation of the N/O and He/H with the stellar temperature exists. The current dredge-up theory for the progenitor AGB stars cannot satisfactorily account for these patterns of chemical enrichment in PN. Furthermore, the correlations of the N/O and He/H with the stellar age and temperature indicate that besides the dredge-ups in the RG and AGB stages, physical pfocesses that happen in the planetary nebula stage may also play a role in forming the observed patterns of chemical enrichment in the planetary nebulae.

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.

Making use of the results from recent infrared and radio surveys of planetary nebulae, we have selected 431 nebulae to form a sample where a number of distance-independent parameters (e.g.,T b , T d , I60μm , and IRE) can be constructed. In addition, we also made use of other distance-independent parameters ne and T* where recent measurements are available. We have investigated the relationships among these parameters in the context of a coupled evolution model of the nebula and the central star. We find that most of the observed data in fact lie within the area covered by the model tracks, therefore lending strong support to the correctness of the model. Most interestingly, we find that the evolutionary tracks for nebulae with central stars of different core masses can be separated in a Tb-T* plane. This implies that the core masses and ages of the central stars can be determined completely independent of distance assumptions. The core masses and ages have been obtained for 302 central stars with previously determined central-star temperatures, We find that the mass distribution of the central stars strongly peaks at 0.6 M⊙, with 66% of the sample having masses

The past decade has seen significant progress in our understanding of spectral energy distribution of planetary nebulae over the entire wavelength range from UV to radio. In this review we show the detailed breakdown of the energy budget for a planetary nebula as a system of the three components, i.e., the central star, the gaseous nebula and the dust shell. This picture of the energy distribution is further discussed in the context of planetary nebula evolution.

High spatial resolution 50 and 100 micron observations of the Ring Nebula reveal a far-infrared emission peak close to the center of the nebula where atomic and molecular emission is at a minimum. Dust in this central peak emission region is heated by direct absorption of radiation from the central star. While the dust temperature in the central peak region is about 50 K (dust emissivity index of 1.5), the temperature of the bulk of the dust located in the regions of the ionized nebular core and the neutral molecular gas is on average lower than 50 K. Ly(alpha) photons are sufficient for heating the dust grains within the nebular core. Far-infrared emission is also found from grains mixed with the molecular gas outside the main ionized nebular core.

ROSAT Position Sensitive Proportional Counter (PSPC) observations of the planetary nebula NGC are reported here. This planetary nebula is here the first discovered to show clearly two components in its X-ray spectrum. A two-component model consisting of a blackbody and a Raymond-Smith thermal plasma is fitted to the observed ROSAT PSPC spectrum. This results in a temperature of T(sub 1) = 1.4 x 10(exp 5) K for the blackbody component and a temperature T(sub 2) = 8.7 x 10(exp 6) K for the hot plasma component, at a hydrogen column density N(sub H) = 1.4 x 10(exp 20)/sq cm. The temperature of the blackbody component is consistent with the helium Zanstra temperature of the central star, indicating that it may be attributed to the photosphere of the central star. The high-temperature component is possibly from a corona around the central star, which may be related to a strong convection in the star. An alternative explanation is that the hot plasma resides in a hot bubble predicted by the interacting wind model. A lower limit of the electron density in the hot plasma is found to be approximately 10/cu cm.

Using existing infrared and radio data on a sample of 432 planetary nebulae, we derived a number of distance-independent parameters for comparison with evolutionary models of planetary nebulae. We find that many of the observed properties of planetary nebulae can be explained by current central star evolutionary models, even if the time scales are subject to significant change by a factor of up to an order of magnitude. Specifically, we find that the evolutionary tracks are well separated in the radio surface brightness-central star temperature plane, therefore allowing us to determine the core mass of individual planetary nebulae. We also obtain the luminosity and gravity of the central stars of individual nebulae, from their temperature and core mass, without relying on the distance assumptions. We find that our results of the core mass are in good agreement with those of Mendez et al. (1992) and Tylenda et al. (1991). A systematic, large discrepancy is found between the luminosity found in this work and that found by Gathier and Pottasch (1986).

The extended infrared emission detected near many young stars can be used to divine the luminosity, apparent color temperature, and thermal emitting mass of the surrounding regions. The dominant dust heating mechanism appears to be ultraviolet radiation, either from stellar wind shocks or directly from hotter stars. The infrared luminosities from stellar winds of low mass stars are an appreciable fraction of the luminosities of the driving star, and the mechanical luminosity in the winds must be even higher.

The extended infrared emission in the Serpens molecular cloud is associated with three B stars, HD 170634, 170739 and 170784. The dust heating is due to a central stellar and an external interstellar radiation field. The Serpens extended emission exhibits apparent small-scale cavities immediately around the three stars. With the assumption that the cavities have been excavated by stellar winds, mass loss ratesfor these stars may be estimated within the range of 10-13 – 10-11 M⊙ yr-1. The IR emission from the core of the cloud has a double-peaked morphology, the grains is mainly heated by ultraviolet photons produced by shock waves.

The IRAS additional observations of planetary nebulae are described. Extended sources can have very wrong integrated flux densities, when compared with the PSC or SSC. Only 7 of the 33 sources observed turn out to be extended. Maps of NGC 6853 and A 21 are presented.

We present an analysis of the radio continuum spectra of a sample of compact planetary nebulae. A correlation is found between the slope of the optically thick portion of the spectrum and linear radius of the nebula. This correlation, and the shapes of individual spectra are well explained by a model in which the radio emission arises in a photon-limited, ionized shell of a stellar wind type envelope. Theoretical fits are used to derive parameters of the wind-shell. Typical mass loss rates inferred for the precursor red giant are a few 10-5 M⊙/yr.