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Coagulation of grains in the solar nebula should result in formation of low-density aggregates with fractal structure. The opacity of the nebula remains much higher than computed for the assumption that particles are compact. Timescales for particle growth, settling to the central plane of the nebula, and formation of planetesimals are lengthened significantly.
The 'standard' accretion-disk model of the solar nebula is examined together with its basic equations, which are solved for the steady-state case. It is shown that, for this model of the solar nebula, the efficiency of mixing the products of thermochemical processing from small to large disk radii depends not on the magnitude of the eddy diffusivity but only on the ratio k between the eddy diffusivity and the eddy viscosity. In most cases, this ratio k is probably of order of unity or less, implying that most of the solar nebula was not contaminated by the consequences of thermochemical equilibria that were established at 'small' disk radii. It is noted that, while this condition is almost certainly satisfied if the physical process responsible for viscosity is thermal convection or waves, it has uncertain validity during the early phases of disk evolution where accretion-induced shear instabilities may dominate.
Formation mechanisms for the two optical bright knots in planetary nebulae, 'ansae', are investigated. Adiabatic two-dimensional numerical simulations of the interacting winds flow at early stages of the planetary nebulae evolution are performed. It is found that no real focusing of the shocked fast wind material toward the symmetry axis occurs, and thus the interaction of winds by itself will not form the ansae. A model is proposed, in which jets from the central star form the ansae. These jets are presumably being formed during the few hundred years in the period between the end of the slow wind and the beginning of the fast wind. It is shown that a companion to the progenitor red giant can lead to the degree of asymmetry observed in elliptical planetary nebulae.
This paper presents the low-resolution spectra of 170 planetary nebulae observed by IRAS, most of which were not included in the Atlas of Low-Resolution Spectra. These have been classified into eight groups based upon the spectral morphology, with emphasis on the dust continuum rather than the spectral lines. The Low-Resolution Spectrometer (LRS) spectra are nearly evenly divided between spectra which show only a dust continuum with no lines, and spectra where there are strong lines with a dust continuum appearing longwards of about 15 microns. A study has been made of the strength of the 11.3-micron PAH emission feature as a function of the nebular C/O ratio, combining ground-based and LRS data. The C/O ratio for IC 2621 is derived from IUE spectra and used in this study. The 11.3-micron feature is present with essentially constant strength in all nebulae with C/O above about 1. Only marginal evidence exists for its presence below C/O about 1 and then at a level about 5 times lower than in carbon-rich nebulae.
Spectrophotometry of 75 large PNe with Shklovsky radii greater than 0.15 pc is presented and used to calculate nebular parameters and compositions, stellar Zanstra temperatures and luminosities, and core masses. Nine new Peimbert type I nebulae are identified. About 40 percent of the stars that are on cooling tracks are above 0.7 solar mass, and over 15 percent are above 0.8 solar mass. The large planetaries demonstrate a clear positive correlation between nitrogen enrichment and core mass. N/O is anticorrelated with O/H. The radii of the nebulae whose stars lie along specific cooling tracks increase monotonically with decreasing central star temperature. For a given central temperature, the nebular radii also increase with increasing core mass, showing that in this part of the log L-log T plane the higher mass cores evolve more slowly in agreement with theoretical prediction. However, theoretical evolutionary rates for the large nebulae stars appear to be much too slow.
The results of a systematic investigation of polycyclic aromatic hydrocarbon (PAH) emission in H II regions, planetary nebulae (PN), and proto-planetary nebulae (PNN), are reported. Data is obtained from the low resolution spectra (LRS) of IRAS. The results show that: PAHs are formed in carbon rich objects; and PAH emission is ubiquitous in general interstellar medium and requires the presence of ultraviolet photons, in planetary and proto-planetary nebulae, PAH emission is seen only where an ionizing flux is present and in carbon rich objects.
A review of current studies of neutral envelopes is presented with particular attention given to the use of the envelopes as test cases for understanding the ionization and thermal structure of photodissociation regions. The study of near-IR H2 emission is discussed with detailed spectra given for a few planetary nebulae, and airborne observations of far-IR atomic lines are discussed. These two methods can discern photodissociation regions with warm gas and UV flux is fairly prominent. The use of resonance-absorption-line spectroscopy is also reviewed with respect to the analysis of the Na D lines, and thereby allows the measurement of integrated columns of material through the shell. The methods provide evidence for the notion that planetary nebulae consist of more than just ionized material; large amounts of neutral and molecular material are being confirmed, which has important implications for the mass-loss episode of the nebulae.
Medium-resolution spectra in the K-window have been obtained for 13 planetary nebulae and one compact H II region in order to study the behavior of the unidentified line at 2.287 microns. The wavelengths of this line and the unidentified 2.199-micron line have been measured in NGC 7027 at high spectral resolution using a Fabry-Perot interferometer. Based on the interferometric measurements and the lack of correlation of the 2.287-micron line with emission in H2 lines, the unidentified lines are not lines of molecular hydrogen. The 2.287-micron line is strongest in medium-high excitation nebulae, and is absent in nebulae of very low and very high excitation, implying that the ionization potential of the parent atom or ion is about 30-40 eV and that the ion itself has an ionization potential of perhaps 40-60 eV.
The Gum Nebula was observed by the A-2 LED proportional counters on the HEAO-1 satellite as part of the all-sky survey. The first detection of X-ray emission associated with the Gum Nebula is reported. Soft X-ray spectra were constructed from the A-2 LED PHA data. Single temperature Raymond-Smith models were fitted to the observed spectra to yield temperature, column density and emission measure. The temperature is 6 x 10 exp 5 K, the column density 4 x 10 exp 20/sq cm, and the emission measure 5 cm exp-6 pc. The X-ray and optical properties of the Gum Nebula are consistent with a supernova remnant in the shell stage of evolution, which was the product of an energetic (3 x 10 exp 51 ergs) supernova explosion which occurred about 2 x 10 exp 6 yr ago.
Using the Faint Object Camera on-board the Hubble Space Telescope, we have obtained images of four planetary nebulae (PNe) in the Magellanic Clouds, namely N2 and N5 in the SMC and N66 and N201 in the LMC. Each nebula was imaged through two narrow-band filters isolating forbidden O III 5007 and H-beta, for a nominal exposure time of 1000 s in each filter. In forbidden O III, SMC N5 shows a circular ring structure, with a peak-to-peak diameter of 0.26 arcsec and a FWHM of 0.35 arcsec while SMC N2 shows an elliptical ring structure with a peak-to-peak diameter of 0.26 x 0.21. The expansion ages corresponding to the observed structures in SMC N2 and N5 are of the order of 3000 yr. LMC N201 is very compact, with a FWHM of 0.2 arcsec in H-beta. The Type I PN LMC N66 is a multipolar nebula, with the brightest part having an extent of about 2 arcsec and with fainter structures extending over 4 arcsec.
New high-quality spectropolarimetry of the post-main-sequence bipolar nebulae GL 618, M2-56, and M1-92 is presented which permits accurate separation of the scattered and unscattered components. Shock emission dominates the optical line spectrum of the three nebulae and probably plays a significant role in their dynamical evolution. The central H II region spectrum for GL 6189 is isolated and T(stellar) of 36,000-40,000 and log (N/O) = 0.0 +/- 0.2 are derived. GL 618 and M2-56 have shock velocities of 40-60 km/s; that of M1-92 is 60-100 km/s. Log (N/O) is derived for different regions of the three nebulae. There is an apparent correlation of outflow velocity with chemical abundance.
UV low-dispersion spectra of the central star of the faint planetary nebula, IW-2, were obtained with the IUE. The apparent large diameter of the very diffuse nebula, about half that of the moon, as seen on the Palomar Sky Survey plates by Ishida and Weinberger (1987), indicates this object to be potentially quite evolved, and nearby. The IUE spectra clearly reveal a hot stellar continuum extending over the entire wavelength range of the short-wavelength prime camera (1200-2000 A). This object with V = 17.7 +/- 0.4 is definitely one of the faintest stars ever successfully observed with the IUE. Comparisons of the IUE observed fluxes with those from white dwarf model atmospheres suggest extinction near E(B - V) = 0.45 for a white dwarf of T(eff) roughly 100,000 K. Constraints from estimates of the nebular emission measure and observed visual magnitude also argue for a white dwarf of T(eff) roughly 100,000 K at a distance of 300 to 350 pc. The nucleus of IW-2 is one of the most evolved stars to be identified with a planetary nebula.
We present a morphological study of the planetary nebula IC 4593, based on our observations in H-alpha, forbidden O III, and forbidden S II. From the H-alpha intensity map, we calculate densities and masses for constituent structures and for the nebula as a whole. We argue that the morphology of IC 4593 suggests that it is moving supersonically through the ISM, and that the ISM shock may be thermally unstable, oscillating between adiabatic and radiative (isothermal) shock conditions. Spectroscopic observations are necessary to further explore the nature of the interaction between IC 4593 and the ISM. An improved understanding of such interactions will greatly expand the potential use of planetary nebulae to probe the ISM.
We study an accretional stage of the formation and early evolution of the solar nebula with relatively small angular momentum. We investigate the evolution of the disk and its vertical structure, particularly the shock front between disk and infalling material. Calculations start at a moment when a low-mass star-like core surrounded by small embryo disk have been formed at the center of the presolar nebula and the bulk of mass remained in the envelope. The forming solar nebula is approximated as a thin viscous disk surrounded by accreting envelope. The distribution of temperature in the infalling envelope is determined by solving spherically symmetric equations of radiative transfer. As the energy source, we take into account all energy released within the centrifugal radius of the infalling matter. Other aspects of this study are discussed.
The 1000-R(sub J)-wide neutral sodium nebula of Jupiter has now been observed on four separate occasions from late 1989 to early 1992. The observations have consisted of both low-dispersion spectroscopy and direct imaging. These observations have allowed us to establish some of the general features of the nebula, such as the largely equatorial confinement of emission, the r(exp -1) intensity behavior in the equatorial plane, and flaring of the emission with respect to the equator. Some additional evidence exists for more subtle morphology, such as a possible variable east-west intensity asymmetry. In this paper, we present a summary of observational evidence of variability in overall intensity, as well as in some of the morphological features of the nebula, over timescales of days, months, and years.
We present the second half of a charge coupled device (CCD) narrow-band imaging survey of galactic Wolf-Rayet stars located in the southern hemisphere as listed by van der Hucht et al. (1981). Images of 50 Wolf-Rayet stars were taken using a wide-field CCD and narrowband interference filters centered on H alpha and (O III) 5007 A wavelengths. The first half of the survey (Marston, Chu, & Garcia-Segura 1993, hereafter Paper I) revealed six new ring nebulae residing around Wolf-Rayet stars. Here we reveal a possible 11 new rings and the existence of multiple rings associated with two previously known nebula, RCW 118 and G2.4+1.4 and around the stars WR 16 and WR 43. Combining our results with those of Miller & Chu (1993) and Paper I, 92% of the van der Hucht catalog of Wolf-Rayet stars have now been surveyed. Of the 38 possible ring nebulae found in our surveys to date, 22 reside around WN subtype Wolf-Rayet stars, 13 around WC stars, one around a triplet of Wolf-Rayet stars and one around a WO star (WR 102). One ring exists around a WN/WC star (WR 98). A bias toward rings being observed around W-R + OB binaries is noted. Such pairings are generally bright, and the detection of a ring around them may merely be a function of their combined luminosity. Several Wolf-Rayet stars are shown to be surrounded by multiple rings (two or three) which suggests that a number of ejections of stellar material have taken place during their evolution.
We present the results of an investigation into the morphology and dynamics of ionized gas in the bulge of the gas-rich S0 galaxy NGC 5102. We show that the bulge of NGC 5102 contains a ring of ionized gas, approximately 1.3 kpc in diameter, which is centered well away from the nucleus. Through spectroscopy and (O III) lambda-5007 imaging, we show that the gas is excited by a low-velocity shock, which varies from approximately 50 to approximately 70 km/s along the ring. Fabry-Perot images in H-alpha confirm that the gas is moving slowly, and suggest that the structure is a supershell, approximately 10(exp 7) yr old. This age is significantly younger than the galaxy's nuclear starburst, which is approximately 2 x 10(exp 8) yr old. We also use our (O III) lambda-5007 images to identify planetary nebulae (PNs) in the bulge and inner disk of NGC 5102. Using the planetary nebula luminosity function, we derive a distance modulus to the galaxy of (m - M)(sub 0) = 27.47(sup +0.18)(sub -0.27), or 3.1(sup +0.3)(sub -0.4) Mpc, confirming its membership in the NGC 5128 group. Our derived value of 47.2(sup +12.2)(sub -9.2) x 10(exp -9) for the bolometric luminosity-specific PN density, alpha(sub 2.5), is higher than that observed for the bulge of M31 or the giant ellipticals of the Virgo Cluster, but not significantly different from that found for the small, normal ellipticals NGC 3377 or M32. The high value for alpha(sub 2.5) suggests that virtually all of NGC 5102's stars will someday evolve through the planetary nebula stage.
We present Hubble Space Telescope (HST) Planetary Camera observations showing the first optically resolved images of the planetary nebula Henize 1357 . The small size of this object has heretofore precluded optical investigations of its morphology. In H-beta, He 3-1357 appears to have an equatorial ring of enhanced density tilted approximately 56 deg from our line of sight. In addition, there are bubbles of gas above and below the ring with areas of decreased brightness near the poles where a fast stellar wind has broken through the red giant envelope. He 3-1357 has been evolving very rapidly, becoming a planetary nebula only within the past few decades. Assuming a distance of 5.6 kpc, the radius of the nebula is 0.02 pc, implying an expansion age of 2650 yr, although it has been ionized for no more than 40 yr. The H-beta flux is now 5.8 x 10(exp -11) ergs/sq cm/s which implies an ionized mass of 0.2 solar mass, log L*/solar luminosity = 3.7, and a core mass of 0.59 solar mass.