Results 221 to 230 of about 55,496 (241)

The Onset of Axial Symmetry in Protoplanetary Nebulae

Astrophysics and Space Science, 1995
We have obtained resolved IR images, at wavelengths from 1.2μm to 12.5μm, of a small sample of protoplanetary nebulae. The results suggest that all PPNe are axially, not spherically, symmetric, that spherical symmetry ends at the tip of the AGB, and that AGB evolution terminates with a burst of remarkably rapid, equatorially concentrated mass-loss.
M. J. Barlow, John F. Arens, Kay Justtanont, C. J. Skinner, Margaret Meixner, J. G. Jernigan   +5 more
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Compact reflection nebulae associated with protoplanetary nebulae

Chinese Astronomy and Astrophysics, 1991
Abstract Optical identifications and slit spectra and CCD direct images show that, of the protoplanetary nebulae (PPN) selected from the IRAS Catalogue of Point Sources, three are associated with compact reflection nebulae. In this paper we use a spherically symmetric dust shell model to calculate the brightness and colour evolution of the central ...
Hu Jing-yao, Tang Qing-quan
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Formation of amorphous ice in the protoplanetary nebula

Planetary and Space Science, 1994
Abstract The criteria for formation of amorphous ice in different models of the protoplanetary nebula are developed. Assuming that comets are composed mostly of amorphous ice, it is shown that the inner boundary of formation of cometary nuclei is about 7 A.U.
Y. Mekler, Morris Podolak
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Thermal cycling and fluctuations in the protoplanetary nebula

Icarus, 1988
Abstract The cooling of the protoplanetary nebula is controlled largely by the opacity of dust grains. Usually nebula structure models employ standard opacity tables calculated for ensembles of micron-sized dust grains, which are assumed to dominate the opacity.
Christoph K. Goertz, Ove Havnes, Gregor E. Morfill   +2 more
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H2 Spectroscopy of Protoplanetary Nebulae

2001
Observations of planetary and protoplanetary nebulae in the spectral line of molecular hydrogen at 2.1218 μm are providing new insights into the processes of formation and evolution of post-AGB stars. In this paper, we present three significant results from the study of H2 toward planetary and protoplanetary nebulae.
Ian Gatley, Joel H. Kastner, David A. Weintraub   +2 more
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Dirty ice grains in the protoplanetary nebula

Planetary and Space Science, 1997
Abstract The model of Mekler and Podolak (1994) for the growth of small ice grains in the nebula is extended to allow for the presence of additional absorbers. The properties of these “dirty” grains are explored. The presence of an additional absorber allows for several interesting effects.
Y. Mekler, Morris Podolak
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ISO Results for Protoplanetary Nebulae

2001
We discuss the results of analysis of Infrared Space Observatory spectra of PPN candidates. In particular for the carbon-rich PPNs in our sample we have analysis of the 21 and 30 μm features as well as the UIR features and plateaus. The 30 μm feature appears to have two sub-components, at about 27 and about 34 μm, which vary from source to source ...
Ryszard Szczerba, Bruce J. Hrivnak, Kevin Volk, Sun Kwok   +3 more
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The distribution of gases in the protoplanetary nebula

Icarus, 1964
Abstract It is demonstrated that the distribution of elements in the solar system may be connected with solar magnetic fields. Relative diffusion of neutral particles and ions in a magnetic field (“ambipolar diffusion”) separates elements of different ionization potential.
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Particle-Gas Dynamics in the Midplane of a Protoplanetary Nebula

Icarus, 1993
Abstract In this paper, we study the stage of planetary formation during which the particulate material is still dispersed as centimeter-to-meter sized primordial aggregates. During this stage, particles are able to settle toward the midplane into a layer of mass density comparable to or much greater than that of the gas.
Jeffrey N. Cuzzi, Joelle M. Champney, Anthony R. Dobrovolskis   +2 more
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OH–IR sources as precursors to protoplanetary nebulae

Nature, 1990
IT has long been suspected1 that OH–IR sources, highly evolved red giant stars that have built up massive, cool gaseous envelopes through heavy mass loss, are precursors to planetary nebulae. The two kinds of object share a similar galactic distribution, and their circumstellar envelopes have comparable masses and expansion velocities2,3.
R. J. Cohen, M. C. Shepherd, M. E. West, Michael J. Gaylard   +3 more
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