Objectives: To understand the nature and importance of cometary knots and other structure within planetary nebula What are they, how do they form and disappear?

Дата канвертавання24.04.2016
Памер16.93 Kb.

PH700 PN project notes Professor Michael Smith

To understand the nature and importance of cometary knots and other structure within planetary nebula.. What are they, how do they form and disappear?
To identify the current major issues in this field. To suggest solutions. To determine the future prospects for progress in this field.

Use virtual observatories to investigate a number of objects.
Conduct a literature review of well-studied PN with cometray knots/globules.
What are the related structures: are small arcs an earlier stage? Are long radial lines an indication of the later stages?
Are the cometray globules visible/detectable at specific wavelengths? In radiation originating from molecules, atoms, dust…..?
Read papers on the the theory. Quantify results, present graphs, histograms, if possible/sensible. i.e. Gather together info you think relevant into a table and then look for correlations.....
How to use Aladin
How to use the ADS
First task: Write Abstract ......... could base it on this:

Evolution of Molecular Gas and the Origin of Cometary Knots in Planetary Nebula


Speck, A.

Planetary Nebulae {PNe}, as the final phase of evolution for intermediate mass stars, are major contributors to the enrichment of the interstellar medium {ISM}. In PNe, a hot central star illuminates a gas and dust shell which was ejected during earlier evolutionary phases. UV radiation from the star creates an ionized region bounded by neutral gas and molecules. A better understanding of the nature of the molecular and ionized gas envelopes of PNe is important to our understanding of the evolution of PNe and their contribution to the enrichment of the ISM. Knots and filaments in the ionized gas images of PNe are common, if not ubiquitous. Additionally, it has been shown that molecular gas exists inside dense condensations within the ionized regions, but the origins of these clumps are not known. We propose to study the morphologies of both molecular and ionized gas for five PNe that have been imaged by both WFPC2 and NICMOS {at the 2.12um H2 line}. The structure and appearance of the knots in ionized and molecular gas for each PNe can be compared to assess the evolutionary status of the molecular clumps and how it is affected by the evolutionary status of the whole PN. This will aid our understanding of the origin of the molecular knots, and the enrichment of the ISM by dying intermediate mass stars.
 Dyson, J et al.

Aims.We have examined a stream-source model for the production of the cometary tails observed in the Helix Nebula NGC 7293 in which a transonic or moderately supersonic stream of ionized gas overruns a source of ionized gas. We have compared the velocity structures calculated with the available observational data. We have also investigated the suggestion that faint striations visible in the nebular gas are the decaying tails of now destroyed cometary globules.
Methods: .We have selected relevant results from extensive hydrodynamic calculations made with the COBRA code.
Results: .The velocities calculated are in good agreement with the observational data on tail velocities and are consistent with observations of the nebular structure. The results also are indicative of a stellar atmosphere origin for the cometary globules. Tail remnants persist for timescales long enough for their identification with striations to be plausible.

Infrared Observations of the Helix Planetary Nebula


Hora et al.


We have mapped the Helix (NGC 7293) planetary nebula (PN) with the IRAC instrument on the Spitzer Space Telescope. The Helix is one of the closest bright PNs and therefore provides an opportunity to resolve the small-scale structure in the nebula. The emission from this PN in the 5.8 and 8 mum IRAC bands is dominated by the pure rotational lines of molecular hydrogen, with a smaller contribution from forbidden line emission such as [Ar III] in the ionized region. The IRAC images resolve the ``cometary knots,'' which have been previously studied in this PN. The ``tails'' of the knots and the radial rays extending into the outer regions of the PN are seen in emission in the IRAC bands. IRS spectra on the main ring and the emission in the IRAC bands are consistent with shock-excited H2 models, with a small (~10%) component from photodissociation regions. In the northeast arc, the H2 emission is located in a shell outside the Halpha emission.

The Dynamical Evolution of Planetary Nebulae after the Fast Wind


García-Segura, G

We explore the dynamics of ionization-bounded planetary nebulae after the termination of the fast stellar wind. When the stellar wind becomes negligible, the hot, shocked bubble depressurizes, and the thermal pressure of the photoionized region, at the inner edge of the swept-up shell, becomes dominant. At this stage the shell tends to fragment, creating clumps with comet-like tails and long, photoionized trails in between, while the photoionized material expands back toward the central stars as a rarefaction wave. Once the photoionized gas fills the inner cavity, it develops a kinematical pattern of increasing velocity from the center outward, with a typical range of velocities starting from the systemic velocity to ~50 km s-1 at the edges. The Helix Nebula is a clear example of a planetary nebula at this late evolutionary stage.

The Origin and Physical Properties of the Cometary Knots in NGC 7293


Capriotti, Eugene R.; Kendall, Anthony D.

On the basis that the cometary knots observed in the Helix Nebula form as a result of larger ``parent clouds'' breaking up due to Rayleigh-Taylor instability induced by radiative acceleration of the clouds, we compute characteristics of the cometary knots and of the parent clouds as well. Present observations place constraints on the positions, velocities, and sizes of the parent clouds. Requiring the clouds to produce cometary knots that are stable places further constraints on the properties of parent clouds. We formulate those constraints and show how they further limit the predicted properties of the cometary knots and bring them into agreement with their observed properties.

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