Astronomy - Research

From Worldtraveller

Contents
Current research	Publications	Theses
Supernovae	Peer-reviewed	PhD
Planetary nebulae	Conference proceedings	MSci

Current research

Currently I work on two topics:

Supernovae

Studies of the very distant universe show that galaxies which are less than a billion years old contain significant amounts of dust. The question is, where did this dust come from? In the beginning, the universe was more or less just hydrogen and helium. Dust is produced by evolved stars - both by low-mass stars like the Sun which produce dust as they evolve into planetary nebulae, and also supernovae - the most massive stars which end their lives in colossal explosions, briefly shining more brightly than entire galaxies.

The dust in the very early universe must have come from supernovae, because low mass stars would not have reached the end of their lives by then - they take a few billion years to start producing a lot of dust. The problem is that the sums don't seem to add up - recent supernovae in nearby galaxies don't seem to have produced as much dust as you'd expect, given the amount in the distant galaxies.

So, what is going on? Is there really that much dust in the early universe? Are the supernovae producing a lot of dust that we are not seeing for some reason? I'm working on this problem using optical and infra-red spectra of nearby supernovae. With modern sensitive infra-red telescopes like the Spitzer Space Telescope, we can better understand the infrared emission from cosmic dust, and simultaneous IR and optical observations should shed some light on the supernova dust problem.

Planetary nebulae

The other part of my work concerns a different type of dying star - planetary nebulae are delicate shells of gas ejected by small-ish stars when they reach the end of their lives. Our Sun, in about 5 billion years time, will form a planetary nebula.

There are two ways of determining how much carbon, nitrogen, oxygen and other elements there is in the gases of a planetary nebula. Unfortunately, there is a considerable discrepancy between these two methods: one method gives you abundances at least a factor of two -- and up to a factor of 100 or more in extreme cases -- higher than the other.

Many theories have been proposed to resolve this discrepancy. Our work at UCL has found strong evidence that the solution lies in the existence of cold, metal-rich clumps within planetary nebulae. The question then to be answered is where did these knots come from? How do they survive within gas ten times as hot and much less dense, without dissipating rapidly? And why can't we see them, even with very high resolution instruments like the Hubble Space Telescope? I'm working on these problems with particular emphasis on the very few planetary nebulae which do contain visible metal-rich clumps.

Some planetary nebulae

Publications

Peer-reviewed

The hydrogen-deficient knot of the `born again' planetary nebula Abell 58 (V605 Aql)

Wesson R., Barlow M.J., Liu X-W., Storey P.J., Ercolano B., De Marco O., 2008, MNRAS, 383, 1639

The abundance discrepancy - recombination line versus forbidden line abundances for a northern sample of galactic planetary nebulae

Wesson R., Liu, X-W., Barlow M.J., 2005, MNRAS, 362, 424

Observations and 3D photoionisation modelling of the Wolf-Rayet planetary nebula NGC1501

Ercolano B., Wesson R., Zhang Y., Barlow M.J., de Marco O., Rauch T., Liu, X-W., 2004, MNRAS, 354, 558

Discovery of a WO star in the Crux-Scutum arm

Drew J.E., Barlow M.J., Unruh Y.C., Parker Q.A., Wesson R., Pierce M.J., Masheder M.R.W., Philipps S., 2004, MNRAS, 351, 206

Electron Temperatures and Densities of Planetary Nebulae determined from the nebular hydrogen recombination spectrum and temperature and density variations

Zhang Y., Liu X-W., Wesson R., Storey P.J., Danziger I.J., 2004, MNRAS, 351, 935

Physical Conditions in the Planetary Nebula NGC6543

Wesson R., Liu X-W., 2004, MNRAS, 351, 1026

Physical Conditions in the Planetary Nebula Abell 30

Wesson R., Liu X-W., Barlow M.J., 2003, MNRAS, 340, 253

Conference proceedings

The Formation of PAHs: A Case Study on Carbon-rich AGB Stars

Caputo D.P., Speck A.K., Barlow M.J., Wesson R., Volk K., Clayton G.C., 2008, American Astronomical Society, AAS Meeting #212, #6.04 (poster)

A nova inside a planetary nebula

Wesson R., the IPHAS consortium, 2008, National Astronomy Meeting, Belfast (talk)

The hydrogen-deficient knot of Abell 58

Wesson R., Barlow M.J., Liu X-W., Storey P.J., Ercolano B., De Marco O., Asymmetric Planetary Nebulae IV, Los Cancajos, La Palma, Spain, 18-22 June 2007 (talk)

Observations and modelling of the hydrogen-deficient knot of Abell 58

Wesson R., Barlow M.J., Liu X-W., Storey P.J., Ercolano B., De Marco O., Deep spectroscopy and modelling of emission line nebulae, Wo Fo Shan Zhuang, Beijing, P.R. China, 16-18 April 2007 (talk)

Probing Nebular Physical Conditions Using the Hydrogen Recombination Spectrum

Zhang Y., Liu X-W., Wesson R., Storey P.J., Liu Y., Danziger I.J., 2006, in Planetary Nebulae Beyond the Milky Way, p. 183 (poster)

Physical Conditions and Abundances in NGC 6543

Wesson R., Liu X-W., 2003, in Planetary Nebulae: Their Evolution and Role in the Universe, IAUS209, p. 379 (poster)

Physical Conditions and Abundances in Abell 30

Wesson R., Liu X-W., Barlow M.J., 2003, in Planetary Nebulae: Their Evolution and Role in the Universe, IAUS209, p. 381 (poster)

Others

Supernova 2008S in NGC 6946

Wesson R., Fabbri J.F., Barlow M.J., Meixner M., 2008, Central Bureau Electronic Telegrams, 1381, Edited by Green, D. W. E.

Theses

PhD (July 2004)

My PhD concerned the nebular abundance discrepancy problem. I carried out detailed studies of three nebulae, and also analysed spectra of a large sample of northern hemisphere planetary nebulae. I was looking at the abundances of elements like carbon, nitrogen and oxygen, trying to reconcile the discrepancy that exists between two ways of measuring how much of these elements there are in planetary nebula. I found that temperature fluctuations within a chemically homogeneous gas could not explain the observed discrepancies, but that there was strong evidence to support the idea that within the hot, rarefied gas of a typical planetary nebula, there are cool, dense clumps of gas which contain almost no hydrogen - the dominant element in the hot gas.

PhD Thesis - July 2004 - Heavy element abundances in emission line nebulae

MSci (September 2000)

The thesis I wrote during the final year of my MSci degree was on the far ultraviolet spectra of hot stars in the Magellanic Clouds, two small irregular galaxies which are orbiting our own galaxy. I calculated masses, radii, stellar wind speed and other parameters, and compared early results from the FUSE satellite with theoretical predictions of what the UV light from these stars would look like.

MSci Thesis - September 2000 - Spectral Synthesis of UV observations of LMC and SMC OB stars - Predictions for FUSE