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: http://star.arm.ac.uk/annrep/annrep99/node13.html
Дата изменения: Fri May 5 20:20:19 2000 Дата индексирования: Tue Oct 2 04:04:37 2012 Кодировка: Поисковые слова: mercury surface |
Stellar corpses are the burnt-out remains of stars which have exhausted their reserves of energy and have ceased to shine. All that may be seen is the glowing cinder as the star gently cools, or the signature of energized material trapped in their enormous gravitational and magnetic fields. Well known examples include black holes, white dwarfs and neutron stars. They are amongst the most exotic objects in the Galaxy -- our mission is to discover how normal and benign stars like the Sun reach such macabre ends. In pursuing it, we also study the origins of elements essential to human life, the physics of matter under extreme conditions and processes that affect the evolution of entire galaxies.
The conventional paradigm has a Sun-like star swell up to become a red giant as it finishes converting hydrogen to helium. Pausing to convert helium to carbon and oxygen, the expansion then continues until the star sheds its outer layers as a planetary nebula and shrinks to become a white dwarf. However many stars do not fit comfortably into this picture, particularly those which have become so mixed up that even their outer layers have no hydrogen left. These are the helium stars and the hot subdwarfs.
The problem demands that we study stars that are in transition between hydrogen burning and death. Such phases frequently do not last long -- a few thousand years or less -- and hence such stars are rare. We must measure their properties in as much detail as possible. For instance, we would like to know their mass, radius and luminosity at the very least. The chemical composition of their surface layers can provide clues to past evolution, particularly if material processed by nuclear reactions in the interior has been exposed at the stellar surface. It is also important to know if the star is or was one of a pair -- a binary star -- because such stars can exchange mass with their companion as they evolve.
The Armagh approach combines high-quality observations with the best possible theoretical models, covering every aspect of stellar structure from the deep interior to the outermost layers of the atmosphere. Our theoretical work involves making hypotheses about the origin of given stars. These define boundary conditions for solving the time-dependent equations of stellar structure. Such solutions show long-term evolution in response to changes in chemical composition at different points within the star, and short-term changes (pulsations) in response to instabilities in the energy flow from the star. Our theoretical work also involves the construction of detailed models of the outermost layers of the star and the spectrum of radiation they emit.
In 1999, Simon Jeffery and Hideyuki Saio (Japan) completed a new evolutionary model for the origin of one particular helium star, V652Herculis, which involves two helium white dwarfs in a binary. The capture of one white dwarf by the other provides a new source of nuclear fuel so that the star is able to expand briefly to become a giant. Unlike most stars which burn from the inside out, nuclear reactions are ignited on the outside of the original white dwarf and then start to burn inwards. As it does so, the star's properties change so that at one point they match the current properties of V652Her almost perfectly.
The analysis of stellar spectra can be a time-consuming and subjective task which involves the fitting of models with many free parameters to observations of varying quality. An important development in 1999 was the initiation of a project to build software for the automatic, efficient and objective analysis of stellar spectra. By the middle of the year, we were able to measure the effective temperatures, apparent diameters and the interstellar extinctions for both single and binary stars from their observed flux distributions. This enabled observational studies of evolution and pulsation in helium stars and of the physical dimensions of hot subdwarfs with cool companions to be rapidly progressed. The new software will be extended to measure other properties of stellar atmospheres during 2000.
The third major achievement of 1999 were two detailed studies of the pulsating star LSS3184 -- a helium star very similar to V652Her. The first was based on observations obtained by Simon Jeffery at the South African Astronomical Observatory in 1995. Although normally impossible to measure for a single non-variable star, radius and distance can be measured for a pulsating star by observing its colours and surface motion through a pulsation cycle. Together with a previous measurement of surface gravity, these observations pointed to a mass too small to be realistic. Dr Vincent Woolf joined the Armagh team as a postdoctoral research assistant in 1999 July, and commenced work on very high-quality observations obtained by Simon Jeffery with the Anglo-Australian and the Hubble Space Telescopes in 1996/1997. These set new and tight constraints on the radius of LSS3184 and partially resolved the mass problem.
In 1900, FGSagittae was an uknown faint blue star. Since then it expanded to become a cool giant helium star. With Schönberner (Potsdam), Simon Jeffery commenced a project to measure changes in its surface composition during the expansion using archival spectra. This complements an ongoing project with Don Pollacco (QUB) to study the chemical evolution of Sakurai's object -- an even more rapidly expanding star which suddenly appeared in 1996.
Short-period non-radial pulsations in hot subdwarfs were discovered during the early 1990's using photometric techniques. Simon Jeffery and Don Pollacco made high-speed measurements of their surface motions; 2600 spectra were have been analyzed and exciting new results will be announced in 2000.
Because they are rare, the identification of new helium stars and helium star classes is an ongoing task of stellar pathology. New surveys and instruments provide thousands of new spectra which require classification and analysis. The definition of a spectral classification system for hot subdwarfs (with John Drilling, Louisiana) is nearly complete. Brian Mahon (Trinity College Dublin) and Simon Jeffery applied neural network software to the automatic classification of subdwarf spectra.
Regina Aznar Cuadrado and Simon Jeffery also made observations of binary subdwarf B stars and extreme helium stars with the Isaac Newton and Jacobus Kapteyn Telescopes in La Palma. An inconclusive search for periodic variations (pulsations) in two hot helium stars was presented at IAU Symposium 176 in Budapest. Regina Aznar Cuadrado is using spectra and photometry of hot subdwarfs to measure the absolute dimensions of those stars with faint companions.
Pilar Montañés Rodríguez studied the effect of projection on the profiles of spectral lines in pulsating helium stars and presented her results at IAU symposium 176. She commenced hydrodynamical calculations of the pulsations in luminous helium stars.
Vincent Woolf made spectroscopic observations of post-AGB stars with the Anglo-Australian Telescope, with a view to exploring the surface exposure of elements produced by the nuclear s-process while the star was a red giant.
Observations made by Simon Jeffery with the International Ultraviolet Explorer of the helium-rich white dwarf HS2253+8023 containing traces of hydrogen and metals were analysed and shown to be consistent with a relatively cool (15000K) white dwarf accreting material from the interstellar medium.
During the year Simon Jeffery also obtained a number of external research grants (including a 3-year PPARC PDRA grant to support Dr Woolf), and presented colloquia locally and abroad, and a number of popular talks.