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Дата изменения: Tue Dec 14 13:44:25 1999
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C.S. Jeffery, Research Astronomer

During the final stages in its life, a star is transformed in ways which remain poorly understood. It is well known that, at the end of hydrogen burning, a star expands to become more luminous and cooler -- a red giant. Thereafter, a low-mass star may contract and expand several times over, becoming alternately hotter/smaller (subdwarfs) and cooler/larger (giants) than the Sun. Each transformation is a response to changes in chemical composition in the stellar interior as nuclear reactions convert one source of fuel into another. In some cases these changes can take less than ten thousand years, in contrast to the Sun's hydrogen-burning lifetime of about ten billion years. Consequently, the number of stars currently in these late stages of stellar evolution is comparatively small.

The goal of our research is to trace how stars progress through these stages until they ultimately become white dwarfs. However, there are major difficulties. For most stars, we do not know how far away they are or precisely how large, luminous or massive they are. Secondly, we cannot look inside stars in order to deduce what chemical changes have taken place. We therefore use a variety of indicators, including the chemical composition of the stellar surface, pulsational properties, and binary companions to deduce this information.

The surfaces of extreme helium giants -- very luminous stars devoid of hydrogen - carry a `fossil record' of the star's previous evolution. An analysis of three such stars, begun in 1997 with the help of summer students Hamill and Jeffers, demonstrated the oxygen and carbon-rich nature of these stars and pointed to large-scale mixing between the surface and a highly-processed interior, these changes occurring sometime in the recent past. This work also led to the improvement of the modelling software used in the analysis and will be crucial in future research at Armagh.

Extreme helium giants evolve rapidly and pulsate. Measurements of their spectra in ultraviolet light are sensitive to small changes in temperature. An analysis of about 150 spectra obtained with the International Ultraviolet Explorer (IUE) was begun with the help of summer student Starling and showed that measurable changes are taking place in a number of helium giants.

The extremely helium-rich subdwarf V652Her was studied intensely. A chemical analysis proved that its surface is predominantly nitrogen-rich, pointing to an evolution quite distinct from that of the helium giants. New observations were obtained with the William Herschel Telescope and will lead to a very accurate description of its pulsation properties, whilst a theoretical study of its pulsations provided additional new insights. Meanwhile, new theoretical models of pulsation in this and other helium-rich subdwarfs identified a potential new class of variable star.

Extreme helium stars and helium-rich subdwarfs are examples of a larger class of stellar remnant broadly identified as hot subdwarfs. Previous work, with Drilling and others, on the spectral classification of hot subdwarfs has continued. Just as the Hertzsprung-Russell diagram for normal stars eventually helped to explain the evolution of young stars, the classification diagram for subdwarfs will help to explain the evolution of stellar remnants. Simon Jeffery and Ph.D. student Pilar Montañés Rodríguez began to calibrate the classification diagram using model atmospheres.

The origin of the largest class of hot subdwarf -- the subdwarf B stars -- remains a puzzle. Essentially naked helium cores, red giant stars stripped of their hydrogen-rich outer layers, the method for the removal of hydrogen is not known. One possibility is that the hydrogen could have been transferred onto a binary companion. The detection of infrared calcium lines in several sdB stars in 1997 demonstrated the unambiguous signature of a cool companion. Simon Jeffery and Ph.D. student Regina Aznar Cuadrado made further observations of sdB binaries in order to measure precise dimensions for both stars.

Much of this work is part of an ongoing programme with substantial results on V652Her due for early completion, and other theoretical work and new observations already carried out.

Other activities included:

1.

Telescope time and research grants: observations of binary and pulsating subdwarf B stars were made with the Isaac Newton and William Herschel Telescopes in La Palma. A PPARC research grant awarded for studies of pulsations in stellar remnants and announced in late 1998, will provide for a 3-year postdoctoral position, commencing 1999 July.

2.

Collaborative visits, conferences, and talks: a parallel session was organised and a presentation made on spectral analysis at the National Astronomy Meeting held in St Andrews in March 1998. Collaborative research visits were made to Japan (under the auspices of a 3-year British Council award) and to La Palma, to work with colleagues Hideyuki Saio and Don Pollacco. Colloquia were given on both occasions.

3.

PUS: The Armagh Astropark interpretation project progressed to the completion of designs for 5 display panels (now installed in the Astropark). Copies of these panels are included in Appendix G. This work was supplemented by a project to refurbish the interior of the Schmidt telescope dome to make it accessible to visitors and to provide an area for scientific displays (realised with the assistance of summer students Marshall and Tipper). Simon Jeffery also made a presentation of new results on the origin of gamma-ray bursts and wrote invited articles for popular astronomy magazines on the evolution of stars and on the origin of water on the Moon. He attended a PPARC-funded training course on Science and the Media.