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Дата изменения: Fri May 5 20:20:19 2000
Дата индексирования: Tue Oct 2 04:04:18 2012
A new PhD student, David García Alvarez, began work with Dr John Butler and Professor Gerry Doyle in October 1999 to continue the analysis and modelling of stellar flares started by Eric Houdebine and John Butler in the late 1980s and early 1990s, subsequently continued by Darko Jevremovic from 1995-1999. Dr Jevremovic completed his thesis (`Hydrogen Balmer Lines in Stellar Flares') and was awarded his degree in 1999.
In his thesis work, Darko Jevremovic developed a new tool for analysing the Balmer decrements which made it possible to estimate the electron density and electron temperature of the flare plasma and the temperature of the underlying background source. Applied to data of flares on Gliese866 he was able to estimate the area of hydrogen emitting flare regions as 1-13% of the stellar surface. The physics of the flare process was taken back a further stage towards its initial cause by the development of a new gas-dynamic radiative transfer code with heating by energetic electron beams. This code satisfactorily predicted the behaviour of the H line during a flare observed on ADLeo with the William Herschel Telescope (WHT) in 1998.
A paper with Dr Armin Theissen, giving full details of the astrometric and photometric observations of the open cluster Stock2, has been accepted for publication. Proposals for follow-up medium-resolution and high-resolution spectroscopic observations of probable members of this cluster did not receive the requisite allocation of telescope time, and as a result it was decided to adopt a different and less observationally demanding approach to our goal of establishing the rotational velocities of cluster members. This involved photometric monitoring using a wide-field CCD camera on a medium-sized telescope. Seven nights were awarded on the 1m Ritchey-Chrétien Telescope at Siding Spring by the Mount Stromlo and Siding Spring Observatory in February 1999 and again in January 2000. In the first run, mediocre weather conditions prevented successful photometric tranfers, but did allow some variability studies to be made. During the second run, in January 2000, two photometric nights allowed photometric transfers to be completed. The data are in the process of reduction and analysis.
The French satellite programme COROT, which will be dedicated to micromagnitude photometric observations of main-sequence stars, has now received a firm commitment for launch. The project has two main objectives: (1) to determine the internal structure of main-sequence stars using asteroseismology techniques; and (2) to search for Earth-like planets around main-sequence stars from optical occultations.
The selection of the target list using high-resolution spectroscopic data of around 1000 candidate stars is a major observational programme which requires international collaboration. As partners in the project, Armagh Observatory has been asked to contribute to the selection process in exchange for priority access to the data which will eventually flow from the satellite. A one-week run on the South African Astronomical Observatory (SAAO) 1.9m telescope using the `Giraffe' Echelle Spectrograph was awarded to this programme. About 80 high-resolution spectra were obtained, which have now been reduced and classified by Claude Catala. A further application for time on the WHT has been made. In addition to the selection of COROT targets, this project will result in a substantial databank of high-resolution spectra which will be made available for stellar atmosphere studies. Eventually, when the satellite is operational, Armagh Observatory will gain priority access to a vast and highly original databank of unparalled photometric accuracy on a wide variety of stars.
It is now widely accepted that solar activity plays some role in climate change, but the relative strength of its influence compared to the enhanced greenhouse effect is still a subject of dispute. The solar activity effect can be divided into two parts: (1) the direct effect of a solar luminosity change associated with solar activity, which many agree can explain roughly 10-20% of the current global warming; and (2) an indirect effect, that is difficult to quantify, but which depends on the influence of solar activity on other less easily identifiable physical processes. One such possible process is the influence of changes in solar activity on cosmic-rays and their effect on the Earth's cloud factor.
An anlaysis of data from the International Satellite Cloud Cover Project (ISCCP), by Enric Pallé Bagó and John Butler has shown that previously identified correlations between the total cloud cover over oceans and cosmic-ray flux for the period 1984-1991, do not continue during the period 1991-1994. However, when the data are separated into low, medium and high clouds, a clear correlation is evident between cosmic-ray flux and low clouds over the whole period covered by the ISCCP data from 1984-1994. Assuming there is no feedback mechanism or other long-term changes, it is then possible to compute the enhanced warming by low clouds and their influence on the Earth's albedo. Preliminary results suggest that much of the global warming since the late nineteenth century can be explained by a combination of the direct and indirect (cosmic-ray induced) effects of solar activity.
John Butler wrote short biographies of several former Armagh Observatory staff (E.M. Lindsay, J.L.E. Dreyer and E.J. Öpik) for the book People and Places in Irish Science, edited by Charles Mollan (Royal Irish Academy, Dublin), and served on several external committees including the Court of the University of Ulster and the Birr Scientific Heritage Trust. He has also been the principal point of contact between Armagh Observatory and Trinity College Dublin (TCD), for the programme in which, each autumn, Armagh Observatory staff develop and supervise undergraduate astronomy projects for final-year TCD students.