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M. de Groot, Consultant Research Associate

Correlation between Spectroscopy and Photometry in PCygni

Data from the Automatic Photoelectric Telescope Service in Arizona continued to be collected. A paper analysing the photometry of PCygni during the 20th century is in preparation in collaboration with C.Sterken, Brussels, and A. van Genderen, Leiden.

An investigation of H$\alpha$ spectroscopy and U!BV photometry of PCygni covering the period 1990 to 1994 has shown that the star varies on a number of different time-scales ranging from a few months to 7 years. The longer time-scale variations are caused by normal SDor variations; non-radial pulsations -- s-mode or g-mode -- are the suspected cause of the shorter time-scale variations. Real variations in the stellar wind are found to be connected to processes in PCygni's photosphere. A paper in collaboration with N. Markova (Bulgaria), S. Scuderi (Italy), and N. Panagia (USA), has recently been submitted.

On the invitation of its Editor, a review article on PCygni, in collaboration with G. Israelian (Tenerife) was produced for Astrophysics and Space Science.

Investigation of the SDor Phenomenon in Luminous Blue Variables

A photometric study of $\eta$Carinae covering the period 1974 to 1998 has provided further evidence that this star is more complex than hitherto believed: a normal SDor variable showing both long-term (a few years) and short-term (a few months) photometric variations seems to be surrounded by a hot luminous disc. The nature of $\eta$Carinae is being debated in the literature and at scientific meetings. We -- de Groot, Sterken, and Van Genderen -- do not find convincing evidence for binarity as proposed by others.

Visual observations of some 20 southern luminous blue variable stars, most of them in the Large Magellanic Cloud, continued to be collected by Albert Jones (New Zealand) and to be made available for analysis by a Nuffield Science Bursary student. Quasi-periodic light variations were found in several of these stars.

Other Activities

During 1999, Mart de Groot served as Co-Editor of The Journal of Astronomical Data. He also delivered a number of popular lectures in various parts of the world, and supervised two Nuffield Science Bursary students. He is currently Chairman of the Scientific and Local Organizing Committee of the international workshop ``PCygni 2000: Four Hundred Years of Progress'', to be held in Armagh from 20-23 August 2000. The conference has been financially supported by sponsorship from the First Trust Bank, Armagh.