AAVSO: Photoelectric Photometry Newsletter Volume 20, No. 1

Most of the stars on the AAVSO PEP program are small-amplitude pulsating red giants. It is apparent, from the 15 years of results from the program, that many of these stars have long-term variations on a time scale of hundreds to thousands of days --- much longer than any radial pulsation period of the star. What is the nature and cause of these long-term variations? I have recently seen two suggestions.

During my visit to New Zealand, I was able to read a recent unpublished 1999 PhD thesis by Irene Cummings, University of Canterbury, in Christchurch. She carried out a long-term spectroscopic and photometric study of a sample of these stars. Her work suggests that the long-term variations may be a rotational phenomenon. The "surface" of the star is not uniform, due perhaps to the presence of large convective cells. The star rotates with a period of hundreds to thousands of days, producing semi-regular variations on this time scale.

Another possibility was presented by Peter Wood at the 1999 colloquium mentioned below. He reported that "a new type of oscillatory thermal mode has been found in (models of) red giant stars. The modes arise because of the interaction of stellar oscillations and convective energy transport. All the modes found are highly damped (stable). However, it is possible that a different treatment of convection could lead to unstable modes, capable of explaining the long secondary periods observed in some red giants."

Therefore it is possible that the long-term photometry of small-amplitude red variables, by the AAVSO PEP program, could help us choose between these hypotheses, and to understand the physical processes in these stars.

Variable Star of the Month

I highly recommend this feature of the AAVSO web site www.aavso.org; go to "variable stars", and then to "variable star of the month". Two stars of interest to the AAVSO photoelectric photometry program have been highlighted recently: R Scuti in July 2000, and CH Cygni in August 2000. R Scuti is not actually on the PEP program, but two other RV Tauri stars are --- AC Her and U Mon. The page on R Scuti includes an excellent mini-review of RV Tauri stars, and some key references. Congratulations to Kerri Malatesta and Kate Davis, of AAVSO HQ, for preparing these.

A Book To Recommend

I try to attend as many conferences on variable stars as possible, both to keep up with the field, and to present the results of the projects which my students carry out --- often using your data. In particular, I try to attend the biennial conferences on pulsating stars. But I missed the last one, in Budapest, in August 1999. It was a dandy, because it coincided with the total solar eclipse. And the skies were clear. And, because of this attraction, there was a very good attendance --- which resulted in lots of good papers.

The Proceedings are now available: The Impact of Large-Scale Surveys on Pulsating Star Research, edited by Laszlo Szabados and Don Kurtz, published in the Astronomical Society of the Pacific's Conference Series, Volume 203. This volume is full of interesting reviews and research reports --- 549 pages worth. Although these are intended for professionals and graduate students, and written in "dense" style, most of them would be understandable by advanced amateurs. And the price is not "out of this world". You can order a copy through the ASP web site: www.aspsky.org

Highlighting Small Telescopes

I strongly recommend to you an article in Science --- one of the two most prestigious general science journals in the world. Author R. Irion (Science, vol. 289, 7 July 2000) presents an excellent overview of the role of small telescopes in astronomy, and the "problem" that so many small professional telescopes are being closed down. Amateurs, of course, are filling the gap. AAVSO Director Janet Mattei is one of several small-telescope experts who are interviewed in this article. Variable star observation, of course, is "front and centre".

Yellow Semi-Regular Variables

Last year, my student David L. Kolin carried out a study of several SRd variables. These are a "mixed bag", ranging from yellow hypergiant stars like rho Cas, to less extreme stars which are more akin to the RV Tauri stars and the Population II Cepheids. His project has now been published in JAAVSO 28, Number 1, 2000, page 1. I refer you to it for another example of how long-term monitoring of slow variable stars can provide information on the nature and evolution of the stars.

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