Photoelectric Photometry Newsletter

Editor's Message

This issue features some new results from the HIPPARCOS satellite database. This year, some of my research students have been using this astrometric and photometric data for projects related to small-amplitude red variables - the stars which figure prominently in the AAVSO PEP program. Metin Guler has been investigating the onset of pulsation among red giants; Branko Miskov has been looking for interesting new multiperiodic variables; Michael Parkes has determined the pulsation mode of many of the best-observed red variables in the AAVSO PEP program. As I imply in the long article below, this photometry complements the photometry which you the AAVSO PEP observers have been doing. It will be exciting to put the two databases together, and also to use the HIPPARCOS results to choose new stars for our program.

We are still working to install the program (i.e. the list of stars) on the AAVSO web site, so potential observers can download the information without going through Howard Landis or me. Of course, we are still here to help.

W Cygni is a bright small-amplitude red variable on the AAVSO PEP program. It has a long and interesting history, which is summarized by J.J. Howarth in JBAA 101, 101-106 (1991). At some times, the star has a period of 131 days; at other times, 234 days. Sometimes the variability is even more complex, and sometimes the star is almost constant. Astronomers do not know why the star cycles between these different forms of behaviour.

It is therefore appropriate that W Cygni is "front and centre" in the AAVSO Hands-On Astrophysics education project. This project uses variable star ob- servation and analysis to develop and integrate a wide range of science, math, and computing skills at the high school and introductory college level. This project - many years in the making - is now complete and available. Contact AAVSO HQ (aavso@aavso.org, www.aavso.org, or the letterhead address) for further details. The project material includes software, a large database of ob- servations, charts, slides, prints, educational videos, and a marvellous students' and teachers' manual to tie the project together.

W Cygni is highlighted in the slides and prints. Students can measure the star on the slides and prints, explore its past behaviour in the database, then go out under the real sky to find out what W Cygni is doing tonight. Perhaps they can find regularities in the star's puzzling behaviour. There are dozens of other variable stars which are represented in the database, and on the charts. The techniques of observation and analysis, which the students learn, can be applied to any other variable star in the sky.

We hope that Hands-On Astrophysics not only enhances science and math education, but also helps to attract a whole new generation of variable star observers.

John R. Percy and Yvonne Tang

RU Cam is a population II Cepheid which "stopped pulsating" in 1965-66. Actually, it did not stop pulsating completely; the amplitude decreased from over a magnitude to about 0.20, and remained stable at that level from 1967 to 1982, according to the work of Bela Szeidl and his colleagues. The period has fluctuated erratically between 17.4 and 26.6 days, but this may be the result of random, cycle-to-cycle fluctuations. As noted below, the HIPPARCOS satellite found a mean period and amplitude of 22.24 days and 0.20 magmitude, during its 3.5-year mission.

Right: The differential photoelectric light curve of the peculiar Population II Cepheid RU Cam, in V light, relative to the comparison star HD 57201. The period is 22.2 days.

RU Cam has been observed by the AAVSO PEP program for about- ten years, but the observations have been very sparse - a few dozen observations of this faint, small-amplitude star. We therefore concentrated on 18 observations during the 1994 season. We used the VSTAR software developed by the AAVSO for its Hands-On Astrophysics education project.

To our pleasant surprise, the best period was 22-20 days (in excellent agree- ment with HIPPARCOS); it gave a good phase diagram with an amplitude of 0.20 magnitude as shown below. It appears that this star has made a clean transition from a large-amplitude pulsator to a small-amplitude pulsator. The explanation for this behaviour is not clear.

A few months ago, I exchanged some correspondence with one of our observers on the topic of d Ser. This star is on the AAVSO PEP program, but is suspected to be non-variable. The HIPPARCOS results support this conclusion. The observer had noted some apparent variability in the (variable - comparison) magnitude. My inspection of the (check - comparison) magnitudes showed a similar degree of scatter, which suggested that the apparent variability of d Ser was instrumental. Which goes to show - it is important to observe the check star as often as possible. It represents your "quality control".

Phil Manker (Americus GA) has sent light curves of these two stars. The cov- erage, especially for 12 Vul, was quite complete. The normal variations of these two stars are a few hundredths of a magnitude, but there are occasional increases or decreases of 0.1 or 0.2 on a time scale of weeks. So don't expect a lot of "ac- tion". They are on the PEP program because they need careful measurement. And don't forget the check star.

R.W. Jones, Fish Hoek, South Africa, continues to provide important observa- tions of southern stars in the AAVSO PEP program. T Cet appears to have a period of about 150 days, in good agreement with the catalogue value. Z Eri and RR Eri have periods of about 100 days, also in good agreement with previous results. The star which is potentially most interesting is AG Cet, which has a light curve with a cycle length of about 45 days, but strongly modulated in amplitude. This may indicate the presence of a secondary period. Double-mode pulsators of all kinds are interesting and important. They are very rare among the pulsating red giants.

Fanie de Villiers, writing from South Africa, where he and R.W. Jones have been very active in observing southern stars in our program, asks: "I am uncertain as to the effect that moonlight may have on the accuracy of PEP observations. Could you please give me some guidance?"

This is a good, but complicated question. It is certainly unwise to observe a variable or comparison star if the moon is within 30 degrees or so. If the moon is further away, its effect depends on the clarity of the sky, and the faintness of the star. Haze will pick up the moonlight, and the sky brightness may swamp the star. If there is any uneven cloud, this will produce uneven sky brightness (and obscuration of the star, of course!), and photoelectric photometry should not be done.

It's always interesting to read about the unique properties of our program stars - especially when they are reported in a journal as prestigious as Science. In the 6 February 1998 issue, it is reported (by Donald E. Jennings and Pedro V. Sada) that there is hot water vapour in the atmospheres of the M supergiants Betelgeuse and Antares. So if you want to take a sauna.....

This item is written in tribute to my late colleague Karl Kamper, who passed away in January after a long and heroic battle with cancer. As well as developing and maintaining first-class spectroscopic instrumentation at the David Dunlap Observatory, University of Toronto, he carried on an active scientific program in astrometry, including precision radial velocity studies of stars such as Polaris.

Polaris is a small-amplitude Cepheid variable, with a period close to four days. In the early 1980's, Armando Arellano Ferro discovered, in the course of his PhD research at the University of Toronto, that the photometric and radial velocity amplitudes of Polaris had decreased significantly. Further studies, especially by Don Fernie and Karl Kamper, confirmed that the amplitude was continuing to decrease, and might well decrease to zero. Was it good-bye to Polaris the Cepheid? And if so, why?

Fernie and Kamper's most recent results were written up by Fernie, shortly before Kamper's death; they allow for two possible interpretations: (i) the pho- tometric amplitude has stabilized at about 0.03 or (ii) if all available data are used, the amplitude will decrease to zero in the next decade.

There was actually an error in the original prediction of the date of demise of Polaris the Cepheid, and it is worth noting the reason. It occurred because of a misunderstanding of the meaning of "amplitude". Is it the peak-to-peak difference in magnitude, or is it the coefficient of the sine curve which fits the data? The latter would be one-half the former. Many of us are guilty of this same ambiguity when we use the term.

Is Polaris evolving out of the Cepheid instability strip in the H-R diagram? Initially, this seemed like a promising explanation. Several years ago, however, Fernie pointed out that Polaris was well inside the instability strip, with similar period and temperature to Cepheids with normal large amplitude. But the Hipparcos satellite has put a new slant on this by showing that Polaris is most likely pulsating in the first overtone mode. Theorist Siobahn Morgan has shown that Polaris may well be at the "red edge" of the instability strip for first overtone pulsation, and may be evolving out of it.

These studies of Polaris - the best known star in the (northern) night sky - illustrate many things: the importance of long-term photometric monitoring of variable stars; the role of photometry of bright stars, using small telescopes; and the new insights which are still to be gained in stellar astronomy. For those of you who have access to the Canadian astronomy magazine "SkyNews", I call your attention to the excellent article on Polaris by Ivan Semeniuk, in the March-April 1998 issue. It is a wonderful remembrance of Karl Kamper's scientific work.