AAVSO: Photoelectric Photometry Newsletter Volume 20, No. 1

August 2000 marked the 400th anniversary of the first (recorded) outburst of P Cygni --- the prototype blue hypergiant variable, and the prototype mass-losing star. This star is almost a million times more luminous than the sun, and its radiation is driving its atmosphere off at a prodigious rate. Astronomers still do not understand most aspects of this star, so a workshop-type conference was in order. The conference was organized by Mart de Groot --- one of the most active researchers on this star. The conference also marked his formal retirement from astronomy, as he takes up a new career in church work.

Armagh is a pleasant little city in Northern Ireland, steeped in history. It is a city of churches, and educational institutions, and the only planetarium in all of Ireland. Armagh Observatory is a professional observatory, with over 200 years of history. It is close to the centre of the city, but surrounded by parkland, and by schools and other public buildings.

The conference was attended by about 50 astronomers from many countries, including Kees de Jager from Holland --- the foremost authority on the most luminous stars. I presented a paper on long-term photometric observations of P Cygni, from a robotic telescope, and from the AAVSO PEP and visual programs. My co-authors were Greg Henry (Tennessee State U), my student Trevor Evans, and Janet Mattei. Here is the abstract of our paper; see Figure 1 below:

Figure 1: Long-term V light curve of the blue hypergiant P Cygni, based on AAVSO PEP observations.

"We report over 13 years of photometric observations of P Cygni from a robotic telescope (VRI:JD 2446512-2451360), from the American Association of Variable Star Observers (AAVSO) photoelectric photometry program (V: JD 2446200-2451500), and from the AAVSO visual program (m(v): JD 2443500- 2451500 and some earlier observations). We have analyzed these observations using light curves, Fourier analysis, and autocorrelation analysis. Light curves of the two photoelectric datasets show variations of up to 0.1 magnitude on time scales of years, months, weeks, and as short as 10 days. Fourier analysis and autocorrelation analysis show that the variations are not periodic, but there are characteristic time scales of 50-200 days and also around 400 days. The visual data, when corrected for a small (0.01) seasonal effect, are capable of revealing variations of 0.1 magnitude on time scales of years, but the evidence for longer-term (decades) variations is weak. The time scales and amplitudes which we have observed are in accord with previous results. Our data provide a useful comparison with data obtained spectroscopically, or at other wavelengths."

The "Spectroscopic Twins" Perform Again

Just before the Armagh conference, and at the Armagh conference, I heard that both rho Cas and V509 Cas (HR 8752) were undergoing large changes in brightness and/or colour. These are yellow hypergiant stars which have been on the AAVSO PEP program for many years. If you are an AAVSO PEP observer, please observe these stars every week or two.

Variable Be Stars

Several bright, active Be stars are on the AAVSO PEP program. This summer, there was a dramatic demonstration of the variability of these stars, and the need to monitor them on a long-term basis. The second-magnitude delta Sco brightened by about 0.3 magnitude to about V = 2.0. The brightening was actually discovered visually, and confirmed photoelectrically. AAVSO observer Sebastian Otero (Buenos Aires) first reported it brightening slowly from its long-time magnitude of 2.3 on 30 June, and reported it at visual magnitude 2.0 by 20 July.

There is also a class of erupting B stars in the Magellanic Clouds, which have been discovered as a by-product of gravitational micro-lensing surveys. They have been styled "bumpers" by the directors of these surveys, but I have been trying to convince them that they are seeing the same thing as we have been seeing among the bright, active Be stars for many decades!

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