Photoelectric Photometry Newsletter

Volume 20, No. 2, May 2001

Contributions to this Newsletter are gratefully received at any time. Please send them to: John Percy, Erindale Campus, University of Toronto, Mississauga ON, Canada L5L lC6; e-mail: jpercy@erin.utoronto.ca

Contents:

• A Tribute to Howard Landis
• Pulsating Red Giants: New Results
• Historical Archive Photometry of MU Cephei
• RS CVn Stars in the AAVSO PEP Program
• Photoelectric Monitoring of Bright Be Stars. IV. 1996-99
• Autocorrelation analysis of RV Tauri Stars
• The Harlow Shapley Visiting Lecturer Program
• 'Partners in Astronomy' Published

A Tribute to Howard Landis

Howard has been honoured by the AAVSO Merit Award, and by the ``Friends" award of the Natural Sciences and Engineering Research Council of Canada. On behalf of the AAVSO, and the astronomical community, I thank Howard for his dedicated and untiring effort, and wish him a happy forthcoming 80th birthday -- and many more beyond.

Pulsating Red Giants: New Results

My collaborators are: Heather Dunlop and Lola Kassim, in the University of Toronto Mentorship Program, which enables outstanding senior high school students to work on research projects at the university; Ryan Kastrukoff and Zoë Nyssa, in the Research Opportunities Program, which enables second-year students in the Faculty of Arts and Science, University of Toronto, to do original research for course credit; Asif Hussain and Joseph Wilson, graduating students in the Astronomy Major Program at the University of Toronto; Greg Henry from Tennessee State University; and the AAVSO Photoelectric Photometry Program. The data for this study came mostly from the AAVSO Photoelectric Program, and from Greg Henry's robotic telescope in Arizona.

Introduction. Red giant stars are large, luminous, cool stars. There is lots of interesting physics in red giants, as AAVSO President Lee Anne Willson knows; she is one of the experts on this subject. At least 10 per cent of the naked-eye stars are cool red giants. The sun will expand and become a red giant in five billion years. It will engulf the inner solar system, thus providing our descendants with the ultimate variable star observing opportunity.

For red giants are also variable (or I would probably not be discussing them at this meeting). The variability of the large-amplitude red giant Mira was discovered in 1596; the AAVSO recently celebrated the 400th anniversary of this discovery. In 1930, Joel Stebbins and Charles Huffer published a landmark study, using one of the first photoelectric photometers to show that almost all cool red giants were variable in brightness. The amplitude increased from M0III (warmer red giants) to M6III (cooler red giants). This work was done at the Washburn Observatory in Madison WI, where this meeting is being held. In the 1970's, the late Olin Eggen discovered, studied, and classified pulsating red giants as Large, Medium, and Small-Amplitude Red Variables -- LARV's, MARV's, and SARV's. When the AAVSO Photoelectric Photometry Program was established in the 1980's, most of the program stars were SARV's, and that is still the case today. In the 1990's, several precision photometry projects showed that even warmer red giants -- K5III to M0III -- were variable with amplitudes of only a few hundredths of a magnitude. For every Mira star in the sky, there are dozens of SARV's.

Methods of Analysis. We have used several different methods of analysis to study the regularity and irregularity of pulsating red giants. (i) We have used light curves, of course, to see visually the relation between magnitude and time; (ii) We have used Fourier or power-spectrum analysis to look for strict periods. This method can detect two or more periods, as long as they are regular, but is prone to so-called `` alias periods" if the data-sampling has periodicities, as it usually does; (iii) We have used autocorrelation analysis to look for characteristic ``time scales" of variability, even if they are not strictly periodic; (iv) We have have usedd wavelet analysis in a trial way: this method can track the changing period(s) and amplitude in a variable star; it has already been used extensively on Mira stars, but not much on SARV's.

We have used two versions of autocorrelation analysis. In most of our studies, we have used a version which, for all pairs of measurements, plots Δ mag versus Δ time; Δ mag will be a minimum when Δ time is a multiple of the characteristic time scale. We have now implemented a second version, based on an algorithm published by Burki et al. (1978 A&A, 65, 363); essentially, this version shifts the data sideways in time, and determines the fit; when the data is shifted by an integral number of characteristic time scales, the fit will be best. We hope to make our programs publicly available on the web, for others to use, later in the year.

Results. Figure 1 shows a selection of long-term and short-term robotic-telescope light curves of five SARV's; TV Psc, EG And, and RZ Ari are normal red giants; Z Psc and 4 Ori are chemically-peculiar red giants.

Click image to enlarge. The long-term (5000 days) and short-term (200 days) light curves of 5 SARV's. The scales are identical for each.

The AAVSO Photoelectric Photometry Program has now amassed the largest collection of long-term photometry on SARV's. Results on two dozen stars were published by Percy et al. (1996 PASP, 108, 139). Now, 25 more stars have been analyzed; the results were recently published in IAU Information Bulletin on Variable Stars, #5041. These are stars which either did not have sufficient observations for analysis in 1996, or were added to the PEP program after the other stars. A second large dataset of photometry has been provided by Greg Henry of Tennessee State University; it includes about 35 stars, with slight overlap with the AAVSO sample. This photometry is in three colours: VRI. Typical results from both datasets include the following:

• Almost all pulsating red giants have a period in the range 20-200 days, which we believe is due to radial pulsation.
• The amplitude of this period can be variable in time.
• Some pulsating red giants have a second period -- often half or twice the first -- which may also be due to radial pulsation.
• Occasionally, stars appear to ``switch modes"; whether one mode switches off while the other switches on, or whether the relative amplitudes of the modes rises and falls (but never becomes zero), is not yet clear.
• Many pulsating red giants have a second period which is about ten times the first one. The cause of this long period is not clear: in EG And, it is half the spectroscopic binary period, so it is presumably due to ellipsoidal variability in this case. This is probably not true for most other long-period red giants. Rotation is one possible mechanism. Another mode of pulsation -- non-radial? -- is another.

Wavelet Analysis. We were interested in the nature of the changes in amplitude in these stars, and also in their apparent mode switches. Wavelet analysis is a potentially good tool to use. There is sophisticated software on the AAVSO web site for this purpose. It was been used extensively for large- and medium-amplitude pulsating red giants, but not for SARV's.

We first used it on two datasets on EU Del -- the ``prototype" SARV. One was the AAVSO photoelectric dataset; the other was 5-day means of the AAVSO visual observations. The latter are less accurate than the photoelectric observations, but there are more of them to ``beat down the error". We found that the well-known 62.5-day period of this star was clearly visible in both datasets, but occasionally the star showed a longer period instead. The times when this occurred seemed to be the same from both datasets, but the value of the longer period was 78 days in the photoelectric dataset, and 90 days in the visual dataset! We must now figure out whether the longer period is real (and what its value is) or whether it is some kind of spurious alias period.

We then used wavelet analysis on the AAVSO photoelectric measurements of W Boo -- a SARV which, from previous measurements and analysis, appears to switch periods: using power spectrum analysis on measurements from different seasons, we found that the star's dominant period was about 25 days from 1985 to 1990, and was about 50 days in 1991, 1992 and 1994, and was about 35 days in 1993. Wavelet analysis appears to confirm that this star shows periods of about 25, 37 and 55 days -- even though the amplitude of this star is less than 0.2 mag.

Another interesting result of the wavelet analysis is that the amplitude of EU Del appears to rise and fall on a time scale of 1000 days. This phenomenon will be investigated further, in the future.

VSARV's: Finally, we studied 11 of the very-small-amplitude variables discovered by Henry et al. (2000 ApJS, 130,201). These are K5-M0III stars with amplitudes of mostly 0.04 to 0.07 in V. We used light curves, power spectra, and autocorrelation analysis. The analysis was very difficult because of the small amplitudes, and the complex behaviour of the stars.

The fundamental periods were between 4.8 and 19.5 days. It was difficult to identify secondary periods which differed by a factor of two, but at least one of the 11 stars seemed to show such a period. Only two of the stars showed any evidence of a second period about 10 times the first and, in each case, the result is uncertain.

The Next Steps. Our goals for the next few months (and these will be carried out by summer students) are: (i) to use new improved temperatures and luminosities of all these stars to determine their pulsation mode; (ii) to use these same temperatures to see whether the periods, amplitudes and/or pulsation modes vary systematically with temperature (or any other property of the stars); (iii) to study the systematics of the very long periods (hundreds to thousands of days) in order to get some clues about their cause.

Historical Archive Photometry of MU Cephei

ftp://ftp.lowell.edu/pub/bas/varseq

RS CVn Stars in the AAVSO PEP Program

My co-authors are Devi Soondarsingh and Vince Velocci; they were participants in the University of Toronto Mentorship Program, which enables outstanding senior high school students to work on research projects at the university.

Abstract: We report V photometry of three RS CVn stars --- HK Lac, SZ Psc, and λ And --- from the AAVSO photoelectric photometry program, and from transformed Hipparcos photometry, over 3500 days. Eighteen PEP observers contributed to this project. The stars vary on periods of days to weeks, due to rotation of a spotted photosphere. The rotational light curves vary on time scales of months to years, due to changes in the area and/or distribution of the spots.

Observers: The following observers contributed the following numbers of measurements to this project: Frank Dempsey (9), Brian Hakes (17), John Isles (11), Robert Johnsson (7), Paul Kneipp (1), George Kohl (13), Kenneth Luedeke (47), Phil Manker (3), Russell Milton (45), Harry Powell (1), Donald Pray (106), Mike Smith (1), Lee Snyder (6), Hans Sorensen (70), Nick Stoikidis (15), Raymond Thompson (24), Thomas Walker (4), Jim Wood (22).

Additional observations were obtained from the Epoch Photometry Database of the Hipparcos satellite, and transformed to the standard V system.

Photoelectric Monitoring of Bright Be Stars. IV. 1996-99

My co-author is Akos Bakos, going into the fourth year of the Astronomy and Physics Specialist Program at the University of Toronto.

Abstract: We report long-term UBV observations of 15 bright, active Be stars, namely: X Per, EW CMa, θ CrB, 4 (V839) Her, 88 (V744) Her, 66 (V2048) Oph, NW Ser, CX Dra, 12 (V395) Vul, 28 (V1624) Cyg, QR Vul, 59 (V832) Cyg, EW Lac, o And, and KX And. The observations were made in 1996-99 through the Automatic Photometric Telescope Service in Arizona, and through the American Association of Variable Star Observers (AAVSO) photoelectric photometry program, and have been added to a database extending back 20 years. We describe the stars' recent behavior, and also comment on the long-term behavior of some of them. They vary photometrically on time scales ranging from about a day, to many years.

Observers: The observers, and the number of observations which they contributed to this project, are as follows: Ted Beresky (74), Bill Barksdale (5), Jack Crast (5), Louis Cox (10), Frank Dempsey (36), Sergio Dallaporta (113), Fanie de Villiers (9), Brian Hakes (10), Robert Johnsson (3), Kenneth Luedeke (70), Phil Manker (64), Mike Smith (6), Hans Sorensen (31), Nick Stoikidis (16), Raymond Thompson (107), David B. Williams (3), and Jim Wood (98).

Additional measurements were obtained using a robotic telescope in Arizona. The AAVSO and robotic measurements tend to be complementary, not a duplication.

Autocorrelation analysis of RV Tauri Stars

AAVSO visual and photoelectric data were used to test a new way of analyzing and classifying RV Tauri stars -- pulsating yellow supergiants whose light curves are characterized by alternating deep and shallow minima. The bulk of the data used in this project was from the MACHO database of variable star observations, obtained as a by-product of a search for gravitational micro-lensing by faint objects in the halo of our galaxy.

The Harlow Shapley Visisting Lecturer Program

Grinnell has one astronomer -- Professor Bob Cadmus -- and a very impressive on-campus observatory, with a well-instrumented 0.6m telescope. Undergraduate students carried out a variety of small and large projects with this telescope, and contributed to Bob Cadmus' research program of long-term photometry of medium-amplitude pulsating red giants. Bob has worked closely with the AAVSO in past years.

I was impressed -- as I usually am on these visits -- by the quality of teaching and supervision at colleges like Grinnell. Classes and labs were small, and students could interact with their instructors on a one-on-one basis. A very pleasant experience!

For more information about the Program, contact shapley@union.edu The co-ordinator is Professor A.G. Davis Philip.

'Partners in Astronomy' Published

Introduction: The AAVSO photoelectric photometry program was established by one of us (JAM) in the early 1980's, to complement the visual observations of some of the stars in the AAVSO visual observing program, which had long-term, large-amplitude (larger than a magnitude) as well as small-amplitude (typically a magnitude) variations, which were more amenable to photoelectric observation. Soon thereafter, JRP assisted in the final choice of program and comparison stars. The program, after an initial ``teething" stage, has grown rapidly. As of the end of 1998, over 2,500 measurements were being received each year, adding up to a total of 24,000.

The program includes about 80 stars, including some which were added to the program later. A few have been dropped as non-variable, or for other reasons. A special survey of suspected small-amplitude red variables (``Project SARV") has observed another 50 stars. Most of the program stars are semi-regular or irregular small-amplitude pulsating red giants. A few semi-regular yellow supergiants, Be stars, and RS CVn stars are also included.

Each star is observed relative to a primary comparison star and a second ``check" star. The difference between the magnitudes of the check and comparison star should be constant; any scatter is due to observational error, or to variability of one of the two stars. The check star thus provides the ``quality control" for the data.

Almost 60 observers have contributed to the program. As of the end of 1998, their total measurements were: Paul Beckmann (46), Ted Beresky (612), Bill Barksdale (246), Jack Crast (255), Wayne Clark (316), Louis Cox (82), Bob Crumerine (12), David Curott (449), Robert DeMartino (4), Frank Dempsey (473), Sergio Dallaporta (1206), Susan D'Amato (5), Fanie deVilliers (215), Ales Dolzan (3), George Fortier (174), Larry M. Gorski (10), Guillermo Gonzalez (2), Brian Hakes (976), John Isles (147), Robert Johnsson (63), R. Win Jones (588), Arthur Koster (84), George Kohl (408), Phil Kuebler (5), Paul Kneipp (96), Gene Lopata (6), Kenneth Luedeke (2933), Howard Landis (1135), Thomas Langhans (266), Frank Mellilo (8), Russell Milton (688), Phil Manker (887), Donald Pray (372), Harry Powell (144), Mike Potter (17), Luciano Pazzi (12), Gordon Ripley (6), Robert Reisenweber (190), Donald Shannon (15), Douglas Slauson (16), Mike Smith (1749), Lee Snyder (202), Jim Soder (10), Hans Sorensen (625), Robert Schmidt (30), Nick Stoikidis (367), Raymond Thompson (4812), Jim Waller (9), Paul Werner (1), David B. Williams (108), Jim Wood (1844), Thomas Walker (38), Rick Wasson (54), Rick Wasatonic (482).

Organization: The AAVSO photoelectric photometry program is co-ordinated by John Percy, with input from Janet Mattei. AAVSO HQ has continued to provide essential encouragement and support. John Percy edits the AAVSO Photoelectric Photometry Newsletter, two or three times a year, and is primarily responsible for the analysis and interpretation of the results. Howard Landis chairs the AAVSO Photoelectric Photometry Committee, serves as Archivist, and works closely with observers --- especially new observers. He was also a very active observer in the early years of the program.¹

Sociology: The stars on the AAVSO photoelectric program require many years of observation, in order to understand the variability of the star. The program has revealed, for example, that most of the small-amplitude pulsating red giants have an underlying period --- though it may be semi-regular. It has also revealed that most of these stars show long-term (years) variations of unknown cause. For both of these reasons, these stars must be monitored systematically over one to two decades. Equally-significant results have been obtained for other types of stars in the program.

Unfortunately, these observations do not provide the kind of instant results and gratification which are sometimes obtained in other photoelectric programs --- the study of starspots on RS CVn stars in the early 1980's, for instance. Similarly, because the program is communal in the sense that many observers contribute to the database (as in the AAVSO visual program), observers are not usually formal co-authors of the resulting scientific publication. We try to ensure, however, that any publication lists the contributing observers, and acknowledges them. Their efforts have led to new scientific results which cannot easily be obtained in any other way.

Howard's Comments: ``In the beginning, I thought that PEP observing should be an interesting, useful activity, and worthwhile to the astronomical community. More than one professional astronomer encouraged me to start observing, and I still feel the same. After working with amateur PEP observers for a few years, I began to wonder --- why, in a high-tech nation, were there so few observers? There are many astronomy clubs with many members, and most are astro-buffs... That has always seemed a shameful situation to me.

I do not yet understand what it takes to succeed in developing effective interest in a person. If one knew the prospective observer's character well, one might learn who to approach. Formal education, or lack of it, does not seem to have anything to do with it. I have seen many people with a wide range of education either fail or succeed. Physicians, engineers, computer experts, business owners, the military, and many more have tried with us to become observers. Some have succeeded and others, if they got started, often became bored with it and dropped out.

In order to make a good observer, we need a person who has lots of curiosity about the sky, the variability of stars, appreciation for a good telescope and photometer, and the ability to keep proper records of their observations --- then to see that their observations go to an organization which will archive the data and make it available to the astronomical community. Only then does it matter whether they do it at all.

After a few years, if the data is good and reliable, a few professional --- or in some cases student --- astronomers will want to use the data in an attempt to solve certain astronomical problems. The observer may need more or less `hand-holding' for a short time or, in some cases, forever. They need the encouragement of a professional who can periodically look at the data, and let them know something about what the data may be showing. It is very good to have, besides regular communication from the Chair, a newsletter that is distributed to all observers. We are fortunate to have Dr. John Percy as editor of the AAVSO PEP Newsletter, from which observers gain much inspiration to observe more often and more carefully. The website which the AAVSO maintains has also been of some help to promote interest in PEP observing. But we have the same problem raised above --- who will fall for it?

E-mail is the most wonderful thing that has ever happened, insofar as the operation of the PEP division is concerned. Communication is much easier, and the workload of the Chair is much lighter. And, of course, the computer has made it all possible."

Acknowledgements. We thank the dozens of photoelectric observers who, by contributing to the AAVSO photoelectric photometry database, have made the program a success. JRP especially acknowledges the pleasant and fruitful collaboration of Howard Landis and Janet Mattei.

¹Howard's work has been absolutely essential to the success of the program, and to the satisfaction which the observers have felt. It has been suitably recognized, both by the AAVSO, and by the Natural Sciences and Engineering Research Council of Canada. --- JRP