PEP Newsletter v18n1
Message from the Editor
Once again, photoelectric photometry was well represented at the AAVSO Fall Meeting, held this year in Chicopee and South Hadley MA, with Tom Dennis and Ron Zissell as hosts.
On Friday October 24, there was a workshop.on "CCD(V), PEP(V) and Visual Magnitudes: How Do They Relate?". The speakers included visual, photoelectric, and CCD observers, chartmaker Chuck Scovil, vision physiologist Peter Hallett, and AAVSO Director Janet Mattei. The main motivation for the workshop was the AAVSO's plan to revise its visual charts by the year 2000, to reflect more realistic magnitudes.
The topic of the workshop is a complex one. CCD(V) and PEP(V) magnitudes should, in principle, agree, if appropriate filters are used, and the magnitudes are carefully transformed through the use of standard stars. Visual magnitudes, however, use a different and complex detector. Most visual observers are not aware of how they make their measurements. Do they glance, or fixate? Do t ey use the central part of the retina, or an off-axis part? And what are the effects of dark adaptation, or background sky brightness?
There is a second problem for some visual observations: some comparison-sequence magnitudes have been derived from photography, from the BD catalog, or from some other source which may have large systematic errors. Workshop participants agreed that a good PEP(V) magnitude was a better approximation to a visual magnitude than these were.
As a first approximation, it appears that the difference between PEP(V) and visual magnitude may be a simple linear function of the (B-V) color, as Richard Stanton had suggested many years ago. it might be possible to derive a more complex transformation equation involving non-linear terms, or terms in the (U-B) color, if enough data were available. It might even be possible to use photoelectric transformation software with these features built in, such as the program HEC-22 developed by my colleague Petr Harmanec in the Czech Republic.
This workshop, with its diverse but well-focussed presentations, was an interesting and useful first step in the AAVSO's "millenium project-".
A webpage for the AAVSO Photoelectric Photometry Program
As many of you have found, there is a page on the AAVSO web site about the photoelectric photometry program - look under "committees". I have been planning/hoping for a year or two to develop and expand this page. Now, with the help of a keen student assistant, it appears that I can do so.
If any readers have suggestions about content for the web page, please let me know.
Gary Frey asks whether there is a more comprehensive list of red-blue pairs (for determining extinction and transformation coefficients) than the four which are recommended by the AAV@0. The most extensive list, as far as I know, is Appendix B in the book "Astronomical Photometry" by Hended and Kaitchuck, published by Willmann-Bell, 1990.
News From Observers
John B. Chouinavas informs us that he is the new observer at the Larissa Observatory, Greece, Nick Stoikidis having retired. We welcome John Chouinavas, and thank Nick Stoikidis for his contributions to the AAVSO over the years. According to my records, he began contributing visual observations in 1974.
Work of Amateur Astronomers Recognized
"Calling All Amateurs" was a one-page article in the popular US newsmagazine TIME, August 11, 1997, page 48. It began by highlighting the work of amateur astronomers in monitoring an occultation of Aldebaran with camcorders and small telescopes. It then described the contributions of amateurf to encryption, epidemiology, ornithology and archaeology. Undoubtedly there are many other fields of science and other scholarship in which skilled amateurs could play a role. But amateur astronomers take the lead!
The New York Times also published a long article on the work of amateur astronomers in its March 18, 1997 edition. "Backyard Astronomers Enlist as Foot Soldiers of Astrophysics" dealt specifically with the Center for Backyard Astrophysics, which is directed by Columbia University astronomer Joe Patterson. There are CBA sites in several countries; some use photoelectric photometers, and others use CCD cameras. Reflecting Joe Patterson's interests, the CBA network observes mainly cataclysmic variables. one goal is to establish sites at many longitudes, so variables can be monitored continuously.
Spectroscopic Monitoring of Variable Stars
Very few amateur astronomers are equipped to monitor variable stars spectroscopically. Yet there is a need for such observations, because very few professional astronomers make them. Be stars, with their bright variable emission lines of hydrogen, are a prime target.
Ernst Pollman, of Leverkusen, Germany, uses a 10cm Maksutov telescope with an objective prism made of flint glass. His detector is a CCD camera with a Philips 386 x 290 chip. In the IAU Newsletter on Active B Stars, #32 (1997), he presents three years of H-alpha measurements of several Be stars. One is Algol, which is not a classical Be star, but has phase-dependent variations in H-alpha due to the accretion disc around the hot star. Another target is P Cygni, which is on the AAVSO photoelectric photometry program. This is also not a classical Be star; its H-alpha emission comes from its dense wind. The H-alpha strength varies on a time scale of about half a year similar to the time scale of the light variations.
Figure 1. Variations in the equivalent width of the H-alpha line in the blue hypergiant P Cygni. Observations by Ernst Pollman.
The IAU Newsletter on Active B Stars is available on-line at: http://www.chara.gsu.edu/BeNews/intro.html
Hipparcos was a European Space Agency satellite whose prime purpose was to measure parallaxes (and therefore distances) of over 100,000 stars with a high degree of accuracy. Another million stars were measured with a lower accuracy. These results have already provided new insights into many areas of astronomy. AAVSO visual observers played a crucial role in this mission, by providing information on the brightness of large amplitude variables such as Miras, so that the exposure times of the measurements would be appropriate. Our director Janet Mattei was the coordinator of this effort. Her excellent work, and that of the AAVSO visual observers, was deeply appreciated and acknowledged by the Hipparcos project team.
As a by-product of these missions, the satellite monitored the brightness of these stars over a 3.5-year period. Up to 300 observations of each star were obtained of each star. These measurements are a "gold mine" of information about stellar variability, which can be used by astronomers and students for years to come.
The Hipparcos data were first released to the scientists who had proposed the measurements, or who had been otherwise involved in the planning and execution of the mission. The first results from the data were presented in May at "Hipparcos Venice 197", and the proceedings of this symposium have just been distributed as ESA Special Publication 402. It is a massive document - 862 pages of frontier science. The Hipparcos data were publicly released in the summer - on CD-ROM, in hard copy, and on the Internet.
All of the AAVSO photoelectric program stars were observed, both photometrically and astrometrically, by Hipparcos. I was interested to see how the Hipparcos results compared with our ground-based results which, in the case of the small-amplitude red variables, were published last year in the Publications of the Astronomical Society of the Pacific.
The Hipparcos "epoch photometry" (as the individual photometric measurements are called) has already been extensively analyzed. For the stars which were periodic - i.e. for which a period could be found - the phase curves were published as part of the Hipparcos database. Stars which were variable, but not obviously periodic, were classified as 'unsolved".
Almost all of the AAVSO SARV's were classified as "unsolved". This was undoubtedly because the variability is complex. For many of the stars, there are both short-term (weeks) and long-term (months to years) variations. Even the most periodic of the SARV's - EU Del with a period of 62.74 days - varies irregularly in amplitude and light curve shape.
This suggests to me that the AAVSO photometry and the Hipparcos photometry are complementary - the AAVSO photometry extends for ten years or more; the Hipparcos photometry was more concentrated, and does not have the unfortunate seasonal gaps that plague ground-based data. This is one more example of the fact that grou-nd-based and space-based astronomy are complementary. Even visual observations have a major role to play in astronomy in the space age - as every AAVSO observer knows!
News About Program Stars (and Others)
New Be Stars
In the previous issue of this newsletter, we introduced several new Be stars to the photoelectric program. We also mentioned that it is desirable (for quality control) to measure the check star as often as the variable star and tlte comparison star. David Williams pointed out that the AAVSP PEP reporting format allows for only 6ne check star observation, as compared with three of the variable and comparison. My advice? One good observation of the check star is probably adequate.
R.W. Jones, of Fish Hoek, South Africa, has begun to monitor some of the neglected small-amplitude red variables (SARV's) in the AAVSO photoelectric photometry program - especially those below the celestial equator. He has provided impressive light curves for the 1997 season. AK Hya varies by 0.6 magnitude; there appear to be two time scales (as in many SARV's), one about a month, and the other an order of magnitude longer. SW Vir varies by 1.6 magnitudes (!) on a time scale of four months. FH Vir varies by 0.5 magnitude on a time scale of two months, and possibly a longer time scale as well. EV Vir varies by 0.4 magnitude, also on time scale of two months but with slower variations superimposed. V533 Oph also varies by 0.4 magnitude, on a time scale of two months. Mr. Jones' observations demonstrate the effectiveness of "adopting" a small number of stars and monitoring them regularly. It will be interesting to see the results of two or three seasons of work.
EG And is a symbiotic star - a pair of stars of unlike type. in this case, we have an M2.4III red giant, and a white dwarf with a mass of 0.4 sun. The binary systeem shows minima whose ephemeris is JD(min) 2445380 + 481 E, where E is the cycle number. Pereira, in Astrophysics and Space Science 240, 1-11 (1996), describes ultraviolet observations of this star, which are used to infer the size of the gaseous emitting region around the white dwarf. The eclipse minima are clearly visible in the ultraviolet flux.
As mentioned in a previous issue of this newsletter, this star was discovered to be a multi-mode pulsator during a special "campaign" in 1996. We also checked out the suspected-variable comparison stars. You can safely use HR 5534 (comparison) and HR 5454 (check).
Miu Cephei is a red supergiant star - one of the most luminous in our galaxy. Brelstaff et al. have recently published a period analysis of this star in the Journal of the British Astronomical Association, 107, 135-140 (1997), using visual observations. They find periods of 850 and 4400 days. They suggest that the 730-day period given in the General Catalogue of Variable Stars, and a period of 920 days which has also been reported, are actually "alias" periods of the 850-day period.
In case any of you had wondered where CH Cyg had gone: the following is the abstract of a paper "Renewed Activity of CH Cyg", presented by Margarita Karovska, Janet Mattei, and Chris Carilli at the recent AAVSO Fall Meeting: "The AAVSO liaht curve of the nearest S-type symbiotic star, CH Cyg, showed an unprecedented drop in the V magnitude in 1996. This event bears striking resemblance to an earlier outburst phase of CH Cyg when, following extreme fading on the V magnitude in 1984, a multi-component jet was ejected. Since October 1996, we carried out multi-frequency monitoring of the radio brightness of this system, using the Very Large Array. We detected a significant increase in the radio flux since May 1997 which may be associated with a jet formation in the system."
CX Dra is one of the new Be stars on the program, referred to above. Phil Manker (Georgia Southwestern State University) has sent a light curve of the 1997 behaviour. The star shows small variations in its 6.696-day binary period, but the main variations occur as it forms an expanding equatorial disc of gas around its equator. These variations have an amplitude of up to 0.3 magnitude, and a time scale of about two months. In 1997, they have been almost continuous.
This is a famous Be/X-ray binary which has been observed both visually and photoelectrically by the AAVSO. When its expanding equatorial disc of gas forms, it is several tenths of a magnitude brighter than when it is "naked". Roche et al. (Astron. Astrophys. 322, 139-146 (1997)) report on the recent (1995) disc loss in the star. After brightening prior to 1995, the star subsequently faded as the disc dispersed.
Figure 2. Recent V light curve of the Be/X-ray binary X Per, taken from the-reference given above.
SAO 031661 = HD 183361
This star was included as a comparison star for CH Cyg in 1989, because it appeared that one of the existing comparison stars might be variable. A few weeks ago, Sergio Dallaporta, in Italy, reported that, on some nights, the star was variable by several tenths of a magnitude. He suggested that it was an EA-type eclipsing binary, with a period of about 2.466 days.
Initially, I was a bit skeptical, because it had not appeared to be variable, and its spectral type (FSV) does not suggest an intrinsic variable. But when I checked the Hipparcos catalogue, which had just arrived in my library, I was interested to find that Hipparcos had discovered it to be variable with a period of 2.47 days, with a range of 7.47 to 7.68 in the Hipparcos system.
Congratulations to Mr. Dailaporta for his independent discovery of the variability and the period!
This summer, I received a message and a short preprint about this star from my colleague Petr Ha=anec in the Czech Republic. HD 6226 is an early B type star which shows occasional brightenings in V, which are accompanied by small reddenings. The star may be an unrecognized Be star - a star which occasionally develops an expanding equatorial ring of gas which produces additional brightness. On the basis of long-te= ground-based photometry, and photometry from the Hipparcos satellite, Petr Harmanec and his colleagues proposed that the brightenings of the star might be periodic, with a period of 481 days. on this basis, a brightening might be expected around the end of November 1997.
Two AAVSO photoelectric observers - Ken Luedeke and Jim Wood - took on the task of monitoring the star, starting in the summer. The results are in: the star was approximately constant until the beginning of November when, over 20 days, it brightened and faded by 0.1-magnitude, just as it had done several times before. The observed brightening came a month or so after the predicted time, but this may indicate that the phenomenon is quasi-periodic, not strictly periodic.
The Mystery of NSV 11271
A few weeks ago, I received a message from Chris Stephan, from Sebring FL, about a visual comparison star for XY Lyr. He suspected, from his visual observations, that the star varied from 6.5 to 6.9 Chuck Scovil, the AAVSO's chartmaster, had been consulted, and had identified the star as NSV 11271 in the New Catalogue of Suspected Variable Stars. It is also HR 7041 and HD 173383 - a sixth-magnitude K5 star, probably a giant.
I immediately went to the Hipparcos Catalogue, which I had recently received. On the basis of 110 photometric observations over 3.5 years (1989 to 1993), the star was completely constant! It is possible, of course, that the star was constant during the Hipparcos mission, but variable after, but it seems more likely to me that the visual scatter is observational. Chris' latest roun of visual observations continues to suggest variability of up to 0.3 magnitude.
This may therefore be a "wild goose chase" but .... if you wish to follow this up, you could measure NSV 11271 (next season), using the comp and check stars for R Lyr. They are HD 175740 (HR 7146) and HD 175884.
A Belated Report on a Meeting About Small Telescopes
The Lowell Observatory has started a series of annual (?) meetings on interesting scientific and educational topics. The 1996 meeting was on the role of small telescopes in astronomical research and education; the 1997 meeting was on "solar analogues". After the 1996 meeting, I had asked Werner Scharlach (Tucson) to prepare a brief report, which he did. But I never included it in this newsletter. The last section of his report is still of great relevance and interest.
"My feelings can be summed up: (1) the individual astronomers showed concern about the current ongoing situation (about issues such as the closing of small telescopes at national observatories); (2) they are anxious to solve the problems; (3) they are eager to co-operate with each other, and thus they hope to (a) pursue their careers; (b) encourage their students, who are the next generation; and (c) carry out what some see as their civic duty in the field of education and service to society.
There was also discussion about educating the public about the relevance of research to society. The next generation of an observatory's trustees will come from the public. Public money is the ultimate source of funding!
I was encouraged because of the enthusiasm I saw in many of the speakers. It was pointed out that, at this critical time, just as the funds are dwindling, astronomy is at a critical juncture because of new technological developments, which hold the prospect of useful new discoveries, which will benefit everyone. Ironic, indeed!"
Reports of the AAVSO PEP Committee
I have included, in this newsletter, two reports by Howard J. Landis, the chair of the AAVSO Photoelectric Photometry Committee. One is the report which he presented at the 1997 Fall Meeting; it is therefore only a few weeks old. The other is the report which Howard presented at the 1997 Spring Meeting, in Switzerland. Because I did not produce a newsletter over the summer, it has not been published. It does, however, provide an excellent overview of the program, and I am pleased to include it here
A A V S O
Photoelectric Photometry, Fall 1997
Howard Landis, Chmn.
As of 1 October 1997, 17 AAVSO PEP observers have reported 2,716 observations in the fiscal year 1996/1997. The number of observations in our archive passed another milestone this year, reaching the 20,323 mark accumulated since 1983.
|Observations Per Observer|
|Beresky, T.||OK||120||Manker, P.||GA||198|
|Crast, J.||PN||49||Smith, M.||AZ||126|
|Clark, W.||MO||32||Sorensen, H.||Denmark||30|
|Cox, L.||Canada||50||Snyder, L.||NV||2|
|Dempsey, F.||Canada||36||Thompson, R.||Canada||535|
|Dalaporta, S.||Italy||430||Williams, D.||IN||8|
|Hakes, B.||IL||210||Wood, J.||CA||208|
|Jones, W.||S. Africa||165||Wasatonic, R.||MD||14|
More PEP data is coming from overseas observers than ever before, this year it totals 640. Sergio Dalaporta contributed 430, R. W. Jones of South Africa contributed 180, and Hans Sorensen of Sweden contributed 30.
As in past years, Ken Luedeke assisted me with my work as archivist by entering data from observers who contribute data in hand written form. He also has done some tedious re- checking of the accuracy of data in STARPARM.DAT file that is used with the AAVSO data reduction program.
Jim Wood also has helped me with the matter of entering data for me. These two colleagues are very much appreciated for their help.
Sergio Dalaporta is a co-discoverer of the variability of SAO 0331661 along with the Hipparcos satellite program. It has been used as one of the field stars on the-CH Cygni PEP chart.
Dr. John Percy is editor of the Photoelectric Photometry Newsletter for which all PEP observers are appreciative. He is a very busy person and we are blessed by the time he gives US. He helps by answering questions about non-program stars or observing problems and gives advice on the relative usefulness of collecting data on them.
If you are interested in contributing PEP data to the AAVSO, please contact me via E-Mail or regular mail, we need you.
Remarks Concerning History and operating Highlights of The AAVSO Photoelectric Program
Presented before AAVSO meeting, opening session Sion,. Switzerland, 26 May, 1997 by Howard Landis
- It began, 14 years ago, in 1983, with 3 observers who in that year contributed 219 observations on 28 stars. Standards were established on how to observe, taking three measures of the variable and one of a check star to monitor the constancy of the check star. These standards have not changed and the reduction program used today is the same one we started with. We do not archive observations with a standard error of greater than 0.02 magnitude. Most observations are much smaller than that.
- We now have 18,757 observations in the Archive. Over the years 37 observers have contributed data, and 16 are currently active. They are mostly US observers, one from Italy, one from Denmark, and one from Union of SA. We have been producing data at a rate of from 2500 to 2900 observations per year.
- As Chair of the committee I communicate with prospective observers, lead them through the training as necessary to help them become successful PEP observers. Then I watch over the data as it comes in and if I see an apparent problem, I communicate with that observer and explain what needs to be changed. I have developed many productive friendships over the years. -
- The AAVSO PEP program has 70 stars including many classifications of small amplitude variables. The finder charts have all the position data, color and magnitudes necessary. All observers are free to observe any star in the catalog, therefore some stars will have observations from many observers. The data is not as uniform as it could be if only one observer observed each star, but we feel it best to allow this. We get more interest in observing if observers are free to take what is most interesting to them.
- Dr. John R. Percy started a program, the "Small Amplitude Red Variables" where he assigns one or more stars to individual observers. Only one observer will observe each star, resulting in cleaner curves for these small amplitude stars. So far over 100 stars have been investigated, some now have been assigned a variable star name and some are now classified as non-variable. Most of these stars are the questionable variables in the Yale Bright star Catalog.
- We have a data reduction program, created by me then developed further by AAVSO HQ which is used by the archivist and many of our observers. A data entry program is provided with which to create data files which will run in the reduction program. A program to calculate epsilon V is also available which uses the RED-BLUE star pair method. We have a few finder charts for these stars. Programs are available for a small fee to AAVSO PEP observers.
- Most observers take the data at the telescope manually, type it into a computer, send it to me, the archivist, by means of computer disk, or via E-mail. Some leave the data on the hand written form and mail it to me or to one of.my assistants, Kenneth Luedeke or Jim Wood.
- Most observers use the OPTEC SSP-3 while a few use the SSP-5. Five observers take the digital output data from the SSP-3 or SSP-5 and feed it into a game port on a PC. With (freeware) software written by Bob Jones, a file is created that is a complete observation without manually writing down anything. (Unfortunately, the columns of data are not AAVSO standard format and must be converted by a program written by me before it can be run through our reduction program.) This operation does save the observer a lot of time and those who use it strongly recommend it.
- There were originally 4 Be type stars in the catalog, and 4 more have been added this year.
Some papers published using AAVSO PEP program star data.
- #3479 DHK-1l,new eclipsing binary, Dan Kaiser
- #3620 V614 Mon
- #3673 FP Virgo analysis.
- #3739 AAVSO PEP data archive 1992
- 101:893-898, October 1989 "The photometric Variability of EU Delphinius"
- 106:611-615, June 1994 "Photoelectric Surveys of Suspected Small amplitude Red Variables. An AAVSO PEP Survey"
Astronomy and Astrophysics
- 191, 248,252 (1988) "Photoelectric and Visual photometry of P Cygnit,
- 246, 441,446 (1991) "Photometry of Yellow Semiregular Variables: rho Cassiopeia"
- V2 #1 p38 X Cygni
- V23 #2 Photometry of EU Delphini in V and I light.
- V25 #1 BE type Stars in the AAVSO PEP Program
- Dr. John Percy is our professional mentor and is the Editor of the AAvso PEP newsletter which is published 3 or 4 times per year. It is free to AAVSO members for the asking and provides much encouragement to PEP observers
- The AAVSO has an award that is given to PEP observers who have submitted over 1,000 observations.