Eclipsing Binary Update Number 12

Number 12 October 2002

Notes from the Chairman

Our ad hoc team of eclipsing binary observers has been very active, participating in the publication of 19 new IBVS numbers.

Charts for all the AAVSO eclipsing binary program stars have been revised to meet the AAVSO chart standards. They can be downloaded from the AAVSO website and are included on the new chart CD available from headquarters.

The seventh listing of "Observed Minima Timings of Eclipsing Binaries" has come together and is expected to be available in time for the Annual Meeting.

At the 2002 Spring Meeting in Hawaii there were just enough clouds to make for terrific sunsets, but somehow it didn't seem to matter that those same clouds interfered with the observation of variable stars -- at least that was the attitude taken by this vacationing participant.

And most recently, Shawn Dvorak has developed an interactive, professional-quality eclipsing binary ephemeris generator at the following website:

http://rollinghillsobs.dyn.dhs.org:8000/cgi-bin/calcEBephem.pl

Enter your longitude and latitude and it will tell you what EB minima are observable on any night from your location. -- MEB

Observers target OW Gem secondary minimum

The deep (8.2-9.7 V) primary minimum was discovered photographically in 1988 by now-AAVSO president Dan Kaiser. The highly displaced secondary minimum, 0.1 magnitude deep, was found photoelectrically, and quite accidentally, near phase 0.23 by Your Editor in 1989. Roger Griffin of the Institute for Astronomy at Cambridge, England, provided a complete radial velocity curve. The 1995 primary minimum received good photometric coverage, and the 2002 primary eclipse was very well observed (above), thanks to the increasing number of amateur and small-college CCD observers who can now provide standardized photometry.

Click image to enlarge

Until now, however, the secondary minimum has been documented only by Your Editor's three photoelectric V measures that showed OW Gem fainter than maximum in January 1989. But as this issue of the Update goes to press, photometers and CCD cameras are recording the secondary minimum expected to begin around October 15. Because of OW Gem's highly elliptical orbit, the secondary eclipse lasts twice as long as the 14-day primary eclipse. Observers will be taking data until the end of November, after which a light curve solution can be combined with the radial velocities to define all the parameters of this remarkable eclipsing system.

The Quest for Eclipsing Binaries in Clusters

According to a report in the June 2002 Sky & Telescope, the Hubble Space Telescope was used to study the young cluster R136 in the Large Magellanic Cloud, revealing three eclipsing binaries. Spectroscopy and photometry was used to determine accurate masses, radii, and surface temperatures. One of the binaries turned out to have an O3 component of 57 solar masses, the highest accurate stellar mass determined so far. The EB investigation confirms that the cluster is very young, only about 1 million years.

Von Braun and Mateo (Astron. J. 123, 279, 2002) searched the field of the southern globular NGC 3201 and found 11 eclipsing binaries within the cluster's tidal radius. Unfortunately, radial velocity measures to compare the systemic velocities of the variables with the systemic velocity of NGC 3201 eliminated all but one candidate from globular membership. The member EB is a W UMa-type "blue straggler," so called because of its displacement from the main sequence in the color-magnitude diagram. Blue stragglers may form from collisions between single stars or the interaction or even coalescence of binary systems. These variables therefore provide a potential tool for investigating the dynamical development and star formation history of the cluster.

Galactic clusters provide a laboratory for investigating star formation and evolution. Because a cluster's age can be determined independently, this information can shed additional light on the evolution of eclipsing binaries in the cluster. The galactic cluster NGC 6819 was targeted for a deep CCD search for variables by Street et al. (Mon. Not. R. Astron. Soc. 330, 737, 2002). They found 25 certain and 13 suspected variables between 17th and 20th magnitude, including 12 eclipsing binaries. Most of the EBs are shorter-period W UMa systems, and three are believed to be cluster members. W UMa systems are thought to form from wider pairs in galactic clusters at around 4-5 Gyr. NGC 6819 has an age of 2.5 Gyr, so the three systems found suggest that W UMa formation is beginning on schedule.

One eclipsing binary found in the search field proved to be of special interest, though not a cluster member. The light curve is that of a completely detached binary with minima of 0.75 and 0.45 magnitude and a period of 1.332 days. With such a short period, the undistorted member stars must be small. The authors conclude that their star 8864 is probably a pair of late-M dwarfs. Very few eclipsing pairs of this spectral type are known, so a detailed study of this system could add significantly to our knowledge of the masses and radii of M dwarfs at the lower end of the hydrogen-burning main sequence.

A CV observer learns EB lessons

"This evening I made the trip out to the observatory eager to catch an eclipse of IY UMa. I had downloaded and printed a copy of an ephemeris posted to VSNET with predicted times of mid-eclipse. I made my first observation to establish a baseline at 10:45 EDT, a good 35 minutes before the next predicted eclipse. I then swung the scope around and nabbed X Leo in outburst, SU, SW and EI UMa, and Z Cam, thinking I had plenty of time.

"Pointed back to IY UMa and began noting observations, expecting an eclipse. What I actually saw was a slight rise at first and then flat line for a half an hour, right through the predicted time and well after. Nothing.… OK, I thought, I'll go get some more CVs, this is boring. So I observe everything else in UMa, CP and DO Dra, check IY UMa here and there just to keep an eye on it, and then go after a few in Virgo.

"It's starting to get a little hazy for Virgo or anything low, so I head back up to UMa to see what's up with our uncooperative friend. It had faded by about 0.4 V since ten minutes before! So *now* I'm seeing an eclipse. I watch it fade to beyond my limit as the sky gets hazier, and then rise back up to pre-eclipse brightness.

"Now I'm having second thoughts about my eyes, the telescope, the haze, my sanity -- because the eclipse happened a good 15 minutes earlier than predicted. But I remember the advise of my good friend and mentor Gene Hanson, "report what you see, not what you think you should be seeing." I repeat this to myself all the way home.

"I type out my report and send it off to AAVSO and VSNET. I also send a note to Taichi Sato [VSNET] stating that my observed eclipse time is 15 minutes ahead of his predictions. I probably missed the first eclipse while doing SU, SW, EI UMa and Z Cam!

"A short while later, Taichi posts a revised ephemeris that matches my eclipse and other observations to within a minute or so. I think the CCD guys are still taking flats or something (dig). And of course it will be days before they can reduce all their data. :-) Never mind, the visual observers have it all straightened out already."

A listing of minima times of the dwarf nova U Gem, dating back to 1965, was included in "AAVSO Observed Minima Timings of Eclipsing Binaries No. 6." The period has been constant for the past 35 years, but the O-C trend in the long-term data permit a fine correction to the period.

WW Cygni: mass transfer plus magnetic cycle?

The O-C plot (next page) for WW Cyg shows a long-term period increase, which can be assigned to mass transfer. But the O-C diagram also shows an alternating series of period increases and decreases superposed on the long-term parabola. The authors were unable to fit the WW Cyg data to the requirements for a third star's light-time effect. They therefore conclude that a magnetic cycle in the late-type secondary component provides the most plausible explanation.

Almost all the minima timings of WW Cyg after 1972 were AAVSO data. Principal author Robert T. Zavala posted the following message on the AAVSO discussion list: "I would like to extend a thank-you to all the AAVSO observers for such excellent work…. Without the AAVSO observations, we would have had holes in our data that would have reduced the reliability of our conclusions."

Click image to enlarge

Featured Star: YY Eridani

YY Eri is also a popular target for professional astronomers, with at least eight photoelectric studies in the literature. The slightly deeper primary minimum occurs when the smaller component is eclipsed, so YY Eri is classed as a W-type W UMa system (in A-type systems, the larger component is more luminous). All the photoelectric light curves also show that Maximum I (following the primary minimum) is brighter than Maximum II. The period varies. This, however, is about as much as the many investigators can agree on.

Click image to enlarge

Three recent papers illustrate the mysteries of YY Eri. Kim et al. (Astron. J. 114, 2753, 1997) present a period study based on PEP and CCD minima timings. They tested three hypotheses: (1) that the O-C diagram can be fitted with several segments of constant period, the abrupt period changes being due to sudden mass ejections, (2) that the minima times can be represented by a cyclical light-time effect due to a possible third body plus a parabola due to continuous mass transfer, and (3) that the O-C diagram can be explained by a cyclical magnetic modulation of the primary star combined with continuous mass transfer (see O-C diagram (right). They conclude that the magnetic activity cycle is most plausible, because there is no supporting evidence for sudden mass ejections or a third star in the system. They do find suggestive, but by no means conclusive, support for magnetic activity in periodic variations of the difference between Maximum II and Minimum II. However, other photometric evidence does not support the magnetic model, and the third-body interpretation cannot be ruled out by the available data.

Yang and Liu (Astron. Astrophys. Suppl. Ser. 136, 139, 1999) present a new photoelectric light curve solution. They find that the best solution requires a dark spot on one component. They also examine the variation in the brightness of the two maxima by comparing data from five of the photoelectric studies, finding that the variations are regular with the rise of one maximum always accompanied by the decline of the other maximum. They conclude that this teeter-totter variation between the two maxima is related to the changing luminosities of the two components without change in the radii, suggesting that the variation is due to spot activity on one or both components.

Karube et al. (IBVS No. 4948, 2000) report new times of minima and take another look at the O-C diagram. They find that recent observations diverge strongly from the periodic + quadratic light elements of Kim et al. and the best fit with this type of equation does not match the data closely. Instead, they re-examine the possibility of fitting the data with several linear periods separated by abrupt period changes. They obtain a good fit with four line segments, which differ from the six segments proposed by Kim et al. They note that evidence of mass ejections isn't required, because abrupt period changes due to cyclic magnetic activity has been postulated for other eclipsing binaries.

So what's happening with YY Eri - linear, cyclical, or parabolic period changes, mass transfer or ejection, a third component, spot activity, magnetic cycles? Or all of the above? As so often in studies of eclipsing binaries, we need more data - high-resolution spectroscopy, UV and X-ray data from satellites, additional multi-filter light curves to monitor the variable maxima, and continued timing of minima to track the changing period. Solving the mysteries of YY Eri could provide answers to questions involving the whole class of W UMa binaries.