Speckle Observations of Binary
Stars
| What is Speckle Imaging?
The WIYN Program Lowell-Tololo Observations For More Information |
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Speckle imaging is a technique that allows astronomers obtain very high-resolution information about astronomical objects. It works by taking many short exposure images in rapid succession. In other words, a speckle observation is simply a movie. In each frame, the blurring effects of the atmosphere spread the light out over a fuzzy patch, but with in that patch there are bright points, called speckles (see movie above). From frame to frame, the position and intensity of the speckles changes, and it turns out that by studying the speckles in each short exposure, you can get the information you need to "reconstruct" a high resolution image. If you just take a long exposure image, the speckle character will wash out and you'll just be left with a fuzzy patch. One use of the much higher resolution that speckle imaging gives you is to resolve very close binary stars that would otherwise be blurred together in normal CCD images taken from the ground. We can also determine the mass of such systems and compare them to theoretical predictions. This process represents one of the most basic checks on what we think we know about how stars really work.
Since 1997, Elliott has been taking speckle observations at the WIYN 3.5-m Telescope at Kitt Peak National Observatory near Tucson, Arizona, in collaboration with Bill van Altena at Yale and Zoran Ninkov at RIT. We are primarily interested in binary stars. Close binary stars that would otherwise be blurred together in normal ground-based observing benefit from speckle observations in two ways: resolution and astrometric precision. The former allows us to obtain high-quality information even on very small-separation binaries, while the latter is important for deriving astrophysical information from the data, such as stellar masses.
In addition to many classic close visual binaries, we have also been observing new double stars discovered by the Hipparcos astrometric satellite in the early 1990's. Although Hipparcos is primarily thought of as measuring distances to many nearby stars, it also discovered about 3400 new double stars and flagged thousands of others as suspected double. At WIYN, we have been trying to determine which of the new discoveries are actually gravitationally bound and which appear close together just by virtue of the earth's line of sight to the two stars (in these cases, one star is far behind the other and the pair is not physically bound). So far we have found three gravitationally bound systems, HDS 17, HDS 318, and CAR 1, all with periods in the range 8 to 20 years.
We have also observed over 500 candidate stars for the Astrometric Grid of the Space Interferometry Mission. The astrometric grid will be a collection of about 3000 stars uniformly distributed over the sky that will help the satellite point and take data properly. Since astrometric stability is very important for the Astrometric Grid, double and multiple stars are undesirable as grid members. A recent summary of the WIYN work was presented by Sally Robinson at the June 2001 meeting of the AAS and is available in pdf format here.
Reed Meyer and Elliott are currently working on
determining relative photometry of binaries observed at WIYN, and Reed's
Ph.D. thesis project involves constructing an improved mass-luminosity
relation (MLR) for main sequence stars.
| The WIYN Telescope: An observing run in winter is a pretty good antidote to Rochester weather!!! |  |
In collaboration with Dr. Otto Franz of Lowell Observatory, Elliott took the RIT fast-readout CCD camera to Cerro Tololo Inter-American Observatory (CTIO) in October of 1999. Otto also particiapted in the observing, and served as telescope operator and "Otto-guider" while Elliott took care of the instrumentation and data archiving. Despite pretty mediocre seeing conditions (1.9 arcsec or so), we obtained very high-quality astrometry of many (neglected) Southern binary systems. We were also able to show that we can get reliable differential photometry of these systems, with random errors of 0.10 to 0.15 magnitudes per 2 minute observation.
Thanks to a small research grant from the AAS, Otto and Elliott will be returning to CTIO in November of 2001 for another run at the Lowell-Tololo Telescope.
Contact Elliott Horch
or check out some of his recent papers.