CFHT, Current Image of the Week


March 13th, 2000

The Pleiades: A Family Portrait

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The Pleiades: A Family Portrait

Credit: Image courtesy of J. Bouvier (Observatoire de Grenoble, France)

(for more details see: Bouvier, Rigaut, Nadeau, 1997, Astronomy & Astrophysics, Vol. 323, p. 139-150.)

The Pleiades, also known as Messier 45, Melotte 22, the Seven Sisters or Makali'i in Hawaiian, look like a small version of the Big Dipper to the naked eye. The seven bright stars form an open cluster that is easily visible in winter, in the Constellation Taurus. For centuries, the Pleiades have been hosts to numerous legends and myths that carry on, even today.

But there is more than meets the eye in the Pleiades. The cluster is actually made of hundreds of stars, too faint to be seen without a telescope. This week's image shows a family portrait of the cluster: many of these stars are twins!!! sometimes triplets!!!

Bouvier and collaborators used PUEO, CFHT's Adaptive Optics bonnette, to have a close look at 144 members of the Pleiades clusters. They could resolve more than 20 stars into close binary systems and 3 into triple systems. The individual images are shown here, in a 4 X 6 mosaic. A variety of Pleiades binaries are visible, with different brightness ratios and different separations, as revealed by the powerful Adaptive Optics system of CFHT.

These observations were needed to measure the fraction of Pleiades stars that are members of multiple systems. They provided an important parameter to understand how stars form and how they evolve with time. The frequency of binaries found in the Pleiades is similar to that found in the solar neighborhood for similar mass stars. However, it seems different (lower) than the fraction found for the much younger and looser clusters like the Taurus star forming region, hinting at the probable role of stellar density to control the binary fraction.

Technical description:
The image presented this week is a mosaic of 24 smaller images, showing the binaries or triples that were found in the Pleiades in October 1996. Each image was obtained with PUEO equipped with MONICA, the Montreal Near-Infrared Camera. The individual images are 3 arc-seconds on a side. The final exposure time was 2.5 minutes, obtained by adding five 30-second images together. A K-band filter was used here (i.e., a filter centered at 2.2 microns). No deconvolution is applied to the images but diffraction rings showing the exquisite image quality of the telescope are visible in most cases.

next week: A Circumstellar disk around a Young star

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.