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May 1st, 2000

The Mystery of Zeta Cancri Revealed

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The Mystery of Zeta Cancri Revealed

Credit: Image courtesy of J.B. Hutchings

Herzberg Institute of Astrophysics, Victoria, Canada

Multiple systems are frequent in the Universe. Most stars form as binaries or multiple systems. Galaxies are often found in pairs or in clusters, Planets have satellites. This week's featured object is also a multiple system, a visual triple system. This means that 3 stars are seen orbiting around each other. Or so we thought...

This week's object, Zeta Cancri, has been studied extensively for the last 200 years or so and many thousands of measurements are available over this period. These measurement come from visual observations, photographic plates, electronic images, spectroscopy, etc... From these data sets, it is clear that three bright stars are detected. But the data sets also allow to trace the stellar motions, their orbits around each other as they dance in the sky. A surprising result of this analysis is that a fourth "object" is needed to explain the observations. Indeed, the tertiary component of the system is "wobbling" in its orbit around the two others.

The three bright stars making up the visual triple are easily observable on this week's near-infrared image. On the right hand side the pair Zeta Cancri AB is visible. The two stars have roughly equal luminosities and their masses, 1.1 and 1.0 solar masses, are very similar to the Sun's mass. They are separated by 0.8arcsec and their orbital period is about 60 years around each other.

Component C, the third bright star in the system, is seen to the left. But for the first time in this image, the long suspected companion of Zeta Cancri C is also detected! This pair is tighter than Zeta Cancri AB as they are separated by 0.3 arcseconds. Previous data told us that Zeta Cancri C and D had an orbital period of about 17 years, with masses estimated to be roughly equal, at 0.99 and 0.93 solar masses for star C and D respectively. So why did Zeta Cancri D remain undetected for so long??? Was it too close, too faint? What is this object anyway?

The most obvious possibility was to suggest that Zeta Cancri D had a low luminosity and was undetectable. An evolved object with low luminosity for example, a white dwarf, was generally accepted until recently.

The image obtained a few months ago with the Adaptive Optics Bonnette at the Canada-France-Hawaii Telescope allowed the group of researchers to measure the colors of Zeta Cancri D and assess its nature. A white dwarf can be ruled out with confidence. Zeta Cancri D is too red, too cold. Rather, by matching the measured colors with computer models, the team of astronomers suggested that Zeta Cancri D is itself another binary! Another pair of stars made of two low mass, very red M stars. The redness of the two stars, and their proximity to Zeta Cancri C explain why they remained undetected in the optical so far. This important result was obtained because of the exquisite image quality provided by adaptive optics. In this case, the ability to resolve the two stars was necessary to solve the long standing puzzle of Zeta Cancri.

The chase is now on to resolve Zeta Cancri D and verify if the system known as Zeta Cancri is indeed a quintuplet... or more?

Technical description:

This weeks image was obtained in February 2000 with PUEO, the Adaptive Optics Bonnette of the CFHT equipped with its near infrared camera KIR. A filter centered at 2.16microns was used. 6 images with exposure time of 1 second were carefully registered and added.

next week: Deuterium in Orion

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.