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?
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.
Technical description: