The Sun's New Exotic Neighbour (Forwarded)

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO
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Contact:

Markus Kasper
ESO, Garching, Germany
Phone: +49 89 3200 6359

Beth Biller, Laird Close
University of Arizona, USA
Phone: +1 520 621 2589, +1 520 626 5992

For immediate release: 22 March 2006

ESO Science Release 11/06

The Sun's New Exotic Neighbour

Very Cool Brown Dwarf Discovered Around Star in the Solar
Neighbourhood

Using ESO's Very Large Telescope in Chile, an international team
of researchers [1] discovered a brown dwarf belonging to the 24th
closest stellar system to the Sun. Brown dwarfs are intermediate
objects that are neither stars nor planets. This object is the
third closest brown dwarf to the Earth yet discovered, and one of
the coolest, having a temperature of about 750 degrees Centigrade.
It orbits a very small star at about 4.5 times the mean distance
between the Earth and the Sun. Its mass is estimated to be
somewhere between 9 and 65 times the mass of Jupiter.

At a time when astronomers are peering into the most distant
Universe, looking at objects as far as 13 billion light-years
away, one may think that our close neighbourhood would be very
well known. Not so. Astronomers still find new star-like objects
in our immediate vicinity. Using ESO's VLT, they just discovered
a brown dwarf companion to the red star SCR 1845-6357, the 36th
closest star to the Sun.

ESO PR Photo 11a/06
New Brown Dwarf in the Solar Neighbourhood (Artist's Impression)

Caption: Artist's impression of the SCR 1845-6357 stellar system.
The small red star is shown in the background while the newly
discovered brown dwarf is at front.

"This newly found brown dwarf is a valuable object because its
distance is well known, allowing us to determine with precision
its intrinsic brightness", said team member Markus Kasper (ESO).
"Moreover, from its orbital motion, we should be able in a few
years to estimate its mass. These properties are vital for
understanding the nature of brown dwarfs."

To discover this brown dwarf, the team used the high-contrast
adaptive optics NACO Simultaneous Differential Imager (SDI [2])
on ESO's Very Large Telescope, an instrument specifically
developed to search for extrasolar planets. The SDI camera
enhances the ability of the VLT and its adaptive optics system
to detect faint companions that would normally be lost in the
glare of the primary star. In particular, the SDI camera provides
additional, often very useful spectral information which can be
used to determine a rough temperature for the object without
follow-up observations.

ESO PR Photo 11b/06
The System SCR 1845-6357 (NACO-SDI/VLT)

Caption: Three-colour image of SCR1845-6357AB generated from
the SDI filter images (blue=1.575 micron, green=1.600 micron,
red=1.625 micron). Since the T-dwarf fades away towards the
longer wavelengths, it appears quite blue in this image. It
is roughly 50 times fainter than the star and is separated
from it by an angle of 1.17 arcsecond on the sky (4.5 times
the Earth-Sun distance).

Located 12.7 light-years away from us, the newly found object is
nevertheless not the closest brown dwarf. This honour goes indeed
to the two brown dwarfs surrounding the star Epsilon Indi, located
11.8 light years away (see ESO PR 01/03).

However, this newly discovered brown dwarf is unique in many
aspects. "Besides being extremely close to Earth, this object is
a T dwarf -- a very cool brown dwarf -- and the only such object
found as a companion to a low-mass star," said Beth Biller, a
graduate student at the University of Arizona and lead author
of the paper reporting the discovery. "It is also likely the
brightest known object of its temperature because it is so close."

The discovery of this brown dwarf hints that, at least close to
the Sun, cool brown dwarfs prefer to be part of a couple with a
star or another brown dwarf, rather than wandering alone in the
cosmic emptiness. Indeed, of the seven cool brown dwarfs that
reside within 20 light years of the Sun, five have a companion.

"This has wide-ranging implications for theories of brown dwarf
formation, which, until now, tend to favour the production of
single brown dwarfs," said team member Laird Close (University
of Arizona).

ESO PR Photo 11c/06
Stars in the Solar Neighbourhood

Caption: Three-dimensional map of all known stellar systems
within 12.7 light-years from the Sun. SCR 1845-6357 appears
towards the bottom right hand corner of the image. This map
was adapted from images by Richard Powell at
atlasoftheuniverse.com.

ESO PR Photo 11d/06
Images of SCR 1845-6357 A and B

Caption: A movie of the SDI camera's images of the low mass
star SCR 1845A and its companion, SCR 1845B. Note how the
brown dwarf companion (B) is clearly changing in brightness
in and out of the SDI methane filters. This proves that B is
methane-rich and is thus a rare cool low-mass brown dwarf
companion. Moreover, it is also clear that the "halo of
speckles" around the much hotter primary star do not change
significantly in the three SDI filters. Hence the star's halo
can be removed by subtracting the different images, showing
that the detection of a faint methane-rich companion (even
10 times closer than B) is made possible with the SDI camera.

High resolution images and their captions are available at
http://www.eso.org/outreach/press-re...hot-11-06.html

More information

The work presented here will appear as a Letter to the Editor
in the Astrophysical Journal ("Discovery of a Very Nearby Brown
Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the
Low Mass Star SCR 1845-6357", by B. Biller et al.).

Notes

[1]: The team is composed of Beth Biller and Laird Close (Steward
Observatory, University of Arizona, Tucson, USA), Markus Kasper
(ESO, Garching, Germany), Wolfgang Brandner (Max-Planck Institute
for Astronomy, Heidelberg, Germany), and Stephan Kellner (W.M.
Keck Observatory, Waimea, Hawaii, USA).

[2]: The NACO SDI camera is a unique type of camera using adaptive
optics, which removes the blurring effects of Earth's atmosphere
to produce extremely sharp images. SDI splits light from a single
star into four identical images and then passes the resulting
beams through three slightly different (methane-sensitive)
filters. Only cool low-mass objects will have methane in their
atmospheres and so only these objects will change brightness in
the SDI filters. When the filtered light beams hit the camera's
detector array, astronomers can subtract the images so the bright
star disappears, revealing a fainter, cooler object otherwise
hidden in the star's scattered light halo ("glare"). The SDI
camera was developed and deployed by Laird Close (Steward
Observatory, University of Arizona) and Rainer Lenzen (Max-
Planck-Institute for Astronomy in Heidelberg) in collaboration
with ESO.

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