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New Technique for Observing Faint Companions (Forwarded)



 
 
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Old June 21st 07, 04:35 AM posted to sci.space.news
Andrew Yee[_1_]
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Default New Technique for Observing Faint Companions (Forwarded)

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO Website at URL:
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Contacts

Niranjan Thatte
University of Oxford, UK
Phone: +44 1865 27 34 12

Laird Close
University of Arizona, USA
Phone: +1 520 626 5992

For Immediate Release: 19 June 2007

ESO Science Release 28/07

Back on Track

New Technique for Observing Faint Companions

Observing the image of a faint object that lies close to a star is a
demanding task as the object is generally hidden in the glare of the star.
Characterising this object, by taking spectra, is an even harder challenge.
Still, thanks to ingenious scientists and a new ESO imaging spectrograph,
this is now feasible, paving the way to an eldorado of many new thrilling
discoveries.

These very high contrast observations are fundamental for directly imaging
unknown extra-solar planets (i.e. planets orbiting a star other than the
Sun), as well as low-mass stars and brown dwarfs, those failed stars that
are too small to start burning hydrogen into helium.

Astronomer Niranjan Thatte and his colleagues developed a new method for
exactly this purpose. The basis of the concept is relatively simple: while
the positions of most of the features associated with the host star and
artefacts produced by the telescope and the instrument scale with the
wavelength, the location of a faint companion does not. So if the image has
an internal reflection of the star masquerading as a planet, this phantom
planet will be in one location in the image when looking in red light, and
another when looking in blue; a real planet will stay at the same place no
matter what colour of light one examines. Therefore, with the combined
detection of spectra and position, one can see what is scaling, subtract it,
and be left with what is fixed, that is the target dim object. Such
observations can be done with specific instruments, called 'integral field
spectrographs', such as the SINFONI instrument on ESO's VLT. This technique,
termed Spectral Deconvolution (SD), although first proposed in 2002 for
space-based applications, has never been applied to obtain spectra of a real
object until now.

"We applied our new technique to a puzzling very small stellar companion --
about twice the size of Jupiter -- known as AB Doradus C and the outcome was
surprising, "says Thatte.

Using SINFONI and this new technique, the astronomers could for the first
time obtain a spectrum of the object that is free from the light of the
brighter companion and that contains all the information necessary for a
complete classification.

The new observations lead to a new temperature for the object and change the
results that some of the same scientists derived in 2005 (ESO PR 02/05).

"This is how science progresses," says Laird Close, leader of the science
team. "New instruments lead to better techniques and measurements, which
often lead to new results, and one must happily change course."

The SINFONI observations were complemented with previous data obtained on
ESO's VLT with the NACO instrument, which were stored in the ESO archive.

AB Doradus is a system of 2 pairs of stars (four stars in total: a quadruple
system), lying 48 light-years away towards the Doradus constellation (the
Swordfish).

AB Doradus A is the young major member of this system and has a faint
companion, AB Dor C, just 3 astronomical units (AU) away, or three times the
distance between the Earth and the Sun. In our Solar System, this would be
within the asteroid belt between the orbits of Mars and Jupiter.

AB Dor C was imaged for the first time, thanks to ESO's VLT, in 2005 (ESO
02/05). The other members of the system are the pair AB Doradus BaBb (also
first imaged in the previous work of 2005) located 133 AU from AB Dor A.
While AB Doradus A has a mass about 85 % that of the Sun, AB Doradus C is
almost 10 times less massive than AB Doradus A and belongs to the category
of cool red dwarfs.

Red dwarfs are extremely interesting because their mass is at the border
with that of brown dwarfs. A precise knowledge of these stars is therefore a
necessary tile in our understanding of the evolution of stars. If AB Doradus
C were only slightly less massive than its 93 Jupiter-mass, it would have
failed to become a star, being instead a brown dwarf. As it is, the centre
of AB Doradus C is slowly heating up, and in about a billion years its core
will become hot enough to begin fusing hydrogen into helium, something a
brown dwarf will never do.

"This red dwarf is 100 million times closer to its brighter companion than
the whole system is from us and about 100 times less bright. It is thus a
perfect example where our very high contrast technique is required," says
team member Matthias Tecza.

From the previous observations this unique star seemed to be cooler than
expected for an object of such a mass and age. The new, more precise
observations show that this is not the case, as the observations are in good
agreement with theory, in particular with the models developed by the group
of Gilles Chabrier from Lyon, France.

With a temperature of about 3 000 degrees (about half as hot as the Sun) and
a luminosity about one thousand times dimmer than the Sun, AB Doradus C lies
on the exact track expected for a 75 million year old star with 9% the Sun's
mass. AB Doradus C is the only such star (young and cool) with an accurate
mass, hence the determination of an accurate temperature is critical for
validating these models.

In the future one can thus use these tracks to extrapolate the mass of small
young stars, once its temperature and luminosity are precisely determined.

"Small stars are back on the expected track," concludes team member Roberto
Abuter.

More Information

"New Photometry and Spectra of AB Doradus C: An Accurate Mass Determination
of a Young Low-Mass Object with Theoretical Evolutionary Tracks", by L.M.
Close, N. Thatte, E.L. Nielsen, R. Abuter, F. Clarke, M. Tecza,
Astrophysical Journal in press. See
http://arxiv.org/abs/astro-ph/0703564

"Very high contrast IFU spectroscopy of AB Doradus C: 9 mag contrast at 0.2"
without coronagraph using spectral deconvolution", by N. Thatte, R. Abuter,
M. Tecza, E.L. Nielsen, F.J. Clarke, L.M. Close, MNRAS in press. It is
available at
http://arxiv.org/abs/astro-ph/0703565

Notes

1. SINFONI is a near-infrared Integral Field Spectrograph (IFS) fed by a
MACAO adaptive optics module. An IFS allows the simultaneous recording of
two dimensional (angular) plus one dimensional (spectroscopic) information.
The end products of integral field spectrographs are cubes of data: two
dimensional images extended with a third spectroscopic dimension. So for
each of a thousand slightly different colors, a full two-dimensional image
is produced. The cube can be cut in slices in order to study the spatial and
spectroscopic features of a given region. Movies showing the observations of
AB Dor C with this instrument are available at
http://www-astro.physics.ox.ac.uk/~thatte/abdorc

2. The advantages of using this newly developed spectral deconvolution (SD)
technique with an Integral Field Spectrograph instead of the NACO SDI
instrument used in the previous study of this object (ESO 02/05) are that
while both techniques can find a faint companion anywhere in the field, this
new SD method does not rely on any intrinsic feature of the hidden
companion. When considering the spectrum, the continuum can be correctly
measured and a high performance is achievable even without a coronagraph.
The SD technique simultaneously detects and characterises any faint
companion with high accuracy; with NACO SDI any detections had to be
followed up by separate observations to take a spectrum. An Integral Field
Spectrograph allows for far more efficient use of telescope time. These
features make it a precious instrument for the quest of exoplanets.

National contacts for the media:

Belgium: Dr. Rodrigo Alvarez, +32-2-474 70 50
Czech Republic: Pavel Suchan, +420 267 103 040
Finland: Ms. Tiina Raivo, +358 9 7748 8369
Denmark: Dr. Michael Linden-Vnle, +45-33-18 19 97
France: Dr. Daniel Kunth, +33-1-44 32 80 85
Germany: Dr. Jakob Staude, +49-6221-528229
Italy: Dr. Leopoldo Benacchio, +39-347-230 26 51
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Spain: Dr. Miguel Mas-Hesse, +34918131196
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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