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A Brown Dwarf Joins the Jet-Set (Forwarded)



 
 
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Old June 25th 07, 04:10 PM posted to sci.space.news
Andrew Yee[_1_]
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Default A Brown Dwarf Joins the Jet-Set (Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Emma Whelan
School of Cosmic Physics
Dublin Institute for Advanced Studies, Ireland
Phone: +353-1-6621333

For Immediate Release: 23 May 2007

ESO Science Release 24/07

A Brown Dwarf Joins the Jet-Set

VLT Finds Smallest Galactic Object with Jets

Jets of matter have been discovered around a very low mass 'failed star',
mimicking a process seen in young stars. This suggests that these 'brown
dwarfs' form in a similar manner to normal stars but also that outflows are
driven out by objects as massive as hundreds of millions of solar masses
down to Jupiter-sized objects.

The brown dwarf with the name 2MASS1207-3932 is full of surprises [1]. Its
companion, a 5 Jupiter-mass giant, was the first confirmed exoplanet for
which astronomers could obtain an image (see ESO 23/04 and 12/05), thereby
opening a new field of research -- the direct detection of alien worlds. It
was then later found (see ESO 19/06) that the brown dwarf has a disc
surrounding it, not unlike very young stars.

Now, astronomers using ESO's Very Large Telescope (VLT) have found that the
young brown dwarf is also spewing jets, a behaviour again quite similar to
young stars.

The mass of the brown dwarf is only 24 Jupiter-masses. Hence, it is by far
the smallest object known to drive an outflow. "This leads us to the
tantalizing prospect that young giant planets could also be associated with
outflows," says Emma Whelan, the lead-author of the paper reporting the
results.

The outflows were discovered using an amazing technique known as
spectro-astrometry, based on high resolution spectra taken with UVES on the
VLT. Such a technique was required due to the difficulty of the task. While
in normal young stars -- known as T-Tauri stars for the prototype of their
class -- the jets are large and bright enough to be seen directly, this is
not the case around brown dwarfs: the length scale of the jets, recovered
with spectro-astrometry is only about 0.1 arcsecond long, that is, the size
of a two Euro coin seen from 40 km away.

The jets stretch about 1 billion kilometres and the material is rushing away
from the brown dwarf with a speed of a few kilometres per second.

The astronomers had to rely on the power of the VLT because the observed
emission is extremely faint and only UVES on the VLT could provide both the
sensitivity and the spectral resolution they required.

"Discoveries like these are purely reliant on excellent telescopes and
instruments, such as the VLT," says Whelan. "Our result also highlights the
incredible level of quality which is available today to astronomers: the
first telescopes built by Galileo were used to observe the moons of Jupiter.
Today, the largest ground-based telescopes can be used to observe a Jupiter
size object at a distance of 200 light-years and find it has outflows!"

Using the same technique and the same telescope, the team had previously
discovered outflows in another young brown dwarf. The new discovery sets a
record for the lowest mass object in which jets are seen [2].

Outflows are ubiquitous in the Universe, as they are observed rushing away
from the active nuclei of galaxies -- AGNs -- but also emerging from young
stars. The present observations show they even arise in still lower mass
objects. The outflow mechanism is thus very robust over an enormous range of
masses, from several tens of millions of solar mass (for AGNs) down to a few
tens of Jupiter masses (for brown dwarfs).
More Information

These results were reported in a Letter to the Editor in the Astrophysical
Journal (vol. 659, p. L45): "Discovery of a Bipolar Outflow from 2MASSW
J1207334-393254 a 24 MJup Brown Dwarf", by E.T. Whelan et al.

The team is composed of Emma Whelan and Tom Ray (Dublin Institute for
Advanced Studies, Ireland), Ray Jayawardhana (University of Toronto,
Canada), Francesca Bacciotti, Antonella Natta and Sofia Randich
(Osservatorio Astrofisico di Arcetri, Italy), Leonardo Testi (ESO), and Subu
Mohanty (Harvard-Smithsonian CfA, USA).

Notes:

[1]: Brown dwarfs are objects whose masses are below those of normal stars
-- the borderline is believed to be about 8% of the mass of our Sun -- but
larger than those of planets. Unlike normal stars, brown dwarfs are unable
to sustain stable nuclear fusion of hydrogen.

[2]: The brown dwarf 2MASS1207-3932 belongs to the TW Hydrae Association and
is therefore about 8 million years old. Albeit this is relatively young,
this also implies that this brown dwarf is one of the oldest Galactic
objects with a resolved jet, highlighting the fact that outflows can persist
for a relatively long time.

Technical information:

Spectro-astrometry is simply Gaussian fitting of the spatial profile of the
continuum and emission line regions of a spectrum in order to very
accurately measure positions. In this way spatial information is recovered
beyond the limitations of the seeing of an observation. For example
spectro-astrometry has been primarily used to investigate binarity in
sources where the binary separation is far less than the seeing and to
confirm outflow activity where the line emission tracing the outflow
originates at a distance again much smaller than the seeing and therefore
appears confined to the source.

The first step is to measure the continuum centroid, i.e. the source
position. The spatial profile of the continuum is extracted at many
positions along the dispersion axis. Each extracted profile is fitted with a
Gaussian to measure the centroid position of the continuum emission and the
result is a position spectrum of the continuum. This map of the continuum
position is easily corrected for curvature or tilting in the spectrum.

Next the continuum is removed and the position of a pure emission line
region is measured (again with Gaussian fitting) with respect to the
continuum position. The presence of the continuum will tend to drag the
position of an emission line region back towards the source so it must be
removed.

The accuracy with which one can measure positions with spectro-astrometry is
strongly dependent on the signal to noise of the observation and is given by
sigma=seeing/[2.3548(sqrt{Np})] where Np is the number of detected photons.
For example, for a seeing of 1 arcsecond and a value of Np of 10,000,
positions can be recovered to an accuracy of less than 5 milliarcseconds.

The forbidden emission lines found in the spectra of some young brown dwarfs
were a strong indication of outflow activity. However the regions were not
extended and therefore that they originated in an outflow could not be
directly confirmed. Using spectro-astrometry the astronomers were able to
show that the line regions were shifted by small amounts with respect to the
brown dwarf continuum (shifts were small relative to the seeing) and
therefore were indeed tracing an outflow.

Please see
http://www.nature.com/nature/journal...ture03598.html
for a more in depth discussion of the spectro-astrometric technique.

National contacts for the media:

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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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