A team of astronomers led by Dr David Pinfield at the University of
Hertfordshire have discovered two of the oldest brown dwarfs in the
Galaxy. These ancient objects are moving at speeds of 100-200 kilometres
per second, much faster than normal stars and other brown dwarfs and
are thought to have formed when the Galaxy was very young, more than 10
billion years ago. Intriguingly the scientists believe they could be
part of a vast and previously unseen population of objects. The
researchers publish their results in the Oxford University Press journal
Monthly Notices of the Royal Astronomical Society.
Brown dwarfs are star-like objects but are much less massive (with
less than 7% of the Sun’s mass), and do not generate internal heat
through nuclear fusion like stars. Because of this brown dwarfs simply
cool and fade with time and very old brown dwarfs become very cool
indeed - the new discoveries have temperatures of 250-600 degrees
Celsius, much cooler than stars (in comparison the Sun has a surface
temperature of 5600 degrees Celsius).
Pinfield’s
team identified the new objects in the survey made by the Wide-field
Infrared Survey Explorer (WISE), a NASA observatory that scanned the
mid-infrared sky from orbit in 2010 and 2011. The object names are WISE
0013+0634 and WISE 0833+0052, and they lie in the Pisces and Hydra
constellations respectively. Additional measurements confirming the
nature of the objects came from large ground-based telescopes (Magellan,
Gemini, VISTA and UKIRT). The infrared sky is full of faint red
sources, including reddened stars, faint background galaxies (large
distances from our own Milky Way) and nebulous gas and dust. Identifying
cool brown dwarfs in amongst this messy mixture is akin to finding
needles in a haystack. But Pinfield's team developed a new method that
takes advantage of the way in which WISE scans the sky multiple times.
This allowed them to identify cool brown dwarfs that were fainter than
other searches had revealed.
The team of scientists then studied the infrared light emitted from
these objects, which are unusual compared to typical slower moving brown
dwarfs. The spectral signatures of their light reflects their ancient
atmospheres, which are almost entirely made up of hydrogen rather than
having the more abundant heavier elements seen in younger stars.
Pinfield comments on their venerable ages and high speeds, “Unlike in
other walks of life, the Galaxy’s oldest members move much faster than
its younger population”.
Stars near to the Sun (in the so-called local volume) are made up of 3
overlapping populations - the thin disk, the thick disk and the halo.
The thick disk is much older than the thin disk, and its stars move up
and down at a higher velocity. Both these disk components sit within the
halo that contains the remnants of the first stars that formed in the
Galaxy.
Thin disk objects dominate the local volume, with thick disk and halo
objects being much rarer. About 97% of local stars are thin disk
members, while just 3% are from the thick-disk or halo. Brown dwarfs
population numbers probably follow those of stars, which explains why
these fast-moving thick-disk/halo objects are only now being discovered.
There are thought to be as many as 70 billion brown dwarfs in the
Galaxy’s thin disk, and the thick disk and halo occupy much larger
Galactic volumes. So even a small (3%) local population signifies a huge
number of ancient brown dwarfs in the Galaxy. "These two brown dwarfs
may be the tip of an iceberg and are an intriguing piece of astronomical
archaeology", said Pinfield. "We have only been able to find these
objects by searching for the faintest and coolest things possible with
WISE. And by finding more of them we will gain insight into the earliest
epoch of the history of the Galaxy."
Media contact
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Science contacts
Prof David Pinfield
University of Hertfordshire
Tel: +44 (0)1707 284171
davidpinfield@googlemail.com
Dr Joana Gomes
University of Hertfordshire
j.gomes@herts.ac.uk
Dr Avril Day-Jones
University of Hertfordshire
avril_day_jones@hotmail.com
Dr Mariusz Gromadski
University of Val Paraiso
Val Paraiso, Chile
mariusz.gromadzki@uv.cl
Further information
The results are published in the paper “A deep WISE search for very
late type objects and the discovery of two halo/thick-disk T dwarfs:
WISE 0013+0634 and WISE 0833+0052”, D. J. Pinfield et al, Monthly
Notices of the Royal Astronomical Society, in press.
Useful links
University of Hertfordshire Centre for Astrophysics Research
http://www.herts.ac.uk/research/stri/research-areas/car
WISE homepage at NASA
http://www.nasa.gov/mission_pages/WISE/main/index.html
The Gemini Telescopes
http://www.gemini.edu/
The Magellan Telescope
http://obs.carnegiescience.edu/Magellan
VISTA homepage
http://www.vista.ac.uk/index.html
The UKIRT Infrared Deep Sky Survey homepage
http://www.ukidss.org/index.html
Notes for editors
The Royal Astronomical Society (RAS, www.ras.org.uk),
founded in 1820, encourages and promotes the study of astronomy,
solar-system science, geophysics and closely related branches of
science. The RAS organizes scientific meetings, publishes international
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(Fellows), a third based overseas, include scientific researchers in
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astronomy and others.
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