| From: VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 05:38:39.78
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Subj: [ASTRO] Updated press release on "A Faint and Lonely Brown Dwarf in the Solar Vicinity"
For immediate release: 28 April 1997
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Text and photos with all links are available on the ESO Website at URL:
http://www.eso.org/outreach/press-rel/pr-1997/
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A Faint and Lonely Brown Dwarf in the Solar Vicinity
Discovery of KELU-1 Promises New Insights into Strange Objects
Brown Dwarfs are star-like objects which are too small to become real stars,
yet too large to be real planets. Their mass is too small to ignite those
nuclear processes which are responsible for the large energies and high
temperatures of stars, but it is much larger than that of the planets we
know in our solar system.
Until now, very few Brown Dwarfs have been securely identified as such. Two
are members of double-star systems, and a few more are located deep within
the Pleiades star cluster. Now, however, Maria Teresa Ruiz of the Astronomy
Department at Universidad de Chile (Santiago de Chile), using telescopes at
the ESO La Silla observatory, has just discovered one that is all alone and
apparently quite near to us. Contrary to the others which are influenced by
other objects in their immediate surroundings, this new Brown Dwarf is
unaffected and will thus be a perfect object for further investigations that
may finally allow us to better understand these very interesting celestial
bodies.
It has been suggested that Brown Dwarfs may constitute a substantial part of
the unseen dark matter in our Galaxy. This discovery may therefore also have
important implications for this highly relevant research area.
Searching for nearby faint stars
The story of this discovery goes back to 1987 when Maria Teresa Ruiz decided
to embark upon a long-term search (known as the Calan-ESO proper-motion
survey) for another type of unusual object, the so-called White Dwarfs, i.e.
highly evolved, small and rather faint stars. Although they have masses
similar to that of the Sun, such stars are no larger than the Earth and are
therefore extremely compact. They are particularly interesting, because they
most probably represent the future end point of evolution of our Sun, some
billions of years from now.
For this project, the Chilean astronomer obtained large-field photographic
exposures with the 1-m ESO Schmidt telescope at La Silla, each covering a
sky area of 5.5 deg x 5.5 deg. When comparing plates of the same sky field
obtained at time intervals of several years [1], she was able to detect,
among the hundreds of thousands of stellar images on the plates, a few faint
ones whose positions had changed a little in the meantime. The search
technique is based on the fact that such a shift is a good indicator of the
object being relatively nearby. It must therefore also be intrinsically
faint, i.e. a potential White Dwarf candidate.
On every pair of plates, approximately twenty faint moving objects were
detected with proper motions [2] of more than 0.25 arcsec per year. Indeed,
follow-up spectroscopic observations showed that about 20 percent of these
or about four per plate were White Dwarfs. Until now, a total of forty new
White Dwarfs have been discovered during this very successful project, i.e.
over ten times more than originally expected.
And then - a Brown Dwarf!
When checking two plates with a time inverval of 11 years, Maria Teresa Ruiz
earlier this year discovered a very faint object in the southern
constellation of Hydra (The Water-Snake), moving at 0.35 arcsec per year
(cf. ESO Press Photo 11/97). In order to establish its true nature, she
obtained its spectrum (in the visual to near-infrared region from
wavelengths 450-1000 nm) on March 15 using the ESO 3.6-m telescope and
the EFOSC1 spectrograph.
To her great surprise, the spectrum was of a type never seen before and
certainly not that of a White Dwarf or any other easily identifiable type of
star (cf. ESO Press Photo 12/97). In particular, there were no signs of
spectral bands of titanium oxide (TiO) or vanadium oxide (VO) which are
common in very cool stars, nor of the spectral lines seen in White Dwarfs.
On the other hand, an absorption line of the short-lived element lithium was
identified, as well as a hydrogen line in emission.
However, when the colour of this mysterious object was measured in different
wavebands, it was found to be very red and quite similar to that of one of
the two known Brown Dwarfs in double star systems. The presence of the
lithium line in the spectrum is also an indication that it might be of that
type.
The astronomer now decided to give the new object the name KELU-1; this word
means `red' in the language of the Mapuche people, the ancient population in
the central part of Chile. Its visual magnitude is 22.3, i.e. more than 3
million times fainter than what can be seen with the unaided eye.
In early April, additional infrared observations with the UKIRT (UK Infrared
Telescope) on Mauna Kea (Hawaii) by Sandra K. Leggett (Joint Astrophysical
Centre, Hilo, Hawaii, USA) confirmed the Brown Dwarf nature of KELU-1, in
particular through the unambiguous detection of methane (CH4) bands in its
spectrum.
The nature of Brown Dwarfs
Brown Dwarfs are first of all characterised by their low mass. When a body
of such a small mass is formed in an interstellar cloud and subsequently
begins to contract, its temperature at the centre will rise, but it will
never reach a level that is sufficient to ignite the nuclear burning of
hydrogen to helium, the process that it is main source of energy in the Sun
and most other stars. The Brown Dwarf will just continue to contract, more
and more slowly, and it will eventually fade from view.
This is also the reason that some astronomers consider Brown Dwarfs in the
Milky Way and other galaxies as an important component of the `dark matter'
whose presence is infered from other indirect measurements but has never
been directly observed.
It is assumed that the mass limit that separates nuclear-burning stars and
slowly contracting Brown Dwarfs is at about 90 times the mass of the giant
planet Jupiter, or 8 percent of that of the Sun.
KELU-1: a great opportunity for Brown Dwarf studies
Assuming that KELU-1 is identical to other known Brown Dwarfs, its measured
characteristics indicate that it must be located at a distance of only 10
parsecs, that is about 33 light-years, from the solar system. Its
temperature is obviously below 1700 degrees C (where TiO and VO condense as
dust grains [3] so that the spectral lines of these molecules are no longer
seen). Its mass can be no more than 75 times that of Jupiter, or 6 percent
of that of the Sun.
During recent years, several Brown Dwarf candidates have been de-masked as
low-mass stars and only recently a few Brown Dwarfs were identified in the
Pleiades star cluster. Those Brown Dwarfs are quite young and therefore
comparatively hotter and brighter.
Contrarily, KELU-1 is most probably somewhat older and its unique location
so close to us greatly facilitates future investigations. Moreover, it is
not at all `disturbed' by the presence of other objects in its immediate
surroundings, as this is the case for all other known objects of this type.
It will now be important to obtain accurate measurements of KELU-1's
parallax, that is, the small annual change of its position in the sky that
is caused by the Earth's motion around the Sun and thus the viewing angle of
an Earth-based observer. This should be possible within the next year.
Moreover, high resolution spectral investigations with large telescope
facilities, soon to include the ESO Very Large Telescope at the Paranal
observatory in northern Chile, will now for the first time enable us to
investigate the processes that take place in the relatively cold upper
layers of Brown Dwarfs. For instance, the observed presence of lithium shows
that its atmosphere must be different from that of low-mass stars.
KELU-1 and the `Dark Matter'
>From the fact that KELU-1 is so faint that it was barely detectable on the
ESO Schmidt plates, it is possible to estimate that the total volume so far
surveyed for this type of objects by this research programme is rather
small, only about 23 cubic parsecs (800 cubic light-years). A further
consideration of the search statistics indicates that less than 10 percent
of the Brown Dwarfs present in the surveyed volume would have been found.
This translates into a local density of about 0.4 such objects per cubic
parsec.
Although the mass density of Brown Dwarfs derived from this estimate is
insufficient to constitute all the `dark matter' in the Milky Way Galaxy, it
is consistent with the most recent estimates of the local mass density, both
observed and as infered from dynamical considerations of the motions of
stars in the solar neighborhood.
Notes:
[1] This is done by means of a so-called blink-comparator, an optical device
in which the two plates are placed. A tilting mirror allows to view the same
sky field alternately on the two plates. Any celestial object that has
changed its position will appear to `jump' back and forth and can thus be
identified.
[2] A proper motion in the sky of 0.25 arcsec/year corresponds to a
transversal speed of about 12 km/sec if the object is located at a distance
of 10 parsec, or 32.6 light-years. The largest known proper motion of an
object outside the solar system is that of Barnard's Star at about 10
arcsec/year.
[3] For instance, as the mineral perovskite.
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ESO Press Photo 11/97
For immediate release: 28 April 1997
The Brown Dwarf KELU-1
This photo shows a small sky area around the newly discovered Brown Dwarf
object KELU-1 in the southern constellation of Hydra (The Water-Snake). It is
indicated with tick marks.
Observations described in ESO Press Release 07/97 (28 April 1997) have shown
that this object is located at a distance of only about 10 parsec from the
Sun. Moreover, it is single and is therefore not disturbed by any other
objects in its neighbourhood.
This image was obtained on March 15, 1997, through a near-infrared `gunn-i'
filter with the EFOSC1 multimode instrument at the ESO 3.6-m telescope at La
Silla. The exposure lasted 40 seconds and was made during good sky
conditions. The field measures approx. 5 x 5 arcmin; North is up and East is
left.
This is the caption to ESO PR Photo 11/97 [JPEG, 144k] which accompanies
ESO Press Release 07/97 (28 April 1997). It is also available in a
high-resolution version [JPEG; 2505 x 3000 pix; 1.3Mb] for reproduction
purposes. It may be reproduced, if credit is given to the European Southern
Observatory.
Copyright Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
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ESO Press Photo 12/97
For immediate release: 28 April 1997
The Spectrum of the Brown Dwarf KELU-1
This photo shows the spectrum of the newly found Brown Dwarf object,
KELU-1, obtained with the EFOSC1 multimode instrument at the ESO 3.6-m
telescope.
A comparison with the spectrum in the visual to near-infrared region (4000 -
10000 A, i.e. 400 - 1000 nm) with that of a normal, cool dwarf star of
spectral type dM6e shows significant differences. In particular, it does not
display the bands of titanium oxide which are seen in the dwarf star.
The insert shows the visual part of the spectrum (4000 - 7000 A) enlarged.
The bands of CaOH and neutral sodium (Na) are seen, and also an absorption
line of neutral lithium. There is a weak H-alpha emission line from neutral
hydrogen. This is the first blue spectrum ever obtained of a Brown Dwarf
object.
This tracing is based on two 30-min spectral exposures in the visual region
and two 20-min exposures in the near-infrared region. The other spectrum is
of another high-proper-motion object identified during the Calan-ESO
proper-motion survey carried out with the ESO 1-m Schmidt telescope and
herewith classified as of the dwarf M-type.
This is the caption to ESO PR Photo 12/97 [GIF, 35k] which accompanies ESO
Press Release 07/97 (28 April 1997). It may be reproduced, if credit is
given to the European Southern Observatory.
Copyright Education & Public Relations Department
Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany
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| From: VBORMC::"[email protected]" "MAIL-11 Daemon" 27-MAY-1997 04:45:59.16
To: [email protected]
CC:
Subj: [ASTRO] Newly Discovered Brown Dwarf is Coolest Object in the Pleiades
Newly Discovered Brown Dwarf is Coolest Object in the Pleiades
A brown dwarf discovered in the Pleiades star cluster by a team of
astronomers at the University of Leicester is the coolest and faintest
object ever found in the cluster, and possibly has the lowest mass of any
known brown dwarf. Given the name PIZ 1 by its discoverers, the new brown
dwarf's mass is estimated at 50 Jupiter masses and its surface temperature
is put at only 2300 degrees K (2000 degrees C). Until now, Gliese 229B was
regarded as the least massive brown dwarf known. But there is considerable
uncertainty about that star's actual mass, which may be anywhere in the
range 20 to 65 times the mass of Jupiter. So PIZ 1 is a definite challenger
for the light-weight title.
The team of astronomers at Leicester are Drs Martin Cossburn, Simon Hodgkin,
Richard Jameson and David Pinfield. David Pinfield will describe the new
discovery at the UK's National Astronomy Meeting at the University of
Southampton on Tuesday 8th April, and a paper on it will be published later
in the Monthly Notices of the Royal Astronomical Society. The team is
involved in an international project to search for brown dwarfs. Astronomers
would like to know just how common these objects are. It is possible that
they may make some significant contribution towards the unseen dark matter
in galaxies, but this is as yet highly uncertain.
PIZ1 was discovered with the 2.5-metre Isaac Newton Newton Telescope on the
island of La Palma during an observing run in December 1995 and January
1996. (Images were taken through filters known as I and Z, hence the origin
of the name.) Further observations were made with the United Kingdom
Infrared Telescope (UKIRT) in Hawaii in October 1996 and spectra were taken
with the 4-metre William Herschel Telescope on La Palma in November 1996.
Brown dwarfs are sometimes referred to as 'failed stars' -- in the sense that
they are not massive enough for the nuclear fusion of hydrogen to take place
in their cores. So once a brown dwarf has formed, it has no ongoing source
of energy and spends the rest of its lifetime cooling down, getting fainter
all the time. So the best time to find a brown dwarf is when it is young and
at its brightest. The Pleiades cluster has proved a particularly good
hunting ground, because the stars in it are relatively young on the
astronomical scale -- only 100 million years, and it is near enough for faint
brown dwarfs to be detectable. Three other brown dwarfs have been discovered
in the Pleiades already.
Even the brightest brown dwarfs are difficult to detect because they give
out so little radiation. The search has gone on over the last 15 years, with
limited success in terms of definite identifications. Recent detections have
been as a direct result of improvements in the sensitivity of detectors and
the use of larger telescopes.
Early indications from the international search suggest that many more new
objects of similar mass to PIZ 1 are being found.
Contacts
All at the Department of Physics and Astronomy, University of Leicester,
Leicester LE1 7RH:
Martin Cossburn, phone (0)116 252 2084, e-mail [email protected]
Simon Hodgkin, phone (0)116 252 2078, e-mail [email protected]
Richard Jameson, phone (0)116 252 2074, e-mail [email protected]
David Pinfield, phone (0)116 252 2084, e-mail [email protected]
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