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Disks around Failed Stars - a Question of Age

August 01, 2002

First Ground-Based Mid-Infrared Observations of Brown Dwarfs [1]

A team of European astronomers [2] have observed eight Brown Dwarfs,
i.e., small and faint objects also known as "failed stars", with the
TIMMI2 infrared sensitive instrument at the ESO 3.6-m telescope on La
Silla.

From two of these, mid-infrared radiation is detected - for the first
time ever from such objects with a ground-based telescope. While the
younger Brown Dwarf, aged a few million years, is found to be
surrounded by a dusty disk, no warm dust is present around the older
ones.

The new observations support the following formation hypothesis for
Brown Dwarfs: they are born in the same way as "real" stars, by
contraction in interstellar clouds of gas and dust. During the later
stages of this process, the infalling material is transferred onto the
star via a gas and dust disk. This disk - in which planets may
possibly form - then disperses with time.

Brown Dwarfs are faint and cool objects

Astronomical objects known as "Brown Dwarfs" are "failed stars". Their
comparatively small mass, less than about 7% of that of our Sun (or
about 75 times the mass of planet Jupiter), is too small to achieve
sufficiently high pressure and temperature at their centre to ignite
energy-producing nuclear processes. Some astronomers also refer to
Brown Dwarfs as a "missing link" between planets and stars, being
neither one nor the other, yet with similarities to both.

They do not burn hydrogen to helium as "real" stars do, but continue
to emit faint light as they slowly contract and cool during millions
of years. They end their inglorious life with a whimper and finally
fade into eternal insignificance.

Although Brown Dwarfs were theoretically predicted already in 1963,
astronomers had to wait until 1995 for the first one to be
discovered. This was mainly due to their extreme faintness as compared
to normal stars - even the most nearby Brown Dwarfs shine so faintly
that they can only be observed with relatively large telescopes. As
they are rather cool objects, they emit mostly in the infrared
spectral region; hence they are best observed with astronomical
instruments that operate at those wavelengths.

With improved techniques, however, more and more Brown Dwarfs have
been found and the count has now reached several hundred. Many of
these are located in the well-known Orion Nebula. Others move through
interstellar space, like the lonely KELU-1 first discovered in 1997 at
the ESO La Silla Observatory by Chilean astronomers, cf. ESO PR
07/97. With a distance of only 33 light-years from the Sun, it was one
of the closest Brown Dwarfs known at that time.

Formation of Brown Dwarfs

Astronomers are still doubtful about the way Brown Dwarfs form. Among
the numerous suggestions are the star-like contraction from an
interstellar cloud of gas and dust and also another based on "ejected
stellar embryos". This latter scenario says that very young stars
that are still accreting material are "kicked out of the nest" by
their more massive brothers in multiple stellar systems. In this
dramatic process, the unlucky objects are stripped of their
surrounding disks. This effectively halts their further growth by
accretion and they end up as underweight Brown Dwarfs.

Recent observations at ESO have shown that the Brown Dwarfs in the
Orion Nebula most likely have formed as stars do, i.e. by contraction
in a cloud of dust and gas, cf. ESO PR 14/01. The clue to this was the
observation of an excess of near-infrared radiation from many of these
objects, interpreted as the presence of dusty disks around them. The
astronomers then argued that if the young Brown Dwarfs possess such
disks exactly like real stars do, then they must also form in the same
way.

Infrared observations of Brown Dwarfs

Those observations were carried out in the near-infrared spectral
region (in the 1.2 - 2.2 micron wavelength interval) with the ESO
3.5-m New Technology Telescope (NTT). However, dusty disks around
young stars (and presumably, those around Brown Dwarfs) radiate mostly
at longer wavelengths. A detailed study of those disks is therefore
best done with instruments that are sensitive to even longer
wavelengths, e.g., in the mid-infrared to the sub-millimetre spectral
region (10 - 1000 micron). This is in fact the only spectral interval
where emission emanating from solid particles can be directly observed
and their (mineral) composition thus be analysed.

Pioneering observations in this wavelength interval of some Brown
Dwarfs were made in mid-1995 by the ESA Infrared Space
Observatory. However, the ISO instruments provided comparatively low
image sharpness and these observations were hampered by confusion with
the radiation from other objects in the same sky field. And the ISO
mission was over before Brown Dwarf objects were discovered in larger
numbers.

Astronomers have therefore long wanted to observe Brown Dwarfs with
large ground-based telescopes in the mid-infrared spectral region. But
these objects are faint and few suitable instruments that work at
these wavelengths are available at the world`s large astronomical
telescopes. Long exposures are necessary to record the faint emissions
and until now, it had not been possible to perform such highly
demanding observations of Brown Dwarfs.

TIMMI2 observes Brown Dwarfs

Now, however, the first ground-based detection of mid-infrared
radiation from two Brown Dwarfs has been achieved by a team of
European astronomers [2], using the Thermal Infrared Multimode
Instrument (TIMMI2) on the ESO 3.6-m telescope at the La Silla
Observatory (Chile).

They pointed the telescope towards a total of eight Brown Dwarf
objects and recorded the emission at three different mid-infrared
wavelengths (5, 9.8 and 11.9 micron). "We were delighted", says team
leader Daniel Apai, "to detect radiation from two of these with
TIMMI2. These are the first observations of their kind with a
ground-based instrument. And although we could only establish upper
limits for the radiation from the five other objects, these results
are highly significant for our attempts to understand the formation
and evolution of Brown Dwarfs."

One of the objects, known as Cha HA 2 and located in the southern
constellation Chamaeleon [3], had earlier been observed with ISO. It
is a bona-fide Brown Dwarf object and an image obtained with the
Hubble Space Telescope indicates that it may possibly be double. It is
a relatively young Brown Dwarf and is a member of the very young Cha I
star-forming region - the age has been estimated at 2 - 4.5 million
years.

The ISO observations hinted at the presence of a dust disk around this
object - this is fully confirmed by the new TIMMI2
observations. Moreover, the mid-IR radiation measured with this
instrument interestingly shows the absence of a strong emission
feature from silicates (at about 10 µm wavelength). According to
the astronomers, this indicates that the disk around Cha HA 2 is
comparatively dense and flat, and without a heated outer layer, cf. PR
Photo 17b/02.

The other Brown Dwarf from which TIMMI2 has now detected mid-infrared
radiation is one of the closest of its type. Designated LP 944-20, it
is located in the southern constellation Fornax (The Oven) at a
distance of only ~15 light-years. It is much older than Cha HA 2,
though, probably 500 - 650 million years. In this case, the age was
determined by measuring the strengths of spectral lines of the element
Lithium; the older the object, the less is the content of Lithium.

The observations show that the radiation from LP 944-20 comes from the
cool star itself - it does not possess a surrounding disk as does the
much younger Cha HA 2 [4].

Evolution of Brown Dwarfs

Daniel Apai explains: "This all fits very nicely into the current
picture of the evolution of Brown Dwarfs. They are born like stars by
contraction in an interstellar cloud of gas and dust. At least some of
them acquire a surrounding disk during this process. But that disk
disperses after some time and we therefore only find it around
relatively young Brown Dwarfs, not around older ones.".

Nobody knows yet whether planets form in those disks around young
Brown Dwarfs (as this was the case in the disk around the young Sun
and other stars), but it might happen. Only future observations with
much more sensitive instruments will be able to cast more light on
this intriguing question.

Future observations

ESO`s Very Large Telescope (VLT) will soon be equipped with the VLT
Mid Infrared Spectrometer/Imager (VISIR), an extremely powerful
mid-infrared sensitive instrument that is well suited for this kind of
studies.

Further into the future, the Atacama Large Millimeter Array (ALMA)
will provide excellent opportunities for in-depth investigations of
Brown Dwarfs. With unequalled sensitivity and very good image
sharpness, ALMA will be able to image disks around the nearest Brown
Dwarfs and possibly, to detect signs of (forming) planets in them.

More information

The information presented in this Press Release is based on a Letter
to the Editor in the research journal "Astrophysical Journal"
("Probing Dust around Brown Dwarfs: The Naked LP 944-20 and the Disk
of Chamaeleon H-alpha 2" by D. Apai and co-authors (Vol. 573,
pp. L115-L117; July 10, 2002). It is available on the web at
http://arXiv.org/abs/astro-ph/0206210.

The full text of this ESO Press Release, with two photos, is available
at:

http://www.eso.org/outreach/press-rel/pr-2002/pr-14-02.html

Notes

[1]: This press release is issued in coordination between ESO and the
Max-Planck-Institut fuer Astronomie (Heidelberg, Germany). A German
version is available at the MPIA website
(http://www.mpia.de/Public/Aktuelles/index_de.html).

[2]: The team consists of Daniel Apai, Ilaria Pascucci and Thomas
Henning (all at Astrophysikalisches Institut und
Universitaetssternwarte, Jena, Germany, and Max-Planck-Institut fuer
Astronomie, Heidelberg, Germany), Michael Sterzik (ESO-Chile), Randolf
Klein and Dimitri Semenov (Astrophysikalisches Institut und
Universitaetssternwarte, Jena, Germany), Eike Guenther and Bringfried
Stecklum (Thueringer Landessternwarte Tautenburg, Germany).

[3]: Cha HA 2 stands for "H-alpha emitting object no. 2 in the
Chamaeleon I Dark Cloud".

[4]: The photosphere of the young Brown Dwarf Cha HA 2 also emits
mid-IR radiation. However, it is quite far away - about 500
light-years, or more than 30 times more distant than LP 944-20) - and
that radiation is too weak to be detected with TIMMI2.

European Southern Observatory (ESO)