"FLYING SAUCER" DUSTY DISK FOUND AROUND YOUNG STAR
14-May-2002
Article courtesy of European Southern Observatory
Image credit: European Southern Observatory
Using the ESO 3.6-m New Technology Telescope and the Very Large Telescope
(VLT), a team of astronomers [1] have discovered a dusty and opaque disk
surrounding a young solar-type star in the outskirts of a dark cloud in
the Milky Way.
It was found by chance during an unrelated research programme and provides
a striking portrait of what our Solar System must have looked like when it
was in its early infancy. Because of its unusual appearance, the
astronomers have nicknamed it the "Flying Saucer".
The new object appears to be a perfect example of a very young star with a
disk in which planets are forming or will soon form, and located far away
from the usual perils of an active star-forming environment. Most other
young stars, especially those that are born in dense regions, run a
serious risk of having their natal dusty disks destroyed by the blazing
radiation of their more massive and hotter siblings in these clusters.
The star at the centre of the "Flying Saucer", seems destined to live a
long and quiet life at the centre of a planetary system, very much like
our own Sun. This contributes to making it a most interesting object for
further studies with the VLT and other telescopes.
The mass of the observed disk of gas and dust is at least twice that of
the planet Jupiter and its radius measures about 45 billion km, or 5 times
the size of the orbit of Neptune.
Circumstellar Disks and Planets
Planets form in dust disks around young stars. This is a complex process of
which not all stages are yet fully understood but it begins when small dust
particles collide and stick to each other. For this reason, observations of
such dust disks, in particular those that appear as extended structures (are
"resolved"), are very important for our understanding of the formation of
solar-type stars and planetary systems from the interstellar medium.
However, in most cases the large difference of brightness between the young
star and its surrounding material makes it impossible to image directly the
circumstellar disk. But when the disk is seen nearly edge-on, the light from
the central star will be blocked out by the dust grains in the disk. Other
grains below and above the disk midplane scatter the stellar light,
producing a typical pattern of a dark lane between two reflection nebulae.
The first young stellar object (YSO) found to display this typical pattern,
HH 30 IRS in the Taurus dark cloud at a distance of about 500 light-years
(140 pc), was imaged by the Hubble Space telescope (HST) in 1996. Edge-on
disks have since also been observed with ground-based telescopes in the
near-infrared region of the spectrum, sometimes by means of adaptive optics
techniques or speckle imaging, or under very good sky image quality, cf. ESO
PR Photo 03d/01 with a VLT image of such an object in the Orion Nebula.
A surprise discovery
Last year, a group of astronomers [1] carried out follow-up observations of
new X-ray sources found by the ESA XMM-Newton and NASA Chandra X-ray
satellites. They were looking at the periphery of the so-called Rho Ophiuchi
dark cloud, one of the nearest star-forming regions at a distance of about
500 light-years (140 pc), obtaining images in near-infrared light with the
SOFI multi-mode instrument on the 3.5-m New Technology Telescope (NTT) at
the ESO La Silla Observatory (Chile).
On one of the NTT photos obtained on April 7, 2001, they discovered by
chance a strange object which by closer inspection turned out to be a
resolved edge-on circumstellar disk, so far unnoticed and displaying
infrared scattered light around a young star. On this photo (PR Photo
12a/02) two characteristic reflection nebulae can barely be seen, flanking a
marginally resolved dark dust lane in the East-West direction in front of
the star.
VLT confirmation
To confirm this discovery and in order to learn more about the object and
the disk, the astronomers obtained additional observations (during
"Director's Discretionary Time") with the 8.2-m VLT ANTU telescope. The
observations were carried out in "service mode" by ESO staff, using the
near-infrared multi-mode Infrared Spectrometer And Array Camera (ISAAC) -
the "father" of the SOFI instrument ("Son OF Isaac").
A series of fine images was obtained on August 15, 2001, under very good
observing conditions (with "seeing" of 0.4 arcsec). Now the two reflection
nebulae are clearly seen (PR Photos 12b-c/02), and the dark dust lane is
well resolved. The leader of the group, Nicolas Grosso, recalls the first
impression when seeing the true shape of the object: "That is when we looked
at each other and, with one voice, immediately decided to nickname it the
`Flying Saucer'!".
The nature of the new object
Seven young stars in the Rho Ophiuchi star-forming region are known to
display similar reflection nebulae surrounding a dark lane (suggesting the
presence of a dusty disk), but these objects are all still deeply embedded
in the dense cores of this dark cloud. They are mostly protostars with ages
of about 100,000 years, surrounded by a remnant infalling envelope.
On the other hand, astronomers think that the newly found object has an age
of about 1 million years and is in a more evolved stage than those in the
neighboring Rho Ophiuchi star-forming region. The new disk is located at the
periphery of the dark cloud and is much less obscured than the younger
objects still embedded in the dense dark cloud nursery, thus allowing a much
clearer view of the dust disk.
The resolved circumstellar dust disk in the "Flying Saucer" has a radius of
about 300 Astronomical Units (45 billion km), or 5 times the size of the
orbit of Neptune (assuming the same distance as the Rho Ophiuchi
star-forming cloud, 500 light-years).
From model calculations, the astronomers find that it is inclined only about
4 deg to the line of sight and therefore seen very nearly from the side. A
lower limit to the total mass of the disk is about twice the mass of planet
Jupiter, or 600-700 times the mass of the Earth. A study of the recorded
(reflected) light from the optical to the near-infrared indicates that the
central young solar-type star has a temperature of about 3000 K and 0.4
times the luminosity of our actual Sun.
A detailed analysis of both reflection nebulae shows an unusual excess of
infrared light from the upper nebula, both visible in the NTT and VLT
images, which cannot be explained by a simple axisymmetrical model. Future
complementary high-resolution observations by the VLT adaptive optics camera
NAOS-CONICA will help the astronomers to understand the origin of this
puzzling phenomenon, and its possible link to the planet-forming mechanism.
Said Nicolas Grosso: "The `Flying Saucer' object presents us with a striking
portrait of our Solar System in its early infancy. With this object, Nature
has provided us a perfect laboratory for the study of both dust and gas in
young circumstellar disks, the raw material of planets."
The next steps
As this disk is located at a dark cloud periphery and not embedded in it,
follow-up studies at millimetre wavelengths with existing antenna arrays
will give a clear view without the complication of unrelated background
emission from dark cloud material. These future observations will provide an
easy mapping of the gas and dust material around this young solar-type star,
and allow a study of the chemical processes at work in this protoplanetary
disk. Moreover, current antenna arrays should be able to detect the
Keplerian rotation of this disk, providing a direct measurement of the mass
of the central star.
Computer simulations predict that baby planets produce measurable structural
changes in circumstellar disks, however such signs of the planet formation
are far from the sensitivity and the spatial resolution of the actual
antenna arrays. The detection of these features are the goal of ALMA, and
there is no doubt that this "planet nursery" object will be a prime target
for this future array of antennas.
More information
The results described in this Press Release have been submitted to the
European research journal Astronomy & Astrophysics ("The `Flying Saucer': a
new edge-on circumstellar dust disk at the periphery of the rho Ophiuchi
dark cloud" by N. Grosso and co-authors).
The text of this ESO Press Release, with three photos and all weblinks,
will be found at
http://www.eso.org/outreach/press-rel/pr-2002/pr-09-02.html
Note
[1]: The team consists of Nicolas Grosso (Max-Planck-Institut fuer
extraterrestrische Physik, Garching, Germany), Joao Alves (ESO, Garching,
Germany), Kenneth Wood (School of Physics & Astronomy, University of St
Andrews, Scotland, UK), Ralph Neuhaeuser (Max-Planck-Institut fuer
extraterrestrische Physik, Garching, Germany), Thierry Montmerle (Service
d'Astrophysique, CEA Saclay, Gif-sur-Yvette, France) and Jon E. Bjorkman
(Ritter Observatory, Department of Physics & Astronomy, University of
Toledo, Ohio, USA).
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