Summary
The star “iota Horologii”, 56 light-years from Earth, possesses
not only an extrasolar planet, but also a dust disk.
This is the exciting result of recent observations with the ADONIS
(ADaptive Optics Near Infrared System) instrument, mounted at
the ESO
3.6-m telescope at the La
Silla Observatory.
Such a disk holds information about the formation of the
exoplanetary system. As this is the fourth known example of a star
with both a disk and a planet, that combination may indeed be
comparatively common among solar-type stars.
Our own Solar system also contains dust. When the dust scatters the
sunlight, this can be observed as “zodiacal light”, a cone of
faint light extending above the western horizon soon after sunset or
the eastern just before sunrise.
The same phenomenon should thus be observable from the planet
orbiting iota Horologii.
PR Photo 27/00: The disk at
iota Horologii.
The exoplanet at iota Horologii
Last year, the star iota Horologii was found to have a
planetary companion, at least twice as heavy as Jupiter, the largest
planet in the Solar System. It was the first exoplanet to be
discovered in an almost earth-like orbit, cf. ESO PR 12/99).
This discovery was based on long-term measurements of the radial
velocity of iota Horologii by means of the 1.4-m Coudé
Auxiliary Telescope (CAT) at La Silla. The extremely accurate
observations were made with the Coude-Echelle-Spectrometer
(CES) which is now connected to the ESO 3.6-m telescope. With the
combination of spectroscopic (CES) and high-angular resolution
(ADONIS) observational facilities at one telescope, the 3.6-m is
uniquely suited for this type of front-line research.
Dust disks and planets around stars
Dust disks around stars still retain information about the
formation processes of the exoplanetary systems as they are formed by
collisions of planetesimals or proto-planets. However, it is still a
somewhat controversial issue exactly how the presence of giant planets
influences these collisions or whether the existence of a planetary
system can be inferred from observed structures in dust disks.
To cast more light on this fundamental issue, it is necessary to
search for systems which have both a planet and a dust disk. Our own
Solar System contains a significant amount of dust particles which can
be seen during very clear evenings and nights by naked eye as a
diffuse band of light in the sky – the “zodiacal light”. Observations
from the Voyager spacecraft have shown that this dust extends quite
far out, well beyond the orbit of planet Pluto.
Observations of stellar disks with ADONIS
The team used the ADONIS instrument with the SHARP
II camera to search for dust disks around the iota
Horologii planetary system. ADONIS corrects the atmospheric
turbulence in real-time by means of a computer-controlled flexible
mirror, allowing the sharpest possible images to be recorded with this
special camera.
In order to detect circumstellar material, it is an absolute
condition that the light that is recorded from the star itself is
reduced to a minimum. The circumstellar dust reflects only a small
fraction of the stellar light and would otherwise be completely
outshone by the intense light from the star in the middle.
This is achieved by inserting in front of the detector a so-called
coronographic mask that blocks the light of the star. The
chosen diameter is a compromise between the desire to detect features
as close as possible to the star and the rapidly increasing amount of
stellar light as the size of the mask is decreased.
For the the present observations of iota Horologii, a mask
with a diameter of 1.0 arcsec was used (about 17 AU, or 2550 million
km at the distance of the star). A series of short exposures were made
through a near-infrared filter (in the H-band that is centred at
wavelength 1.64 µm), a spectral region where the disk/star light
intensity ratio and the instrument efficiency are optimal.
In the course of the extensive data analysis the exposures are
combined to produce the resulting image of the star. Moreover, to
correct for stray light in the instrument, it is necessary to
“subtract” the image of a reference star which is known to be free of
any circumstellar material. This procedure effectively reduces the
unavoidable halo of instrumentally introduced stray light from the
star that – despite the mask – is still significantly brighter than
the light coming from the disk.
The dust disk around iota Horologii
PR Photo 27/00 displays a dusty disk around the star
iota Horologii (left) as compared with that of a “reference”
star (right). At a distance of 56 light-years from Earth, iota
Horologii was already known to possess an extrasolar planet. The
discovery of the disk may help to better understand how this
exoplanetary system was formed. The observations were obtained with
the ADONIS adaptive optics instrument at the ESO 3.6-m
telescope on La Silla. The strong stellar light in the central
area of the two images has been blocked with an instrumental
mask. While the image processing still leaves some unavoidable stray
light around the reference star (mostly due to reflections within the
instrument), a much brighter, diffuse disk around iota
Horologii is clearly visible. Technical
information about this photo is available below.
The observations were obtained during the night of September 6-7,
2000, during a period of very good weather conditions (seeing about
0.6 arcsec). They consisted of 150 individual exposures, each lasting
4 seconds.
After subtraction of the recorded image of a reference star, a
wide, somewhat elongated structure around iota Horologii is
clearly visible, cf. PR Photo 27/00 (left image). Although this
cannot be seen directly on the photo, various arguments [3] indicate that this is indeed a dust disk that
extends in NE-SW direction.
In order to check that this structure is not an artefact of the
image processing, exactly the same procedure was applied to images of
another star without circumstellar material (right image). In this
case, there is no indication of a similar, extended structure. The
“noise residuals” in the right image are about 10 times weaker than
the light from the disk around iota Horologii, i.e. there is no
doubt that the disk is real.
Size and inclination of the disk
Even though these new images are quite sensitive, a direct
detection of the planet near iota Horologii is not possible with
this technique as its light is several thousand times fainter than
that from the disk. Moreover, the planet orbits the star at a
distance of only 1 AU and is thus completely hidden behind the
coronographic mask which has a radius of 8.5 AU.
The dust disk around iota Horologii is quite extended; it is
detected to a distance of about 65 AU, i.e. 10,000 million km, from
the star. This corresponds to more than twice the distance of Neptune
from the Sun. It is also much larger and denser than the dust disk
now observed in the Solar System. From the elongation, it appears
that the inclination of the disk is about 42°.
More observations to follow
By means of future observations at different wavelengths, it will
become possible to measure some of the physical properties of the dust
grains in the disk, e.g., their temperature, sizes and chemical
composition.
In this context, great progress in this exciting research field is
expected when the NAOS adaptive
optics facility at the ESO Very Large Telescope
(VLT) enters into operation next year. This instrument will
have a much higher sensivity and will thus be able to detect fainter
disks as well as small structures in known ones that may hint at the
presence of orbiting planets.
Notes
[1]: The team searching for dust disks around
southern exoplanet systems consists of: Sebastian Els (Institut
für Theoretische Astrophysik, Universität Heidelberg, Germany
and ESO-Chile), Eric Pantin (Service d’Astrophysique, CEA,
Saclay, France), Franck Marchis (Institut d’Astrophysique
Spatiale, Université Paris Sud, France, and ESO-Chile),
Michael Endl (Institut für Astronomie, Universität
Wien, Austria, and ESO-Chile), and Martin Kürster and
Michael Sterzik (both ESO-Chile).
[2]: In addition to iota Horologii, the
following stars – all in the northern celestial hemisphere – are also
known to possess both a disk and a planet: rho Corona Borealis,
HD 210277 and epsilon Eridani. 55 Cancri is
another possible candidate under investigation.
[3]: Several arguments speak in favour of a
disk structure, rather than a (near-)spherical dust halo. A disk is a
more stable configuration than a halo. It is also rather unlikely that
there would be a dust halo around an otherwise seemingly normal G0
zero-age-main-sequence star like iota Horologii. Moreover,
mid-infrared observations with the Infrared Space
Observatory (ISO) show an excess of infrared radiation. In
the case of a star like iota Horologii, this is indicative of a
circumstellar dust disk (the dust emits thermal radiation in the
infrared part of the spectrum – hence the radiation from a star with a
dust disk is usually relatively stronger in the infrared, as compared
to stars without circumstellar dust). Furthermore, radio observations
with the SEST
at La Silla did not find any molecular carbon monoxide (CO) emission;
thus the star did not retain a part of its parent cloud. It is also
not located in the proximity of a star-forming region and it is
extremely unlikely that there would be material along the
line-of-sight that may mimic a halo. The circumstellar dust around
iota Horologii is therefore most likely to be arranged in a
disk.
Technical information about the photo
PR Photo 27/00: The intensity scale ranges from 0.1 (deep
red) to 100 (white) mJy/arcsec2 (surface brightness). The
diameter of the mask is 1.0 arcsec, corresponding to 17 AU (2550
million km) at the distance of iota Horologii (56
light-years). The images are based on 150 integrations of 4 seconds
each, i.e. a total exposure time of 10 min. The observing conditions
were excellent (0.6 arcsec seeing) and the achieved image resolution
by the adaptive optics system (approx. 0.11 arcsec) is near the best
possible (the “diffraction limit”) at this wavelength (H-band at 1.64
µm).
This is the caption to ESO PR Photo 27/00. It may be
reproduced, if credit is given to the European Southern
Observatory.
