Figure 1:
Near-infrared (1.6 micron) image of the debris ring around the star HR
4796A. An astronomical unit (AU) is a unit of length that corresponds
to the average distance between the Earth and Sun, almost 92 million
miles (over 149 million km).
The
ring consists of dust grains in a wide orbit (roughly twice the size of
Pluto's orbit) around the central star. Its edge is so precisely
revealed that the researchers could confirm a previously suspected
offset between the ring's center and the star's location. This "wobble"
in the dust's orbit is most likely caused by the unbalancing action of –
so far undetected – massive planets likely to be orbiting within the
ring. Furthermore, the image of the ring appears to be smudged out at
its tips and reveals the presence of finer dust extending out beyond the
main body of the ring.
For high resolution versions of the above image, click on the following links: Image only or Image with labels.
The SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru
Telescope/HiCIAO) project, a five-year international collaboration
launched in 2009 and led by Motohide Tamura of NAOJ (National
Astronomical Observatory of Japan) has yielded another impressive image
that contributes to our understanding of the link between disks and
planet formation. Researchers used Subaru's planet-finder camera, HiCIAO
(High Contrast Instrument for the Subaru Next Generation Adaptive
Optics), to take a crisp high-contrast image of the dust ring around HR
4796A, a young (8-10 million years old) nearby star, only 240 light
years away from Earth. The ring consists of dust grains in a wide orbit,
roughly twice the size of Pluto's orbit, around the central star. The
resolution of the image of the inner edge of the ring is so precise that
an offset between its center and the star's position can be measured.
Although data from the Hubble Space Telescope led another research group
to suspect such an offset, the Subaru data not only confirm its
presence but also reveal it to be larger than previously assumed.
What
caused the wheel of dust around HR 4796A to run off its axis? The most
plausible explanation is that the gravitational force of one or more
planets orbiting in the gap within the ring must be tugging at the dust,
thus unbalancing their course around the star in predictable ways.
Computer simulations have already shown that such gravitational tides
can shape a dust ring into eccentricity, and findings from another the
eccentric dust ring around the star Formalhaut may be observational
evidence for the process. Since no planet candidates have been spotted
near HR 4796A yet, the planets causing the dust ring to wobble are
probably simply too faint to detect with current instruments.
Nevertheless, the Subaru image allows scientists to infer their presence
from their influence on the circumstellar dust.
The
Subaru Telescope's near-infrared image is as sharp as the Hubble Space
Telescope's visible-light image, thus enabling accurate measurements of
its eccentricity. While Subaru Telescope's mirror is much larger than
Hubble's, light from the HR 4796A system must first pass through the
turbulent air layers of Earth's atmosphere before Subaru's instruments
can measure it. Subaru's adaptive optics system (AO188) allows it to
correct for most of the atmosphere's blurring effects in order to take
razor-sharp images. The application of an advanced image processing
technique, angular differential imaging, to the data suppressed the
star's bright glare and enhanced the faint light reflected from the ring
so that it was more visible.
This image gives
scientists more information about the relationship between a
circumstellar disk and planet formation. Planets are believed to form in
the disks of gas and dust that remain around young stars as the
by-products of star formation. As the material is swept up by the
newborn planets or blown out of the system by the star's radiation, such
(primordial) disks soon disappear in a few tens of million years.
Nevertheless, some stars are surrounded by a debris or secondary disk,
mainly composed of dust long after the primordial disk should have
dispersed. Collisions between small solid bodies ("planetesimals") left
over from planet formation may continuously replenish the dust in these
disks. The dust ring around HR 4796A is such a debris disk and provides
essential information for studying planet formation and possible formed
planets in such debris disk systems.
References:
Refer to the following article for the published report of the results:
Thalmann et al., The Astrophysical Journal Letters, Volume 743, Issue 1, (2011)
Articles describing other results from the SEEDS project can be found at the following locations:
Direct Images of Disks Unravel Mystery of Planet Formation, February 17, 2011. (Subaru Telescope Press release: 17 Feb. 2011)
Discovery of an Exoplanet Candidate Orbiting a Sun-Like Star, December 3, 2009. (Subaru Telescope Press release: 3 Dec. 2009)
Source: https://www.naoj.org/
