Figure 1: The protoplanetary disk around TW Hya. (Left)
The near infrared image obtained by the HST shows the ring-like gap at
80 AU. (Right) The new image taken by the Subaru telescope is shown with
the observed HST radius of 80 AU represented by a dashed circle. The
orbital radius of Uranus represented by the thin solid line circle is
superposed on the image as a reference. The dark filled circle at the
center indicates a software mask with radius of 11 AU. The ring-like gap
was newly discovered at 20 AU from the central star. The surface
brightness is multiplied by r2 in both of the images for emphasizing the
gaps, where r is the distance from the central star. (Credit: NAOJ)
Figure 2: Artist rendition of the TW Hydrae showing double ringed structure with flare up toward the outer edge. (Credit: NAOJ)
The Strategic Exploration of Exoplanets and Disks (SEEDS, Note 1)
team of astronomers, led by the National Astronomical Observatory of
Japan (NAOJ), has found a close-in ring-like gap in the protoplanetary
disk of gas and dust around the nearby sun-like young star, TW Hydrae
(TW Hya). New Subaru Telescope images of the gap, including an earlier
ring-like gap found by Hubble Space Telescope, suggest that ongoing
planet formation is occurring in the disk, and provides a good picture
of how the early formation of our own solar system might have looked.
TW Hya is about 7 to 10 million years old and has about half the mass
of the Sun. It is located 180 light-years away in the constellation
Hydra, making it the closest T Tauri star to the Sun. This proximity
makes TW Hya a particularly accommodating target for high-resolution
studies at several wavelength ranges. In near-infrared observations by
the Hubble Space Telescope (HST) in 2013, a ring-like gap structure was
discovered at 80 astronomical units (AU, Note 2) from the central star (Fig. 1 left).
The newly discovered gap lies only 20 AU from the star in a region
that has only recently been observed. The spectral energy distribution
(information about the geometrical structure of the innermost part of
the disk which is unobservable due to its closeness to the central star)
between observed optical and mid-infrared wavelengths suggests that a
thin disk with the diameter of 4 AU exists closest to the star, but it
has not yet been imaged.
The SEEDS project team observed the small dust component (0.1 – 1
micron size) of the disk around TW Hya in H-band (1.6 μm), which allowed
them to find the gap structure at a distance of 20 AU from the central
star (Fig. 1 right).
In our solar system, this would correspond to a gap lying at about the
orbit of the planet Uranus (which lies at 19.19 AU). The new image from
Subaru Telescope also includes the gap structure first imaged by HST.
Such ring-like gaps provide strong observational evidence to support
the presence of planets around other stars. Their presence in the disk
around TW Hya suggests multiple planetesimals building blocks are
forming from materials in the dusk disk at different distances from the
central star. Eventually, they will combine to form fully grown planets.
TW Hya as an Early Solar System Analog
Since the first extrasolar planet was discovered around the sun-like
star 51 Pegasi in 1995, more than 5,000 exoplanet candidates have been
found (as of April 2015). As stars with multiple planets make up a large
number of these discoveries, many astronomers think that planetary
systems like our solar system exist in the universe. This makes their
formation a topic of interest.
As TW Hya is a young star, it may still be in the middle of the
planetary formation process, giving astronomers a look at what our own
solar system looked like some 4.5 billion years ago. Theoretical
simulations have long predicted a ring-like gap structure opened by a
protoplanet, so the discovery of multiple gaps at TW Hya marks an
important step toward understanding the mystery of planetary system
formation (Figure 2).
Future Work
As part of the observation of TW Hya with Subaru Telescope, the team
found a distribution of dust at 0.1 – 1 micron size on the disk surface.
Since dust grain growth is important in planetary formation processes,
it is necessary to understand the distribution of large-size (millimeter
or larger) dust grains in a disk's interior.
Radio interferometry
observations using the Atacama Large Millimeter Array (ALMA) have
successfully detect multiple ring-like gaps around another young star,
HL Tauri, which is younger than TW Hya. That observation provided the
distribution of large-size dust grains in the disk around that star (Note 3).
The SEEDS group plans millimeter and sub-millimeter radio observations
that are capable of tracing large-size dust grains in the interior of
the disk around TW Hya. In addition, radio observations can detect gas
that is main component of gas-giant planets similar to Jupiter, as well
as atmospheres enveloping rocky planets. The combination of Subaru
Telescope and ALMA observations will reveal the 3-dimensional structures
of protoplanetary disks and help guide our understanding of planetary
formation mechanisms. Farther in the future, the ALMA and
next-generation Thirty Meter Telescope (TMT) will provide convincing
information about the origin of gaps in protoplanetary disks and shed
new light on planet formation.
About SEEDS
The Strategic Exploration of Exoplanets and Disks with the Subaru
(SEEDS) group began in 2009 and consists of about 30 universities and
research institutes around the world, led by Motohide Tamura, director
of the Extra-Solar Planet Detection Project at NAOJ. The group has
discovered and studied a number of protoplanetary disks, including the
discovery of the spiral arm structure in the disk around the star AB
Aurigae, and detailed observations of disks around other stars. For
observations of TW Hya, the team used its high performance planet and
disk imager, HiCIAO, mounted on the Subaru Telescope. This instrument
improves images by correcting atmospheric aberration, which allowed the
high-contrast camera to focus in on the central region of the disk near
the TW Hya. Slight variations in luminosity in this region are difficult
to observe, due to the brightness of the nearby star.
This press release is based on the research paper "DISCOVERY OF A DISK
GAP CANDIDATE AT 20 AU IN TW HYDRAE " by Akiyama et al., which was
published in Astrophysical Journal Letters, April 2015 (ApJ, 802, L17,
2015). This work is partially supported by KAKENHI 22000005.
Notes
- The Strategic Exploration of Exoplanets and Disks with the Subaru (SEEDS) group used its high performance planet and disk imager, HiCIAO, mounted on the Subaru Telescope to conduct its research. The SEEDS Project began in 2009, with observations planned for a five-year period using 120 observing nights at Subaru Telescope.
- Astronomical Unit (AU) is the average distance between Earth and Sun. It is approximately 150 million kilometers (93 million miles).
- The Atacama Large Millimeter/submillimeter Array (ALMA) is
the largest radio interferometer in the world. It is currently under
construction at an altitude of 5,000 meters above sea level in Atacama
desert in Chile. The full ALMA installation will have 66 antennas
working together and separately across a 20 km section of desert. It
will provide high spatial resolution and sensitivity. The observations
at millimeter and submillimeter wavelengths that are capable of tracing
large dust distributions conducted toward the protoplanetary disk around
HL Tauri and shows the clear multiple gaps probably created by planets
sweeping up the surrounding material in this disk.
http://www.almaobservatory.org/en/press-room/press-releases/771-revolutionary-alma-image-reveals-planetary-genesis
Source: Subaru Telescope
