An international team of astronomers that are members of the Strategic
Exploration of Exoplanets and Disks with Subaru Telescope (SEEDS)
Project has used Subaru Telescope's High Contrast Instrument for the
Subaru Next Generation Adaptive Optics (HiCIAO) to observe a disk around
the young star RY Tau (Tauri). The team's analysis of the disk shows
that a "fluffy" layer above it is responsible for the scattered light
observed in the infrared image. Detailed comparisons with computer
simulations of scattered light from the disk reveal that this layer
appears to be a remnant of material from an earlier phase of stellar and
disk development, when dust and gas were falling onto the disk.
Figure 2: (left) An image in the near infrared (1.65 μm) around RY Tau, using a special mode of the HiCIAO coronagraph, the polarized intensity image. This type of observation is preferred for faint emissions associated with scattered light around planet-forming disks, as there is less light from the much brighter star. The colors indicate the strength of the emission (blue, yellow and red from faint to bright). A coronagraphic mask in the telescope optics blocks the central star, with its position marked at the center. A white ellipse shows the position of the midplane of the disk, which is observed at millimeter wavelengths. Scattered light observed in the near infrared is offset to the top of the image compared with the denser millimeter disk.
(right) Schematic view of the observed infrared light. The light from the star is scattered in the upper dust layer, and it makes the observed light offset from the midplane. (Credit: NAOJ)
Figure 1: Artist’s
rendition of the "fluffy" layer associated with the protoplanetary disk
of RY Tau, including jets coming from the star. Although typical young
stars like RY Tau are often associated with jets, they are not visible
in the HiCIAO observations at this time. (Credit: NAOJ)
Since 2009, the five-year SEEDS Project (Note)
has focused on direct imaging of exoplanets, i.e., planets orbiting
stars outside of our Solar System, and disks around a targeted total of
500 stars. Planet formation, an exciting and active area for
astronomical research, has long fascinated many scientists. Disks of
dust and gas that rotate around young stars are of particular interest,
because astronomers think that these are the sites where planets
form--in these so-called "protoplanetary disks." Since young stars and
disks are born in molecular clouds, giant clouds of dust and gas, the
role of dust becomes an important feature of understanding planet
formation; it relates not only to the formation of rocky, Earth-like
planets and the cores of giant Jupiter-like planets but also to that of
moons, planetary rings, comets, and asteroids.
As a part of the SEEDS Project, the current team of
researchers used HiCIAO mounted on the Subaru Telescope to observe a
possible planet-forming disk around the young star RY Tau. This star is
about 460 light years away from Earth in the constellation Taurus and is
around half a million years old. The disk has a radius of about 70 AU
(10 billion kilometers), which is a few times larger than the orbit of
Neptune in our own Solar System.
Astronomers have developed powerful instruments to
obtain images of protoplanetary disks, and Subaru Telescope's HiCIAO is
one of them. HiCIAO uses a mask to block out the light of the central
star, which may be a million times brighter than its disk. They can then
observe light from the star that has been reflected from the surface of
the disk. The scattered light will reveal the structure of the surface
of the disk, which is very small in scale and difficult to observe, even
with large telescopes. Observers use HiCIAO with a 188 element adaptive
optics system to reduce the blurring effects of the Earthʼs atmosphere,
making the images significantly sharper.
This team succeeded in capturing a near-infrared
image (1.65 μm) associated with the RY Tau disk. Unlike many other
protoplanetary disks, the disk emission is offset from the centre of the
star (Figure 2,
left). In contrast to longer wavelength observations, which are
associated with the midplane of the disk, near-infrared, scattered light
coming from the surface of the disk produced this offset (Figure 2, right), which provides information about the vertical structure of the disk.
Figure 2: (left) An image in the near infrared (1.65 μm) around RY Tau, using a special mode of the HiCIAO coronagraph, the polarized intensity image. This type of observation is preferred for faint emissions associated with scattered light around planet-forming disks, as there is less light from the much brighter star. The colors indicate the strength of the emission (blue, yellow and red from faint to bright). A coronagraphic mask in the telescope optics blocks the central star, with its position marked at the center. A white ellipse shows the position of the midplane of the disk, which is observed at millimeter wavelengths. Scattered light observed in the near infrared is offset to the top of the image compared with the denser millimeter disk.
(right) Schematic view of the observed infrared light. The light from the star is scattered in the upper dust layer, and it makes the observed light offset from the midplane. (Credit: NAOJ)
Changes in structure perpendicular to the surface of a disk are much harder to investigate because there are few good examples to study. Therefore, the information about vertical structure that this image provides is a contribution to understanding the formation of planets, which depends strongly on the structure of the disk, including structures such as spirals and rings, as well as height.
Figure 3: Computer
simulation for dust scattering for RY Tau. The color indicates the
strength of the modeled flux (blue, yellow and red for faint to bright).
The white contours show the image observed using Subaru Telescope's
HiCIAO. This modeled disk has a disk with a fluffy layer and closely
matches the image in shape and brightness. (Credit: NAOJ)
Figure 4: Schematic
views of the structure of the protoplanetary disk. The disk is
transparent at millimeter wavelengths, and as a result, the observed
millimeter emission is associated with the densest region (the
midplane). In contrast, the disk is opaque in the infrared in even at
the upper layer. Researchers often assume that the near-infrared
emission is due to scattered light from its surface like figure (a).
Figure (b) shows the revised schematic view through this study for RY
Tau. There is another layer above the two layers in (a). This layer is
almost transparent in the near-infrared, but not completely. The team
concludes that the scattered emission observed using Subaru Telescope's
HiCIAO is mainly due to scattering in this layer. (Credit: NAOJ)
Why is this fluffy layer observed in this disk, but
not in many other possible planet-forming disks? The team suspects that
this layer is a remnant of the dust that fell onto the star and the disk
during earlier stages of formation. In most stars, unlike RY Tau, this
layer dissipates by this stage in the formation of the star, but RY Tau
may still have it because of its youth. It may act as a special
comforter to warm the inside of the disk for baby planets being born
there. This may affect the number, size, and composition of the planets
being born in this system.
The Atacama Large Millimeter/Submillimeter Array
(ALMA), a superb international millimeter/submillimeter telescope, will
soon be making extensive observations of protoplanetary disks, which
will allow scientists to directly observe ongoing planet formation in
the midplane of a disk. By comparing SEEDS and ALMA observations
scientists may be able to understand the details of how planets form,
something that has raised fascinating questions for centuries.
References:
Takami, M. et al, 2013, Astrophysical Journal, Vol.
772, paper 145, "High-Contrast Near-Infrared Imaging Polarimetry of the
Protoplanetary Disk around RY Tau"
Core members of this research team are: M. Takami, J.L. Karr, J. Hashimoto, H. Kim, J. Wisniewski, T. Henning, C. Grady, R. Kandori, K.W. Hodapp, T. Kudo, N. Kusakabe, M.-Y. Chou, Y. Itoh, M. Momose, S. Mayama, and M. Tamura.
Core members of this research team are: M. Takami, J.L. Karr, J. Hashimoto, H. Kim, J. Wisniewski, T. Henning, C. Grady, R. Kandori, K.W. Hodapp, T. Kudo, N. Kusakabe, M.-Y. Chou, Y. Itoh, M. Momose, S. Mayama, and M. Tamura.
Note:
The SEEDS Project began in 2009 for a five-year
period, using 120 observing nights at Subaru Telescope, located at the
summit of Mauna Kea on the island of Hawaii. The goal of the project is
to explore hundreds of nearby stars in an effort to directly image
extrasolar planets and protoplanetary/debris disks that surround less
massive stars like the Sun. Principal investigator Motohide Tamura
(University of Tokyo and NAOJ) leads the project.
Acknowledgements:
This research was supported in part by the following:
- National Science Council grant 100-2112-M-001-007-MY3
- National Science Foundation (U.S.A.) grants 1008440 1009203 and 1009314
- Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan) Grants-in-Aid for Scientific Research in a Priority Area 2200000, 23103004.
- The Center for the Promotion of Integrated Sciences (CPISS) of The Graduate University for Advanced Studies (SOKENDAI, Japan)