Discovery of Multiple Ring-Like Gaps in a Protoplanetary Disk

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

1. 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.
2. Astronomical Unit (AU) is the average distance between Earth and Sun. It is approximately 150 million kilometers (93 million miles).
3. 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

A Fluffy Disk Around a Baby Star

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 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)

The team performed extensive computer simulations of the scattered light, for disks with different masses, shapes, and types of dust (Figure 3). They found that the scattered light is probably not associated with the main surface of the disk, which is the usual explanation for the scattered light image (Figure 4a). Instead, the observed infrared emission can be explained if the emission is associated with a fluffy upper layer, which is almost transparent and not completely transparent (Figure 4b). The team estimated the dust mass in this layer to be about half the mass of Earthʼs Moon.

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.

Source: Subaru Telescope

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)

 Source: Subaru Telescope

Subaru Telescope’s Imaging Discovery of a Second Jupiter Shows the Power and Significance of the SEEDS Project

Astronomers in the Strategic Explorations of Exoplanets and Disks with Subaru (SEEDS) Project have recently discovered and captured an image of the least massive planet ever imaged so far--a so-called "second Jupiter" (Figure 1). This discovery marks an important step toward the direct imaging of much fainter Earth-like planets in the future and may lead to new models of planet formation. It also illustrates the important role that the SEEDS project plays in observational astronomy.

Figure 1: Near-infrared color composite images of a "second Jupiter" around the Sun-like star GJ 504. A coronagraph and differential techniques suppress the bright light from the central star. On the left is the intensity image, which shows the radiant power passing through the area, while on the right is the signal-to-noise ratio image, which shows the weakest signal that the detecting system can recognize. (Credit: NAOJ)

Observations of Exoplanets

Exoplanets are planets orbiting stars other than our Sun, outside of our Solar System. As of July 2013, most of the 890 exoplanets reported thus far have been discovered by indirect observation techniques, e.g. monitoring the host star for radial velocity variation or planetary transits (Note 1). Such techniques require observations over at least one orbital period and are impractical for detecting planets that are widely separated from their host stars and have long orbital periods. In contrast, direct imaging may be the most important way to observe exoplanets, because it yields information about the planet's luminosity, temperature, atmosphere, and orbit. However, this method is technically challenging, because these distant planets are not only faint but also close to their bright central stars, the glare of which easily obscures an indication of a planet. It's like trying to see a firefly around a distant lighthouse. Only a dozen planets with a location similar in scale to that of the Solar System have been discovered. In spite of these technical challenges, 8 m class telescopes with adaptive optics and coronagraphs that block the light of the central star have reached the high performance necessary to image massive planets at large orbital separations. The Subaru SEEDS project is in the forefront of using direct observation to discover and explore the features of exoplanets.

Discovery of a "Second Jupiter"

Since 2009, SEEDS, an international project approved by the National Astronomical Observatory of Japan (NAOJ) and led by Motohide Tamura (University of Tokyo and NAOJ), has been probing this new frontier with its five-year direct imaging survey of exoplanets and disks around a targeted total of 500 stars. The team is composed of more than 120 members (two-thirds from Japan and one-third in the United States and Europe) and over 25 institutes.

Video: An explanation of what an exoplanet is and why direct imaging of a "second Jupiter" around the Sun-like star GJ 504 is so significant. (Credit: NAOJ)

During the SEEDS survey, a team led by members from the Tokyo Institute of Technology (TiTech), the University of Tokyo, and NAOJ used the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) (Note 2) mounted on the Subaru Telescope to discover and image planet candidate GJ 504 b that orbits a Sun-like star GJ 504, which is in the constellation Virgo about 60 light years from Earth (Figure 2). They used a mature (160 Myr) host star as a natural guide star for the AO 188 (adaptive optics with 188 elements). GJ 504 is bright and visible to the naked-eye (~5 magnitudes), but the planet is very dim (17-20 magnitudes at infrared wavelengths).

Since a single direct imaging cannot distinguish planet candidates from background stars that happen to be within the small field of view of the camera, the team conducted seven observations, including the use of IRCS (Infrared Camera and Spectrograph) with AO 188, to confirm that GJ 504 b orbits around GJ 504 and is NOT a background star.

What is the mass of this planet? Planet mass is estimated from age and luminosity. However, mass estimates for planets suffer from the uncertainties of cooling models for giant planets. Planetary mass estimates usually assume a hot start, in which the planet is initially in a hot-temperature and luminous state. However, recent theoretical models suggest that giant planets could be much colder initially. The difference between the relation of mass to luminosity decreases with an exoplanet's age and converges after 100 Myr for a five-Jupiter-mass planet. Regardless of which model is used to calculate the mass of this cold, dim object, GJ 504 b, the central star GJ 504 is mature enough (160 Myr) to predict a planetary rather than brown dwarf mass.

Based on the relation of its observed luminosity and estimated age in comparison with the theoretical model, scientists can infer that GJ 504 b has a mass as small as three Jovian masses. If so, it is the lightest-mass planet ever imaged. The apparent distance between the central star and planet is 44 AU (astronomical unit), which is larger than Neptune's orbit and comparable to Pluto's, and the team concludes that they have finally imaged a "second Jupiter". This is an important step toward the direct imaging of much fainter Earth-like planets in future.

The question remains: How did such a giant planet form around a Sun-like star? The standard core accretion model based on our Solar System does not provide an adequate explanation for the formation of these outer planets situated so far from their central stars. This model maintains that the core reaches a critical mass from the accretion of bodies of rock or ice (planetesimals) and subsequently increases with the rapid and direct accumulation of gas from its protoplanetary disk. However, it falls short of explaining how planets beyond 30 AU have developed.

Perhaps another model like gravitational instability of formation sites could supply the answer. This model maintains that a massive protoplanetary disk becomes gravitationally unstable in its outer regions. Consequently, the outer disk fragments, collapsing directly into one or more giant planets. However, gravitational instability has its own challenge. The theory requires that Sun-like stars have very massive disks around them, and it is unlikely that they could obtain enough mass in such outer regions. 

At this point scientists cannot definitely say what led to the formation of GJ 504 b. Its features provide the basis for continuing investigation. Subsequent observations reveal that the planet has a unique blue color (Figure 2), which indicates that its atmosphere is less cloudy than that of other imaged planets.

Figure 2: Near-infrared color and magnitude of the planet GJ 504 b, compared with other imaged planets and brown dwarfs. (Credit: NAOJ)

Its relatively cold temperature, less cloudiness, and light mass place it in a physically distinct category from previously imaged planets, which are hotter and have cloudier atmospheres. Additional interesting features include its age (oldest among all directly imaged planets) as well as its wide separation and orbit around a main sequence star with a Sun-like mass.

Other Fruits of the SEEDS Project

In line with the project's aim of using the Subaru Telescope to directly image young planetary-mass objects and circumstellar structures located at relatively large distances from central stars, observers have used HiCIAO (Notes 2 & 3) with the Subaru Telescope to directly image two exoplanets and provide detailed images of more than ten protoplanetary and two debris disks (Figure 3). Observations have yielded reports of the following;

• Giant planet candidates in GJ 758 and Kappa Andromedae ("Direct Imaging of Super-Jupiter Around A Massive Star")
• Detailed structures of disks in AB Aur and LkCa15 ("Direct Images of Disks unravel Mystery of Planet Formation")
• HD 169142, SAO 206462 ("Spiral Structure of Disk May Reveal Planets")
• PDS 70 ("Discovery of a Giant Gap in the Disk of a Sun-like star May Indicate Multiple Planets")
• UX Tau ("Dust Grains Highlight the Path to Planet Formation")
• U Sco J 1604 ("Direct Infrared Image of an Arm in Disk Demonstrates Transition to Planet Formation")
• HR 4796 A ("Subaru's Sharp Eye Confirms Signs of Unseen Planets in the Dust Ring of HR 4796 A")
• HIP 79977

They have also detected interesting fine structures of the disks around many young stars, which show gaps, spiral arms, and rings; these structures may be "signposts of planets". In addition, the image of a companion around HAT-P-7 may explain the retrograde motion of its planet HAT-P-7 b ("The Origin and Maintenance of a Retrograde Exoplanet")

Figure 3: Gallery of disks imaged in the near-infrared by Subaru SEEDS. Note the spirals and gaps in the disks that the images reveal. (Credit: NAOJ)

The Promise of Future Research

The SEEDS Project has finished more than three-quarters of its allotted 120 observation nights. About 500 stars will eventually be observed, and the statistical results on the frequency of outer planets will provide clues to planet formation. Scientists can address such questions as: "How numerous are outer planets?"; "Where and how did they form?"; "What do their atmospheres look like ?"

The team has detected many gaps and spiral-arm structures in disks at the same radial distance where outer planets were discovered. Planets within the disks could explain the formation of these structures and allow scientists to hypothesize that outer planets have formed or are close to their birth. At the very least, the SEEDS results demonstrate that it is now time to revisit current planet formation theory and include the findings about outer planets and disk structures. 

SEEDS also will explore the inner regions near the central star with a very advanced adaptive optics system called the Subaru Telescope Coronagraphic Extreme Adaptive Optics (SCExAO, Note 4), which will become available soon. Observations with this technology promise to contribute to better understandings of exoplanetary systems.

Since observations from SEEDS include data on very young stars as well as relatively old ones, these data are and will be used to explore the links between planets and disks as well as the evolution of protoplanetary systems and debris disks. The SEEDS data set will become a dominant source of information in this important field of research for many years to come.

Notes:

1. Indirect observations detect exoplanets by noting the central star's velocity shift (radial velocity method) or the dimming of stellar light as the planet passes by (transit method). In contrast, direct observations directly image the exoplanet rather than inferring its presence by indirect means.
2. HiCIAO is an acronym for High Contrast Instrument for the Subaru Next Generation Adaptive Optics, which has various differential imaging modes (wavelengths, polarizations). This technologically adaptable system replaced the Coronographic Imager with Adaptive Optics (CIAO) in 2007 and has been used with AO 188 for the SEEDS project since October, 2009. Scientists use it to directly observe exoplanets and circumstellar disks. Since the bright light from a central star can hinder the detection of planets, the instrument uses a coronagraph to suppress the bright light of the central star.
3. Adaptive Optics (AO) is a technique for obtaining a high resolution image by correcting for atmospheric distortion during the observation. It measures the atmospheric distortion and then compensates for and corrects the wave plane by using a deformable mirror in real time.
4. SCExAO will utilize much more actuators than the current AO system to obtain stellar images that are almost comparable to those obtained in space. It will also utilize a new coronagraphic technique to obtain a higher contrast by about two orders of magnitude.

References:

• The scientific paper on which this article is based will appear in the Astrophysical Journal.
• Other findings from the SEEDS project are posted on the Subaru Telescope website:
  • February 7, 2013, "Direct Infrared Image of an Arm in Disk Demonstrates Transition to Planet Formation"
  • January 24, 2013, "The Origin and Maintenance of a Retrograde Exoplanet"
  • December 19, 2012, "Spiral Structure of Disk May Reveal Planets"
  • November 27, 2012, "Dust Grains Highlight the Path to Planet Formation"
  • November 19, 2012, "Direct Imaging of a Super-Jupiter Around A Massive Star"
  • November 8, 2012, "Discovery of a Giant Gap in the Disk of a Sun-like Star May Indicate Multiple Planets"
  • December 29, 2011, "Subaru's Sharp Eye Confirms Signs of Unseen Planets in the Dust Ring of HR 4796 A"
  • February 17, 2011, "Direct Images of Disks Unravel Mystery of Planet Formation"

Acknowledgements:

• The SEEDS team acknowledges the continuous support given to them by the Subaru Telescope and its instrument teams. This research was supported in part by Grants-in-Aid (# 22000005, 23103002, 23103004), Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Direct Infrared Image of an Arm in Disk Demonstrates Transition to Planet Formation

An international team of astronomers led by Satoshi Mayama (The Graduate University for Advanced Studies, Japan) and Ruobing Dong (Princeton University, U.S.A.) has made observations with the Subaru Telescope and captured the first vivid infrared image of a curved arm of dust extending over a hole on a disk around a young star--2MASS J16042165-2130284 (J 1604). This feature indicates the probable existence of unseen planets within the hole. The image shows the dynamic environment in which planets may be born and gives information about constraints on the distance at which planets can form from a central star.

Figure 1: Subaru Telescope's near-infrared (1.6 μm) image of the protoplanetary disk around the young star J 1604. A black circular mask covers the bright, saturated light from the central star. The gauges for distance are in astronomical units and arc seconds. (Abbreviated as AU, an astronomical unit is the distance between the Sun and Earth. Abbreviated as arcsec, an arc second is 1/3600 of a degree.) Prominent features include the hole (white dotted line) in the disk; the arm extending over the hole (on the right); and the asymmetric dip (on the left). Click here for the image without labels. (Credit: The Graduate University for Advanced Studies and the National Astronomical Observatory of Japan)

Research over the past two decades has confirmed that new stars are often surrounded by disks of dense gas and dust ("protoplanetary disks") from which planets form. A central star enters an active phase of planet building when it is a few million years old. During this period, newborn planets may deplete some of the gas and dust in the disk, producing a hole within it, although the outer ring remains. However, the debatable origins of the hole require direct observation to confirm this process. Direct imaging of the structures that indicate planet building inside of the hole have rarely occurred—until now. The current team's research, a part of the Strategic Explorations of Exoplanets and Disks with Subaru (SEEDS) Project (Note 1) is filling in the observational gaps in this relatively unexplored area.

The team used the high-resolution infrared camera HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics) mounted on the 8.2-m Subaru Telescope in April 2012 to observe the young star J 1604, which has a mass similar to the Sun's; it is located in the Upper Scorpius star-forming region at a distance of 470 light years and is estimated to be 3.7 million years (Myr) old. The researchers captured a very high-resolution (0.07 arc seconds) near-infrared image of its protoplanetary disk, which shows dust particles that scatter the light from the central star. The disk has some interesting features: a large hole with an asymmetric dip in the disk and a curved arm extending over the hole. This is the first vivid infrared image of such an arm in observations of the disks around young stars, and it also marks the first detection of an arm of dust that could lead to the formation of Earth-like rocky planets (Note 2). The arm emerges from the inner edge of the western side of the disk, begins to extend inward, and then curves to the northeast. Based on their detailed modeling, the team estimates that the radius of the disk's inner edge is 63 AU; its inclination is 10 degrees; and the length of the arm is 50 AU. Their measurements of the surface brightness of the gap show that it drops by a factor of five when compared with the rest of the disk.

Characteristics of the hole in the disk and the arm over it indicate the possible presence of unseen planets within the hole. The width and depth of the observed hole conform to the size of a hole that planets would create according to current theories of planet formation. The researchers' calculations suggest that the hole in this disk might mark the presence of at least one planet at 40 – 50 AU from the central star. Current theories also predict that the gravity of a planet could produce a curved arm in a disk. Because the shape of the arm in the Subaru Telescope image shared features in line theoretical predictions, the team concluded that unseen planets could explain its structure. Overall, these findings identify constraints on planet formation at certain distances from the central star.

Figure 2: Artist's rendition of the protoplanetary disk around J 1604

Credit: The Graduate University for Advanced Studies, Japan)

The high-resolution image from these scientists' research at Subaru Telescope clearly illustrates the dynamic context in which planets are born. Providing these kinds of detailed images of a face-on disk object becomes a perfect laboratory for astronomers to test and refine their theoretical models of planet formation.

References:
Mayama, S. et al. December 2012, Astrophysical Journal Letter 760, L26,
"Subaru Imaging of Asymmetric Features in a Transitional Disk in Upper Scorpius".

Core members of this research are:
S. Mayama, J. Hashimoto, T. Muto, T. Tsukagoshi, N. Kusakabe, M. Kuzuhara, Y. Takahashi, T. Kudo, R. Dong, M. Fukagawa, M. Takami, M. Momose, J. P. Wisniewski, K. Follette, and M. Tamura.

Notes:

1. The SEEDS Project begun 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 (NAOJ) leads the project.
2. A January 2, 2013 article in Nature reported that the ALMA Telescope resolved a submillimeter image of the disk of another object, HD142527, which showed similar arm-like structures in its disk. However, the image displays streams of gas rather than dust particles flowing across the gap in the disk. These researchers suggest that the arm may indicate the presence of unseen planets, because giant planets may be using the gas as they grow within the disk.

Acknowledgements:
This research was supported in part by the following:

• National Science Foundation (U.S.A.) grants 1009314 and 1009203
• Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan) Grants-in-Aid for Scientific Research in a Priority Area 2200000, 23103004, 24103504, 24840037
• Center for the Promotion of Integrated Sciences (CPISS) of The Graduate University for Advanced Studies (SOKENDAI, Japan)

Spiral Structure of Disk May Reveal Planets

An international team of astronomers has used HiCIAO (High Contrast Instrument for the Subaru Next Generation Optics) (Note 1) to observe a disk around the young star SAO 206462. They succeeded in capturing clear, detailed images of its disk, which they discovered has a spiral structure with two discernable arms. On the basis of their observations and modeling according to spiral density wave theory, the team suspects that dynamic processes, possibly resulting from planets in the disk, may be responsible for its spiral shape. This research may provide the basis for another indirect method of detecting planets.

Scientists have known that planets form in a broad disk of dust and gas surrounding a star, a so-called "protoplanetary disk." However, the composition of these special disks as well as the process by which they give rise to planets have remained a mystery. The bright light of a central star makes it difficult to detect fainter objects around it or to capture a detailed image of the composition of the disk itself. Recent research with HiCIAO, Subaru Telecope's "planet-hunter", has overcome some of those obstacles. By masking the bright light from the central star, the instrument can then detect more detailed features of the star's disk and the objects that it contains.

As part of the SEEDS project (Strategic Exploration of Exoplanets and Disks with the Subaru Telescope) (Note 2), the researchers in the current study used HiCIAO to conduct observations of the disk around the young star SAO 206462 (sometimes referred to as HD 135344B). This star is about 460 light years away from Earth in the constellation Lupus ("the wolf") and is some 9 million years old. The radius of the disk is 20 billion kilometers (12.4 billion miles), about five times greater than Neptune's distance from the Sun in our Solar System.

The researchers captured images of SAO 206462's disk (Figure 1) that clearly reveal its spiral structure and indicate some features of its composition. They then were able to analyze its spiral structure by using density wave theory to infer the properties of the disk. This process allows a productive interface between observational data and a theoretical model.

Figure 1: An image of the disk around SAO 206462 captured with HiCIAO. A coronagraph blocks the direct light of the central star, which appears as the black, circular area in the image. Arrows show the two arms of the spiral structure around the star. (Credit: NAOJ) 

Density wave theory has been applied to explain the spiral arm structure of spiral galaxies. It proposes that a rotating disk of matter would "naturally" develop regions of enhanced density, so-called "spiral density waves", due to differential rotation. The wave-like concentration of dense material grows and forms a spiral pattern. A similar process may be at work in SAO 206462's disk. When the team compared their model with the observational data, they found that it was useful in revealing the features of the disk. (Figure 2)

Figure 2: A comparison of the fit between the theoretical model and observational data. The red dashed line represents the shape of the disk based on modeling from density wave theory. The image shows that the data conform to the predictions of the theory and supports an explanation for the development of the structure in terms of this theory's model. (Credit: NAOJ)  
The team was able to use the model to estimate the temperature of the disk based on dynamic processes and predict the evolution of the spiral structures. The observational data conform to the model. Although they could not specifically identify the origin of the spirals, it is possible that planets embedded in the disk may be the catalysts for the development of its shape. If a planet has already been formed in a disk, its gravity can produce a density wave, which then may result in the creation of a spiral structure in the protoplanetary disk. (Figure 3).

 Figure 3: A representation of how interaction between a protoplanetary disk and planet makes a density wave and affects the disk's structure. A planet in the disk may be one explanation for the formation of the disk's spiral structure. The basis for the simulation is a code in computational fluid dynamics called FARGO that simulates the flow of gases in motion. Colors indicate the surface density of the disk; the darkest colors show the areas with the least density while the white shows those with the greatest density. (Credit: NAOJ)   

Although the observed image does not necessarily show the existence of a planet, the possibility remains that a planet in the disk causes the density wave. This is the first time that density wave theory has been applied to measuring the features of a protoplanetary disk. The research takes an important step in explaining how a spiral disk could form and may mark the development of another indirect means of discovering planets.

References

The research paper entitled "Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory" by Muto et al. was published in Astrophysical Journal Letters, April 2012 (ApJ, 748, L22, 2012)

Note:

1. HiCIAO (High Contrast Instrument for the Subaru Next Generation Adaptive Optics) is designed to block out the bright direct light from a central star so that it can image nearby faint objects such as planets and detect faint dust disks around the central star.


2. SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru Telescope) is a large-scale project led by Motohide Tamura at NAOJ (National Astronomical Observatory of Japan). Researchers conduct observations at the Subaru Telescope that focus on the direct imaging and examination of exoplanets and disks to better understand the formation of planetary systems. Over 100 scientists and 25 institutions belong to the international consortium supporting the project . Since 2009, the SEEDS project has yielded a set of impressive findings, including imaging of the detailed structure of disks in AB Aur, LkCa15, HR4796A, and HD 169142.

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