UKIRT discovers 'impossible' binary stars

This artist's impression shows the tightest of the new record breaking binary systems. Two active M4 type red dwarfs orbit each other every 2.5 hours, as they continue to spiral inwards. Eventually they will coalesce into a single star. Credit: J. Pinfield, for the RoPACS network.

A team of astronomers have used the United Kingdom Infrared Telescope (UKIRT) on Hawaii to discover four pairs of stars that orbit each other in less than 4 hours. Until now it was thought that such close-in binary stars could not exist. The new discoveries come from the telescope's Wide Field Camera (WFCAM) Transit Survey, and appear in the journal Monthly Notices of the Royal Astronomical Society.

About half of the stars in our Milky Way galaxy are, unlike our Sun, part of a binary system in which two stars orbit each other. Most likely, the stars in these systems were formed close together and have been in orbit around each other from birth onwards. It was always thought that if binary stars form too close to each other, they would quickly merge into one single, bigger star. This was in line with many observations taken over the last three decades showing the abundant population of stellar binaries, but none with orbital periods shorter than 5 hours.

For the first time, the team have investigated binaries of red dwarfs, stars up to ten times smaller and a thousand times less luminous than the Sun. Although they form the most common type of star in the Milky Way, red dwarfs do not show up in normal surveys because of their dimness in visible light.

For the last five years, UKIRT has been monitoring the brightness of hundreds of thousands of stars, including thousands of red dwarfs, in near-infrared light, using its state-of-the-art Wide-Field Camera (WFC). This study of cool stars in the time domain has been a focus of the European (FP7) Initial Training Network 'Rocky Planets Around Cool Stars' (RoPACS) which studies planets and cool stars.

"To our complete surprise, we found several red dwarf binaries with orbital periods significantly shorter than the 5 hour cut-off found for Sun-like stars, something previously thought to be impossible", said Bas Nefs from Leiden Observatory in the Netherlands, lead author of the paper. "It means that we have to rethink how these close-in binaries form and evolve."

Since stars shrink in size early in their lifetime, the fact that these very tight binaries exist means that their orbits must also have shrunk as well since their birth, otherwise the stars would have been in contact early on and have merged. However, it is not at all clear how these orbits could have shrunk by so much.

One possible answer to this riddle is that cool stars in binary systems are much more active and violent than previously thought.

It is possible that the magnetic field lines radiating out from the cool star companions get twisted and deformed as they spiral in towards each other, generating the extra activity through stellar wind, explosive flaring and star spots. Powerful magnetic activity could apply the brakes to these spinning stars, slowing them down so that they move closer together.

"Without UKIRT's superb sensitivity, it wouldn't have been possible to find these extraordinary pairs of red dwarfs", said David Pinfield. He adds: "The active nature of these stars and their apparently powerful magnetic fields has profound implications for the environments around red dwarfs throughout our Galaxy."


Image and caption

An image accompanying the release can be downloaded from http://star-www.herts.ac.uk/~dpi/close_mdwarfs.png


Media contact

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 / 4582 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk


Science contacts

Bas Nefs
Leiden Observatory
Tel: +31 (0)71 527 8439
nefs@strw.leidenuniv.nl

Dr Jayne Birkby
Leiden Observatory
Tel: +31 (0)71527 5832
birkby@strw.leidenuniv.nl

Dr David Pinfield
University of Hertfordshire
Leads the European ROPACS network: http://star.herts.ac.uk/RoPACS/ and is co-PI of the WFCAM Transit Survey (WTS).
Tel: +44 (0)1707 284171
d.j.pinfield@herts.ac.uk

Dr Simon Hodgkin
Institute of Astronomy
University of Cambridge
(Co-PI of WTS)
Tel: +44 (0)1223 766657
sth@ast.cam.ac.uk
(http://www.ast.cam.ac.uk/)


Further information

The team publish their work in the paper, "Four ultra-short period eclipsing M-dwarf binaries in the WFCAM Transit Survey", S. V. Nefs et al, Monthly Notices of the Royal Astronomical Society, in press. A preprint of the paper can be downloaded from http://arxiv.org/abs/1206.1200


Notes for editors

With a 3.8 metre diameter mirror, the UK Infrared Telescope (UKIRT: http://www.jach.hawaii.edu/UKIRT/) is the second largest dedicated infrared telescope in the world. Sited at an altitude of 4200 m on the top of the volcano Mauna Kea on the island of Hawaii, it began operations in 1979. UKIRT is carrying out the UKIRT Deep Sky Survey (UKIDSS: http://www.ukidss.org/) searching for objects from nearby brown dwarfs to distant quasars. In 2012 the UKIDSS team received the RAS Group Award.

The Royal Astronomical Society (RAS, www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

Follow the RAS on Twitter via @royalastrosoc

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Unique Kuiper Belt Binary Has Unscathed Orbit

Figure 1: Gemini Multi-Object Spectrograph (on Gemini North, GMOS-N) observation of the “Plutino” binary system 2007 TY430, which is located nearly 40 astronomical units (AU) from the Sun. (The average distance between the Earth and Sun is 1 AU.) The pair is separated by about 42,000 km, which appears here at maximum separation of only 0.7 arcsecond on the sky.

Figure 2: The smooth line shows the model of the orbit of the 2007 TY430 binary, displayed as the motion of one body around the other. Individual observations are marked with crosses (the Subaru discovery and subsequent Gemini observations) and a circle (from the Hubble Space Telescope).

Small icy bodies are the remnant leftovers from the formation of planets in the Solar System. Ongoing observations made with Gemini of an extremely red pair of such Kuiper Belt objects (KBOs) in an orbiting binary system offer an indirect glimpse into the past. Scott Sheppard (Carnegie Institution of Washington), Darin Ragozzine (Harvard-Smithsonian Center for Astrophysics), and Chad Trujillo (Gemini Observatory) obtained nearly monthly observations of the pair, named 2007 TY430, to yield precise measurements of their orbital motion. Uniquely, compared with other Kuiper Belt binaries, this pair’s current mutual orbit is likely primordial, unchanged since the formation of the system. The primordial orbit reveals the binary’s formation mechanism, and therefore provides hints of past conditions during the formation of the Solar System itself. The research team concludes that this system may have formed in a more complex interaction involving yet another body. The composition of the bodies’ ultra-red material is unknown, but it may be associated with organic material and it depends upon the pair’s formation site.

The members of the pair otherwise have the characteristics similar to ordinary KBOs. They are roughly equally sized (at a radius of about 50 km each), with a total mass of nearly 1018 kg, and their mutual orbits are nearly circular. However, the overall location of the pair is somewhat closer to the Sun than the classical Kuiper Belt boundaries. The system likely moved out of the classical Kuiper Belt and then got stuck in their current location, which is favored because of a resonance with Neptune’s gravitational pull. For every three orbits around the Sun that Neptune makes, 2007 TY430 will complete two. This so-called “3:2 resonance” is the same relationship Pluto has with Neptune, so such objects are also known as “Plutinos.”

A widely separated binary pair like 2007 TY430 is not very stable, and indeed this is the first wide, equal-sized binary found in this resonance with Neptune. Most other similar systems have likely disintegrated. Thus, 2007 TY430 is possibly one of the few remaining examples of the type.

Members of the team previously reported the discovery of this system based on observations from the Subaru Telescope. The new work appearing in the March 2012 Astronomical Journal takes advantage of Gemini’s capabilities, using the Gemini Multi-Object Spectrograph to measure the system precisely, despite the apparent separation of the bodies by only 0.7 arcseconds or less on the sky (Figures 1 and 2). This corresponds to the apparent size of a dime at a distance of 3 miles. Capturing such fine measurements requires excellent image quality, which Gemini delivers.

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NASA'S RXTE Helps Pinpoint Launch of 'Bullets' in a Black Hole's Jet

Using observations from NASA's Rossi X-ray Timing Explorer (RXTE) satellite and the National Science Foundation's (NSF) Very Long Baseline Array (VLBA) radio telescope, an international team of astronomers has identified the moment when a black hole in our galaxy launched super-fast knots of gas into space.

X-ray and radio data let astronomers pinpoint when the black hole system H1743-322 ejected powerful gas 'bullets' during its mid-2009 outburst. In this animation, an X-ray hot spot in the gas around the black hole produced signals of rising frequency as the spot moved closer to the black hole. When the bullets were ejected June 3, the hot spot vanished. Download high-res video from NASA Goddard's Scientific Visualization Studio

Racing outward at about one-quarter the speed of light, these "bullets" of ionized gas are thought to arise from a region located just outside the black hole's event horizon, the point beyond which nothing can escape.

"Like a referee at a sports game, we essentially rewound the footage on the bullets' progress, pinpointing when they were launched," said Gregory Sivakoff of the University of Alberta in Canada. He presented the findings today at the American Astronomical Society meeting in Austin, Texas. "With the unique capabilities of RXTE and the VLBA, we can associate their ejection with changes that likely signaled the start of the process."

Radio imaging by the Very Long Baseline Array (top row), combined with simultaneous X-ray observations by NASA's RXTE (middle), captured the transient ejection of massive gas "bullets" by the black hole binary H1743-322 during its 2009 outburst. By tracking the motion of these bullets with the VLBA, astronomers were able to link the ejection event to the disappearance of X-ray signals seen in RXTE data. These signals, called quasi-periodic oscillations (QPOs), vanished two days earlier than the onset of the radio flare that astronomers previously had assumed signaled the ejection. (Credit: NRAO and NASA's Goddard Space Flight Center). Larger image

The research centered on the mid-2009 outburst of a binary system known as H1743–322, located about 28,000 light-years away toward the constellation Scorpius. Discovered by NASA's HEAO-1 satellite in 1977, the system is composed of a normal star and a black hole of modest but unknown masses. Their orbit around each other is measured in days, which puts them so close together that the black hole pulls a continuous stream of matter from its stellar companion. The flowing gas forms a flattened accretion disk millions of miles across, several times wider than our sun, centered on the black hole. As matter swirls inward, it is compressed and heated to tens of millions of degrees, so hot that it emits X-rays.

Some of the infalling matter becomes re-directed out of the accretion disk as dual, oppositely directed jets. Most of the time, the jets consist of a steady flow of particles. Occasionally, though, they morph into more powerful outflows that hurl massive gas blobs at significant fractions of the speed of light.

This 327-MHz radio view of the center of our galaxy highlights the position of the black hole system H1743-322, as well as other features. (Credit: J. Miller-Jones, ICRAR-Curtin Univ.; C. Brogan, NRAO). Larger image - Larger image (no labels)

In early June 2009, H1743–322 underwent this transition as astronomers watched with RXTE, the VLBA, the Very Large Array near Socorro, N.M., and the Australia Telescope Compact Array (ATCA) near Narrabri in New South Wales. The observatories captured changes in the system's X-ray and radio emissions as the transformation occurred.

From May 28 to June 2, the system's X-ray and radio emissions were fairly steady, although RXTE data show that cyclic X-ray variations, known as quasi-periodic oscillations or QPOs, gradually increased in frequency over the same period. On June 4, ATCA measurements showed that the radio emission had faded significantly.

Astronomers interpret QPOs as signals produced by the interaction of clumps of ionized gas in the accretion disk near the black hole. When RXTE next looked at the system on June 5, the QPOs were gone.

The same day, the radio emission increased. An extremely detailed VLBA image revealed a bright, radio-emitting bullet of gas moving outward from the system in the direction of one of the jets. On June 6, a second blob, moving away in the opposite direction, was seen.

Until now, astronomers had associated the onset of the radio outburst with the bullet ejection event. However, based on the VLBA data, the team calculated that the bullets were launched on June 3, about two days before the main radio flare. A paper on the findings will be published in the Monthly Notices of the Royal Astronomical Society.

"This research provides new clues about the conditions needed to initiate a jet and can guide our thinking about how it happens," said Chris Done, an astrophysicist at the University of Durham, England, who was not involved in the study.

A super-sized version of the same phenomenon occurs at the center of an active galaxy, where a black hole weighing millions to billions of times our sun's mass can drive outflows extending millions of light-years.

"Black hole jets in binary star systems act as fast-forwarded versions of their galactic-scale cousins, giving us insights into how they work and how their enormous energy output can influence the growth of galaxies and clusters of galaxies," said lead researcher James Miller-Jones at the International Center for Radio Astronomy Research at Curtin University in Perth, Australia.

The Rossi X-ray Timing Explorer, which operated from Dec. 1995 to Jan. 2012, was managed by NASA's Goddard Space Flight Center in Greenbelt, Md. The VLBA, the world's largest and highest-resolution astronomical instrument, is controlled from the National Radio Astronomy Observatory's Domenici Science Operations Center.

Related Link: More about RXTE

The Very Long Baseline Array is a system of ten radio telescopes spanning 5,500 miles that work together as the world's largest dedicated astronomical instrument. Each station consists of an 82-foot-diameter, 240-ton dish antenna and an adjacent control building. Credit: NASA's Goddard Space Flight Center. Larger image - Larger image (no labels)

Technicians work on RXTE in 1995
Credit: NASA's Goddard Space Flight Center. Larger image

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NASA's RXTE Detects 'Heartbeat' of Smallest Black Hole Candidate

This animation compares the X-ray 'heartbeats' of GRS 1915 and IGR J17091, two black holes that ingest gas from companion stars. GRS 1915 has nearly five times the mass of IGR J17091, which at three solar masses may be the smallest black hole known. A fly-through relates the heartbeats to hypothesized changes in the black hole's jet and disk. Credit: NASA/Goddard Space Flight Center/CI Lab. Download this video and related content from NASA Goddard's Scientific Visualization Studio

An international team of astronomers has identified a candidate for the smallest-known black hole using data from NASA's Rossi X-ray Timing Explorer (RXTE). The evidence comes from a specific type of X-ray pattern, nicknamed a "heartbeat" because of its resemblance to an electrocardiogram. The pattern until now has been recorded in only one other black hole system.

Named IGR J17091-3624 after the astronomical coordinates of its sky position, the binary system combines a normal star with a black hole that may weigh less than three times the sun's mass. That is near the theoretical mass boundary where black holes become possible.

Gas from the normal star streams toward the black hole and forms a disk around it. Friction within the disk heats the gas to millions of degrees, which is hot enough to emit X-rays. Cyclical variations in the intensity of the X-rays observed reflect processes taking place within the gas disk. Scientists think that the most rapid changes occur near the black hole's event horizon, the point beyond which nothing, not even light, can escape.

Astronomers first became aware of the binary system during an outburst in 2003. Archival data from various space missions show it becomes active every few years. Its most recent outburst started in February and is ongoing. The system is located in the direction of the constellation Scorpius, but its distance is not well established. It could be as close as 16,000 light-years or more than 65,000 light-years away.

The record-holder for wide-ranging X-ray variability is another black hole binary system named GRS 1915+105. This system is unique in displaying more than a dozen highly structured patterns, typically lasting between seconds and hours.

"We think that most of these patterns represent cycles of accumulation and ejection in an unstable disk, and we now see seven of them in IGR J17091," said Tomaso Belloni at Brera Observatory in Merate, Italy. "Identifying these signatures in a second black hole system is very exciting."

In GRS 1915, strong magnetic fields near the black hole's event horizon eject some of the gas into dual, oppositely directed jets that blast outward at about 98 percent the speed of light. The peak of its heartbeat emission corresponds to the emergence of the jet.

Changes in the X-ray spectrum observed by RXTE during each beat reveal that the innermost region of the disk emits enough radiation to push back the gas, creating a strong outward wind that stops the inward flow, briefly starving the black hole and shutting down the jet. This corresponds to the faintest emission. Eventually, the inner disk gets so bright and hot it essentially disintegrates and plunges toward the black hole, re-establishing the jet and beginning the cycle anew. This entire process happens in as little as 40 seconds.

While there is no direct evidence IGR J17091 possesses a particle jet, its heartbeat signature suggests that similar processes are at work. Researchers say that this system's heartbeat emission can be 20 times fainter than GRS 1915 and can cycle some eight times faster, in as little as five seconds.

Astronomers estimate that GRS 1915 is about 14 times the sun's mass, placing it among the most-massive-known black holes that have formed because of the collapse of a single star. The research team analyzed six months of RXTE observations to compare the two systems, concluding that IGR J17091 must possess a minuscule black hole.

"Just as the heart rate of a mouse is faster than an elephant's, the heartbeat signals from these black holes scale according to their masses," said Diego Altamirano, an astrophysicist at the University of Amsterdam in The Netherlands and lead author of a paper describing the findings in the Nov. 4 issue of The Astrophysical Journal Letters.

The researchers say this analysis is just the start of a larger program to compare both of these black holes in detail using data from RXTE, NASA's Swift satellite and the European XMM-Newton observatory.

"Until this study, GRS 1915 was essentially a one-off, and there's only so much we can understand from a single example," said Tod Strohmayer, the project scientist for RXTE at NASA's Goddard Space Flight Center in Greenbelt, Md. "Now, with a second system exhibiting similar types of variability, we really can begin to test how well we understand what happens at the brink of a black hole."

Launched in late 1995, RXTE is second only to Hubble as the longest serving of NASA's operating astrophysics missions. RXTE provides a unique observing window into the extreme environments of neutron stars and black holes.

Related links

Dutch press release

http://www.astronomie.nl/nieuws\2362/astronomen_observeren_de_hartslag_van_een_piepklein_zwart_gat.html

Italian press release
http://www.media.inaf.it/2011/12/12/batticuore-buco-nero/

GRS 1915+105: Taking the Pulse of a Black Hole System
http://chandra.harvard.edu/photo/2011/g1915/

RXTE Homes in on a Black Hole's Jets
http://www.nasa.gov/topics/universe/features/black-hole-jets.html


Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Md.

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Trans-Neptunian Binaries and the History of the Outer Solar System

Full Resolution MP4 | 4.2M

Figure 1. Frame from a movie showing the binaries as they are observed (and extrapolated) from Earth over the period from 2000-2013. The resulting on-sky behavior is somewhat complex, due to the varying viewing geometry from Earth's and the binary system's orbital motions. During the period of this animation, all illustrated binaries except 2001 QW322 will have completed at least one mutual orbit (2001 QW322 has a mutual orbit period of approximately 17.4 years). Animation is rendered with a 10-day timestep in simulated 0.35 arcsecond seeing (roughly the best seeing achieved during our observational campaign). Lower right bars are one arcsecond on a side and point North and East. Nominal best-fit orbit is illustrated. One-sigma astrometric uncertainty reflected in size of data points, while the color of the data points indicates data taken from Gemini North (red) or other facilities (blue). Note: the original version of this movie includes audible tones which correspond to each observation.

Figure 2. Original GMOS observation in the r band clearly shows both members of the trans-Neptunian binary system 2006 JZ₈₁.

An international team of astronomers, using data from the Gemini North telescope, are revealing the history of the outer Solar System by measuring the mutual orbits of extremely widely separated binaries located beyond Neptune in the icy Kuiper Belt. The researchers conclude that these “trans-Neptunian binaries” formed close to their current locations, and—contrary to previous suggestions—that they may have formed from direct collapse in the disk of material that produced the planets and other solid bodies of the Solar System.



Binary systems are found in most minor planet populations, such as asteroids, but binaries in the Kuiper Belt are notable due to the frequency of very widely-separated systems and those with nearly equal-sized primary and secondary bodies. A team of researchers led by Alex Parker (Harvard-Smithsonian Center for Astrophysics; formerly of the University of Victoria, Canada) followed some of the most widely-separated Kuiper Belt binaries known, using a combination of archival and new data from the Magellan Telescope, the Very Large Telescope and Gemini North telescope (Figure 1, animation).



Over four semesters at Gemini North, the team acquired high-resolution optical images of these systems with the Gemini Multi-Object Spectrograph in its imaging mode (Figure 2). The program required extremely good resolution, only possible under very stable atmospheric conditions (which astronomers call good “seeing”). With the short exposure times feasible given Gemini’s light-gathering capacity and the flexibility of queue scheduling, the observations could be obtained in the strict conditions they required. The data enabled high-precision measurement of the relative positions of the binary components without the use of adaptive optics or space-based imaging. The level of precision achieved was comparable to measuring the width of a human hair from over a kilometer away.



The team determined the properties of the mutual orbits of these binary systems for the first time, and several systems set new records. For example: 2001 QW322 is the most widely-separated binary minor planet known, with an average separation exceeding 100,000 kilometers. Another extreme is 2006 CH69, which has the most eccentric (highly elliptical) mutual orbit known—at e=0.9 the two components of this system are separated by only approximately 2,800 kilometers at closest approach, while at their most distant they have over 52,000 kilometers between them. The smallest object in the sample, 2000 CF105, has the lowest mass of any measured Kuiper Belt object—roughly twice as massive as Mauna Kea, the long-dormant volcano that is the site of Gemini North.



Their wide separations make these systems very sensitive to perturbations; encounters with massive objects like the giant planets or direct collisions with very small impactors can disrupt the binaries and send their components into solitary orbits around the Sun. Their continued existence suggests that these objects formed near their current locations in the outer Solar System and were not subject to significant migration, as some hypotheses of the origin of the Kuiper Belt have suggested. Additionally, the current Kuiper Belt cannot have a very large population of small objects since collisions would have blown apart these systems long ago.



The full study revealed further surprises about the binaries’ formation history. Unlike more tightly-bound Kuiper Belt binaries, the widely-separated systems appear to prefer low mutual inclinations, with orbital planes that are nearly aligned with the rest of the Solar System. Interestingly, about half of the systems orbit each other in the same direction as they orbit the Sun (prograde), while the other half orbit each other in the opposite sense (retrograde). Previous formation theories hold that if the binaries have low mutual inclinations, they must have formed through a pathway that tends to create only retrograde systems. The researchers suggest that the fact that these binaries have equal numbers of prograde and retrograde systems, yet also prefer low mutual inclinations, may indicate that a novel, recently-proposed formation process was at play. In this scenario, instead of building solitary Kuiper Belt objects through slow "hierarchical" accretion and then combining into binaries later, the binaries may have formed from rapid, direct collapse of solids in the protoplanetary disk.



The complete work will appear in The Astrophysical Journal, and a preprint is available now.

Source: Gemini Observatory

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Duo of Big Telescopes Probes the Depths of Binary Star Formation

Figure 1: A composite image toward Taurus FS A binary system. The green color shows the intensity of visible light, based on optical data from the Hubble Space Telescope. The red color displays near-infrared data from the Subaru Telescope. The ellipse marks an artifact from data processing of the central bright star. The dashed lines denote the directions of the support of the secondary mirror of the telescope. The field of view is 16.5" x 17.5". North is up, and east is to the left.

Figure 2: The polarization distribution in visible light, overlaid on the visible image (top) and near-infrared image (bottom). The color-coding refers to the offset from the expected centro-symmetric pattern around FS A star and FS B star. The blue and red colors show the circular pattern around FS A while the green encircles FS B. The area toward the southeast of FS A is bright in the near infrared, and the offset, denoted by the yellow color, is larger. The fields of view are 19" x 31" (top) and 14" x 17" (bottom), respectively. North is up and east is to the left.

A team of researchers from four Japanese universities (Kobe, Saitama, Osaka, and Tokyo) has been able to delineate the intricate structure of the circumbinary disk that surrounds a young binary star system from the observation with the Subaru Telescope and the Hubble Space Telescope. By using different wavelengths to examine the system's internal structure, they succeeded in demonstrating a distinct color difference between its northern and southern portions (figure 1). The researchers are now prepared to apply their approach of combining optical and near-infrared observations to other regions of binary formation.

Previous observations have demonstrated that protoplanetary disks, composed of a ring of dense gas surrounding a star like our Sun, not only accompany many infant stars but also are sites that generate planetary systems such as the one to which our Earth belongs. Therefore, these disks provide important information about the formation of stars and planets.

Past observations have focused on the protoplanetary disks of single stars. However, stellar research reveals that the majority of stars are members of binary or multiple star systems rather than ones composed of a single star. The research team addressed the issue of limited research on binary systems by pointing the Subaru Telescope toward the FS star system in the constellation Taurus. The separation between the primary (A) and companion (B) stars is 20 arc seconds (2800 AU; astronomical unit, the distance between the Sun and the Earth) and the FS A star itself is also a binary system with only 0.2 arc seconds (30 AU) of separation between its stars. The research team succeeded in detecting a circumbinary disk by using the Subaru Telescope's near-infrared camera CIAO (Coronagraphic Imager with Adaptive Optics), which blocks out the bright light of the central star. The disk's size of 630 AU is equivalent to the aphelion (the furthest point from the Sun in its orbit) of Sedona, one of the trans-Neptunian objects.

The team then compared its near-infrared image with the optical image taken by the Advanced Camera System (ACS) aboard the Hubble Space Telescope (HST). The area north of the FS A binary is brighter in the visible light (optical), while that south of the binary stands out in the near-infrared. In other words, the north is blue, and the south is red. The protoplanetary disk reflects the visible or the infrared light from the central star, but it does not emit light by itself. The highly distinct color contrast between the northern and southern portions of FS A's protoplanetary disk is a very unique characteristic of the system.

The question becomes why the color is different in these regions around the binary. Part of the answer relates to the degree of the polarization dispersed from the surface of the disk. Regardless of whether the light is visible or near-infrared, it displays the properties of a wave as well as a particle, and its reflection shows polarization. The degree and the direction of the polarization provide information about the object that reflects the light. This is why measurement of the polarization is important for understanding the structure of the protoplanetary disk. The observation with the Hubble Space Telescope included information on polarization, and figure 2 shows the distribution of the polarized light. The majority of the disk shows a typical concentric pattern around the central star. Other protoplanetary disks show similar patterns.

In addition to its circular pattern, the outer region to the north reflects the light from the FS B star, which is much further away from the FS A system. The research team interpreted this feature as an effect of abundant interstellar material in front of the FS A circumbinary disk and the influence of the FS B. However, the polarization data show that the inner region to the north is part of the protobinary disk surrounding the FS A binary. The mystery of color difference remains.

In sum, the research team established that there is a distinct color difference between the areas to the north and south of the circumbinary disk of the FS A star. They want to continue observations of protoplanetary disks so that they can identify their common characteristics and chronicle their evolution. Their ultimate goal is to understand the planetary formation process in circumstellar/circumbinary disks.

Reference:
"High-Resolution Optical and Near-Infrared Images of the FS Tauri Circumbinary Disk", Tomonori Hioki, Yoichi Itoh, Yumiko Oasa, Misato Fukagawa, Masahiko Hayashi. June 2011 issue of the Publications of the Astronomical Society of Japan.

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