A Fluffy Disk Around a Baby Star

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)

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.

(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)

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 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. 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, which provides information about the vertical structure of the disk.

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.

Image : 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. 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. 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. The team estimated the dust mass in this layer to be about half the mass of Earthʼs Moon.

Image : 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.

Source : National Astronomical Observatory of Japan.
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New theory points to ‘zombie vortices’ as key step in star formation

Artist concept of a brown dwarf, spotted by NASA's Spitzer Space Telescope, surrounded by a spinning protoplanetary disk. UC Berkeley researchers have developed a model that shows how vortices help destabilize the disk so that gas can spiral inward toward a forming star. (Image courtesy of NASA/JPL-Caltech)

A new theory by fluid dynamics experts at the University of California, Berkeley, shows how “zombie vortices” help lead to the birth of a new star. 

Reporting today (Tuesday, Aug. 20) in the journal Physical Review Letters, a team led by computational physicist Philip Marcus shows how variations in gas density lead to instability, which then generates the whirlpool-like vortices needed for stars to form.

Illustration of the near stellar environment of the star Beta Pictoris. This image is based upon observations made with the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope. (Image by Dana Berry, Space Telescope Science Institute)

Astronomers accept that in the first steps of a new star’s birth, dense clouds of gas collapse into clumps that, with the aid of angular momentum, spin into one or more Frisbee-like disks where a protostar starts to form. But for the protostar to grow bigger, the spinning disk needs to lose some of its angular momentum so that the gas can slow down and spiral inward onto the protostar. Once the protostar gains enough mass, it can kick off nuclear fusion.

“After this last step, a star is born,” said Marcus, a professor in the Department of Mechanical Engineering.

What has been hazy is exactly how the cloud disk sheds its angular momentum so mass can feed into the protostar.

Destabilizing forces

The leading theory in astronomy relies on magnetic fields as the destabilizing force that slows down the disks. One problem in the theory has been that gas needs to be ionized, or charged with a free electron, in order to interact with a magnetic field. However, there are regions in a protoplanetary disk that are too cold for ionization to occur.

“Current models show that because the gas in the disk is too cool to interact with magnetic fields, the disk is very stable,” said Marcus. “Many regions are so stable that astronomers call them dead zones – so it has been unclear how disk matter destabilizes and collapses onto the star.”

The researchers said current models also fail to account for changes in a protoplanetary disk’s gas density based upon its height.

“This change in density creates the opening for violent instability,” said study co-author Pedram Hassanzadeh, who did this work as a UC Berkeley Ph.D. student in mechanical engineering. When they accounted for density change in their computer models, 3-D vortices emerged in the protoplanetary disk, and those vortices spawned more vortices, leading to the eventual disruption of the protoplanetary disk’s angular momentum.

“Because the vortices arise from these dead zones, and because new generations of giant vortices march across these dead zones, we affectionately refer to them as ‘zombie vortices,’” said Marcus. “Zombie vortices destabilize the orbiting gas, which allows it to fall onto the protostar and complete its formation.”

The researchers note that changes in the vertical density of a liquid or gas occur throughout nature, from the oceans – where water near the bottom is colder, saltier and denser than water near the surface – to our atmosphere, where air is thinner at higher altitudes. These density changes often create instabilities that result in turbulence and vortices such as whirlpools, hurricanes and tornadoes. Jupiter’s variable-density atmosphere hosts numerous vortices, including its famous Great Red Spot.

Illustration of a protoplanetary disk based upon observations from the Keck II telescope. (Image courtesy of W. M. Keck Observatory)

Connecting the steps leading to a star’s birth

This new model has caught the attention of Marcus’s colleagues at UC Berkeley, including Richard Klein, adjunct professor of astronomy and a theoretical astrophysicist at the Lawrence Livermore National Laboratory. Klein and fellow star formation expert Christopher McKee, UC Berkeley professor of physics and astronomy, were not part of the work described in Physical Review Letters, but are collaborating with Marcus to put the zombie vortices through more tests.

Klein and McKee have worked over the last decade to calculate the crucial first steps of star formation, which describes the collapse of giant gas clouds into Frisbee-like disks. They will collaborate with Marcus’s team by providing them with their computed velocities, temperatures and densities of the disks that surround protostars. This collaboration will allow Marcus’s team to study the formation and march of zombie vortices in a more realistic model of the disk.

Source : University of California - Berkeley.

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ALMA Takes Close Look at Drama of Starbirth

Starbirth Surprisingly Energetic: New Insights Into Protostars

Stunning ALMA and NTT image of Newborn Star

 Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have obtained a vivid close-up view of material streaming away from a newborn star. By looking at the glow coming from carbon monoxide molecules in an object called Herbig-Haro 46/47 they have discovered that its jets are even more energetic than previously thought. The very detailed new images have also revealed a previously unknown jet pointing in a totally different direction.

Young stars are violent objects that eject material at speeds as high as one million kilometres per hour. When this material crashes into the surrounding gas it glows, creating a Herbig-Haro object . A spectacular example is named Herbig-Haro 46/47 and is situated about 1400 light-years from Earth in the southern constellation of Vela (The Sails). This object was the target of a study using ALMA during the Early Science phase, whilst the telescope was still under construction and well before the array was completed.

ALMA’s view of the outflow associated
with the Herbig-Haro object HH 46/47

The new images reveal fine detail in two jets, one coming towards Earth and one moving away. The receding jet was almost invisible in earlier pictures made in visible light, due to obscuration by the dust clouds surrounding the new-born star. ALMA has not only provided much sharper images than earlier facilities but also allowed astronomers to measure how fast the glowing material is moving through space.

These new observations of Herbig-Haro 46/47 revealed that some of the ejected material had velocities much higher than had been measured before. This means the outflowing gas carries much more energy and momentum than previously thought.

The team leader and first author of the new study, Héctor Arce (Yale University, USA) explains that "ALMA's exquisite sensitivity allows the detection of previously unseen features in this source, like this very fast outflow. It also seems to be a textbook example of a simple model where the molecular outflow is generated by a wide-angle wind from the young star."

The Herbig-Haro object HH 46/47 seen
 with ESO’s New Technology Telescope

 The observations were obtained in just five hours of ALMA observation time – even though ALMA was still under construction at the time – similar quality observations with other telescopes would have taken ten times longer.

"The detail in the Herbig-Haro 46/47 images is stunning. Perhaps more stunning is the fact that, for these types of observations, we really are still in the early days. In the future ALMA will provide even better images than this in a fraction of the time," adds Stuartt Corder (Joint ALMA Observatory, Chile), a co-author on the new paper.

Diego Mardones (Universidad de Chile), another co-author, emphasises that "this system is similar to most isolated low mass stars during their formation and birth. But it is also unusual because the outflow impacts the cloud directly on one side of the young star and escapes out of the cloud on the other. This makes it an excellent system for studying the impact of the stellar winds on the parent cloud from which the young star is formed."

Wide-field view of the star-forming region around the Herbig-Haro object HH 46/47

The sharpness and sensitivity achieved by these ALMA observations also allowed the team to discover an unsuspected outflow component that seems to be coming from a lower mass companion to the young star. This secondary outflow is seen almost at right angles to the principal object and is apparently carving its own hole out of the surrounding cloud.

The Herbig-Haro object HH 46/47 in the constellation of Vela

Arce concludes that "ALMA has made it possible to detect features in the observed outflow much more clearly than previous studies. This shows that there will certainly be many surprises and fascinating discoveries to be made with the full array. ALMA will certainly revolutionise the field of star formation!"

Source : European Southern Observatory - ESO

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Waking Up to a New Year: Exoplanet Orbits Its Star in 8.5 Hours

Artist's illustration. Researchers have discovered an Earth-sized exoplanet named Kepler 78b that whips around its host star in a mere 8.5 hours -- one of the shortest orbital periods ever detected. (Credit: Image courtesy of Cristina Sanchis Ojeda)

In the time it takes you to complete a single workday, or get a full night's sleep, a small fireball of a planet 700 light-years away has already completed an entire year.

Researchers at MIT have discovered an Earth-sized exoplanet named Kepler 78b that whips around its host star in a mere 8.5 hours -- one of the shortest orbital periods ever detected. The planet is extremely close to its star -- its orbital radius is only about three times the radius of the star -- and the scientists have estimated that its surface temperatures may be as high as 3,000 degrees Kelvin, or more than 5,000 degrees Fahrenheit. In such a scorching environment, the top layer of the planet is likely completely melted, creating a massive, roiling ocean of lava.

What's most exciting to scientists is that they were able to detect light emitted by the planet -- the first time that researchers have been able to do so for an exoplanet as small as Kepler 78b. This light, once analyzed with larger telescopes, may give scientists detailed information about the planet's surface composition and reflective properties.

Kepler 78b is so close to its star that scientists hope to measure its gravitational influence on the star. Such information may be used to measure the planet's mass, which could make Kepler 78b the first Earth-sized planet outside our own solar system whose mass is known.

The researchers reported their discovery of Kepler 78b in The Astrophysical Journal.

In a separate paper, published in Astrophysical Journal Letters, members of that same group, along with others at MIT and elsewhere, observed KOI 1843.03, a previously discovered exoplanet with an even shorter orbital period: just 4 1/4 hours. The group, led by physics professor emeritus Saul Rappaport, determined that in order for the planet to maintain its extremely tight orbit around its star, it would have to be incredibly dense, made almost entirely of iron -- otherwise, the immense tidal forces from the nearby star would rip the planet to pieces.

"Just the fact that it's able to survive there implies that it's very dense," says Josh Winn, an associate professor of physics at MIT, and co-author on both papers. "Whether nature actually makes planets that are dense enough to survive even closer in, that's an open question, and would be even more amazing."

Dips in the data

In their discovery of Kepler 78b, the team that wrote the Astrophysical Journal paper looked through more than 150,000 stars that were monitored by the Kepler Telescope, a NASA space observatory that surveys a slice of the galaxy. Scientists are analyzing data from Kepler in hopes of identifying habitable, Earth-sized planets.

The goal for Winn and his colleagues was to look for Earth-sized planets with very short orbital periods.

"We've gotten used to planets having orbits of a few days," Winn says. "But we wondered, what about a few hours? Is that even possible? And sure enough, there are some out there."

To find them, the team analyzed light data from thousands of stars, looking for telltale dips indicating that a planet may periodically pass in front of a star.

Picking out these tiny dips among tens of thousands of light curves is typically a time-intensive ordeal. To speed the process along, the group devised a more automated approach, applying a basic mathematical method known as the Fourier transform to the large dataset. The method essentially whittles the field to those light curves that are periodic, or that exhibit a repetitive pattern.

Stars that host orbiting planets may display periodic dips of light each time a planet crosses, or transits, the star. But there are other periodic stellar phenomena that can affect light emission, such as a star eclipsing another star. To pick out those signals associated with actual planets, physics graduate student Roberto Sanchis-Ojeda searched through the set of periodic light curves, looking for frequent smaller dips in the data midway between the planetary transits.

The group was able to detect the light given off by the planet by measuring the amount by which the overall light dimmed each time the planet passed behind the star. The researchers posit that the planet's light is possibly a combination of radiation from its heated surface and light reflected by surface materials, such as lava and atmospheric vapor.

"I was just looking by eye, and all of a sudden I see this extra drop of light right when it was expected, and it was really beautiful," Sanchis-Ojeda recalls. "I thought, we're actually seeing the light from the planet. It was a really exciting moment."

Living on a lava world

From their measurements of Kepler 78b, the team determined that the planet is about 40 times closer to its star than Mercury is to our sun. The star around which Kepler 78b orbits is likely relatively young, as it rotates more than twice as fast as the sun -- a sign that the star has not had as much time to slow down.

While it is about the size of Earth, Kepler 78b is most certainly not habitable, due to its extreme proximity to its host star.

"You'd have to really stretch your imagination to imagine living on a lava world," Winn says. "We certainly wouldn't survive there."

But this doesn't entirely rule out the possibility of other habitable, short-period planets. Winn's group is now looking for exoplanets that orbit brown dwarfs -- cold, almost-dead stars that somehow failed to ignite.

"If you're around one of those brown dwarfs, then you can get as close in as just a few days," Winn says. "It would still be habitable, at the right temperature."

Co-authors on the two papers are Alan Levine of MIT, Leslie Rogers of the California Institute of Technology, Michael Kotson of the University of Hawaii, David Latham of the Harvard-Smithsonian Center for Astrophysics, and Lars Buchhave of the University of Copenhagen. This research was supported by grants from NASA.

Planetary 'Runaway Greenhouse' More Easily Triggered, Research Shows

Venus. In the runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium. As a result, the world overheats, boiling its oceans and filling its atmosphere with steam, which leaves the planet glowing-hot and forever uninhabitable, as Venus is now. (Credit: NASA/JPL)

It might be easier than previously thought for a planet to overheat into the scorchingly uninhabitable “runaway greenhouse” stage, according to new research by astronomers at the University of Washington and the University of Victoria published July 28 in the journal Nature Geoscience.

In the runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium. As a result, the world overheats, boiling its oceans and filling its atmosphere with steam, which leaves the planet glowing-hot and forever uninhabitable, as Venus is now.

One estimate of the inner edge of a star’s “habitable zone” is where the runaway greenhouse process begins. The habitable zone is that ring of space around a star that’s just right for water to remain in liquid form on an orbiting rocky planet’s surface, thus giving life a chance.

Revisiting this classic planetary science scenario with new computer modeling, the astronomers found a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought.

“The habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse,” said Tyler Robinson, a UW astronomy postdoctoral researcher and second author on the paper. The lead author is Colin Goldblatt of the University of Victoria.

Though further research is called for, the findings could lead to a recalibration of where the habitable zone begins and ends, with some planets having their candidacy as possible habitable worlds revoked.

“These worlds on the very edge got ‘pushed in,’ from our perspective — they are now beyond the runaway greenhouse threshold,” Robinson said.

Subsequent research, the astronomers say, is needed in part because their computer modeling was done in a “single-column, clear-sky model,” or a one-dimensional measure averaged around a planetary sphere that does not account for the atmospheric effect of clouds.

The findings apply to planet Earth as well. As the sun increases in brightness over time, Earth, too, will move into the runaway greenhouse stage — but not for a billion and a half years or so. Still, it inspired the astronomers to write, “As the solar constant increases with time, Earth’s future is analogous to Venus’s past.”

Source : University of Washington 

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