ALMA Reveals Planetary Construction Sites

PR Image eso1549a

Artist’s impression of a transitional disc around a young star

PR Image eso1549b

Schematic view of a transitional disc around a young star
 

PR Image eso1549c

ALMA imaging of the transitional disc HD 135344B

PR Image eso1549d

ALMA imaging of the transitional disc DoAr 44

Videos

 

PR Video eso1549a

Artist’s impression of a transitional disc around a young star

PR Video eso1549b

Artist’s impression of a transitional disc around a young star

---

New evidence for young planets in discs around young stars

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found the clearest indications yet that planets with masses several times that of Jupiter have recently formed in the discs of gas and dust around four young stars. Measurements of the gas around the stars also provide additional clues about the properties of those planets.

Planets are found around nearly every star, but astronomers still do not fully understand how — and under what conditions — they form. To answer such questions, they study the rotating discs of gas and dust present around young stars from which planets are built. But these discs are small and far from Earth, and the power of ALMA was needed for them to reveal their secrets.

A special class of discs, called transitional discs, have a surprising absence of dust in their centres, in the region around the star. Two main ideas have been put forward to explain these mysterious gaps. Firstly, the strong stellar winds and intense radiation could have blown away or destroyed the encircling material [1]. Alternatively, massive young planets in the process of formation could have cleared the material as they orbit the star [2].

The unparalleled sensitivity and image sharpness of ALMA have now allowed the team of astronomers, led by Nienke van der Marel from the Leiden Observatory in the Netherlands to map the distribution of gas and dust in four of these transitional discs better than ever before [3]. This in turn has allowed them to choose between the two options as the cause of the gaps for the first time.

The new images show that there are significant amounts of gas within the dust gaps [4]. But to the team’s surprise, the gas also possessed a gap, up to three times smaller than that of the dust.

This could only be explained by the scenario in which newly formed massive planets have cleared the gas as they travelled around their orbits, but trapped the dust particles further out [5].

“Previous observations already hinted at the presence of gas inside the dust gaps,” explains Nienke van der Marel. “But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc.”

Remarkably, these observations were conducted utilising just one tenth of the current resolving power of ALMA, as they were performed whilst half of the array was still under construction on the Chajnantor Plateau in northern Chile.

Further studies are now needed to determine whether more transitional discs also point towards this planet-clearing scenario, although ALMA’s observations have, in the meantime, provided astronomers with a valuable new insight into the complex process of planetary formation.

“All the transitional discs studied so far that have large dust cavities also have gas cavities. So, with ALMA, we can now find out where and when giant planets are being born in these discs, and compare these results with planet formation models,” says Ewine van Dishoeck, also of Leiden University and the Max Planck Institute for Extraterrestrial Physics in Garching [6]. “Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look.”

---

Notes

[1] This process, which clears the dust and gas from the inside out, is known as photoevaporation. 

[2] Such planets are difficult to observe directly (eso1310) and previous studies at millimetre wavelengths (eso1325) have failed to achieve a sharp view of their inner, planet-forming zones where these different explanations could be put to the test. Other studies (eso0827) could not measure the bulk of the gas in these discs. 

[3] The four targets of these investigations were SR 21, HD 135344B (also known as SAO 206462), DoAr 44 and Oph IRS 48. 

[4] The gas present in transitional discs consists primarily of hydrogen, and is traced through observations of the carbon monoxide — or CO — molecule. 

[5] The process of dust trapping is explained in an earlier release (eso1325). 

[6] Other examples include the HD 142527 (eso1301 and here) and J1604-2130 transitional discs.

---

More Information

This research was presented in a paper entitled “Resolved gas cavities in transitional disks inferred from CO isotopologs with ALMA”, by N. van der Marel, et al., to appear in Astronomy & Astrophysics in December 2015.

The team is composed of N. van der Marel (Leiden University, Leiden, the Netherlands; Institute for Astronomy, University of Hawaii, Honolulu, USA), E. F. van Dishoeck (Leiden University, Leiden, the Netherlands; Max Planck Institute for Extraterrestrial Physics, Garching, Germany), S. Bruderer (Max-Planck Institute for Extraterrestrial Physics, Garching, Germany), S. M. Andrews (Harvard-Smithsonian Center for Astrophysics, Massachusetts, USA), K. M. Pontoppidan (Space Telescope Science Institute, Baltimore, Maryland, USA), G. J. Herczeg (Peking University, Beijing, China), T. van Kempen (Leiden University, Leiden, the Netherlands) and A. Miotello (Leiden University, Leiden, the Netherlands).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes.

VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

---

Links

• Research paper in Astronomy & Astrophysics 
• Photos of ALMA

---

Contacts

Nienke van der Marel
Institute for Astronomy, University of Hawaii
Honolulu, USA
Email: nmarel@ifa.hawaii.edu

Ewine van Dishoeck
Leiden Observatory
Leiden, The Netherlands
Tel: +31 71 527 5814
Email: ewine@strw.leidenuniv.nl

Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Source: ESO

Spirals in Dust Around Young Stars May Betray Presence of Massive Planets

MWC 758

[Right] — Observations taken by the European Southern Observatory's Very Large Telescope show a protoplanetary disk around the young star MWC 758. The disk has two spiral arms that extend over 10 billion miles from the star.

[Left] — A computer model reproduces the two-spiral-arm structure; the "x" is the location of a putative planet. The planet, which cannot be seen directly, probably excites the two spiral arms.

Photo Credit: NASA, ESA, ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z. Zhu (Princeton University).  Release Images

A team of astronomers is proposing that huge spiral patterns seen around some newborn stars, merely a few million years old (about one percent our sun's age), may be evidence for the presence of giant, unseen planets. This idea not only opens the door to a new method of planet detection, but also could offer a look into the early formative years of planet birth.

Though astronomers have cataloged thousands of planets orbiting other stars, the very earliest stages of planet formation are elusive because nascent planets are born and embedded inside vast, pancake-shaped disks of dust and gas encircling newborn stars, known as circumstellar disks.

The conclusion that planets may betray their presence by modifying circumstellar disks on large scales is based on detailed computer modeling of how gas-and-dust disks evolve around newborn stars, which was conducted by two NASA Hubble Fellows, Ruobing Dong of Lawrence Berkeley National Laboratory, and Zhaohuan Zhu of Princeton University. Their research was published in the Aug. 5 edition of The Astrophysical Journal Letters.

"It's difficult to see suspected planets inside a bright disk surrounding a young star. Based on this study, we are convinced that planets can gravitationally excite structures in the disk. So if you can identify features in a disk and convince yourself those features are created by an underlying planet that you cannot see, this would be a smoking gun of forming planets," Dong said.

Identifying large-scale features produced by planets offers another method of planet detection that is quite different from all other techniques presently used. This approach can help astronomers find currently forming planets, and address when, how, and where planets form.

Gaps and rings seen in other circumstellar disks suggest invisible planets embedded in the disk. However gaps, presumably swept clean by a planet's gravity, often do not help show the location of the planet. Also, because multiple planets together may open a single common gap, it’s very challenging to estimate their numbers and masses.

Ground-based telescopes have photographed two large-scale spiral arms around two young stars, SAO 206462 and MWC 758. A few other nearby stars also show smaller spiral-like features. "How they are created has been a big mystery until now. Scientists had a hard time explaining these features," Dong said. If the disks were very massive, they would have enough self-gravity to become unstable and set up wave-like patterns. But the disks around SAO 206462 and MWC 758 are probably just a few percent of the central star's mass and therefore are not gravitationally unstable.

The team generated computer simulations of the dynamics of a disk and how a star's radiation propagates through a disk with embedded planets. This modeling created spiral structures that very closely resemble observations. The mutual gravitational interaction between the disk and the planet creates regions where the density of gas and dust increases, like traffic backing up on a crowded expressway. The differential rotation of the disk around the star smears these over-dense regions into spiral waves. "Although it had been speculated that planets can produce spiral arms, we now think we know how," said Zhu.

"Simulations also suggest that these spiral arms have rich information about the unseen planet, revealing not only its position but also its mass," Zhu said. The simulations show that if there were no planet present, the disk would look smooth. To make the grand-scale spiral arms seen in the SAO 206462 and MWC 758 systems, the unseen planet would have to be bulky, at least 10 times the mass of Jupiter, the largest planet in our solar system.

The first planet orbiting a normal star was identified in 1995. Thanks to ground-based telescopes and NASA's Kepler mission, a few thousand exoplanets have been cataloged to date. But because the planets are in mature systems, many millions or a few billion years old, they offer little direct clues as to how they formed.

"There are many theories about how planets form but very little work based on direct observational evidence confirming these theories," Dong said. "If you see signs of a planet in a disk right now, it tells you when, where, and how planets form."

Astronomers will use the upcoming NASA James Webb Space Telescope to probe circumstellar disks and look for features, as simulated by the modeling, and will then try to directly observe the predicted planet causing the density waves.

Contacts

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu

Ruobing Dong
Lawrence Berkeley National Laboratory, and University of California, Berkeley, California
rdong2013@berkeley.edu

Zhaohuan Zhu
Princeton University, Princeton, New Jersey
zhzhu@astro.princeton.edu

Source: HubbleSite

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.

1.  

Spiral Arms Point to Possible Planets in a Star's Dusty Disk

Two spiral arms emerge from the gas-rich disk around SAO 206462, a young star in the constellation Lupus. This image, acquired by the Subaru Telescope and its HiCIAO instrument, is the first to show spiral arms in a circumstellar disk. The disk itself is some 14 billion miles across, or about twice the size of Pluto's orbit in our own solar system. (Credit: NAOJ/Subaru)

Larger image | Unlabeled version

"Detailed computer simulations have shown us that the gravitational pull of a planet inside a circumstellar disk can perturb gas and dust, creating spiral arms. Now, for the first time, we're seeing these dynamical features," said Carol Grady, an astronomer with Eureka Scientific, Inc., who is based at NASA's Goddard Space Flight Center in Greenbelt, Md. She revealed the image today at the Signposts of Planets meeting hosted this week at the center.

Simulations of young stellar systems suggest that planets embedded in a circumstellar disk can produce many distinctive structures, including rings, gaps and spiral arms. This video compares computer simulations of hypothetical systems to the Subaru image of SAO 206462. (Credit: NASA's Goddard Space Flight Center/NCSA)

Download this video in broadcast quality from NASA Goddard's Scientific Visualization Studio

Grady's research is part of the Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS), a five-year-long near-infrared study of young stars and their surrounding dust disks using the Subaru Telescope atop Mauna Kea in Hawaii. The international consortium of researchers now includes more than 100 scientists at 25 institutions.

"What we're finding is that once these systems reach ages of a few million years, their disks begin to show a wealth of structure -- rings, divots, gaps and now spiral features," said John Wisniewski, a collaborator at the University of Washington in Seattle. "Many of these structures could be caused by planets within the disks."

The newly imaged disk surrounds SAO 206462, an 8.7-magnitude star located about 456 light-years away in the constellation Lupus. Astronomers estimate that the system is only about 9 million years old. The gas-rich disk spans some 14 billion miles, which is more than twice the size of Pluto's orbit in our own solar system.

The Subaru near-infrared image reveals a pair of spiral features arcing along the outer disk. Theoretical models show that a single embedded planet may produce a spiral arm on each side of a disk. The structures around SAO 206462 do not form a matched pair, suggesting the presence of two unseen worlds, one for each arm.

However, the research team cautions that processes unrelated to planets may give rise to these structures.

The view was made possible by the High Contrast Instrument for the Subaru Next Generation Adaptive Optics, or (HiCIAO, pronounced "HI-chow"), which is designed to block out harsh direct starlight.

"Together with improvements to Subaru's adaptive optics system, which counteracts the blurring effects of Earth's atmosphere, the telescope is operating near its theoretical performance limits," said SEEDS principle investigator Motohide Tamura at National Astronomical Observatory of Japan, which operates the telescope. "We are just beginning to see what it will do."

"The Signposts of Planets meeting is all about understanding these kinds of patterns,” said NASA Goddard’s Marc Kuchner, who organized the conference. “It’s a new kind of planet-hunting technique that is just now coming to fruition, and this new image from SEEDS is the perfect example of how it can work."

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