A planet-forming disk still fed by the mother cloud

This false-colour image shows the filaments of accretion around the protostar [BHB2007] 1. The large structures are inflows of molecular gas (CO) nurturing the disk surrounding the protostar. The inset shows the dust emission from the disk, which is seen edge-on. The "holes" in the dust map represent an enormous ringed cavity seen (sideways) in the disk structure. © MPE 

Two different observations of the protoplanetary disk show signatures of the formation of a companion to the protostar . The grey scale represents the dust thermal emission from the disk, same as in the inset of Fig. 1. The red/blue contours show the molecular CO brightness emission levels from the northern/southern side of the dust cavity observed with ALMA. The brighter CO emission from the south indicates that the gas is hotter there. This location coincides with a zone of non-thermal emission tracing ionised gas (green contours) observed with the VLA (middle), which is observed in addition to the protostar (centre of the image). The team proposes that both the ionised gas and the hot molecular gas are due to the presence of a protoplanet or a brown dwarf in the cavity. The configuration of such a system is shown in the sketch on the right. © MPE; illustration: Gabriel A. P. Franco 

Stellar systems like our own form inside interstellar clouds of gas and dust that collapse producing young stars surrounded by protoplanetary disks. Planets form within these protoplanetary disks, leaving clear gaps, which have been recently observed in evolved systems, at the time when the mother cloud has been cleared out. ALMA has now revealed an evolved protoplanetary disk with a large gap still being fed by the surrounding cloud via large accretion filaments. This shows that accretion of material onto the protoplanetary disk is continuing for times longer than previously thought, affecting the evolution of the future planetary system.

A team of astronomers led by Dr. Felipe Alves from the Center for Astrochemical Studies (CAS) at the Max Planck Institute for Extraterrestrial Physics (MPE) used the Atacama Large Millimeter/submillimeter Array (ALMA) to study the accretion process in the stellar object [BHB2007] 1, a system located at the tip of the Pipe Molecular Cloud. The ALMA data reveal a disk of dust and gas around the protostar, and large filaments of gas around this disk. The scientists interpret these filaments as accretion streamers feeding the disk with material extracted from the ambient cloud. The disk reprocesses the accreted material, delivering it to the protostar. The structure observed is very unusual for stellar objects at this stage of evolution — with an estimated age of 1,000,000 years — when circumstellar disks are already formed and matured for planet formation. “We were quite surprised to observe such prominent accretion filaments falling into the disk”, said Alves. “The accretion filament activity demonstrates that the disk is still growing while simultaneously nurturing the protostar.”

The team also reports the presence of an enormous cavity within the disk. The cavity has a width of 70 astronomical units, and it encompasses a compact zone of hot molecular gas. In addition, supplementary data at radio frequencies by the Very Large Array (VLA) point to the existence of non-thermal emission in the same spot where the hot gas was detected. These two lines of evidence indicate that a substellar object — a young giant planet or brown dwarf — is present within the cavity. As this companion accretes material from the disk, it heats up the gas and possibly powers strong ionized winds and/or jets. The team estimates that an object with a mass between 4 and 70 Jupiter masses is needed to produce the observed gap in the disk.

“We present a new case of star and planet formation happening in tandem,” states Paola Caselli, director at MPE and head of the CAS group. “Our observations strongly indicate that protoplanetary disks keep accreting material also after planet formation has started. This is important because the fresh material falling onto the disk will affect both the chemical composition of the future planetary system and the dynamical evolution of the whole disk.” These observations also put new time constraints for planet formation and disk evolution, shedding light on how stellar systems like our own are sculpted from the original cloud.

 Source:  Max Planck Institute for Extraterrestrial Physics

Contacts

Dr. Felipe De Oliveira Alves
postdoc
+49 (0)89 30000-3897
0049(0)17637101004
+49 (0)89 30000-3950

Prof. Dr. Paola Caselli
acting director
+49 (0)89 30000-3400
+49 (0)89 30000-3399

Original publication

1. Felipe O. Alves, L. Ilsedore Cleeves, Josep M. Girart, Zhaohuan Zhu, Gabriel A. P. Franco, Alice Zurlo and Paola Caselli

A case of simultaneous star and planet formation
Astrophysical Journal Letters, 904 L6
DOI

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Hubble Captures 'Shadow Play' Caused by Possible Planet

TW Hydrae

Credits: NASA, ESA, and J. Debes (STScI)

Hi-re images

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Searching for planets around other stars is a tricky business. They're so small and faint that it's hard to spot them. But a possible planet in a nearby stellar system may be betraying its presence in a unique way: by a shadow that is sweeping across the face of a vast pancake-shaped gas-and-dust disk surrounding a young star.

The planet itself is not casting the shadow. But it is doing some heavy lifting by gravitationally pulling on material near the star and warping the inner part of the disk. The twisted, misaligned inner disk is casting its shadow across the surface of the outer disk.

A team of astronomers led by John Debes of the Space Telescope Science Institute in Baltimore, Maryland, say this scenario is the most plausible explanation for the shadow they spotted in the stellar system TW Hydrae, located 192 light-years away in the constellation Hydra, also known as the Female Water Snake. The star is roughly 8 million years old and slightly less massive than our sun. The researchers uncovered the phenomenon while analyzing 18 years' worth of archival observations taken by NASA's Hubble Space Telescope.

"This is the very first disk where we have so many images over such a long period of time, therefore allowing us to see this interesting effect," Debes said. "That gives us hope that this shadow phenomenon may be fairly common in young stellar systems."

Debes will present his team's results Jan. 7 at the winter meeting of the American Astronomical Society in Grapevine, Texas.

Debes' first clue to the phenomenon was a brightness in the disk that changed with position. Astronomers using Hubble's Space Telescope Imaging Spectrograph (STIS) first noted this brightness asymmetry in 2005. But they had only one set of observations, and could not make a definitive determination about the nature of the mystery feature.

Searching the archive, Debes' team put together six images from several different epochs. The observations were made by STIS and by Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS).

STIS is equipped with a coronagraph that blocks starlight to within about 1 billion miles from the star, allowing Hubble to look as close to the star as Saturn is to our sun. Over time, the structure appeared to move in counterclockwise fashion around the disk, until, in 2016, it was in the same position as it was in images taken in 2000.

This 16-year period puzzled the researchers. They originally thought the feature was part of the disk, but the short period meant that the feature was moving way too fast to be physically in the disk.

Under the laws of gravity, disks rotate at glacial speeds. The outermost parts of the TW Hydrae disk would take centuries to complete one rotation.

"The fact that I saw the same motion over 10 billion miles from the star was pretty significant, and told me that I was seeing something that was imprinted on the outer disk rather than something that was happening directly in the disk itself," Debes said. "The best explanation is that the feature is a shadow moving across the surface of the disk."

The research team concluded that whatever was making the shadow must be deep inside the 41-billion-mile-wide disk, so close to the star it cannot be imaged by Hubble or any other present-day telescope. The most likely way to create a shadow is to have an inner disk that is tilted relative to the outer disk. In fact, submillimeter observations of TW Hydrae by the Atacama Large Millimeter Array (ALMA) in Chile suggested a possible warp in the inner disk.

But what causes disks to warp? "The most plausible scenario is the gravitational influence of an unseen planet, which is pulling material out of the plane of the disk and twisting the inner disk," Debes explained. "The misaligned disk is inside the planet's orbit."

Given the relatively short 16-year period of the clocklike moving shadow, the planet is estimated to be about 100 million miles from the star — about as close as Earth is from the sun. The planet would be roughly the size of Jupiter to have enough gravity to pull the material up out of the plane of the main disk. The planet's gravitational pull causes the disk to wobble, or precess, around the star, giving the shadow its 16-year rotational period.

Recent observations of TW Hydrae by ALMA in Chile add credence to the presence of a planet. ALMA revealed a gap in the disk roughly 9 million miles from TW Hydrae. A gap is significant, because it could be the signature of an unseen planet clearing away a path in the disk.

This new Hubble study offers a unique way to look for planets hiding in the inner part of the disk and probe what is happening very close to the star, which is not reachable in direct imaging by current telescopes. "What is surprising is that we can learn something about an unseen part of the disk by studying the disk's outer region and by measuring the motion, location, and behavior of a shadow," Debes said. "This study shows us that even these large disks, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn't imagine."

The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

Contact

Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov

Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu

John Debes
Space Telescope Science Institute, Baltimore, Maryland
410-338-4782
debes@stsci.edu

Source: HubbleSite