Exciting Structures Discovered in a Young Protoplanetary Disk Lend Strong Support to Planet Formation in the Disk

(ASIAA Science Highlight, June 20, 2018)

An international research team, led by Dr. Ruobing Dong at University of Arizona in USA and in Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) in Taiwan, has discovered the tale-telling signs of planets in the young, likely planet-forming disk around the star MWC 758. Obtained with the Atacama Large Millimeter and submillimeter Array (ALMA), the “ultra” high resolution image revealed the disk having not only an off-centered cavity but also a spiral arm corresponding to one previously seen in reflected light.

“What drives the various kinds of structures we see in planet forming young disks around protostars? This is a question that has baffled astronomers for over a decade!”, pointed out by Dr. Dong, “Our observations reveal a great amount of exciting new details in this system and provide crucial clues on their origins.”

ALMA image of the 0.87 mm continuum emission from the MWC 758 disk. Credit: ALMA (ESO/NAOJ/NRAO)/Dong et al.

ALMA image of the 0.87 mm continuum emission from the MWC 758 disk. High resolution observation with ALMA shows various structures: elliptical cavity, dust clumps, rings, and a spiral arm. Credit: ALMA (ESO/NAOJ/NRAO), Dong et al.

Since the early 2000, rich structures, including gaps and rings, dust clumps, and spiral arm-like features, have been discovered in a few tens of disks surrounding newborn stars. With the belief that planets are forming inside, astronomers named these disks “protoplanetary disks”.

The origin of these structures is in hot debate among astronomers. In one scenario, they are thought to be produced by unseen planets forming inside and gravitationally interacting with the host disks, as planets open gaps, shepherd dust clumps, and excite spiral arms.

Alternative ways to produce observed disk structures that do not invoke planets have also been raised. For examples, large central cavities may be the outcome of photoevaporation, as high energy radiations from the central star evaporate the inner disk. Also, under certain conditions shadows in disks may mimic the spiral arms seen in reflected light.

The protoplanetary disk around a young star MWC 758 is located at 500 light years from us. In 2012, a pair of near symmetric giant spiral arms was discovered in reflected light. In dust thermal and molecular gas line emission at millimeter wavelengths, a big inner hole and two major dust clumps have been found, too.

Now with the new ALMA image, the previously known cavity of MWC 758 is shown to be off-centered from the star with its shape well described by an ellipse with one focus on the star. Also, a millimeter dust emission feature corresponds nicely with one of the two spiral arms previously seen in reflected light. Both discoveries are the first among protoplanetary disks.

“MWC 758 is a rare breed!”, says Dr. Sheng-Yuan Liu at ASIAA, “All major types of disk structures have been found in this system. It reveals to us one of the most comprehensive suites of evidence of planet formation in all protoplanetary disks.”

Previously in 2015, Dr. Dong and his collaborators proposed that the two arms in the MWC 758 disk can be explained as driven by a super-Jupiter planet just outside the disk.

“Our new ALMA observations lend crucial support to planet-based origins for all the structures.”, says Dr. Takayuki Muto at Kogakuin University, Japan, “For example, it’s exciting to see ellipses with one focus on the star — that’s Kepler’s first law! It’s pointing to a dynamical origin, possibly interactions with planets.”

The off-centered cavity strongly, on the other hand, disfavors alternative explanations such as photoevaporation, which do not have an azimuthal dependence.

The fact that the south spiral arm is present in the millimeter emission tracing the dust rules that the arm is a density arm. Other scenarios, such as shadows, which view the spiral arms as surface features, are not expected to reproduce the observations. The ultra-high resolution achieved in the new ALMA dataset also enables the detection of a slight offset between the arm locations in reflected light and in dust emission, which is consistent with models of planet-induced density wave.

“These fantastic new details are only made possible thanks to the amazing angular resolution delivered by ALMA”, says Dr. Eiji Akiyama at Hokkaido University, Japan, “We took full advantage of ALMA’s long baseline capabilities, and now the MWC 758 disk joins the elite club of “ultra-high resolution ALMA disks” alongside only a handful of others.”

Paper and research team

This research was presented in a paper titled “The Eccentric Cavity, Triple Rings, Two-Armed Spirals, and Double Clumps of the MWC 758 Disk” by Dong et al. appeared in the Astrophysical Journal.

The team is composed of Ruobing Dong (U. of Arizona, USA; ASIAA, Taiwan), Sheng-yuan Liu (ASIAA, Taiwan), Josh Eisner (University of Arizona, USA), Sean Andrews (Harvard-Smithsonian Center for Astrophysics, USA), Jeffrey Fung (UC Berkeley, USA), Zhaohuan Zhu (UNLV, USA) Eugene Chiang (UC Berkeley, USA), Jun Hashimoto (Astrobiology Center, NINS, Japan), Hauyu Baobab Liu (European Southern Observatory, Germany), Simon Casassus (University of Chile, Chile), Thomas Esposito (UC Berkeley, USA), Yasuhiro Hasegawa (JPL/Caltech, USA), Takayuki Muto (Kogakuin University, Japan), Yaroslav Pavlyuchenkov (Russian Academy of Sciences, Russia), David Wilner (Harvard-Smithsonian Center for Astrophysics, USA), Eiji Akiyama (Hokkaido University, Japan), Motohide Tamura (The University of Tokyo; Astrobiology Center, NINS, Japan), and John Wisniewski (U. of Oklahoma, USA)