Figure 1: Jet and disk in the HH 212 protostellar system: (a) A
composite image for the jet in different molecules, produced by
combining the images from the Very Large Telescope (McCaughrean et al.
2002) and ALMA (Lee et al. 2015). Orange image around the center shows
the dusty envelope+disk at submillimeter wavelength obtained with ALMA
at 200 AU resolution. (b) A zoom-in to the very center for the dusty
disk at 8 AU resolution. Asterisks mark the possible position of the
central protostar. A dark lane is seen in the equator, causing the disk
to appear as a "hamburger". A size scale of our solar system is shown in
the lower right corner for size comparison. (c) An accretion disk model
that can reproduce the observed dust emission in the disk. Credit: ALMA (ESO/NAOJ/NRAO)/Lee et al.
An international research team, led by Chin-Fei Lee in Academia Sinica
Institute of Astronomy and Astrophysics (ASIAA, Taiwan), has made a new
high-fidelity image with the Atacama Large Millimeter/submillimeter
Array (ALMA), catching a protostar (baby star) being fed with a dusty
"Hamburger", which is a dusty accretion disk. This new image not only
confirms the formation of an accretion disk around a very young
protostar, but also reveals the vertical structure of the disk for the
first time in the earliest phase of star formation. It not only poses a
big challenge on some current theories of disk formation, but also
potentially brings us key insights on the processes of grain growth and
settling that are important to planet formation.
"It is so amazing to see such a detailed structure of a very young
accretion disk. For many years, astronomers have been searching for
accretion disks in the earliest phase of star formation, in order to
determine their structure, how they are formed, and how the accretion
process takes place. Now using the ALMA with its full power of
resolution, we not only detect an accretion disk but also resolve it,
especially its vertical structure, in great detail", says Chin-Fei Lee
at ASIAA.
"In the earliest phase of star formation, there are theoretical
difficulties in producing such a disk, because magnetic fields can slow
down the rotation of collapsing material, preventing such a disk from
forming around a very young protostar. This new finding implies that the
retarding effect of magnetic fields in disk formation may not be as
efficient as we thought before," says Zhi-Yun Li at University of
Virginia.
HH 212 is a nearby protostellar system in Orion at a distance of
about 1300 ly. The central protostar is very young with an age of only
~40,000 yrs (which is about 10 millionth of the age of Our Sun) and a
mass of ~0.2 Msun. It drives a powerful bipolar jet and thus must
accrete material efficiently. Previous search at a resolution of 200 AU
only found a flattened envelope spiraling toward the center and a hint
of a small dusty disk near the protostar. Now with ALMA at a resolution
of 8 AU, which is 25 times higher, we not only detect but also spatially
resolve the dusty disk at submillimeter wavelength.
The disk is nearly edge-on and has a radius of about 60 AU.
Interestingly, it shows a prominent equatorial dark lane sandwiched
between two brighter features, due to relatively low temperature and
high optical depth near the disk midplane. For the first time, this dark
lane is seen at submillimeter wavelength, producing a
"hamburger"-shaped appearance that is reminiscent of the scattered-light
image of an edge-on disk in optical and near infrared. The structure of
the dark lane clearly implies that the disk is flared, as expected in
an accretion disk model.
Our observations open up an exciting possibility of directly
detecting and characterizing small disks around the youngest protostars
through high-resolution imaging with ALMA, which provides strong
constraints on theories of disk formation. Our observations of the
vertical structure can also yield key insights on the processes of grain
growth and settling that are important to planet formation in the
earliest phase.
Paper and research team
This research was presented in a paper "First Detection of Equatorial
Dark Dust Lane in a Protostellar Disk at Submillimeter Wavelength," by
Lee et al. to appear in the journal Science Advances.
The team is composed of Chin-Fei Lee (ASIAA, Taiwan; National Taiwan
University, Taiwan), Zhi-Yun Li (University of Virginia, USA), Paul T.P.
Ho (ASIAA, Taiwan; East Asia Observatory), Naomi Hirano (ASIAA,
Taiwan), Qizhou Zhang (Harvard-Smithsonian Center for Astrophysics,
USA), and Hsien Shang (ASIAA, Taiwan).
