An international research team using the Atacama Large
Millimeter/submillimeter Array (ALMA) revealed the distribution of heavy
hydrogen, or deuterium, in planet formation sites with the highest
resolution ever achieved. This provides clues to understand the physical and
chemical conditions during the formation of exoplanets and Solar System
objects.
“The various bodies in our Solar System have a variety of chemical
compositions,” says Yuri Aikawa, a professor at the University of Tokyo.
“This variety could be due to differences in the chemical composition and
physical state at their formation sites. Revealing the chemical variation
within the planet-forming disks is thus fundamental to the study of planet
formation.”
Protoplanetary disks around young stars contain a variety of molecules, each
of which emits radio waves of specific wavelengths. In this study,
researchers utilized the superb resolution and sensitivity of ALMA to
understand the physical and chemical conditions in planet forming disks.
Gianni Cataldi, a postdoc at the University of Tokyo and the National
Astronomical Observatory of Japan, and his team focused on deuterium, the
heavy brother of hydrogen, in protoplanetary disks. Although there is only
one deuterium atom for every 100,000 hydrogen atoms, it is known that the
ratio is higher in certain molecules. This deuterium enrichment can be used
as a footprint to infer where an object was formed in a disk.
The team analyzed ALMA data and measured the spatial distribution of the
deuterium abundance ratio in protoplanetary disks. They found that the
deuterium abundance ratios differed by a factor of about 100 among different
locations within a single disk, with the abundance ratios becoming smaller
closer to the central star.
“Two major reactions are thought to be responsible for the deuterium
enrichment; one is active in very low-temperature regions and the other
remain effective even in the relatively warm regions. Our observations show
that both play an important role in disks,” says Cataldi.
Comparing the deuterium abundance ratios observed in protoplanetary disks
with those of Solar System objects can provide information on the origin of
the objects. For example, the deuterium abundance ratio in HCN molecules was
measured for Comet Hale-Bopp, which approached the Sun around 1997 and could
be seen brightly from Earth. The value for Comet Hale-Bopp was smaller than
the one measured in the protoplanetary disks this time.
“This may suggest that Comet Hale-Bopp formed in the inner part of the disk,
close to the young Sun (within 30 au),” says Yoshihide Yamato, a graduate
student at the University of Tokyo and a co-author of the research paper.
“Another possibility is that the HCN molecules in the comet originated from
ices that condensed from the gas cloud at a much earlier stage of the
formation of the disk, and were not affected by the deuterium enrichment in
the disk.”
These observations are part of an ALMA Large Program, “Molecules with ALMA
at Planet-forming Scales,” or MAPS, to detect radio waves emitted by
molecules in protoplanetary disks with high spatial resolution. In this
program researchers observed protoplanetary disks around five young stars,
IM Lupi, GM Aurigae, AS 209, HD 163296, and MWC 480 with ALMA to infer the
distribution of about 20 molecules, including deuterated molecules such as
DCN and N2D+.
“With ALMA we were able to see how molecules are distributed where
exoplanets are currently assembling,” said Karin Öberg, an astronomer at the
Center for Astrophysics | Harvard & Smithsonian (CfA) and the Principal
Investigator for MAPS. “One of the really exciting things we saw is that the
planet-forming disks around these five young stars are factories of a
special class of organic molecules, so-called nitriles, which are implicated
in the origins of life here on Earth.”
Scientists also observed complex organic molecules like HC3N, CH3CN, and
c-C3H2; notably these contain carbon, and therefore are most likely to act
as the feedstock of larger, prebiotic molecules. Although these molecules
have been detected in protoplanetary disks before, MAPS is the first
systematic study across multiple disks at very high spatial resolution and
sensitivity, and the first study to find the molecules in such significant
quantities at small scales. “We found more of the large organic molecules
than expected, a factor of 10 to 100 more, located in the inner disks on
scales of the Solar System, and their chemistry appears similar to that of
Solar System comets,” said John Ilee, an astronomer at the University of
Leeds and the lead author of a MAPS paper. “The presence of these large
organic molecules is significant because they are the stepping-stones
between simpler carbon-based molecules such as carbon monoxide, which is
found in abundance in space, and the more complex molecules that are
required to create and sustain life.”
Aikawa and the MAPS team also revealed the spatial distribution of ionized
molecules in the disks. They found that ionized molecules are less abundant
in the region inside the 100-au radius of disks. If ionized, the gas in the
disk is more susceptible to magnetic fields, which can cause gas to start
outflowing or, conversely, allow gas to flow into the central star, greatly
affecting the growth of stars and planets. The observation also suggests
that the ionization rate in the disk midplane might vary from object to
object, which indicates that the physical conditions of planet-forming disks
are quite complicated.
“I believe that we can approach the mystery of the formation process of our
Solar System by combing the observations of protoplanetary disks using ALMA,
observations and analysis of Solar System material, and predictions based on
theoretical research,” summarizes Aikawa.
These observation results are presented as Gianni Cataldi et al. “Molecules
with ALMA at Planet-forming Scales (MAPS) X: Studying deuteration at high
angular resolution towards protoplanetary disks” and Yuri Aikawa et al.
“Molecules with ALMA at Planet-forming Scales (MAPS) XIII: HCO+ and disk
ionization structure” and other 18 papers in the MAPS special issue of the
Astrophysical Journal Supplement Series.
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