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Unexpected Changes in Star-System Chemistry Discovered in Taurus Molecular Cloud




An international research team observed the young protostar L1527 in the Taurus molecular cloud shown above at a high spatial resolution with the ALMA Observatory, and discovered an unexpected chemical change in the transition zone between the infalling envelope and the gas disk.

So far, it has been believed that interstellar matter is smoothly delivered to the gas disk around the protostar without any significant chemical changes. However, it is now found to be oversimplified. The infalling gas is jammed up due to centrifugal force at the outer edge of the gas disk, where local heating causes a drastic chemical change. This chemical change highlights the outer edge of the gas disk which is still growing

This research highlights “chemistry in the formation of protoplanetary disks” which is not anticipated before. It is still unknown whether this sharp transition is seen around many protostars or not, and it will be uncovered by future observational studies. This is also important for understanding of the formation process of our own solar system.

Stars are formed by the contraction of interstellar gas and dust. Around a protostar, gas and dust form a disk in which planets are eventually formed. Then, are the chemical compositions of the interstellar cloud and the disk identical? The new ALMA observations show that the answer is 'no.' This finding has a large impact on understandings of the formation process of planets and protoplanetary disks.

The international research team, led by Dr. Nami Sakai, an assistant professor at the Department of Physics, The University of Tokyo, observed a baby star L1527 in the constellation Taurus with ALMA (image below). The team observed radio emission from cyclic-C3H2 and sulfur monoxide (SO) molecules to analyze the motion and temperature of the gas around the baby star. Cyclic-C3H2 consists of three carbon atoms connected to form a loop with two hydrogen atoms attached.





L1527 is a well-known protostar (baby star) and many astronomers have pointed telescopes at it. For example, NASA's Spitzer Space Telescope took infrared images of the star. The stellar light escapes through a cavity excavated by a powerful bipolar gas flow from the star and illuminates the surrounding gas, which makes a butterfly-shaped nebula extending in the east-west direction. Past radio observations revealed that gas is circling around the star to form a disk and we see the disk edge-on.

Radio observations by ALMA have the advantage of being able to see the gas directly, which is invisible in infrared light. Various molecules in the gas emit characteristic radiation as radio waves under characteristic conditions (temperature, density, chemical compositions). Therefore astronomers can investigate the nature of the gas by observing various molecules. Also astronomers measure the motion of the gas with the Doppler Effect.

ALMA's unprecedented sensitivity enables us to detect weak radio emissions which are undetectable by other telescopes. In fact, because of their low abundances, cyclic-C3H2 and SO emissions are much weaker than, for example, the CO emission observed in detail around L1527 in earlier studies.

Observations of cyclic-C3H2 with ALMA show that the gas forms a disk with a radius of 500 AU (1 AU corresponds to the distance between the Sun and Earth. The distance from the Sun to Neptune is 30 AU) circling around the protostar. Beyond 100 AU, as gas rotates around the protostar it is also infalling towards the star. Inside 100 AU, the emission from cyclic-C3H2 is very weak, which indicates chemical differentiation between the inner and outer disk. The team estimated the gas temperature at -240 to -250 degrees Celsius from the emission strength. On the other hand, SO has a completely different distribution: a ring-like structure with a radius of 100 AU. The temperature of the SO molecules is estimated to be -210 degrees Celsius, which is clearly higher than that of cyclic-C3H2.

L1527 observed by Spitzer (Left) and the distributions of cyclic-C3H2 (center) and SO (right) observed by ALMA. ALMA reveals the gas distribution just close to the protostar. Emission from cyclic-C3H2 is weak toward the protostar but strong at the northern and southern parts. Meanwhile, SO has its emission peak near the protostar.



What causes the drastic chemical composition change at 100 AU from the star? Simple simulations show that the infalling gas is inhibited due to the centrifugal force and piles up. This boundary is called the "centrifugal barrier". The infalling gas collides with the barrier and is warmed up. SO molecules frozen on the surface of cold dust grains are liberated into the gas phase. The temperature decreases inside the barrier and the SO molecules are frozen again. This is the formation process of the SO ring at 100 AU.

Rotating motion dominates inside the centrifugal barrier. Hence, the barrier is the edge of the disk formation region in which eventually a planetary system will be formed. There has been little consideration of the chemical differences between the interstellar clouds and the protoplanetary disks. This is the first evidence for a drastic change in the chemical composition during the formation of a protoplanetary disk.

This research highlights "chemistry in the formation of protoplanetary disks" which was not anticipated before. It is still unknown whether this sharp transition is seen around many protostars or not, and that will be uncovered by future observational studies. This is also important for understanding the formation process of our own Solar System.

The image at the top of the page from the (Atacama Pathfinder Experiment) telescope in Chile (APEX), of part of the Taurus Molecular Cloud, shows a sinuous filament of cosmic dust more than ten light-years long, where newborn stars are hidden, and dense clouds of gas are on the verge of collapsing to form yet more stars. It is one of the regions of star formation closest to Earth. The cosmic dust grains are so cold that observations at wavelengths of around one millimeter, such as these made with the camera on APEX, are needed to detect their faint glow.

The image shows two regions in the cloud: the upper-right part of the filament shown here is Barnard 211, while the lower-left part is Barnard 213.The submillimeter-wavelength observations from the camera on APEX, which reveal the heat glow of the cosmic dust grains, are shown here in orange tones. They are superimposed on a visible-light image of the region, which shows the rich background of stars. The bright star above the filament is φ Tauri.

The Taurus Molecular Cloud, in the constellation of Taurus (The Bull), lies about 450 light-years from Earth. This image shows two parts of a long, filamentary structure in this cloud, which are known as Barnard 211 and Barnard 213. Their names come from Edward Emerson Barnard's photographic atlas of the "dark markings of the sky", compiled in the early 20th century. In visible light, these regions appear as dark lanes, lacking in stars. Barnard correctly argued that this appearance was due to "obscuring matter in space".

We know today that these dark markings are actually clouds of interstellar gas and dust grains. The dust grains — tiny particles similar to very fine soot and sand — absorb visible light, blocking our view of the rich star field behind the clouds. The Taurus Molecular Cloud is particularly dark at visible wavelengths, as it lacks the massive stars that illuminate the nebulae in other star-formation regions such as Orion. The dust grains themselves also emit a faint heat glow but, as they are extremely cold at around -260 degrees Celsius, their light can only be seen at wavelengths much longer than visible light, around one millimeter.

These clouds of gas and dust are not merely an obstacle for astronomers wishing to observe the stars behind them. In fact, they are themselves the birthplaces of new stars. When the clouds collapse under their own gravity, they fragment into clumps. Within these clumps, dense cores may form, in which the hydrogen gas becomes dense and hot enough to start fusion reactions: a new star is born. The birth of the star is therefore surrounded by a cocoon of dense dust, blocking observations at visible wavelengths. This is why observations at longer wavelengths, such as the millimeter range, are essential for understanding the early stages of star formation.

Observations show that Barnard 213 has already fragmented and formed dense cores — as illustrated by the bright knots of glowing dust — and star formation has already happened. However, Barnard 211 is in an earlier stage of its evolution; the collapse and fragmentation is still taking place, and will lead to star formation in the future. This region is therefore an excellent place for astronomers to study how Barnard's "dark markings of the sky" play a crucial part in the lifecycle of stars.

The observations were made by Alvaro Hacar (Observatorio Astronomico Nacional-IGN, Madrid, Spain) and collaborators. The LABOCA camera operates on the 12-metre APEX telescope, on the plateau of Chajnantor in the Chilean Andes, at an altitude of 5000 meters.

The Daily Galaxy via http://www.nao.ac.jp/en/news/ and the ESO

Image credit: J. Tobin/NASA/JPL-Caltech, N. Sakai/The University of Tokyo



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