Espacio Chile , Atacama, Jueves, 13 de febrero de 2014 a las 09:24

Astronomers discovered a drastic chemical change in the birth of a planetary system

As a result of the observations, Sakai et al. have found that carbon-chain molecules and their related species such as cyclic-C3H2 almost completely disappear in the gas phase

ALMA/DICYT So far, the interstellar matter was believed to be smoothly brought into a gas disk forming a planetary system as it is. Using the Atacama Large Millimeter/submillimeter Array (ALMA), a team led by Dr. Nami Sakai has been able to observe a drastic chemical change associated with the formation of the disk around the young protostar L1527 in the Taurus molecular cloud. In-falling gas is jammed up due to centrifugal force around the outer edge of the disk, where local heating causes the drastic chemical change to happen. This discovery was published by Nature Journal with the title "Change in the chemical composition of in-falling gas forming a disk around a protostar" on february 12th, 2014.

 

A new star and its planetary system are formed through gravitational collapse of interstellar gas, mostly H2, and dust. Even after the birth of a protostar (a baby star), the gas and dust in the envelope are still in-falling onto it. At the same time, the gas disk grow up around the baby star, and eventually evolves into a planetary system. Until Dr. Sakai et al. work, formation processes of the gas disk as well as the associated chemical change were left unexplored by the observatories.

 

Dr. Nami Sakai, assistant profesor of physics in the University of Tokyo, and her global team, observed the young protostar L1527 in the Taurus molecular cloud at a high spatial resolution and high sensitivity with ALMA Observatory, newly constructed in the Atacama desert in Chile, and investigated the formation process of the disk in spectral lines of several molecules. They discovered an unexpected chemical change in the transition zone between the in-falling envelope and the gas disk, contrary to what was believed so far to be a smooth delivery without any significant chemical change.

 

As a result of the observations, Sakai et al. have found that carbon-chain molecules and their related species such as cyclic-C3H2 almost completely disappear in the gas phase inward of the radius of 100 AU from the protostar (figure 1, top left; top right). The motion of the gas was investigated on the basis of the precise measurement of the Doppler shift of their spectral lines, and it was revealed that the 100 AU radius corresponds to the centrifugal barrier (figure 2). At this radius, in-falling gas is jammed up due to the centrifugal force, and is gradually transferred to the inner disk. Namely, this is the front of the disk forming region which has been clearly identified with the spectral line of cyclic-C3H2.

 

On the other hand, the distribution of sulfur monoxide molecules (SO) is found to be localized in a ring structure located at the radius of the centrifugal barrier (100 AU) (Figure 1, bottom left; bottom right). Furthermore, the temperature of the SO molecules is found to be higher than the one of the in-falling gas. This means that the in-falling gas probably causes weak shocks when it inrushes into the outer edge of the disk around the centrifugal barrier. The gas temperature then raises around this radius, and the SO molecules frozen on dust grains are liberated into the gas phase. Hence, the spectral lines of SO also highlight the disk-formation front. Since the density of the disk happens to be 108 cm-3 or higher, most of the molecules are frozen out onto dust grains in the disk after they pass through the front.

 

Hence, the chemical change associated to the disk formation have been successfully detected by the observation of two chemical species: cyclic-C3H2 and SO molecules. It was not anticipated at all that such a drastic chemical change occurred in the transition zone between the in-falling envelope and the inner disk.

 

This success was possible thanks to the high sensitivity and high spatial resolution capacity of ALMA. Therefore, the study will be extended toward various star-forming regions to examine how widely is applicable the phenomenon seen in L1527. This study novelty is the focus on the chemical change and, by extending this new method to various solar-type protostars, diversity and generality of the chemical evolution from interstellar matter to planetary matter could be unveiled within the next few yeas with ALMA, and finally critically examining whether the Solar System experienced the same drastic chemical change, tracing the origin of the matter found in microanalyses of meteorites and spectroscopy of comets.