Drastic chemical change occurring in birth of planetary system: Has the solar system also experienced it?

Feb 13, 2014
Implications of protoplanetary disc chemistry for the solar system
Illustration of rotating-infalling gas toward a protostar. The abundance of sulfermonooxcyde is enhanced at the outer edge (colored blue) of a protoplenatry disk. Credit: The University of Tokyo

A new star is formed by gravitational contraction of an interstellar molecular cloud consisting of gas and dust. In the course of this process, a gas disk (protoplanetary disk), whose size is on the order of 100 AU, forms around the protostar and evolves into a planetary system. The solar system was also formed in this way about 4.6 billion years ago, and life was eventually born on the Earth. How unique in the universe is the situation which happened for the solar system?

In order to answer this question, understanding the formation of protoplanetary disks as well as the associated chemical evolution in various star forming regions is essential. There have been extensive observational efforts made toward this goal. So far, most of them have focused on changes in the physical structure and the kinematics during the formation process. However, it was very difficult to distinguish the from the infalling envelope clearly with such conventional approaches.

On the other hand, the chemical evolution associated with disk formation has scarcely been studied observationally because of the insufficient sensitivity and spatial resolution of previous radio telescopes. As a result, a chemical model calculation with many assumptions is the only approach. Naturally, the physical and chemical changes in the disk formation should be coupled with each other. The disk formation around a young protostar has been explored from a novel point of view looking at physics and chemistry simultaneously.

L1527 in the Taurus is a molecular cloud core which harbors a young protostar. A global team led by Dr. Nami Sakai, the University of Tokyo, conducted high-sensitivity, high-spatial-resolution observations of L1527 with ALMA (Atacama Large Millimeter/submillimeter Array) newly constructed in the Atacama desert in Chile, and investigated the disk formation process by using the spectral lines of several molecules.

As a result, Sakai al. have found that carbon-chain molecules and their related species such as cyclic-C5H2 almost completely disappear from the gas phase inside a raius of 100 AU around the protostar (Figure1, top left; top right).

Figure 1. An infrared image of the protostar L1527 taken by the Spitzer Space Telescope. Credit: J. Tobin/NASA/JPL-Caltech

Precise measurements of the motion of the gas using the Doppler shift in the spectral lines of the gas components revealed that 100 AU corresponds to the radius of the centrifugal barrier (Figure 2).

Figure 2. 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. Credit: J. Tobin/NASA/JPL-Caltech, N. Sakai/The University of Tokyo

At this radius, infalling gas is stopped and accumulated due to the centrifugal force, and then is gradually transferred to the inner disk. Namely, this is the edge of the disk forming region. It has clearly been indentified with the spectral line of cycle-C5H2.

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 that of the infalling gas. This means that the infalling gas probably causes a weak shock when in rushes into the outer edge of the disk at the centrifugal barrier. The gas temperature is raised around this radius, and the SO molecules frozen on dust grains are liberated into the gas phase. Hence, the spectral line of SO also highlights the disk-formation front. Since the density of the disk is 108 cm-5 or higher, most of the molecules are frozen out onto dust grains in the disk after they pass through the front.

It is not at all anticipated that such a drastic chemical change occurs in the transition zone between the infalling envelope and the inner disk. The disk formation and the associated chemical change have successfully been detected by observations of the two chemical species, cyclic-C5H2 and SO.

This study has demonstrated a drastic change in chemical composition associated with disk formation around the young protostar (cf; Figure 3). With a coupled view of physics and chemistry, it has also succeeded in highlighting the outermost part of the disk where the is still accreting. This success was realized by high-sensitivity and high-spatial-resolution observations with ALMA, and such a study will be extended to other various star-forming regions. In particular, it is very interesting to examine how widely applicable the picture seen in L1527 is to other star-forming regions.

Although many observational efforts aimed at understanding planetary-system formation have been made, this study is novel in focusing on the chemical change. By extending this new method to various solar-type protostars using ALMA, the diversity and generality of the chemical evolution from interstellar matter to planetary matter will be unveiled within the next few years. Then, we can critically examine whether the solar system experienced this drastic chemical change. In parallel to the astronomical approach, the origin of the solar system is being investigated by exploring the itself through microanalyses of meteorites, spectroscopy of comets, sample return missions to the asteroids, and so on. The present study will also have a strong impact on these studies by tracing the origins back to interstellar clouds.

Explore further: ALMA discovers a formation site of a giant planetary system

More information: Nami Sakai, Takeshi Sakai, Tomoya Hirota, Yoshimasa Watanabe, Cecilia Ceccarelli, Claudine Kahane, Sandrine Bottinelli, Emmanuel Caux, Karine Demyk, Charlotte Vastel, Audrey Coutens, Vianney Taquet, Nagayoshi Ohashi, Shigehisa Takakuwa, Hsi-Wei Yen, Yuri Aikawa & Satoshi Yamamoto, "Change in the chemical composition of infalling gas forming a disk around a protostar", Nature, DOI: 10.1038/nature13000.

Related Stories

A fluffy disk around a baby star

Aug 23, 2013

An international team of astronomers that are members of the Strategic Exploration of Exoplanets and Disks with Subaru Telescope (SEEDS) Project has used Subaru Telescope's High Contrast Instrument for the ...

A rare snapshot of a planetary construction site

Oct 24, 2013

(Phys.org) —Planets are formed in disks of gas and dust around nascent stars. Now, combined observations with the compound telescope ALMA and the Herschel Space Observatory have produced a rare view of ...

Recommended for you

Image: Chandra's view of the Tycho Supernova remnant

Jul 25, 2014

More than four centuries after Danish astronomer Tycho Brahe first observed the supernova that bears his name, the supernova remnant it created is now a bright source of X-rays. The supersonic expansion of ...

Satellite galaxies put astronomers in a spin

Jul 24, 2014

An international team of researchers, led by astronomers at the Observatoire Astronomique de Strasbourg (CNRS/Université de Strasbourg), has studied 380 galaxies and shown that their small satellite galaxies almost always ...

Video: The diversity of habitable zones and the planets

Jul 24, 2014

The field of exoplanets has rapidly expanded from the exclusivity of exoplanet detection to include exoplanet characterization. A key step towards this characterization is the determination of which planets occupy the Habitable ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

Dr_toad
not rated yet Feb 13, 2014
"sulfermonooxcyde" and "raius" are words I am unfamiliar with. Do you clowns *ever* read what you mangle?

Or can you read?
Fleetfoot
not rated yet Feb 14, 2014
The text talks of cyclic-C5H2 but the graphics ddescribe cyclic-C3H2.

A replacement of "3" by "5" might also explain the density units: "the density of the disk is 108 cm-5 or higher".

The figure descriptions are nonsense too:

".. a raius of 100 AU around the protostar (Figure1, top left; top right)."

".. the radius of the centrifugal barrier (100 AU) (Figure 1, bottom left; bottom right)."

versus:

"Figure 1. An infrared image of the protostar L1527 taken by the Spitzer Space Telescope."

There is no indication of the radius in that and it is a single image. Then we have:

".. 100 AU corresponds to the radius of the centrifugal barrier (Figure 2)."

versus:

"Figure 2. L1527 observed by Spitzer (Left) and the distributions of cyclic-C3H2 (center) and SO (right) observed by ALMA."

And finally:

".. disk formation around the young protostar (cf; Figure 3)."

There is no figure 3!