Disilicon Carbide Molecules Found In Interstellar Space

Interstellar space is not empty space.

The space in between stars contains an extensive accumulation of dispersed material, making up around 5-10% of the total mass of our Milky Way galaxy.

Most of that material is gaseous in form, but about 1% of this mass, a relatively large portion in astronomical terms, takes the form of tiny dust grains made predominantly of silicates.

These dust grains contain a large fraction of many important elements in the universe like silicon, carbon, and iron. They additionally play several crucial roles.

For example, they are essential to the chemistry that takes place in the interstellar medium by providing gas molecules with a surface on which to react with other molecules.

They also absorb ultraviolet and optical light, re-emitting the energy as infrared light, and thus they both constrain what astronomers can see and control much of the energy balance in the interstellar medium.

And in the early stages of a star’s evolution the dust can form into large clumps, the first step towards forming planets.

Interstellar grains, formed from molecular seeds, are produced in two main types of sources.

  • The inner winds of a class of evolved stars
  • The ejecta of supernovae

One of the likely molecular seeds is predicted by astronomers to be the molecule Si-C-Si (disilicon carbide), although it had never been identified in space.

Similar molecules have been found, like Si-C-C (SiC_2), and both SiC molecules and grains are known, and so the search for disilicon carbide has been underway for several decades.

Now, Harvard-Smithsonian Center for Astrophysics astronomers have reported detecting 112 transitions of the disilicon carbide in the extended atmosphere of the evolved, carbon-rich star RW Leo.

Their success was mostly because of their own, new laboratory measurements which determined more accurate values for the line frequencies. The team then used the Submillimeter Array and the IRAM submillimeter telescope to search in RW Leo, which was already well-known for hosting a rich family of carbon-bearing molecules.

These observations of disilicon carbide are in resonable, but not perfect agreement with the chemical model expectations, giving support to the theory but highlighting some required corrections to the model of the source.

The molecule is roughly ten times less abundant in this source as is its cousin, SiC_2. These two molecules are thought to be the most abundant silicon-carbon species in the dust-forming part of the stellar environment, and they surely play a key role in making dust grains.

Discovery of SiCSi in IRC+10216: A missing link between gas and dust carriers of SiC bonds
J. Cernicharo, M. C. McCarthy, C. A. Gottlieb, M. Agundez, L. Velilla Prieto, J. H. Baraban, P. B. Changala, M. Guelin, C. Kahane, M. A. Martin-Drumel, N. A. Patel, N. J. Reilly, J. F. Stanton, G. Quintana-Lacaci, S. Thorwirth, K. H. Young
arXiv:1505.01633 [astro-ph.GA]

Illustration: A deep optical image of the carbon star RW Leo, showing traces of its surrounding envelope. The important dust-forming molecule Si-C-Si (disilicon carbide) has been discovered in space after decades of speculation and searching, found in the envelope of this source. Credit: Izan Leao; the Very Large Telescope