Gardel’s tango “Volver” sounds, the one that I like so much and to which I have resorted on occasion for its temporal implications, and I arrive at that part in which he sings that “twenty years is nothing”. And hearing it I can’t help but remember the SiCSi molecule and silicon carbide (SiC, not to be confused with sic). All because, in the 1990s, scientists were already talking about SiCSi as a missing link in the formation of silicon carbide and that there should be a lot in the most intimate and hidden regions of carbon-rich dying star envelopes. But it took twenty years to be able to confirm this by astronomical observations.
The SiCSi molecule is formed by two silicon atoms and one carbon atom. The fact that it has been called a ‘missing link’ is simple, although we have to explain it step by step.
For starters, we go back to our fetish envelope, which is providing a huge amount of information to astrochemists. We’re talking about the envelope of the star CW Leonis, also known as IRC+10216. Let us remember that it is an evolved star, a star that has begun the final phase of its life, ejecting into the environment the matter that composes it in the form of layers. At the end of its life it will form a planetary nebula and, at its center, there will be a white dwarf star. But there´s still a long way to go to reach that point.
About 400 light years from us, this star is one of the brightest infrared sources in the sky and, thanks to its proximity, we can study its envelope in great detail (and more since we have tools and data with Herschel and ALMA). IRC+10216 has been found to be exceptionally rich in molecular species (in fact, half of the known interstellar and circumstellar species have been observed in this carbon-rich envelope). But astrochemistry has an interdisciplinary way of working: observations are not enough; you have to corroborate the information in a laboratory.
Before interpreting astronomical observations, astrochemistry looks for answers in laboratories. This is where they work to characterize molecular species and chemical processes, emulating the conditions that occur in very specific environments (such as the envelopes of evolved stars or the interstellar medium).
Experts had been trying to “catch” the SiCSi molecule for many years, but it took a long time. After numerous attempts, McCarthy et al. (2015, Harvard University) managed to synthesize and characterize it in the laboratory, allowing the community to confirm that what was seen in the astronomical observations was, indeed, the molecule SiCSi. It was finally possible to put a name to those lines obtained by the team of astronomers of Astromol/Nanocosmos: in total, 112 lines of this molecule in the spectrum of IRC + 10216 using data from the IRAM 30m radio telescope .
By obtaining such an accurate spectroscopic characterization, it was possible to validate the hypothesis that this molecule is abundant in this type of environment, where it is very likely to play a fundamental role in the early stages of formation of silicon carbide (SiC) powder grains. However, many unknowns remain about that role.
How are dust grains formed?
We know that dust grains, an ubiquitous component in the interstellar medium of galaxies, are synthesized mainly in two types of sources: in the internal winds of AGB stars (Asymptotic Giant Branch)  and in the ejection of massive stars when they explode as supernovae.
In supernovae different types of dust can form depending on the degree of enrichment in heavy elements, although the effectiveness of dust formation in supernovae is still a matter of debate (in fact, recent studies with the Herschel Space Telescope suggest that the masses of dust formed are much greater than previously thought).
The formation of dust grains can be simplified by explaining it as a two-stage process: first the “seeds”, called nucleation seeds, are formed from gas-phase species composed of chemical elements of a high refractory character. When we talk about refractory character we refer to those chemical elements that, below a certain temperature (the higher the temperature the greater the refractory character), tend to disappear from the gas phase and participate in the formation of solid condensates.
The second stage involves the growth of the grain: on the nuclei formed, compounds of a certain refractory character are condensed (in theory, less refractory than the elements that have formed the nucleus). That is, after forming a small nucleus, atoms and molecules are glued to it.
There are still many mysteries in this sequence of events, starting with the fundamental step of the formation of the seeds of nucleation, in which simple species in the gas phase are added to form nanoparticles. In fact, that’s one of the goals of Nanocosmos – to recreate this process in a lab to learn more about how those dust grains are born.
Silicon carbide (SiC): grain versus molecule
Silicon carbide (SiC) grains are abundant in space. In fact, their presence in micrometeorites is proof of the importance of the chemistry of these compounds (SiC, Si2C and SiC2) in the environment of red giant stars .
That’s because micrometeorites are a repository of primordial matter, since we assume that these remains formed in the same molecular cloud from which the entire Solar System emerged. If that is the case, the primordial molecular cloud could, in turn, be made up of material ejected by a red giant star  into the interstellar medium. This would close, for now, the cycle of silicon carbide: from a dying star to the interstellar medium, where a cloud would form from which, in turn, stars would be born that would eventually die, either as supernovae or as red giant stars, and so on. But knowing the process does not explain how silicon carbide was born.
On the one hand, it would be logical to think that the molecular precursors from which these grains of silicon carbide are born were the gas-phase molecules of silicon carbide themselves, but that does not seem to be the case.
In fact, the gas-phase silicon carbide (SiC) molecule has been detected in the outer regions of the IRC +10216 envelope (and not in the inner ones). Therefore, if it is not in the internal ones, it does not participate in the formation of the grains. In contrast, in the inner layers, where the dust is formed, the most abundant molecules containing Si−C bonds are SiC2 and the newly discovered SiCSi.
Both molecules should play a key role in the formation of grains of silicon carbide (SiC) dust. Add to this the fact that the abundance of SiCSi decreases as we move away from the star, and the data suggest that these molecules could be progressively incorporated into silicon carbide grains as they move into a colder environment.
Further deepening in this process of birth and formation of silicon carbide grains will require high-resolution interferometric observations. We still don’t know how that nucleation seed is born, whether it’s silicon carbide or any other grain of dust. Again, laboratories and frontier projects such as Nanocosmos will help us to obtain more data to draw more clearly the path of the formation of the dust grains that populate the interstellar medium and that are so important in the process of star and planet formation. Perhaps, recalling Gardel again, we should not wait another twenty years to uncover this mystery.
 Nine of these lines have also been detected with the Submillimeter Array (SMA).
 AGB stars are stars with masses less than 8 solar masses that are in their last evolutionary stages. Since the early twentieth century it has been known that, in the photospheres of AGB stars, there are molecules such as TiO, VO, ZrO, CN, C2 and C3, among others.
 The presence of silicon carbide in carbon-rich AGB stars was first confirmed in the 1970s.
Article based on astromol’s press release (in Spanish): “How silicon carbide is formed“.
Image 1: Large-scale structure of the circumstellar envelope IRC+10216, seen through the brightness of the J=2-1 carbon monoxide (CO) line. The observations have been made with the IRAM 30m radio telescope (Granada) and are described in the article (Cernicharo et al 2015, A&A, 575, A91). The region in which the SiCSi molecule has been found and where the formation of the dust grains takes place corresponds to the innerst region.
Originally published in Spanish on the Naukas website: Veinte años no son nada para el carburo de silicio (2015/06/16).