First identification of two species in space, HCS and HSC
Over the past few decades, various research teams working to study different areas of space have had the same question: where is the missing sulfur? This is what happens, for example, with the study of some protoplanetary disks and interstellar clouds. That is because sulfur chemistry outside the Earth, especially in the dark and cold clouds, on which we will focus today, presents some unknowns such as the detection of less sulfur than expected.
Sulfur in the gas phase can be found as part of different molecules (called sulfur “carrier” species). However, these species are less than 0.1% of the estimated cosmic abundance of sulfur for dark clouds, that is, only a tiny amount of what is supposed to be there has been detected.
The case of HCS and HSC in the cold cloud L483
The team led by Marcelino Agúndez, Ramón y Cajal researcher at the Institute of Fundamental Physics of the CSIC, who is among those looking for “lost sulfur”, detected two new molecules carrying sulfur: the HCS radical and its less stable isomer, HSC. They identified them in the dense cloud L483, a source with a very rich chemistry and where new molecules from other families (such as HCCO, NCCNH+ and NS+) have also been discovered.
L483 is a molecular cloud, located in the Aquila Rift, which houses the IRAS 18148-0440 protostar. This protostar is in full transition, going from being a protostar of class 0 to class I, that is, the dust and gas around it are taking the form of a disk and begin to distinguish its layers (although it does not yet have reactions in the core, that will come after going through phases II and III, after which it will end up being a full-right star).
The presence of the protostar in L483 involves a great deal of activity, with material from the cloud falling by gravity on it, fattening it, as well as a powerful jet of matter emanating from it, taking away much of the angular momentum and favoring the process of star formation. Given the environment, there should be a lot of sulfur, so it must be identified among the observations’ data.
The sulfur molecules discovered, HCS and HSC, are not abundant enough to explain the problem of lost sulfur, but their detection has revealed several peculiarities about how sulfur chemistry works in interstellar clouds. Since sulfur is in the same column of the periodic table as oxygen, both elements are considered to have similar chemical properties. However, the detection of HCS and HSC has revealed that sulfur and oxygen chemistry behave significantly differently than expected. Let’s see why.
To understand this we can compare the abundances of sulfur molecules with those of their oxygen analogues. That is, if we talk about the molecule with hydrogen, carbon and sulfur (HCS), its analog with oxygen is HCO (hydrogen, carbon and oxygen). In this way we can compare the relative abundances of the species carrying sulfur H2CS /HCS (being H2CS the stable form and HCS the unstable radical) with the relative amounts of H2CO /HCO.
The result is that in L483, H2CO (formaldehyde) is ten times more abundant than HCO (formyl radical). But in the case of its sulfur analogue the same does not happen, since H2CS (Thioformaldehyde) is as abundant as HCS. In addition, in L483 HCS is even more abundant than HCO, which is surprising, given that oxygen is more abundant than sulfur in the cosmos.
On the other hand, the metastable isomer HSC is found with a low abundance, while its oxygen analogue HOC has not yet been observed in space, mainly because there is a total lack of experimental information about this species (it has not been possible to characterize it in laboratory experiments).
Delving a little into the title of this report, we wonder, again, where is the sulfur that we don’t see. It is interesting to note a recent study, by Vidal et al., “On the reservoir of sulphur in dark clouds: chemistry and elemental abundance reconciled“, which concludes that most of the sulfur in dark cold clouds should be in the form of H2S and SH ice on the surface of dust grains.
This study also indicated that, to detect HCS in gas phase, it would be necessary more than a thousand hours of observation with the IRAM 30 m telescope, concluding that it was difficult to locate HCS in gas phase in dark and cold clouds. However, this work, in the words of Agúndez, “shows that, although there are many unknowns to be solved, we have managed to detect the presence of HCS with much fewer hours of observation“.
Then, where is the sulfur missing from the dark, cold clouds? It is possible that sulfur is deposited only on dust grains, although it is not clear how. Part of it could get trapped in the core of the grains as refractory compounds and another part could be in the form of ice. Although it is also possible that molecules of the gas phase that have not yet been identified may contain a significant amount of sulfur. It may even be in atomic form.
So, we assume that sulfur is there, but we haven’t been able to detect it yet with our instruments. On the other hand, studies of this region have not only revealed its “lack” of sulfur… but that will be in the next chapter of this trilogy about L483, the dark cloud that has so many things to reveal about the chemical complexity of the universe.
This work has been published in the paper “Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry”, A&A 611, L1 (2018). By M. Agúndez, N. Marcelino, J. Cernicharo (Institute of Fundamental Physics, CSIC, Spain) and M. Tafalla, (Observatorio Astronómico Nacional, IGN, Spain). Based on observations carried out with the IRAM 30m Telescope.
IMAGE: Image of the L483 region captured by NASA’s Spitzer Space Telescope. http://www.spitzer.caltech.edu/images/3132-sig10-006e-Protostellar-Envelope-and-Jet-L483
Originally published in Spanish on the Naukas website: “En busca del azufre perdido. Trilogía de L483 (Primera parte)” (2019/06/18).