Tracing Chemical Reactions of Interstellar Molecules Using Abundance Variations of Their Isotopic Species
CCS is a linear carbon chain molecule, and is known as an abundant interstellar molecule. The spectral lines of its two 13C isotopic species are detected in interstellar molecular clouds by very sensitive observations of their rotational spectral lines with the 45m radio telescope of Nobeyama Radio Observatory and the 100m radio telescope of National Radio Astronomy Observatory in US. The abundances of the two isotopic species, 13CCS and C13CS, are found to be different by a factor of 4, the former being less abundant. This shows that the two carbon atoms in CCS are not equivalent in the dominant formation process in interstellar clouds. Although CCS has long been believed to be produced from C2H2 (acetylene), this process is proved to be inefficient. Alternatively, it is now established that CH + CS reaction is the main production pathway for CCS. Furthermore, the abundance of 13CCS is lower than that of the atomic 13C abundance, indicating that 13CCS is selectively diluted. This is xplained by the dilution of 13CH, which is caused by isotope selective photodissociation. Since the spectral lines of CCS are frequently used for tracing the early evolutionary stage of star formation, detailed understanding of its formation pathway would give a substantial impact not only on astrochemstry, but also on astrophysics.
Figure 1: CCS and its 13C Isotopic Species
In interstellar space, there exists a low-density cloud consisting of molecular gas and dust, called interstellar molecular clouds. Molecular clouds are birthplaces of new stars, and our sun and the solar system had been formed from one of molecular clouds about 4.6 billion years ago.
Although the main constituent of molecular clouds is H2, other elements such as C, N, O and S are also involved with a fractional concentration of less than 10-4. So far more than 130 interstellar molecules are known to exist in molecular clouds mainly by radio astronomical observations. They include simple molecules like CO and HCN and also fairly complex organic molecules like alcohols, carbonic acids, and esters. Understanding of the formation and destruction of these molecules is important in order to explore the chemical evolution from interstellar molecular clouds to protoplanetary nebula, and to planets.
One of the peculiar feature of interstellar chemistry is the existence of various carbon chain molecules (HCnN, CnH, CnH2, CnS and so on). The longest one so far detected is HC11N (HCCCCCCCCCCCN). Although these carbon chains do not exist in nature on the earth because of their high chemical reactivity, they can survive in the low density and low temperature condition of molecular clouds. In particular, CCS and CCCS are abundant interstellar molecules which were discovered with the 45m telescope of Nobeyama Radio Observatory 20 years ago. Since then, the spectral lines of CCS have extensively been used by many astronomers as an important tracer to study the chemical evolution along the star formation processes. In the present study, the formation pathway of this important molecule, CCS, has definitively been determined by sensitive observations of its 13C isotopic species (Figure 1).
Observations and Results
Figure 2: Spectral Lines of C13CS and 13CCS Observed with the 100m telescope of National Radio Astronomy Observatory
The rotational spectral lines of 13CCS and C13CS were first searched for in the 30GHz region with the 45m telescope of Nobeyama Radio Observatory in 1999 and 2002. Although the lines of C13CS were detected with the expected intensity, those of 13CCS were not detected at all (Figure 2). This indicates that the abundances of the two 13C isotopic species are much different from each other. Since this is a very surprising result, we further carried out more sensitive observations with the 100m telescope of National Radio Astronomy Observatory at Greenbank in US in 2006. Then the spectral lines of 13CCS and C13CS were successfully detected in the 20GHz region (Figure 2). From this observation, the abundance of C13CS is found to be normal in comparison with the interstellar 13C/12C ratio (1/60), whereas the abundance of 13CCS is found to be 1/230, being much lower than the interstellar ratio. Such a big abundance variation among the isotopic species of the same molecule has never been recognized.
Figure 3: Production Pathway of CCS
The important conclusion derived from the above result is that the two carbon atoms are nonequivalent in the production process. Among the production pathways considered for this molecules (Figure 3), the reaction of C2H2 or C2 can be excluded, because the two carbon atoms are equivalent by symmetry. The reaction of C2H2 has long been believed to be the most important pathway for the CCS production, but it is now turned out to be ineffective. In addition, the production pathways from C2H can also be ruled out as the main pathway, because the abundances of the two 13C isotopic species of CCH are not so different. As a result, the CH + CS reaction is now determined to be the most probable pathway. This is very reasonable, because both CH and CS are abundant interstellar species.
As mentioned above, 13CCS is significantly diluted in comparison with the interstellar 13C abundance. If CCS is produced from CH and CS, the dilution of 13CCS originates from the dilution of 13CH. The dilution of 13CH would be due to the isotope selective photodissociation.
As demonstrated here, the production of interstellar molecules can be traced by measuring the abundances of their isotopic species. This is a very unique method exploring the interstellar chemical reactions in a molecule-to-molecule base, and will be more and more important in future observations, as sensitivity of observations will increase.
This result is published in Astrophysical Journal 663, 1174 (2007). The authors and the title are as follows:
- Nami Sakai, Masafumi Ikeda, and Masaru Morita(Department of Physics, The University of Tokyo)
- Takeshi Sakai and Shuro Takano(Nobeyama Radio Observatory, National Astronomy Observatory of Japan)
- Yoshihiro Osamura(Kanagawa Institute of Technolocy)
- Satoshi Yamamoto(Department of Physics, The University of Tokyo)
“Production Pathways of CCS and CCCS Inferred from Their 13C Isotopic Species”