Production Pathways of CCS and CCCS Inferred from Their¹³C Isotopic Species

Abstract

The rotational spectral lines (J N ¼ 3 2 Y2 1 and J N ¼ 2 1 Y1 0 ) of 13 CCS and C 13 CS have been observed toward a cold dark cloud, TMC-1. The strongest hyperfine component lines of 13 CCS and C 13 CS (J N ¼ 2 1 Y1 0 , F ¼ 5/2Y3/2) have successfully been detected. The C 13 CS ½ / 13 CCS ½ abundance ratio is determined to be 4:2 AE 2:3 (3 ). The CCS ½ / 13 CCS ½ ratio is evaluated to be 230 AE 130 (3 ), and hence, 13 CCS is found to be significantly diluted. Such a difference between the 13 CCS and C 13 CS abundances is also found in L1521E, which is a very young core with rich carbon-chain molecules. Therefore, the anomaly is not specific to TMC-1, but seems to be common for the CCS-rich clouds. Furthermore, we have also observed the J ¼ 4Y3 transition of 13 CCCS and CCC 34 S in TMC-1 and L1521E and have found that the CCC 34 S ½ / 13 CCCS ½ ratio is larger than 8.4 (3 ). This lower limit is considerably larger than the interstellar 12 C ½ 34 S ½ / 13 C ½ 32 S ½ ratio of 3, indicating that 13 CCCS is diluted as in the case of 13 CCS. These results give us strong constraints on the main pathways to produce CCS and CCCS.

Abundance anomaly of the13C species of CCH

“…We recently reported on the abundance anomaly of the 13 C species of CCS in the young starless cores TMC-1 and L1521E (Sakai et al 2007a). The abundance ratio of [CCS]/[C 13 CS] in TMC-1 is determined to be 54 ± 5 (3σ), which is almost consistent with the interstellar [ 12 C]/[ 13 C] ratio of 60 (Lucas & Liszt 1998).…”

“…Although we made a more detailed integration for 13 CCH than for C 13 CH toward L1527 to improve the S/N ratio, we confirmed that the line intensities of 13 CCH observed simultaneously with the C 13 CH lines are consistent with those finally obtained after the extended observation. Hence it is clearly established that the abundances of the two 13 C isotopic species are different, as in the case of HC 3 N (Takano et al 1998) and CCS (Sakai et al 2007a). Since the H 2 densities of TMC-1 and L1527 are ∼3×10 4 (Sect.…”

“…As described in the introduction, we recently discovered the abundance difference between C 13 CS and 13 CCS in TMC-1 and L1521E (Sakai et al 2007a). The [C 13 CS]/[ 13 CCS] ratio is 4.2 ± 2.3(3σ) in TMC-1, indicating that the two carbon atoms in CCS are not equivalent in the main production pathways.…”

“…If the reactions including CCH are mainly responsible, the abundances of 13 CCH and C 13 CH must be different as in the case of 13 CCS and C 13 CS. However, Sakai et al (2007a) could not confirm this because of the lack of observational data of 13 CCH and C 13 CH, so that the reactions including CCH were not completely ruled out as the main production process. Since the CCS molecule is now widely used to study the chemical evolution of starless cores (e.g.…”

Tracing the evolutionary stage of Bok globules: CCS and NH3

“…While ammonia was the first polyatomic molecule to be discovered in the interstellar medium (review e.g. by Ho & Townes 1983), the CCS radical was first identified far more recently (Saito et al 1987) and the main pathways leading to its formation remain a subject of discussion (Suzuki et al 1992;Petrie 1996;Sakai et al 2007), although it is frequently observed along with other carbon-chain molecules (e.g., Hirota et al 2009).…”