Anomalous 13C Isotope Abundances in C3S and C4H Observed

Jun 21, 2013 - the Cold Interstellar Cloud, Taurus Molecular Cloud‑1. Nami Sakai,* ... 12C/13C ratio is around 60−70 in the solar neighborhood,1,2...
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Anomalous 13C Isotope Abundances in C3S and C4H Observed toward the Cold Interstellar Cloud, Taurus Molecular Cloud‑1 Nami Sakai,*,† Shuro Takano,‡ Takeshi Sakai,§ Shoichi Shiba,† Yoshihiro Sumiyoshi,∥ Yasuki Endo,⊥ and Satoshi Yamamoto*,† †

Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan Nobemaya Radio Observatory, National Astronomical Observatory of Japan, and Department of Astronomical Science, The Graduate University for Advanced Studies, Minamimaki, Minamisaku, Nagano 384-1305, Japan § Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan ∥ Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Aramaki, Maebashi, Gunma 371-8510, Japan ⊥ Department of Basic Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ‡

ABSTRACT: We have studied the abundances of the 13C isotopic species of C3S and C4H in the cold molecular cloud, Taurus Molecular Cloud-1 (Cyanopolyyne Peak), by radioastronomical observations of their rotational emission lines. The CCCS/13CCCS and CCCS/C13CCS ratios are determined to be >206 and 48 ± 15, respectively. The CC13CS line is identified with the aid of laboratory microwave spectroscopy, and the range of the CCCS/CC13CS ratio is found to be from 30 to 206. The abundances of at least two 13C isotopic species of C3S are thus found to be different. Similarly, it is found that the abundances of the four 13C isotopic species of C4H are not equivalent. The CCCCH/13CCCCH, CCCCH/C13CCCH, CCCCH/CC13CCH, and CCCCH/CCC13CH ratios are evaluated to be 141 ± 44, 97 ± 27, 82 ± 15, and 118 ± 23, respectively. Here the errors denote 3 times the standard deviation. These results will constrain the formation pathways of C3S and C4H, if the nonequivalence is caused during the formation processes of these molecules. The exchange reactions after the formation of these two molecules may also contribute to the nonequivalence. In addition, we have confirmed that the 12C/13C ratio of some species are significantly higher than the interstellar elemental 12C/13C ratio of 60−70. The observations of the 13C isotopic species provide us with rich information on chemical processes in cold interstellar clouds.

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found that the CCS/13CCS and CCS/C13CS ratios are 230 ± 130 and 54 ± 5, respectively, the C13CS/13CCS ratio being 4.2 ± 2.3. Here, the errors represent 3 times the standard deviation. This result was surprising, because such a large abundance difference among the different 13C isotopic species of a molecule had not been recognized so far. Sakai et al.5 also found the abundance difference between the two 13C isotopic species of CCH: The CCH/13CCH and CCH/C13CH ratios are found to be higher than 170 and 250 in TMC-1 (CP), respectively, the C13CH/13CCH ratio being 1.6 ± 0.4. All these results indicate that nonequivalent carbon atoms in a molecule can have different 12C/13C ratios. This anomaly could reflect formation pathways of molecules. In this case, the two carbon atoms of CCS have to be nonequivalent in its

INTRODUCTION Rapidly increasing sensitivity of radio astronomical observations has made it possible to detect faint spectral emission lines of less abundant molecules in interstellar molecular clouds. A typical example is the 13C isotopic species. The interstellar 12 C/13C ratio is around 60−70 in the solar neighborhood,1,2 and hence, the spectral lines of the 13C species are much weaker than those of the normal species as long as the normal species lines are not optically thick (saturated). Nevertheless, spectral lines of the 13C isotopic species of moderately abundant molecules have been detected in molecular clouds.3−5 Such observations have posed interesting problems in astrochemistry. Takano et al.3 reported the abundance anomaly of the 13C species of HC3N toward the cold dark cloud, Taurus Molecular Cloud-1 Cyanopolyyne Peak (TMC-1 (CP)). They observed spectral lines of the three 13C isotopic species and found that HCC13CN is more abundant by a factor of 1.4 than H13CCCN and HC13CCN. Motivated by this work, Sakai et al.4 observed spectral lines of 13CCS and C13CS toward TMC-1 (CP) and © 2013 American Chemical Society

Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 27, 2012 Revised: June 18, 2013 Published: June 21, 2013 9831

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formation mechanism. For instance, the C2H2 + S+ reaction cannot be the main production pathway, because the two carbon atoms of C2H2 are equivalent. Alternatively, the anomaly could be caused by isotope exchange reactions after formation of molecules. For CCH and CCS, the reactions such as H + 13CCH → C13CH + H and H + 13CCS → C13CS + H may contribute to the anomaly, although the rate coefficients of these reactions are uncertain.5,6 In any case, an understanding of the origin of the anomaly is a good target for molecular science and physical chemistry. Another interesting aspect is dilution of the 13C species in cold molecular clouds. The interstellar 12C/13C ratio has been investigated through molecular line observations. Lucas and Liszt1 reported the 12C/13C ratio in the solar neighborhood to be 59 ± 2 on the basis of observations of HCO+, HCN, and HNC. Milam et al.2 later reported the average 12C/13C ratio in the local interstellar medium to be 68 ± 15 by using the CO, CN, and H2CO data. Although the 12C/13C ratio depends on the distance from the Galactic center,8 the 12C/13C ratio in the local interstellar medium is mostly 60−70. The molecular 12 C/13C ratios for various carbon-chain molecules are generally higher than the interstellar ratio. This is particularly significant for CCH, as noted above. Such a high 12C/13C ratio (dilution) for carbon-chain molecules is qualitatively consistent with the theoretical prediction by Langer et al.7 On the basis of the above results, a natural question is whether the anomaly and the dilution can be seen in longer chains like C3S and C4H. We have already pointed out that the CCCS/13CCCS ratio is higher than 160.4 However, the corresponding ratios for C13CCS and CC13CS remain unexplored. In particular, there were no laboratory data for rotational transitions for CC13CS, because the frequencies of the CC13CS lines are too close to those of the normal species lines. We have thus conducted laboratory Fourier transform microwave (FTMW) spectroscopy for CC13CS. On the basis of the result, we have detected its emission line in TMC-1 (CP). In addition to C3S, we have also observed the 13C species of C4H in TMC-1 (CP).

MHz and 1.5 kHz for the 22 GHz observation, respectively, and 50 MHz and 12 kHz for the 47 GHz observation, respectively. The channel spacing corresponds to velocity widths of 0.02 and 0.08 km s−1 at 22 and 47 GHz, respectively. For the 22 GHz data, three successive channels are summed to improve the signal-to-noise ratio, and the resultant velocity resolution is 0.06 km s−1. The intensity scale was calibrated by using a noise injection diode. We took the weighted average of the spectra of right- and left-handed circular polarizations to obtain the final spectrum. Because the baseline of the raw spectra is very stable in the GBT observation, a linear baseline is subtracted to obtain the final spectra. Laboratory Spectroscopy. Rotational spectra of C3S and its isotopic species were measured by a Balle-Flygare-type FTMW spectrometer combined with a pulsed discharge nozzle.9 C3S was produced by discharging a mixture of CS2 (0.3%) and C2H2 (0.3%) diluted in Ar. We optimized the production condition by using the J = 4−3 line of C3S. C3S has the 1Σ+ ground electronic state. The rotational spectrum of C3S was first detected by Yamamoto et al.10 Then, Ohshima and Endo11 measured frequencies of low J lines of C3S, C334S, 13CCCS, and C13CCS by FTMW spectroscopy. They measured lines of the 13C isotopic species in the natural abundance, where, however, the CC13CS lines were not identified due to the disturbance of the nearby normal species lines. In the present study, a 13C isotope enriched sample of CS2 was used for the measurements of the 13C species. The J = 4−3 lines of the three 13C species of C3S were readily detected by using 13CS2. In particular, the CC13CS line was clearly detected near the normal species line (Figure 1). The CC13CS

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EXPERIMENTAL SECTION Astronomical Observation. All the astronomical observations were carried out with the Robert C. Byrd Green Bank Telescope (GBT) of National Radio Astronomy Observatory in 2008 March, 2010 May, and 2011 December. [The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.] The observed source is TMC-1 (CP) (α(J2000) = 4h41m42.88s, δ(J2000) = 25d41′27.0″). A Kband receiver was used for observations of the C3S lines, whereas a Q-band receiver was used for observations of the C4H lines. The instantaneous bandwidth of both receivers is 4 GHz. The beam size of the telescope is 33.6″ and 15.7″ at 22 and 47 GHz, respectively. The main beam efficiency is 0.85 and 0.79 at 22 and 47 GHz, respectively. The pointing of the telescope was checked by observing nearby continuum sources every hour, and the maximum pointing error was 6″. The frequency-switching mode with frequency offsets of ±1.0 and ±2.5 MHz was employed for the 22 and 47 GHz observations, respectively. These correspond to the velocity differences of ±14 and ±16 km s−1, respectively. The system temperature during the observations ranged from 25 to 50 K at 22 GHz, and from 80 to 110 K at 47 GHz. We used a bank of autocorrelators as back ends, whose bandwidth and channel spacing are 12.5

Figure 1. J = 4−3 lines of CCCS and CC13CS observed by FTMW spectroscopy. The upper trace shows the spectrum observed with CS2 and C2H2, whereas the lower trace shows the spectrum observed with 13 CS2 and C2H2. In the lower trace, the line of CC13CS is observed in addition to that of the normal species. The line shows a doublet feature due to the Doppler effect, because the direction of the jet beam is parallel to the cavity axis.

line is so close in frequency to the normal species line that it was difficult to resolve it without using an isotope enriched sample. In addition to the monosubstituted species, we also measured doubly substituted species except for 13C13CCS. Measurements were done for the J = 2−1, 3−2, 4−3, and 5−4 lines. The rotational constants and centrifugal distortion constants were determined by least-squares fits. Observed 9832

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Table 2. Molecular Constantsa Table 1. Observed Frequenciesa J′

a

J″

2 3 4 5

1 2 3 4

2 3 4 5

1 2 3 4

2 3 4 5

1 2 3 4

2 3 4 5

1 2 3 4

3 4 5

2 3 4

3 4 5

2 3 4

3 4 5

2 3 4

3 4 5

2 3 4

3 4 5

2 3 4

2 3 4 5

1 2 3 4

νobs (MHz)

νobs − νcalc (MHz)

CCCS 11561.5099 17342.2564 23122.9836 28903.6913 13 CCCS 11132.2395 16698.3481 22264.4418 27830.5140 C13CCS 11445.4767 17168.2027 22890.9123 28613.6016 CC13CS 11561.5334 17342.2863 23123.0214 28903.7389 CCC34S 16922.1924 22562.8997 28203.5870 13 CCC34S 16288.8381 21718.4244 27147.9958 C13CC34S 16745.7441 22327.6388 27909.5093 CC13C34S 16921.7207 22562.2722 28202.8005 13 CC13CS 16698.5262 22264.6808 27830.8134 C13C13CS 11445.5916 17168.3712 22891.1380 28613.8877

−0.0018 0.0013 0.0003 −0.0003 −0.0007 0.0001 0.0007 −0.0003

species

B0 (MHz)

D0 (MHz)

CCCS 13 CCCS C13CCS CC13CS CCC34S 13 CCC34S C13CC34S CC13C34S 13 CC13CS C13C13CS

2890.37959(29) 2783.061702(138) 2861.370962(52) 2890.385095(157) 2820.3691561(98) 2714.80987(48) 2790.96122(42) 2820.29073(26) 2783.09143(26) 2861.39905(34)

0.2086(75) 0.2053(35) 0.21603(133) 0.2260(50) 0.20908(24) 0.2069(118) 0.2047(104) 0.2131(63) 0.2010(66) 0.2071(88)

a

The numbers in parentheses represent one standard deviation in units of the last significant digits.

corresponding carbon atom to the center of mass. The vibration−rotation constant can be calculated if the quadratic and cubic force constants are available. However, we do not go into details, because such a calculation is beyond the scope of this paper. Nevertheless, our identification of the spectral lines of CC13CS is firm, because we used the 13C enriched sample.

−0.0003 0.0003 −0.0001 0.0000

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0.0002 0.0002 −0.0015 0.0010

OBSERVATIONAL RESULTS C3S. In the present study, the J = 4−3 line of C13CCS was clearly detected toward TMC-1 (CP), as shown in Figure 2. The line parameters are listed in Table 3. The integrated intensity is found to be 10 ± 1 mK km s−1 in the TMB scale (i.e., intensity corrected for atmospheric attenuation and efficiency of the telescope). On the other hand, the corresponding line of 13 CCCS was not detected; the upper limit to the integrated intensity is 2 mK km s−1 (3 σ) in the TMB scale. This is consistent with the previous result (2 mK km s−1 (3 σ) in the Ta* scale) reported by Sakai et al.4 Hence, the intensity of C13CCS is higher than that of 13CCCS at least by a factor of 3, where the error of the integrated intensity of the C13CCS line is taken into account. Because the normal species lines of C3S may be optically thick, we employed the CCC34S line to derive the column density ratio of CCCS/C13CCS, as in the case of the estimate of the lower limit of the column density ratio of CCCS/13CCCS.4 The C13CCS and CCC34S lines can be regarded as optically thin, because the stronger C13CS lines are optically thin in TMC-1 (CP) judging from the intensity ratio of the hyperfine components.4 Hence, the integrated intensity ratio of CCC34S/C13CCS well represents the abundance ratio under the assumption of local thermodynamic equilibrium (LTE). This is because the transition frequencies for both species are close to each other, and because the energy level structures are similar to each other, ensuring similar excitation temperature. The integrated intensity ratio of CCC34S/C13CCS is evaluated to be 2.5 ± 0.8, and the resultant column density ratio of CCCS/C13CCS is 48 ± 15, if we assume the 32S/34S ratio of 19.1 The error denotes 3 times the standard deviation. The 3σ lower limit to the CCCS/13CCCS ratio is also derived to be 206 from the 3σ upper limit to the integrated intensity of the 13CCCS line. The CCCS/C13CCS ratio is indeed lower than that of the CCCS/13CCCS ratio. Although the CC13CS line is very close to the normal species line, it was also detected as a shoulder of the normal species line (Figure 2). The rest frequency difference between the CC13CS and normal species lines for J = 4−3 is only 37.8 kHz, corresponding to the velocity shift of 0.49 km s−1. Because the

0.0000 0.0000 0.0000 0.0012 −0.0016 0.0006 −0.0011 0.0014 −0.0005 −0.0006 0.0009 −0.0003 −0.0006 0.0009 −0.0003 0.0020 −0.0007 −0.0014 0.0007

A typical frequency measurement error is 1 kHz.

frequencies and derived molecular constants are summarized in Tables 1 and 2, respectively. The molecular constants for 13 CCCS and C13CCS are consistent with those reported previously11 but are more precise. Note that hyperfine splittings due to the neuclear-spin rotation interaction of the 13C neucleus were not observed in this study. It should be noted that the rotational constant of CC13CS is slightly larger than that of the normal species in spite of the 13C substituted species, which seems to be originated from the vibration−rotation interaction as well as proximity of the 9833

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Figure 2. Spectral line profiles of C3S and its isotopic species (J = 4−3) observed toward TMC-1(CP). Gray dashed lines indicates Vlsr of 5.85 km s−1, which is the average velocity of the C334S line weighted by intensity of each channel.

Table 3. Observed Parameters of J = 4−3 Lines of C3S and Its Isotopic Speciesa species 13

CCCS C13CCS CC13CSe CCC34S CCCSe

rest Frequencyb (GHz)

rmsc (mK)

TMB (mK)

22.2644418 22.8909123 23.1230214 22.5628997 23.1229836

4.5 5.0 4.2 5.8 4.2

71 (3σ) (54 ± 8)−(72 ± 10) (1σ)a 26 (43 ± 6)−(57 ± 9) (1σ)b 26 68 ± 5 (1σ)c 26 71 ± 5 (1σ)d 26 79 ± 26 (1σ)e 26 >2505 >1705 230 ± 130 (3σ)4 54 ± 5 (3σ)4 >206 (3σ) 48 ± 15 (3σ) 30−206 141 ± 44 (3σ) 97 ± 27 (3σ) 82 ± 15 (3σ) 118 ± 23 (3σ) 79 ± 11 (1σ)3 75 ± 10 (1σ)3 55 ± 7 (1σ)3 27 90+31 −21 (1σ) 27 82+26 (1σ) −19 +32 103−23 (1σ)27 27 86+24 −17 (1σ) 27 79+24 (1σ) −18 f 28 87+35 (1σ) −19

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AUTHOR INFORMATION

Corresponding Author

*E-mail: N.S., [email protected]; S.Y., [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.S. thanks Sumitomo Foundation for financial support. This study is supported by a Grant-in-Aid from the Japan Society of Promotion of Science (21224002 and 21740132).

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REFERENCES

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a

Derived from H15NC, assuming the 14N/15N ratio of 250−330 and optically thin limit in the HN13C line. bDerived from H15NC, assuming the 14N/15N ratio of 250−330 and optical depth of 0.6 in the HN13C line. cDerived from C34S, assuming the 32S/34S ratio of 22.7 and optically thin limit in the C34S line. dDerived from C34S, assuming the 32S/34S ratio of 22.7 and optical depth of 0.15 in the C34S line. e Derived from H2C34S, assuming the 32S/34S ratio of 22.7 and optically thin limit in the H2C34S line. fThe observed position slightly differs (∼10″) to east from TMC-1(CP).

in CH by an observation of the 9 cm lines of CH and 13CH with the Effelesberg 100 m telescope (Figure 6). [Details for the observation are reported by Sakai et al.25] The CH/13CH ratio is derived to be higher than 71 (3σ). The averaged ratios

Figure 6. Spectral line profiles of the J = 1/2 Λ-type doubling transition of 13CH and CH observed toward TMC-1(CP) with the Effelsberg 100 m telescope. The arrow in the 13CH spectrum means the expected intensity for the CH/13CH ratio of 59. 9838

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