production pathways of ccs and cccs inferred from their ... - IOPscience

The Astrophysical Journal, 663:1174Y1179, 2007 July 10 # 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.

PRODUCTION PATHWAYS OF CCS AND CCCS INFERRED FROM THEIR 1

1

1

13

C ISOTOPIC SPECIES

2

Nami Sakai, Masafumi Ikeda, Masaru Morita, Takeshi Sakai, Shuro Takano,3 Yoshihiro Osamura,4 and Satoshi Yamamoto1 Received 2007 January 16; accepted 2007 April 2

ABSTRACT The rotational spectral lines (JN ¼ 32 Y21 and JN ¼ 21 Y10 ) of 13CCS and C13CS have been observed toward a cold dark cloud, TMC-1. The strongest hyperfine component lines of 13CCS and C13CS (JN ¼ 21 Y10 , F ¼ 5/2Y3/2) have successfully been detected. The ½C 13 CS
/½ 13 CCS
abundance ratio is determined to be 4:2  2:3 (3
). The ½CCS
/½ 13 CCS
ratio is evaluated to be 230  130 (3
), and hence, 13CCS is found to be significantly diluted. Such a difference between the 13CCS and C13CS 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 13CCCS and CCC34S in TMC-1 and L1521E and have found that the ½CCC 34 S
/½ 13 CCCS
ratio is larger than 8.4 (3
). This lower limit is considerably larger than the interstellar ½ 12 C
½ 34 S
/½ 13 C
½ 32 S
ratio of 3, indicating that 13CCCS is diluted as in the case of 13CCS. These results give us strong constraints on the main pathways to produce CCS and CCCS. Subject headingg s: ISM: molecules — ISM: individual ( TMC-1, L1521E) 1. INTRODUCTION

This clearly shows that the three-carbon atoms are nonequivalent in the formation process. From this result, it was suggested that the ion molecular reaction between cyclic C3 Hþ 3 and N, which had been thought to be most important, is not the main route. Alternatively, the C2 H2 þ CN reaction was recognized as an important process in the formation of HC3N. Since CCS has two carbon atoms, we expect to obtain useful information on the production pathways from their relative abundances. On the basis of these motivations, we have observed the JN ¼ 32 Y21 and JN ¼ 21 Y10 lines of 13CCS and C13CS toward TMC-1 and the JN ¼ 21 Y10 lines toward L1521E. Furthermore, we have also observed the J ¼ 4Y3 transitions of 13CCCS and CCC34S to explore the production pathways of CCCS, a longer member of Cn S (Yamamoto et al. 1987).

The CCS radical is an abundant and widespread interstellar molecule, which was first discovered as a carrier of a strong unidentified line, U45379 (Suzuki et al. 1984; Kaifu et al. 1987; Saito et al. 1987). Since the spectral lines of CCS are intense toward various clouds, they have widely been used to study the chemical composition and its evolution in molecular clouds. Suzuki et al. (1992) carried out an extensive survey of CCS, HC3N, HC5N, and NH3 toward 49 dense cores and found that the abundance ratio of ½NH3
/½CCS
is a good indicator of chemical evolution of dense cores. Scappini & Codella (1996) obtained a similar result toward Bok globules. Furthermore, Benson et al. (1998) observed CCS and N2H + toward 60 dense cores and found the ½N2 Hþ
/½CCS
ratio is related to physical evolution of the cores. Ohashi et al. (1999) observed the CCS line toward a starless core L1544 with the Berkeley-Illinois-Maryland Association ( BIMA) array and found that the CCS distribution has a central hole with a size of 7500 AU. Such a distribution is successfully interpreted in terms of chemical evolution and depletion by Aikawa et al. (2003). Now, CCS is widely recognized as an important tracer for studying chemical evolution of molecular clouds. In addition, CCS is also used to measure magnetic fields in molecular clouds (e.g., Levin et al. 2001; Uchida et al. 2001) and to investigate the complex velocity structure of starless cores (Peng et al. 1998). In spite of importance in astrochemistry and astrophysics, the production pathways of CCS have not been well understood. Several ion molecular reactions and neutral-neutral reactions are proposed ( Millar & Herbst 1990; Petrie 1996; Yamada et al. 2002), but their relative importance is still unknown. A powerful technique for testing these processes is the study of the 13C isotopic species. Takano et al. (1998) observed the three 13C substituted species of HC3N and found that the relative abundance ratio of H13CCCN, HC13CCN, and HCC13CN is 1.0:1.0:1.4.

2. OBSERVATIONS AND RESULTS 2.1. Observation with Nobeyama 45 m Telescope An observation of the JN ¼ 32 Y21 transitions of 13CCS and C CS was carried out in 1999 March and April and 2002 April with the Nobeyama 45 m radio telescope.5 The frequencies of the transitions measured by Ikeda et al. (1997) are listed in Table 1. We observed the 13CCS and C13CS lines simultaneously by using the high electron mobility transistor (HEMT) amplifier receiver (H30). The beam size of the telescope is 6000 , and the main beam efficiency is 0.72. We used the accousto-optical radio spectrometers, whose bandwidth and resolution each are 40 MHz and 37 kHz, respectively. The observed position was the cyanopolyyne peak of TMC-1, (2000 ; 2000 ) ¼ (04h 41m 42:88s ; 25 41 0 27:0 00 ). In the 1999 season, we employed the position-switching mode, where the off-source position was set to be 100 in the right ascension, whereas we used the frequency-switching mode in the 2002 season. We averaged the data of the two seasons to obtain the final spectrum, where the typical rms noise is 3.8 mK (TA ). In this observation, the intensity difference between the C13CS and 13CCS lines was first recognized by one of the authors (Ikeda). As shown in Figure 1, the two hyperfine component lines of C13CS were successfully detected. On the other hand, the corresponding 13

1

Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033. Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 3841305, Japan. 3 Nobeyama Radio Observatory and Graduate University for Advanced Studies, Minamimaki, Minamisaku, Nagano 384-1305, Japan. 4 Kanagawa Institute of Technology, Shimo-ogino, Atsugi 243-0402, Japan. 2

5

Nobeyama Radio Observatory is a branch of the National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Japan.

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PRODUCTION PATHWAYS OF CCS AND CCCS

1175

TABLE 1 Molecular Lines Observed toward TMC-1 and L1521E

Species CC34S .............. C13CS ..............

13

CCS ..............

CCC34S ........... 13 CCCS ...........

Transition

JN JN JN JN JN JN JN JN

JN ¼ 21 Y10 ¼ 32 Y21 , F ¼ 7/2Y5/2 ¼ 32 Y21 , F ¼ 5/2Y3/2 ¼ 21 Y10 , F ¼ 5/2Y3/2 ¼ 21 Y10 , F ¼ 3/2Y1/2 ¼ 32 Y21 , F ¼ 7/2Y5/2 ¼ 32 Y21 , F ¼ 5/2Y3/2 ¼ 21 Y10 , F ¼ 5/2Y3/2 ¼ 21 Y10 , F ¼ 3/2Y1/2 J ¼ 4Y3 J ¼ 4Y3

Sa

 ( MHz)

TA b (mK)

TMC-1 R  dvb TA dv (3
) 1 ( K km s1) ( km s )

1.98 3.40 2.38 2.37 1.32 3.40 2.38 2.37 1.32 4.00 4.00

21930.476 33613.545 33615.179 22254.734 22256.611 32443.951 32440.193 21498.660 21494.411 22562.894 22264.440

182(4) 34(3) 17(3) 40(3) 19(3) ... ... 8(2) ... 43(4) ...

0.36(1) 0.61(6) 0.75(16) 0.34(3) 0.29(6) ... ... 0.37(11) ... 0.42(5) ...

0.074(3) 0.024(4) 0.014(4) 0.016(2) 0.010(2) 0.005 0.004 0.004(2) 0.002 0.020(4) 0.002

rmsc (mK)

TA b (mK)

L1521E R  dvb TA dv (3
) 1 ( K km s1) ( km s )

2.4 2.2 2.6 2.0 2.2 2.3 2.0 1.9 1.9 3.0 1.8

121(7)

0.31(2)

0.041(5)

5.0

32(4) 14(4)

0.30(4) 0.32(11)

0.010(2) 0.005(3)

2.8 2.7

... ... 53(11) ...

... ... 0.27(7) ...

0.002 0.002 0.015(3) 0.003

2.5 2.7 4.2 2.8

rmsc (mK)

Note.—The numbers in parentheses represent the errors in unit of last significant digits. a Line strength. b Obtained by the Gaussian fit. c The rms noise averaged over the line width. In the case of 13CCS and 13CCCS, the line widths of the corresponding C13CS and CCC34S lines, respectively, are assumed.

lines of 13CCS were not detected, although the rms noise for the 13 CCS lines is comparable to that for the C13CS lines. Since the intensity ratio of the hyperfine components is close to the theoretical ratio, the C13CS line is optically thin. Then, the abundance ratio, ½C 13 CS
/½ 13 CCS
, is estimated to be greater than 3.8 (3
) from the integrated intensity of the C13CS line and the rms noise for the 13CCS line. This is remarkable, because such a highintensity contrast among the 13C isotopic species of the same molecule has never been reported. However, a further observation was thought to be necessary, because small possibilities like instrumental problems or incorrect assignments of the laboratory spectrum of 13CCS still remained. 2.2. Observation with Green Bank Telescope Then we conducted a very sensitive observation of the JN ¼ 21 Y10 transitions toward TMC-1 with the Robert C. Byrd Green Bank Telescope (GBT) of the National Radio Astronomy Observatory6 in 2006 December. We simultaneously observed the lines of all the isotopic species listed in Table 1 (Ikeda et al. 1997; Yamamoto et al. 1990) by using the K-band receiver. The beam size of the telescope is 3700 , and the main beam efficiency is 0.88 at 20 GHz. We used the autocorrelators as back ends, whose bandwidth and resolution each are 50 MHz and 12 kHz, respectively. We observed the cyanopolyyne peak of TMC-1 and L1521E [(2000 ; 2000 ) ¼ (04h 29m 16:43s; 26 13 0 49:7 00 )] with the frequencyswitching mode. We took the weighted average of the spectra of the right- and left-handed circular polarizations to obtain the final spectrum. In the GBT observation, we definitively found the difference in the C13CS and 13CCS intensities (as shown in Fig. 1). In particular, we successfully detected the stronger hyperfine component (F ¼ 5/2Y3/2) of 13CCS. Although the weaker component (F ¼ 3/2Y1/2) is still below the noise level, a hint of the line can be seen in the spectrum at the right LSR velocity. This verifies that the laboratory spectroscopic data of 13CCS reported by Ikeda et al. (1997) are essentially correct. We derive the isotopic abundance ratio by use of the JN ¼ 21 Y10 data obtained at GBT, whose quality is higher than that of the JN ¼ 32 Y21 data. The JN ¼ 21 Y10 lines of C13CS, 13CCS, 6

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

and CC34S can be regarded as optically thin, because their intensities are much weaker than that of the normal species. In fact, the intensity ratio of the F ¼ 5/2Y3/2 and F ¼ 3/2Y1/2 components of C13CS is close to the theoretical line strength ratio, corresponding to the optically thin limit. Then the integrated intensity ratios well represent the abundance ratios. The ½CC 34 S
/½C 13 CS
ratio is derived to be 2:8  0:3 (3
), which is close to the interstellar ½ 12 C
½ 34 S
/½ 13 C
½ 32 S
ratio of 3 (Lucas & Liszt 1998). On the other hand, the ½CC 34 S
/½ 13 CCS
ratio is determined to be 12  7 (3
). If we adopt the interstellar ½ 32 S
/ ½ 34 S
ratio of 19 ( Lucas & Liszt 1998), then the ½CCS
/½C 13 CS
and ½CCS
/½ 13 CCS
ratios are 54  5 and 230  130 (3
), respectively. Therefore, C13CS is normal or slightly fractionated, whereas 13CCS is diluted. The ½C 13 CS
/½ 13 CCS
ratio is determined to be 4:2  2:3 (3
), which is consistent with the lower limit obtained from the JN ¼ 32 Y21 observation at NRO. In addition to TMC-1, we detected the two hyperfine component lines of C13CS in L1521E (Fig. 1). This cloud is known to be in a very early stage of chemical evolution, where NH3 and N2H+ are deficient and no signature of depletion of molecules onto dust grains is seen (Hirota et al. 2002; Tafalla & Santiago 2004). The ½CC 34 S
/½C 13 CS
ratio is found to be 2:7  0:7 (3
). This value is close to that found in TMC-1. In contrast, the lines of 13CCS were not detected in L1521E, because the rms noise is not as low as that for TMC-1. The ½CC 34 S
/½ 13 CCS
ratio is thus evaluated to be greater than 6.8 (3
), and the ½C 13 CS
/½ 13 CCS
ratio is greater than 3.5 (3
). Therefore, the significant difference can also be seen between the two 13C species even toward L1521E. If we adopt the interstellar ½ 32 S
/½ 34 S
ratio, then the ½CCS
/½C 13 CS
and ½CCS
/½ 13 CCS
ratios are 51  13 (3
) and greater than 130 (3
), respectively. Hence, 13CCS is diluted as in the case of TMC-1. This result indicates that the anomalous abundance difference between 13CCS and C13CS is not specific to TMC-1, but seems to be common for the CCS-rich clouds. We also observed the 13CCCS and CCC34S lines (Ohshima & Endo 1992) simultaneously with the CCS lines (Fig. 2). Although we detected the J ¼ 4Y3 line of CCC34S with the intensity of 42:9  4:3 mK (TA ) in TMC-1, the corresponding line of 13CCCS was not detected with the rms noise of 1.9 mK. We assume that the lines of CCC34S and 13CCCS are optically thin, and the ½CCC 34 S
/½ 13 CCCS
ratio is evaluated to be greater than 8.4 (3
). Then, we can say that 13CCCS is diluted as in the case

Fig. 1.— Spectral line profiles of the 13C isotopic species of CCS observed toward TMC-1 and L1521E. Dashed lines in the 13CCS spectra represent the VLSR value of each source (5.85 km s1 for TMC-1 and 6.8 km s1 for L1521E).

PRODUCTION PATHWAYS OF CCS AND CCCS

1177

Fig. 2.— Spectral line profiles of CCC34S and 13CCCS toward TMC-1 and L1521E. Dashed lines in the 13CCCS spectra represent the VLSR value of each source (5.85 km s1 for TMC-1, and 6.8 km s1 for L1521E). The arrows represent the line intensity expected from the normal interstellar ½ 12 C
½ 34 S
/½ 13 C
½ 32 S
ratio of 3.

of 13CCS. Furthermore, such a dilution of 13CCCS is also seen in L1521E, where the ½CCC 34 S
/½ 13 CCCS
ratio is greater than 4.7 (3
). 3. DISCUSSION

On the other hand, the importance of the neutral-neutral reactions has been pointed out by Petrie (1996) and Yamada et al. (2002). In particular, Yamada et al. (2002) systematically studied various routes using the ab initio calculations and proposed the reactions

3.1. CCS In the framework of the ion-molecule reaction scheme, the following production routes have been proposed for CCS (e.g., Millar & Herbst 1990; Suzuki et al. 1992): Sþ þ C2 H2 ! HCCSþ þ H;

C2 H þ S ! CCS þ H;

ð3Þ

SH þ C2 ! CCS þ H;

ð4Þ

CH þ CS ! CCS þ H;

ð5Þ

C þ HCS ! CCS þ H;

ð6Þ

CH þ HCS ! CCS þ H2 ;

ð7Þ

C2 þ H2 S ! CCS þ H2 :

ð8Þ

ð1Þ

and Sþ þ C2 H ! C2 Sþ þ H

ð2aÞ

C2 Sþ þ H2 ! HCCSþ þ H;

ð2bÞ

where the HCCS+ ion is recombined with an electron to give CCS.

and

1178

SAKAI ET AL.

In the present study, we have shown that the abundances of CCS and C13CS are different from each other. This indicates that the two carbon atoms in CCS are nonequivalent in the main production pathway. Therefore, the reactions (1), (4), and (8) can be excluded as the main route. Although the reaction (1) has been regarded as an important route, it has now turned out to be ineffective. Next we consider the reactions (2) and (3) including CCH. In this case, the nonequivalence of the 13C isotope abundances in CCS originates from that of CCH. It seems unlikely that 13CCH and C13CH have different abundances, because CCH is thought to be produced mainly by the dissociative recombination reaction of C2 Hþ 2 . Therefore, these reactions would probably be ruled out as the main production pathway for CCS. It should be noted, however, that CCH may be also produced by the CH2 þ C reaction (e.g., Turner et al. 2000). This may contribute the nonequivalent abundances of the 13C species of CCH, although the migration of the hydrogen atoms in the reaction intermediate, H2CC, would reduce the nonequivalence. Sensitive observations of 13CCH and C13CH in TMC-1 would be important for the evaluation of the contribution of the reactions (2) and (3). The reactions (6) and (7) involve HCS as a parent molecule. However, HCS is not an abundant interstellar molecule in TMC-1. Although the rotational spectrum of HCS was reported by Habara et al. (1998), the spectral lines of HCS have not yet been detected in molecular clouds, in spite of extensive astronomical searches. Furthermore, the chemical models predict low abundance of HCS (e.g., Lee et al. 1996). Hence, the reactions (6) and (7) seem unlikely as the main production route. Under these considerations, the remaining reaction between CH and CS (reaction [5]) would be the most probable route to produce CCS. Both the CH and CS molecules are known to be abundant in TMC-1 ( Irvine et al. 1987; Kaifu et al. 2004), and the chemical models (e.g., Lee et al. 1996) predict their high abundances. In this production scheme, the carbon atom of CH must react with CS without breaking the CS bond. According to Yamada et al. (2002), there are two possible pathways in the CH þ CS reaction. The CH radical attacks the carbon atom of CS to form an intermediate, linear HCCS, or it attacks the C¼S bond to form another intermediate, cyclic HCCS. The CS bond is conserved in the former case, whereas it may not always be in the latter case. The relative importance of these two pathways is unknown, but the former case is preferable, because the latter case requires isomerization to linear HCCS before forming CCS. Further theoretical and laboratory studies of this reaction with particular emphasis of the conservation of the CS bond are required. Since the ½C 13 CS
/½ 13 CCS
ratio is determined to be 4.2 in TMC-1, the minimum contribution of the nonequivalent route including the reaction (5) must be larger than 76%. When the minimum fraction is the case, the nonequivalent route must produce only C13CS. If the fractionation of 13CCS and C13CS by exchange reactions with C+ or C would occur, then it might make the ½C 13 CS
/½ 13 CCS
ratio lower. In this case, the original ½C 13 CS
/ ½ 13 CCS
ratio after the formation process would be higher than 4.2, requiring in the higher contribution of the nonequivalent route. These results would be useful for a critical test of the chemical model simulations for dense cores. If reaction (5) is the main pathway for the nonequivalent route, the abundance ratio of ½CH
/½ 13 CH
must be at least 4.2 times larger than the interstellar value. Such a dilution of the 13C species may be possible, for instance, by the isotope-selective photodissociation. The CH radical has a relatively low dissociation energy of 3.47 eV, which is lower than CS (7.36 eV) (Huber &

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Vol. 663

Herzberg 1979). The predissociation is observed in the C 2
þ (32,000 cm1) and higher states (Herzberg & Johns 1969). Therefore, CH can easily be photodissociated by UV photons, and the mechanism of the isotope-selective photodissociation may work in the cloud peripheries. When we consider the contracting cloud, 13 CH produced in the diffuse stage may still survive in the central part. Although the detailed mechanisms of the isotope-selective photodissociation of CH are unclear, we expect two possibilities as in the case of CO, the self-shielding and the mutual shielding (van Dishoeck & Black 1988). The column density of CH in TMC-1 is reported to be 2 ; 1014 cm2 (Irvine et al. 1987), which would be high enough for self-shielding. In addition, the cosmicrayYinduced UV (Sternberg et al. 1987) may play an important role in the isotope-selective photodissociation. For example, the H2 emission lines excited by the cosmic ray would selectively photodissociate 13CH. The present estimate of the ½CH
/½ 13 CH
ratio would provide us with a useful constraint on the future photodissociation models. 3.2. CCCS As described before, the abundance of 13CCCS relative to CCC34S is lower than the interstellar value. The 13CCCS, C13CCS, and CC13CS molecules are more stable than CCCS by 33, 54, and 59 K, respectively, due to the differences in the zero-point vibrational energies. Therefore, it is difficult to realize the dilution of heavy isotopes by exchange reactions with C+ or C, and the dilution reflects the formation process of CCCS. So far several pathways are proposed for the CCCS production in molecular clouds. As for the ion-molecule reaction scheme, the reactions of S+ and hydrocarbons are considered (Millar & Herbst 1990) Sþ þ (c-C3 H2 ; l-C3 H2 ) ! HCCCSþ þ H;

ð9Þ

where ‘‘c’’ and ‘‘l’’ represent the cyclic and linear isomers, respectively. The HCCCS+ ion produced above is recombined with an electron to give CCCS. Alternatively, Yamada et al. (2002) proposed the neutral-neutral reaction as C2 þ H2 CS ! CCCS þ H2 :

ð10Þ

In reaction (9), the dilution of the carbon atom originates from c-C3 H2 or l-C3 H2. If these molecules are produced by the dissociative recombination of the c-C3 Hþ 3 ion, all the three-carbon atoms are equivalent. In this case, all the 13C of CCCS species must be diluted in comparison with the interstellar value. However, the 13C species of c-C3 H2 seems to have a normal interstellar abundance judging from the line survey data toward TMC-1 by Kaifu et al. (2004). Therefore, reaction (9) would not contribute to the production of CCCS as the main pathway. If l-C3 H2 is produced from the dissociative recombination of the l-C3 Hþ 3 ion, the nonequivalence among three-carbon atoms seems possible. However, it would be difficult to realize very low abundance of 13 CCCS observed in the present study, because the contribution from the c-C3 Hþ 3 ion would mitigate the dilution. In reaction (10), C2 must have lower 13C abundance than the interstellar value. If this is the case, the heavy dilution is not the specific case of CH, but is a more general phenomenon, which seems unlikely. Then we propose the neutral-neutral reaction of CCS and CH as the main production route for CCCS. This reaction is exothermic by 369 kJ mol1, according to our density functional theory (DFT) calculation with the B3LYP/6-311G(d,p)

No. 2, 2007

PRODUCTION PATHWAYS OF CCS AND CCCS

method (Frisch et al. 1998). In this case, the dilution of 13CCCS can be naturally explained as far as 13CH is diluted. Unfortunately, we did not observe the C13CCS and CC13CS lines in the present study. However, their observations are essential in order to confirm the above scenario. If 13CH is responsible for the dilution of 13CCCS and 13CCS, the isotopic abundances of the ‘‘CCS’’ part of CCCS should be similar to those of CCS. Namely, we expect a significant dilution of C13CCS and a normal abundance of CC13CS. Furthermore, such a scheme may be extended to the formation of the larger member of Cn S. Theoretical

1179

and/or laboratory studies on the reaction between Cn S and CH would be very important.

The authors thank an anonymous referee for valuable comments. The authors are grateful to the staff of NRO and GBT for their excellent support in the observations. This study is supported by Grants-in-Aid from the Ministry of Education, Culture, Sports, Science, and Technologies (19-6825, 14204013, and 15071201).

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