Analysis of the column densities of C4H using the revised dipole moment
T. Oyama,1 H. Ozaki,2 M. Araki,1 Y. Sumiyoshi,2 S. Takano,3 A. Ohsugi,1A. Ubagai,1 Y. Minami,1 N. Kuze4 and K. Tsukiyama1
1

2Department of Chemistry, Tokyo University of Science, Japan
Division of Pure and Applied Science, Graduate School of Science and Technology,Gunma University, Japan
3Department of Physics, College of Engineering, Nihon University, Japan
4Department of Materials and Life Sciences, Sophia University, Japan

A series of CnH molecules are the simplest linear carbon chains. They are crucial for not only
traces of young clouds but also benchmarks of chemical reaction network calculations.
However, their abundances occasionally show anomaly. For example, observed column
densities of C4H in various sources are one order of magnitude higher than theoreticallyestimated values. Herbst et al. suggested that these excesses of C4H come from thetheoretically determined dipole moment of C4H [1]. Based on the simple theory, the 2+ground state of this molecule has the small dipole moment of 0.87 D [2]. However, themixing of wavefunctions between the ground state and the low-lying 2excited state having
the large dipole moment of 4.4 D occurs, giving a higher dipole moment to the ground state.
By using a higher dipole moment, a smaller column density is derived via observed line
intensities. In the present study, we re-calculated the dipole moment of C4H by quantum
chemical calculations including the mixing. The calculations were carried out by the multireference configuration interaction (MRCI) level of ab initio theory using the aug-cc-pVQZ
basis set. The new dipole moment was derived to be 2.366 D, which is three times higher thanthe value of 0.87 D used so far. Reported lines of C4H were analyzed to revise column
densities by using the new dipole moment. Revised column densities are one order ofmagnitude lower than those in the previous works, as listed in Table 1. The column densities
of CnH molecules in IRC+10216 are described in Figure 1. Using the revised column densityof C4H, abundances of the CnH series show a linearity. Trends of the other sources listed inTable 1 are also similar. These trends might be results of sequential formation of CnH
molecules in the sources.

Objects
L483a
TMC-1 CPbBarnard 1aL134Na
Horseheada
Orion Bara
L1527cLupus-1Ad
IRC+10216e

Previous work
N / cm−2 T / K
6.9E+13 10
2.9E+146.7
2.5E+145
6.1E+135
3.0E+13 15
2.5E+13 15
1.0E+14 14.3
5.0E+147.3
3.0E+15 35

Re-analysis
N / cm−2
T/K
1.4(2)E+137.9(8)4.2E+136.7
3.4E+135
8.3E+125
4.1E+1215
3.2E+1215
2.37(8)E+13 14.5(9)6.7E+137.3
4.1E+1435

a Ref. 3. b Ref. 4. c Ref. 5. d Ref. 6. e Ref. 7.

References

1017

Cloumn density in IRC+10216 (cm−2)

Table 1: The column densities of C4H.

1016

C2H

Previous work
This work

1015

C4 H
C6H

1014

C8H

1013
1012

23

4567
Number of carbon atom

8Figure 1: The column densities of CnH
molecules in IRC+10216.

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