2020ApJ...890...39O

Abnormally large column densities of the carbon-chain molecule, C₄H, have been reported in various sources. The main reason for this was supposed to be an underestimated value of the permanent dipole moment of C₄H. C₄H has a low-lying electronic excited state of ²Π with a large dipole moment in the previous quantum chemical calculations. However, the mixing of the excited state with the ground state ²Σ⁺ has not been taken into consideration. In the present study, we evaluated this mixing by introducing a multireference configuration interaction. The effective dipole moment of C₄H in the ground state was calculated to be 2.10 D, which is about 2.4 times larger than the values used so far. Revised column densities of C₄H in dark clouds, low-mass star-forming regions, and the circumstellar envelope IRC+10216 based upon the new value are about a factor of 6 smaller than the previous values. With the revised column densities, exponential smooth decreases are found for the abundances of the C_2nH (n = 1-4) molecules with carbon-chain length in these sources, suggesting high similarities among individual formation mechanisms of the C_2nH molecules.