Reevaluation of C4H Abundance Based on the Revised Dipole Moment

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 2 with a large dipole moment in the previous quantum chemical calculations. However, the mixing of the excited state with the ground state 2 + 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 C2nH (n = 1-4) molecules with carbon-chain length in these sources, suggesting high similarities among individual formation mechanisms of the C2nH molecules.