Transfer of radiation in the formic acid: A precursor for amino acids

Abstract

Formic acid (HCOOH), a simplest carboxylic acid, has great importance as it is a precursor for amino acids (constituents of life). It has two rotameric isomers: trans-HCOOH and cis-HCOOH, each of which lies in a plane due to the delocalization of $\pi$-electrons over the heavy atom chain. In each of the isomers, the electric dipole moment is aligned such that there are both a and b type rotational transitions. Further, the energy levels in each type of transitions can be classified into two groups. Thus, there are 8 groups in which the rotational transitions of formic acid may be classified. The trans-HCOOH is detected in Sgr B2, cold dark cloud L134N, Sgr A, comet Hale-Bopp, Orion KL, W51, IRAS 16293-2422 through its a-type transitions. Because of very small value of b-component of electric dipole moment, the b-type transitions of trans-HCOOH may not be detected. To our knowledge, no transitions of cis-HCOOH are yet detected in the interstellar medium, though a and b components of its electric dipole moment are quite large. Using spectroscopic data of trans-HCOOH and cis-HCOOH, we have calculated energies of 100 rotational levels for each of the 8 groups, and the radiative transition probabilities (Einstein A and B coefficients) for radiative transitions between the levels. Since the rate coefficients for collisional transitions between the levels are not available, by using the scaled values for them along with the radiative transition probabilities, we have solved a set of 100 statistical equilibrium equations coupled with the equations of radiative transfer for each group. We have investigated intensities of 16 observed a-type transitions and 12 b-type transitions of trans-HCOOH. We have also found six transitions, $1_{1 0} - 1_{1 1}$ (1.405 GHz), $2_{1 2} - 3_{0 3}$ (7.545 GHz), $3_{1 2} - 3_{0 3}$ (79.744 GHz), $3_{2 1} - 3_{1 2}$ (222.287 GHz), $1_{1 1} - 2_{0 2}$ (30.843 GHz) and $4_{1 3} - 4_{0 4}$ (82.740 GHz) of cis-HCOOH showing anomalous absorption and nine transitions $4_{1 4} - 3_{1 3}$ (85.042 GHz), $5_{1 5} - 4_{1 4}$ (106.266 GHz), $3_{0 3} - 2_{0 2}$ (65.840 GHz), $4_{0 4} - 3_{0 3}$ (87.694 GHz), $5_{0 5} - 4_{0 4}$ (109.470 GHz), $5_{0 5} - 4_{1 4}$ (40.778 GHz), $7_{0 7} - 6_{1 6}$ (90.910 GHz), $4_{0 4} - 3_{1 3}$ (16.350 GHz) and $6_{0 6} - 5_{1 5}$ (65.661 GHz) of cis-HCOOH showing emission feature. These transitions of cis-HCOOH in addition to those of trans-HCOOH may help in the identification of HCOOH in a cosmic object.