Potential lines of formic acid for its detection in cosmic objects

Abstract

Being precursor for amino acids (constituents of life), the simplest carboxylic acid, formic acid (HCOOH) has great importance in astrobiology. It has two rotameric isomers: trans-HCOOH and cis-HCOOH, each of which, due to the delocalization of $\pi$-electrons over the heavy atom chain, lies in a plane. The electric dipole moment of each isomer has a and b components and energy levels for both components are common. It is, therefore, more scientific to consider both components of electric dipole moment together. The stronger isomer trans-HCOOH has been detected in several cosmic objects through its 16 a-type transitions, because its $\mu _b$ is very small as compared to $\mu _a$. The cis-HCOOH has been detected in Orion Bar through 12a and b types of transitions.

For each isomer, we have calculated energies of rotational levels and radiative transition probabilities for radiative transitions between the levels. We have considered 200 rotational levels of each isomer connected through 1437 radiative transitions in cis-HCOOH and 1481 radiative transitions in trans-HCOOH. A scaling law has been used for calculation of collisional rate coefficients for collisional transitions between the levels. These collisional rate coefficients plus the radiative transition probabilities have been used in the Sobolev LVG analysis for each isomer. We have found, 64 weak MASER lines and 17 anomalous absorption transitions, for cis-HCOOH, and 55 weak MASER lines and 5 anomalous absorption transitions for the trans–HCOOH. Out of observed 16 + 12 transitions, two transitions $1_{1,0} - 1_{1,1}$ and $2_{1,1} - 2_{1,2}$ of trans–HCOOH, and one transition $7_{0,7} - 6_{1,6}$ of cis-HCOOH have been found as weak MASERs. Rest of the observed transitions have been found neither MASER lines nor anomalous absorption transitions. In addition to the observed lines, these $66 + 55$ weak MASER lines, and $17 + 5$ anomalous absorption transitions may play key role in the identification of formic acid in the interstellar medium.