Observations of Formic Acid in Hot Molecular Cores

We present BIMA Array observations of formic acid (HCOOH) in Galactic hot molecular cores. It has been found that among nearly 120 interstellar and circumstellar molecular species identified to date, the more complex and saturated organic species are usually observed in hot molecular cores—dense and warm molecular condensations associated with active star formation regions inside molecular clouds. Formic acid, one of the molecules in this category, shares common structural elements with both methyl formate (HCOOCH₃) and acetic acid (CH₃COOH). In this study, we successfully mapped HCOOH emission in three regions: Orion KL, Sgr B2, and W51. Column densities of HCOOH are above 10¹⁵ cm^-2 in these sources. The derived HCOOH column density in Sgr B2(N-LMH) is comparable to the CH₃COOH column density found by Mehringer et al. in 1997. Ethyl cyanide (C₂H₅CN) and HCOOCH₃ emission spectra were also detected in several sources. The distribution of HCOOH emission is consistent with a surface chemistry origin for the species. The abundance ratios of HCOOH to C₂H₅CN and to HCOOCH₃ vary by nearly 2 orders of magnitude from source to source.