Observations of Interstellar Formamide: Availability of a Prebiotic Precursor in the Galactic Habitable Zone

摘要

We conducted a study on interstellar formamide, NH_2CHO, toward star-forming regions of dense molecular clouds, using the telescopes of the Arizona Radio Observatory (ARO). The Kitt Peak 12 m antenna and the Submillimeter Telescope (SMT) were used to measure multiple rotational transitions of this molecule between 100 and 250GHz. Four new sources of formamide were found [W51M, Ml7 SW, G34.3, and DR21(OH)], and complementary data were obtained toward Orion-KL, W3(OH), and NGC 7538. From these observations, column densities for formamide were determined to be in the range of 1.1×l0~(12) to 9.1×10~(13) cm~(-2), with rotational temperatures of 70-177 K. The molecule is thus present in warm gas, with abundances relative to H_2 of 1 × 10 ~(-11) to 1×10~(-10). It appears to be a common constituent of star-forming regions that foster planetary systems within the galactic habitable zone, with abundances comparable to that found in comet Hale-Bopp. Formamide's presence in comets and molecular clouds suggests that the compound could have been brought to Earth by exogenous delivery, perhaps with an infall flux as high as ～ 0.1 mol/km~2/yr or 0.18 mmol/m~2 in a single impact. Formamide has recently been proposed as a single-carbon, prebiotic source of nucleobases and nucleic acids. This study suggests that a sufficient amount of NH_2CHO could have been available for such chemistry.