Observations of interstellar formamide: availability of a prebiotic precursor in the galactic habitable zone

Abstract

We conducted a study on interstellar formamide, NH2CHO, 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 250 GHz. Four new sources of formamide were found [W51M, M17 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 1.1×10(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 H2 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 NH2CHO could have been available for such chemistry.

FIG. 1.

Chemical structure of formamide. NH₂CHO is a nearly flat prolate asymmetric top. The principal moments of inertia a and b are indicated.

FIG. 2.

Schematic representation of the Milky Way Galaxy. The Sun is indicated by the cross. The galactic habitable zone is indicated by the dashed, gray circles, based on work of Gowanlock et al. (2011). The approximate galactic positions of the molecular clouds where formamide has been detected are indicated by filled circles. The circle size indicates the relative amount of formamide. The figure summarizes the current work and past observations (e.g., Cummins et al., ; Bisschop et al., ; Halfen et al., 2011). The distances are in kiloparsecs, or kpc (1 kpc ∼ 3259 light-years).

FIG. 3.

Example spectra of formamide transitions observed toward G34.3 (left panels) and W51M (right panels) at 3 and 2 mm in wavelength with the ARO 12 m telescope. The spectral resolution is 500 kHz for all data, except the J=7_0,7→6_0,6 transition in W51M, where it is 1 MHz. The quantum numbers for each transition are shown on each panel. The vertical line under each spectrum indicates the average cloud LSR velocity, near 58 km/s for both regions. Other organic molecules can be seen in the data, such as C₂H₅CN and HCOOCH₃.

FIG. 4.

Example spectra of formamide transitions observed toward DR21(OH) (left panels) and M17 SW (right panels) at 3 and 2 mm with the ARO 12 m telescope. The spectral resolution is 500 kHz. The quantum numbers for each transition are shown on each panel. The vertical line under each spectrum indicates the cloud average LSR velocity: −3 km/s for DR21(OH) and 18 km/s for M17 SW.

FIG. 5.

Example spectra of formamide transitions observed toward Orion-KL (left panels), NGC 7538, and W3(OH) (right panels) at 3 and 2 mm with the ARO 12 m telescope. The spectral resolution is 500 kHz. The quantum numbers for each transition are shown on each panel. The vertical line under the data indicates the cloud average LSR velocity: 8 km/s for Orion-KL, −58 km/s for NGC 7538, and −48 km/s for W3(OH). Other molecular features are present in these spectra.

FIG. 6.

Rotational diagrams for Orion-KL, W51M, G34.3 (upper panels), and NGC 7538, W3(OH) (lower panels). The y axis represents log I, where . The rotational diagrams for NGC 7538 and W3(OH) were constructed by combining our measurements with those of Bisschop et al. (2007). Rotational temperatures derived from the plots lie in the range of 70–177 K, indicating that formamide is present in warm gas in molecular clouds.