First mapping of prebiotic molecule CH2NH in a pre-stellar core

We present the first spatially resolved map of methanimine CH2NH in the prestellar core L1544 using the IRAM 30m telescope. The 2${0,2}$-1${0,1}$ line at 127 GHz was mapped with 20" resolution ($\sim$2800 au), revealing extended CH2NH emission across the core. The peak line intensity coincides with the well-known c-C3H2 peak, while the integrated intensity peaks between the HNCO and dust continuum peaks due to broader linewidths in the latter region. Column densities of CH2NH are $\sim$(0.5-1.4$\times$)10$^{12}$ cm$^{-2}$, corresponding to fractional abundances of $5\times10^{-11}$-$1\times10^{-10}$, with a trend decreasing from the southern, carbon-chain rich region to the dust and HNCO peak in the north. Comparison with complementary molecular maps and the gas-grain chemical model of Sipilä et al. suggests that neutral-neutral gas-phase reactions and dissociative recombination dominate in the outer carbon-chain shell. This study demonstrates that CH2NH, a simple nitrogen- and carbon-bearing molecule previously detected with pointed observations in other cold cores, is present and spatially extended in the evolved pre-stellar core L1544. This indicates that prebiotic nitrogen-carbon chemistry continues efficiently up to the onset of gravitational collapse, providing key constraints for astrochemical models and the early stages of chemical complexity leading to amino acids.

💡 Research Summary

This paper presents the first spatially resolved map of the prebiotic molecule methanimine (CH₂NH) in the pre-stellar core L1544, a cold, dense cloud on the verge of star formation. Using the IRAM 30m telescope, the team observed the 2₀,₂–1₀,₁ rotational transition of CH₂NH at 127.856795 GHz, mapping its emission across the core with a resolution of 20" (approximately 2800 Astronomical Units).

The observations revealed extended emission of CH₂NH throughout the core. A key finding is the spatial differentiation of its emission properties: the peak line intensity coincides with the known peak of the carbon-chain molecule c-C₃H₂, located in the southern part of the core. However, the integrated intensity, which accounts for the line width, peaks in a region between the dust continuum peak and the HNCO (isocyanic acid) peak in the north. This shift is attributed to significantly broader line widths (~0.25-0.30 km/s) in this northern region compared to the narrower lines (~0.11 km/s) at the c-C₃H₂ peak, suggesting the influence of local gas dynamics.

Under the assumption of Local Thermodynamic Equilibrium (LTE) and an excitation temperature of 10 K, the derived column densities of CH₂NH range from (0.4–1.4)×10¹² cm⁻². When compared to the H₂ column density from Herschel data, the fractional abundance of CH₂NH is between 5×10⁻¹¹ and 1×10⁻¹⁰. This abundance shows a clear gradient, decreasing from the carbon-chain-rich southern region (c-C₃H₂ peak) towards the dust and HNCO peak in the north.

Comparing this value to other astronomical environments shows that while lower than in some cold dark clouds like L183, it is comparable to upper limits in the low-mass protostar IRAS16293-2422B and orders of magnitude lower than in massive hot cores. This indicates that the formation mechanisms for CH₂NH are environment-dependent.

The core of the analysis involves comparing the observational results with state-of-the-art gas-grain chemical models (the pyRate model). The models simulate chemistry at conditions relevant to L1544 (densities of 10⁵ to 5×10⁵ cm⁻³, temperature of 10 K). They predict that gas-phase CH₂NH is an “early-time” species, peaking in abundance around 10⁴–10⁵ years via neutral-neutral reactions such as CH + NH₃ → H + CH₂NH, before declining due to adsorption onto dust grains. The observed spatial correlation of CH₂NH with the c-C₃H₂ peak—a region known for enhanced UV-driven radical chemistry—strongly supports the model prediction. It suggests that the formation of CH₂NH in this cold core is dominated by gas-phase photochemistry in the outer envelope, rather than by grain-surface processes which are more important in warmer regions.

In conclusion, this study successfully demonstrates that CH₂NH, a simple but crucial nitrogen- and carbon-bearing molecule and a potential precursor to amino acids like glycine, is not only present but also spatially extended in an evolved pre-stellar core. This provides direct evidence that prebiotic chemistry, leading to chemical complexity necessary for life, operates efficiently even in the cold, quiescent environments preceding gravitational collapse and star formation. The research offers vital constraints for astrochemical models, bridging our understanding of molecular evolution from diffuse clouds to the eventual emergence of planetary system ingredients.