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

Yuxin Lin, Silvia Spezzano, Olli Sipilä, Jaime E. Pineda, Paola Caselli

TL;DR

This work delivers the first spatially resolved map of the prebiotic molecule CH2NH in the pre-stellar core L1544, revealing extended emission with peak intensity near the c-C3H2-rich zone and a peak in integrated emission between the HNCO and dust peaks. Derived CH2NH column densities are $(0.4-1.4) \times 10^{12}$ cm$^{-2}$, corresponding to fractional abundances of $(0.3-1.2) \times 10^{-10}$, with a south-to-north gradient from carbon-chain–rich outer layers toward the dense center. By comparing to the pyRate gas–grain chemical model, the authors find that gas-phase neutral–neutral and ion–mastro reactions, aided by mild UV irradiation in the outer envelope, dominate CH2NH formation, while adsorption onto grains becomes important in the inner core; CH2NH abundance peaks early in the gas phase and declines, consistent with an outer-layer origin. The results imply that prebiotic nitrogen–carbon chemistry remains active before gravitational collapse and can be inherited during protostellar evolution, providing essential constraints for astrochemical models of early chemical complexity and amino-acid precursors in star-forming regions.

Abstract

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.

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

TL;DR

This work delivers the first spatially resolved map of the prebiotic molecule CH2NH in the pre-stellar core L1544, revealing extended emission with peak intensity near the c-C3H2-rich zone and a peak in integrated emission between the HNCO and dust peaks. Derived CH2NH column densities are cm, corresponding to fractional abundances of , with a south-to-north gradient from carbon-chain–rich outer layers toward the dense center. By comparing to the pyRate gas–grain chemical model, the authors find that gas-phase neutral–neutral and ion–mastro reactions, aided by mild UV irradiation in the outer envelope, dominate CH2NH formation, while adsorption onto grains becomes important in the inner core; CH2NH abundance peaks early in the gas phase and declines, consistent with an outer-layer origin. The results imply that prebiotic nitrogen–carbon chemistry remains active before gravitational collapse and can be inherited during protostellar evolution, providing essential constraints for astrochemical models of early chemical complexity and amino-acid precursors in star-forming regions.

Abstract

We present the first spatially resolved map of methanimine CH2NH in the prestellar core L1544 using the IRAM 30m telescope. The 2-1 line at 127 GHz was mapped with 20" resolution (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 (0.5-1.4)10 cm, corresponding to fractional abundances of -, 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.

Paper Structure

This paper contains 6 sections, 6 figures.

Table of Contents

  1. Introduction
  2. Observation and data reduction
  3. Analysis and Results
  4. Comparison with other sources
  5. Comparison with chemical models
  6. Conclusion

Figures (6)

  • Figure 1: Top: Integrated intensity map of CH2NH 2$_{\rm{0,2}}$-1$_{\rm{0,1}}$ line toward L1544. The velocity range for integration is 6.8-7.6 km s$^{-1}$ (system velocity, $V_{\mathrm{LSR}}$ = 7.2 km s$^{-1}$). Gray contours indicate molecular hydrogen column density ($N_{\mathrm{H2}}$) derived from Herschel (Spezzano16) from 3.0$\times$10$^{20}$-2.7$\times$10$^{22}$ cm$^{-2}$ with six uniform intervals. The three molecular peaks and the dust peak are indicated with cross markers (Spezzano17). Beam sizes of the line (20$"$, in blue) and of $N_{\mathrm{H2}}$ map (38$"$, in gray) are indicated in the bottom left corner. Middle: Signal-to-noise (S/N) ratio map based on integrated intensity maps divided by $\sigma_{\rm rms} \times \sqrt{N_{\rm chan}} \times \Delta V$; markers follow same definition as in the top panel. Bottom: Average spectrum from S/N$>$3 pixels; $V_{\mathrm{LSR}}$ is shown as a vertical dashed line and 2$\sigma$ noise level as a horizontal dotted line.
  • Figure 2: The average CH2NH 2$_{\rm{0,2}}$-1$_{\rm{0,1}}$ spectrum towards the three molecular peaks and the dust peak (Fig. \ref{['fig:mom0']}). The area used for spectrum averaging is one beam size, 20$"$. The vertical dashed line indicates $V_{\mathrm{lsr}}$ and the horizontal pointed line shows the 2$\sigma$ noise level.
  • Figure 3: The Gaussian fitted parameters of the CH2NH line, showing the peak intensity, centroid velocity and velocity linewidth. The three molecular peaks and dust peak are marked.
  • Figure 4: Upper panels: The column density $N(\ce{CH2NH})$ and column density uncertainty map of L1544 assuming $T_{\mathrm{ex}}$ = 10 K. Lower panels: The CH2NH abundance map and abundance uncertainty map, calculated with respect to $N_{\mathrm{H_2}}$ of 38$"$ angular resolution. Gray contours indicate molecular hydrogen column density ($N_{\mathrm{H2}}$) from 3.0$\times$10$^{20}$-2.7$\times$10$^{22}$ cm$^{-2}$ with six uniform intervals. The three molecular peaks and the dust peak are indicated with cross markers.
  • Figure 5: The abundance variation of CH2NH (panel (a)) in the gas phase and on the grain surface. Three density setups (10$^{5}$ cm$^{-3}$, 3$\times$10$^{5}$ cm$^{-3}$, 5$\times$10$^{5}$ cm$^{-3}$) are shown. The shaded region indicates the observed $X(\ce{CH2NH})$ in L1544 above $N_{\mathrm{H_2}}$ of 5$\times$10$^{21}$ cm$^{-2}$. The variation of gas-phase abundance of related species (panel (b)) as a function of time, for density of 10$^{5}$ cm$^{-3}$.
  • ...and 1 more figures