Insights into surface hydrogenation in the interstellar medium: Observations of methanimine and methyl amine in SGR B2(N)

Multiple observations of methanimine (CH₂NH) and methyl amine (CH₃NH₂) have been performed toward Sgr B2(N) at 1, 2, and 3 mm using the Submillimeter Telescope and the 12 m antenna of the Arizona Radio Observatory. In the frequency range 68-280 GHz, 23 transitions of CH ₂NH and 170 lines of CH₃NH₂ have been observed as individual, distinguishable features, although some are partially blended with other lines. For CH₂NH, the line profiles indicate V _LSR = 64.2 ± 1.4 km s^-1 and ΔV _1/2 = 13.8 ± 2.8 km s^-1, while V _LSR = 63.7 ± 1.6 km s^-1 and ΔV _1/2 = 15.1 ± 3.0 km s ^-1 for CH₃NH₂, parameters that are very similar to those of other organic species in Sgr B2(N). From these data, rotational diagrams were constructed for both species. In the case of CH₂NH, a rotational temperature of T _rot = 44 ± 13 K and a column density of N _tot = (9.1 ± 4.4) × 10¹⁴ cm ^-2 were determined from the analysis. For CH₃NH ₂, T _rot = 159 ± 30 K and N _tot = (5.0 ± 0.9) × 10¹⁵ cm^-2, indicating that this species is present in much warmer gas than CH₂NH. The fractional abundances for CH₂NH and CH₃NH₂ were established to be f (H₂) ≈ 3.0 × 10^-10 and f (H₂) ≈ 1.7 × 10^-9, respectively. It has been proposed that CH₂NH is formed on grains via hydrogenation of HCN; further hydrogenation of CH₂NH on surfaces leads to CH ₃NH₂. However, given the dissimilarity between the rotational temperatures and distributions of CH₂NH and CH ₃NH₂ in Sgr B2, it is improbable that these species are closely related synthetically, at least in this source. Both CH₂NH and CH₃NH₂ are more likely created by neutral-neutral processes in the gas phase.