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No statistical study of COMs toward a large sample of high-mass protostarswith ALMA has been carried out so far. We aim to study six N-bearing species:CH$_3$CN, HNCO, NH$_2$CHO, C$_2$H$_5$CN, C$_2$H$_3$CN and CH$_3$NH$_2$ in alarge sample of high-mass protostars. From the ALMAGAL survey, 37 of the mostline-rich hot molecular cores are selected. Next, we fit their spectra and findcolumn densities and excitation temperatures of the above N-bearing species, inaddition to CH$_3$OH. We (tentatively) detect CH$_3$NH$_2$ in $\sim32%$ of thesources. We find three groups of species when comparing their excitationtemperatures: hot (NH$_2$CHO; Tex > 250 K), warm (C$_2$H$_3$CN, HN$^{13}$CO andCH$_{3}^{13}$CN; 100 K < Tex < 250 K) and cold species (CH$_3$OH andCH$_3$NH$_2$; Tex < 100 K). This temperature segregation reflects the trendseen in their sublimation temperatures and validates the idea of onion-likestructure of COMs around protostars. Moreover, the molecules studied here showconstant column density ratios across low- and high-mass protostars withscatter less than a factor $\sim3$ around the mean. The constant column densityratios point to a common formation environment of COMs or their precursors,most likely in the pre-stellar ices. The scatter around the mean of the ratios,although small, varies depending on the species considered. This spread caneither have a physical origin (source structure, line or dust optical depth) ora chemical one. Formamide is most prone to the physical effects as it istracing the closest regions to the protostars, whereas such effects are smallfor other species. Assuming that all molecules form in the pre-stellar ices,the scatter variations could be explained by differences in lifetimes orphysical conditions of the pre-stellar clouds. If the pre-stellar lifetimes arethe main factor, they should be similar for low- and high-mass protostars.

astronomy and astrophysics

N-bearing complex organics toward high-mass protostars: Constant ratios  pointing to formation in similar pre-stellar conditions across a large mass  range