Chemical segregation in hot cores with disk candidates - An investigation with ALMA

摘要

Context. In the study of high-mass star formation, hot cores are empirically defined stages where chemically rich emission is detected toward a massive YSO. It is unknown whether the physical origin of this emission is a disk, inner envelope, or outflow cavity wall and whether the hot core stage is common to all massive stars. Aims. We investigate the chemical makeup of several hot molecular cores to determine physical and chemical structure. We use high spectral and spatial resolution submillimeter observations to determine how this stage fits into the formation sequence of a high-mass star. Methods. The submillimeter interferometer ALMA (Atacama Large Millimeter Array) was used to observe the G35.20-0.74N and G35.03+0.35 hot cores at 350 GHz in Cycle 0. We analyzed spectra and maps from four continuum peaks (A, B1, B2 and B3) in G35.20-0.74N, separated by 1000–2000 AU, and one continuum peak in G35.03+0.35. We made all possible line identifications across 8 GHz of spectral windows of molecular emission lines down to a 3 σ line flux of 0.5 K and determined column densities and temperatures for as many as 35 species assuming local thermodynamic equilibrium (LTE). Results. In comparing the spectra of the four continuum peaks, we find each has a distinct chemical composition expressed in over 400 different transitions. In G35.20, B1 and B2 contain oxygen- and sulfur-bearing organic and inorganic species but few nitrogen-bearing species whereas A and B3 are strong sources of O-, S-, and N-bearing organic and inorganic species (especially those with the CN bond). Column densities of vibrationally excited states are observed to be equal to or greater than the ground state for a number of species. Deuterated methyl cyanide is clearly detected in A and B3 with D/H ratios of 8 and 13 % , respectively, but is much weaker at B1 and undetected at B2. No deuterated species are detected in G35.03, but similar molecular abundances to G35.20 were found in other species. We also find co-spatial emission of isocyanic acid (HNCO) and formamide (NH _(2) CHO) in both sources indicating a strong chemical link between the two species. Conclusions. The chemical segregation between N-bearing organic species and others in G35.20 suggests the presence of multiple protostars surrounded by a disk or torus.