The ALMA-PILS survey: complex nitriles towards IRAS 16293–2422

Calcutt, H.; Jørgensen, J. K.; Müller, H. S. P.; Kristensen, L. E.; Coutens, A.; Bourke, T. L.; Garrod, R. T.; Persson, M. V.; van der Wiel, M. H. D.; van Dishoeck, E. F.; Wampfler, S. F. (2018). The ALMA-PILS survey: complex nitriles towards IRAS 16293–2422. Astronomy and astrophysics, 616, A90. EDP Sciences 10.1051/0004-6361/201732289

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Context. Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. Their detection is crucial to understanding the chemical evolution of the Universe and exploring the link between the early stages of star formation and the formation of solar system bodies, where complex organic molecules have been found in abundance. In particular, molecules that contain nitrogen are interesting due to the role nitrogen plays in the development of life and the compact scales such molecules have been found to trace around forming protostars.
Aims. The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a –C≡N functional group) towards two hot corino sources in the low-mass protostellar binary IRAS 16293–2422. This work explores the abundance differences between the two sources, the isotopic ratios, and the spatial extent derived from molecules containing the nitrile functional group.
Methods. Using data from the Protostellar Interferometric Line Survey (PILS) obtained with ALMA, we determine abundances and excitation temperatures for the detected nitriles. We also present a new method for determining the spatial structure of sources with high line density and large velocity gradients – Velocity-corrected INtegrated emission (VINE) maps.

Results. We detect methyl cyanide (CH₃CN) as well as five of its isotopologues, including CHD₂CN, which is the first detection in the interstellar medium (ISM). We also detect ethyl cyanide (C₂H₅CN), vinyl cyanide (C₂H₃CN), and cyanoacetylene (HC₃N). We find that abundances are similar between IRAS 16293A and IRAS 16293B on small scales except for vinyl cyanide which is only detected towards the latter source. This suggests an important difference between the sources either in their evolutionary stage or warm-up timescales. We also detect a spatially double-peaked emission for the first time in molecular emission in the A source, suggesting that this source is showing structure related to a rotating toroid of material.
Conclusions. With high-resolution observations, we have been able to show for the first time a number of important similarities and differences in the nitrile chemistry in these objects. These illustrate the utility of nitriles as potential tracers of the physical conditions in star-forming regions.

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