The ALMA-PILS survey: Complex nitriles towards IRAS 16293--2422

TL;DR

This study uses ALMA data to analyze complex nitriles in the IRAS 16293-2422 protostellar system, revealing molecular inventories, spatial structures, and isotopic ratios, with a new method for source structure analysis.

Contribution

It introduces a new VINE mapping technique for high line density sources and provides the first detection of CHD$_2$CN in the ISM.

Findings

Detection of methyl cyanide and five isotopologues, including CHD$_2$CN.

Similar abundances of nitriles in both sources except vinyl cyanide, which is only in IRAS 16293B.

First observation of double-peaked emission indicating a rotating toroid.

Abstract

Complex organic molecules are readily detected in the inner regions of the gaseous envelopes of forming protostars. 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. The goal of this work is to determine the inventory of one family of nitrogen-bearing organic molecules, complex nitriles (molecules with a $-$CN 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. 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. We detect methyl cyanide (CH$_3$CN) as well as 5 of its isotopologues, including the detection of CHD$_2$CN which is the first detection in the ISM. We also detect ethyl cyanide (C$_2$H$_5$CN), vinyl cyanide (C$_2$H$_3$CN), and cyanoacetylene (HC$_3$N). 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.