L. M. Ziurys, S. N. Milam, A. J. Apponi & N. J. Woolf
The interstellar medium is enriched
primarily by matter ejected from old, evolved stars. The outflows from these
stars create spherical envelopes, which foster gas-phase chemistry. The
chemical complexity in circumstellar shells was originally thought to be
dominated by the elemental carbon to oxygen ratio6.
Observations have suggested that envelopes with more carbon than oxygen have a
significantly greater abundance of molecules than their oxygen-rich analogues.
Here we report observations of molecules in the oxygen-rich shell of the red
supergiant star VY Canis Majoris (VY CMa). A variety of unexpected chemical
compounds have been identified, including NaCl, PN, HNC and HCO+.
From the spectral line profiles, the molecules can be distinguished as arising
from three distinct kinematic regions: a spherical outflow, a tightly
collimated, blue-shifted expansion, and a directed, red-shifted flow. Certain
species (SiO, PN and NaCl) exclusively trace the spherical flow, whereas HNC
and sulphur-bearing molecules (amongst others) are selectively created in the
two expansions, perhaps arising from shock waves. CO, HCN, CS and HCO+
exist in all three components. Despite the oxygen-rich environment, HCN seems
to be as abundant as CO. These results suggest that oxygen-rich shells may be
as chemically diverse as their carbon counterparts.
