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Chemical complexity in the winds of the oxygen-rich supergiant star VY Canis Majoris


The interstellar medium is enriched primarily by matter ejected from old, evolved stars1,2. The outflows from these stars create spherical envelopes, which foster gas-phase chemistry3,4,5. 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 analogues7. 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.

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Figure 1: Model of the molecular outflows in VY CMa, superimposed on the HST infrared image.
Figure 2: Sample molecular spectra from VY CMa, measured with the Arizona Radio Observatory’s Sub-millimeter Telescope (SMT), showing the variation in line profiles.
Figure 3: Comparison of spectra for the metastable isomers HCN and HNC in VY CMa.


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We thank the National Radio Astronomy Observatory for the loan of the ALMA Band 6 mixer system, and A. Lichtenberger and the University of Virginia Microfabrication Laboratory for supplying the mixer junctions. This research is partly supported by the NSF Astronomy and NASA Astrobiology programmes.

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Correspondence to L. M. Ziurys.

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Supplementary Information 1

This file contains Supplementary Discussion with a brief description of the model employed for molecular abundance derivations and additional references. (PDF 187 kb)

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Ziurys, L., Milam, S., Apponi, A. et al. Chemical complexity in the winds of the oxygen-rich supergiant star VY Canis Majoris. Nature 447, 1094–1097 (2007).

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