Red supergiants as potential type IIn supernova progenitors: Spatially resolved 4.6 μm CO emission aroung VY CMa and betelgeuse

We present high-resolution 4.6 μm CO spectra of the circumstellar environments of two red supergiants (RSGs) that are potential supernova (SN) progenitors: Betelgeuse and VY Canis Majoris (VY CMa). Around Betelgeuse, ¹²CO emission within 3″ (12 km s^-1) follows a mildly clumpy but otherwise spherical shell, smaller than its 55″ shell in K I λ7699. In stark contrast, 4.6 μm CO emission around VY CMa is coincident with bright K I in its clumpy asymmetric reflection nebula, within 5″ (40 km s^-1) of the star. Our CO data reveal redshifted features not seen in K I spectra of VY CMa, indicating a more isotropic distribution of gas punctuated by randomly distributed asymmetric clumps. The relative CO and K I distribution in Betelgeuse arises from ionization effects within a steady wind, whereas in VY CMa, K I is emitted from skins of CO cloudlets resulting from episodic mass ejections 500-1000 yr ago. In both cases, CO and K I trace potential pre-SN circumstellar matter: we conclude that an extreme RSG like VY CMa might produce a Type IIn event like SN 1988Z if it were to explode in its current state, but Betelgeuse will not. VY CMa demonstrates that luminous blue variables are not necessarily the only progenitors of SNe IIn, but it underscores the requirement that SNe IIn suffer enhanced episodic mass loss shortly before exploding.