Cool, evolved stars have copious, enriched winds. The structure of these
winds and the way they are accelerated is not well known. We need to improve
our understanding by studying the dynamics from the pulsating stellar surface
to about 10 stellar radii, where radiation pressure on dust is fully effective.
Some red supergiants have highly asymmetric nebulae, implicating additional
forces. We retrieved ALMA Science Verification data providing images of sub-mm
line and continuum emission from VY CMa. This enables us to locate water masers
with milli-arcsec precision and resolve the dusty continuum. The 658-, 321- and
325-GHz masers lie in irregular, thick shells at increasing distances from the
centre of expansion. For the first time this is confirmed as the stellar
position, coinciding with a compact peak offset to the NW of the brightest
continuum emission. The maser shells (and dust formation zone) overlap but
avoid each other on tens-au scales. Their distribution is broadly consistent
with excitation models but the conditions and kinematics appear to be
complicated by wind collisions, clumping and asymmetries.Comment: Letter 4 pages, 5 figures plus appendix with 3 figures. Accepted by
Astronomy and Astrophysics Letter