ALMA Compact Array observations of the Fried Egg nebula: Evidence for
  large-scale asymmetric mass-loss from the yellow hypergiant IRAS 17163-3907
  Yellow hypergiants are rare and represent a fast evolutionary stage of
massive evolved stars. That evolutionary phase is characterised by a very
intense mass loss, the understanding of which is still very limited. Here we
report ALMA Compact Array observations of a 50$"$-mosaic toward the Fried Egg
nebula, around one of the few Galactic yellow hypergiants IRAS 17163-3907. The
emission from the $^{12}$CO J=2-1 line, H30$\alpha$ recombination line, and
continuum is imaged at a resolution of $\sim$8$"$, revealing the morphology of
the molecular environment around the star. The continuum emission is unresolved
and peaks at the position of the star. The radio recombination line H30$\alpha$
shows unresolved emission at the star, with an approximately gaussian spectrum
centered on a velocity of 21$\pm$3~km/s with a width of 57$\pm$6~km/s. In
contrast, the CO 2-1 emission is complex and decomposes into several components
beyond the contamination from interstellar gas in the line of sight. The CO
spectrum toward the star is a broad plateau, centered at the systemic velocity
of +18 km/s and with an expansion velocity of 100$\pm$10 km/s. Assuming
isotropic and constant mass-loss, we estimate a mass-loss rate of 8$\pm$1.5
$\times10^{-5}$~M$_\odot$ yr$^{-1}$. At a radius of 25$"$ from the star, we
detect CO emission associated with the dust ring previously imaged by {\it
Herschel}. The kinematics of this ring, however, is not consistent with an
expanding shell, but show a velocity gradient of $v_{sys} \pm$20 km/s. In
addition, we find a puzzling bright feature radially connecting the star to the
CO ring, at a velocity of +40 km/s relative to the star. This spur feature may
trace a unidirectional ejection event from the star. Our ACA observations
reveal the complex morphology around IRAS 17163 and illustrate the
breakthroughs that ALMA will bring to the field of massive stellar evolution.
