CO in the C1 globule of the Helix nebula with ALMA
  We present and analyse 12CO, 13CO and C18O(2-1) ALMA observations of the C1
globule inside the Helix nebula in order to determine its physical properties.
Our findings confirm the molecular nature of the globule with a multi-peak
structure. The 12CO line has a high optical depth of about 10. The derived
12C/13C ratio of 10 and 16O/18O ratio of 115 are not in agreement with the
expected isotopic ratios of carbon-rich AGB stars. Assuming that the 12CO
optical depth has been underestimated, we can find a consistent fit for an
initial mass of 2 Msol. We obtain a molecular mass of 2x10-4 Msol for the C1
globule, which is much higher than its mass in the literature. Clumping could
play a role in the high molecular mass of the knot. The origin of the tail is
discussed. Our findings show that the most probable model appears to be
shadowing. The kinematics and molecular morphology of the knot are not
consistent with a wind-swept model and the photoevaporation model alone is not
enough to explain the nature of the globule. We propose an integrated model
where the effects of the photoevaporation, the stream and shadowing models are
all considered in the tail shaping process.
