We present and analyse ^ { 12 } CO , ^ { 13 } CO and C ^ { 18 } O ( 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 ^ { 12 } CO line has a high optical depth \tau \sim 10 .
The derived ^ { 12 } C/ ^ { 13 } C \sim 10 and ^ { 16 } O/ ^ { 18 } O \sim 115 ratios are not in agreement with the expected isotopic ratios of carbon-rich AGB stars .
Assuming that the ^ { 12 } CO optical depth has been underestimated , we can find a consistent fit for an initial mass of 2 M _ { \odot } .
We obtain a molecular mass of \sim 2 \times 10 ^ { -4 } \textup { M } _ { \odot } 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 .