Filament S242 is 25 pc long with massive clumps and YSO clusters concentrated in its end regions ; it is considered a good example of edge collapse .
We mapped this filament in the ^ { 12 } CO ( 1-0 ) and ^ { 13 } CO ( 1-0 ) lines .
A large-scale velocity gradient along filament S242 has been detected ; the relative velocity between the two end-clumps is \sim 3 km s ^ { -1 } , indicating an approaching motion between them .
These signatures are consistent with the filament S242 being formed through the collapse of a single elongated entity , where an effect known as “ gravitational focusing ” drives the ends of the filament to collapse ( edge collapse ) .
Based on this picture , we estimate a collapse timescale of \sim 4.2 Myr , which is the time needed for a finite and elongated entity evolving to the observed filament S242 .
For the whole filament , we find that increases in surface densities lead to increases in velocity dispersion , which can be consistently explained as the result of self-gravity .
We also calculated the contribution of longitudinal collapse to the observed velocity dispersion and found it to be the dominant effect in driving the gas motion near the end-clumps .
We propose that our filament S242 is formed through a two-stage collapse model , where the edge collapse of a truncated filament is followed by a stage of longitudinal accretion toward the dense end-clumps .