We have obtained spectra of the J=2-1 and J=10-9 transitions of cyanoacetylene ( HC _ { 3 } N ) toward a collection of positions in the most prominent filament , B213 , in the Taurus molecular cloud .
The analysis of the excitation conditions of these transitions reveals an average gas H _ { 2 } volume density of ( 1.8 \pm 0.7 ) \times 10 ^ { 4 } cm ^ { -3 } .
Based on column density derived from 2MASS and this volume density , the line of sight dimension of the high density portion of B213 is found to be \simeq 0.12 pc , which is comparable to the smaller projected dimension and much smaller than the elongated dimension of B213 ( \sim 2.4 pc ) .
B213 is thus likely a true cylinder–like filament rather than a sheet seen edge-on .
The line width and velocity gradient seen in HC _ { 3 } N are also consistent with Taurus B213 being a self-gravitating filament in the early stage of either fragmentation and/or collapse .