The Serpens filament: at the onset of slightly supercritical collapse

The Serpens filament, as one of the nearest infrared dark clouds, is regarded as a pristine filament at a very early evolutionary stage of star formation. In order to study its molecular content and dynamical state, we mapped this filament in seven species. Among them, HCO$^{+}$, HNC, HCN, and CS show self-absorption, while C$^{18}$O is most sensitive to the filamentary structure. A kinematic analysis demonstrates that this filament forms a velocity-coherent (trans-)sonic structure, a large part of which is one of the most quiescent regions in the Serpens cloud. Widespread C$^{18}$O depletion is found throughout the Serpens filament. Based on the Herschel dust-derived H$_{2}$ column density map, the line mass of the filament is 36--41~M$_{\odot}$~pc$^{-1}$, and its full width at half maximum is 0.17$\pm$0.01~pc, while its length is ~1.6~pc. The inner radial column density profile of this filament can be well fitted with a Plummer profile with an exponent of 2.2$\pm$0.1, a scale radius of $0.018\pm 0.003$ pc, and a central density of $(4.0\pm 0.8)\times 10^{4}$~cm$^{-3}$. The Serpens filament appears to be slightly supercritical. The widespread blue-skewed HNC and CS line profiles and HCN hyperfine line anomalies across this filament indicate radial infall in parts of the Serpens filament. C$^{18}$O velocity gradients also indicate accretion flows along the filament. The velocity and density structures suggest that such accretion flows are likely due to a longitudinal collapse parallel to the filament's long axis. Both the radial infall rate and the longitudinal accretion rate along the Serpens filament are lower than all previously reported values in other filaments. This indicates that the Serpens filament lies at an early evolutionary stage when collapse has just begun, or that thermal and non-thermal support are effective in providing support against gravity.