Physical and chemical structure of the Serpens filament: Fast formation and gravity-driven accretion

Context. The Serpens filament, a prominent elongated structure in a relatively nearby molecular cloud, is believed to be at an early evolutionary stage, so studying its physical and chemical properties can shed light on filament formation and early evolution.
Aims: The main goal is to address the physical and chemical properties as well as the dynamical state of the Serpens filament at a spatial resolution of ~0.07 pc and a spectral resolution of ≲0.1 km s^-1.
Methods: We performed ¹³CO (1-0), C¹⁸O (1-0), C¹⁷O (1-0), ¹³CO (2-1), C¹⁸O (2-1), and C¹⁷O (2-1) imaging observations toward the Serpens filament with the Institut de Radioastronomie Millimétrique 30-m and Atacama Pathfinder EXperiment telescopes.
Results: Widespread narrow ¹³CO (2-1) self-absorption is observed in this filament, causing the ¹³CO morphology to be different from the filamentary structure traced by C¹⁸O and C¹⁷O. Our excitation analysis suggests that the opacities of C¹⁸O transitions become higher than unity in most regions, and this analysis confirms the presence of widespread CO depletion. Further we show that the local velocity gradients have a tendency to be perpendicular to the filament's long axis in the outskirts and parallel to the large-scale magnetic field direction. The magnitudes of the local velocity gradients decrease toward the filament's crest. The observed velocity structure can be a result of gravity-driven accretion flows. The isochronic evolutionary track of the C¹⁸O freeze-out process indicates the filament is young with an age of ≲2 Myr.
Conclusions: We propose that the Serpens filament is a newly-formed slightly-supercritical structure which appears to be actively accreting material from its ambient gas.

The reduced datacubes are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/646/A170