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Characterizing the properties of nearby molecular filaments observed with Herschel

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Authors : Doris Arzoumanian, Philippe André, Vera Konyves, Pedro Palmeirim, Arabindo Roy, Nicola Schneider, Milena Benedettini, Pierre Didelon, James Di Francesco, Jason Kirk, and Bilal Ladjelate
Context. Molecular filaments have received special attention recently thanks to new observational results on their properties. In particular, our early analysis of filament properties from Herschel imaging data in three nearby molecular clouds revealed a narrow distribution of median inner widths centered at a characteristic value of about 0.1 pc.

Aims. Here, we extend and complement our initial study with a detailed
analysis of the filamentary structures identified with Herschel in
eight nearby molecular clouds (at distances <500 pc). Our main goal is
to establish statistical distributions of median properties averaged
along the filament crests and to compare the results with our earlier
work based on a smaller number of filaments.

Methods. We use the column density (N_H2) maps derived from Herschel
data and the DisPerSE algorithm to trace a network of individual
filaments in each cloud. We analyze the density structure along and
across the main filament axes in detail. We build synthetic maps of
filamentary clouds to assess the completeness limit of our extracted
filament sample and validate our measurements of the filament
properties. These tests also help us to select the best choice of
parameters to be used for tracing filaments with DisPerSE and fitting
their radial column density profiles.

Results. Our analysis yields an extended sample of 1310 filamentary
structures and a selected sample of 599 filaments with aspect ratios
larger than 3 and column density contrasts larger than 0.3. We show
that our selected sample of filaments is more than 95 % complete for
column density contrasts larger than 1, with only 5 % spurious
detections. On average, more than 15 % of the total gas mass in the
clouds, and more than 80 % of the dense gas mass (at N_H2 > 7x1021 cm-2
), is found to be in the form of filaments. Analysis of the radial
column density profiles of the 599 filaments in the selected sample
indicates a narrow distribution of crest- averaged inner widths, with a
median value of 0.10 pc and an interquartile range of 0.07 pc. In
contrast, the extracted filaments span wide ranges in length, central
column density, column density contrast, and mass per unit length. The
characteristic filament width is well resolved by Herschel
observations, and a median value of 0.1 pc is consistently found
using three distinct estimates based on (1) a direct measurement of the
width at half power after background subtraction, as well as (2)
Gaussian and (3) Plummer fits. The existence of a characteristic
filament width is further supported by the presence of a tight
correlation between mass per unit length and central column density for
the observed filaments.

Conclusions. Our detailed analysis of a large filament sample confirms
our earlier result that nearby molecular filaments share a common mean
inner width of 0.1 pc, with typical variations along and on either
side of the filament crests of about ±0.06 pc around the mean
value. This observational result sets strong constraints on possible
models for the formation and evolution of filaments in molecular
clouds. It also provides important hints on the initial conditions of
star formation.
Journal : Astronomy&Astrophysics ArXiv