Letter to the Editor

The typical width of Herschel filaments

¹ Laboratoire d’Astrophysique (AIM), Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
e-mail: pandre@cea.fr
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: pedro.palmeirim@astro.up.pt
³ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
e-mail: doris.arzoumanian@nao.ac.jp

Received: 19 July 2022
Accepted: 10 October 2022

Abstract

Context. Dense molecular filaments are widely believed to be representative of the initial conditions of star formation in interstellar clouds. Characterizing their physical properties, such as their transverse size, is therefore of paramount importance. Herschel studies suggest that nearby (d <  500 pc) molecular filaments have a typical half-power width of ∼0.1 pc, but this finding has been questioned recently on the ground that the measured widths tend to increase with distance to the filaments.

Aims. Here we revisit the dependence of measured filament widths on distance or, equivalently, spatial resolution in an effort to determine whether nearby molecular filaments have a characteristic half-power width or whether this is an artifact of the finite resolution of the Herschel data.

Methods. We perform a convergence test on the well-documented B211/213 filament in Taurus by degrading the resolution of the Herschel data several times and reestimating the filament width from the resulting column density profiles. We also compare the widths measured for the Taurus filament and other filaments from the Herschel Gould Belt Survey to those found for synthetic filaments with various types of simple, idealized column density profiles (Gaussian, power law, and Plummer-like).

Results. We find that the measured filament widths do increase slightly as the spatial resolution worsens and/or the distance to the filaments increases. However, this trend is entirely consistent with what is expected from simple beam convolution for filaments with density profiles that are Plummer-like and have intrinsic half-power diameters of ∼0.08–0.1 pc and logarithmic slopes 1.5 <  p <  2.5 at large radii, as directly observed in many cases, including for the Taurus filament. Due to the presence of background noise fluctuations, deconvolution of the measured widths from the telescope beam is difficult and quickly becomes inaccurate.

Conclusions. We conclude that the typical half-power filament width of ∼0.1 pc measured with Herschel in nearby clouds most likely reflects the presence of a true common scale in the filamentary structure of the cold interstellar medium, at least in the solar neighborhood. We suggest that this common scale may correspond to the magnetized turbulent correlation length in molecular clouds.