The Snake filament: A study of polarization and kinematics

¹ Centre for Astrochemical Studies, Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße1, 85748 Garching bei München, Germany
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
³ Departamento de Física–ICEx–UFMG, Caixa Postal 702, 30.123-970 Belo Horizonte, Brazil

^★ Corresponding author.

Received: 14 February 2025
Accepted: 10 July 2025

Abstract

Context. Understanding the role of magnetic fields in the formation of dense filamentary structures in molecular clouds is critical for understanding the star formation process. The Snake filament, which is in or close to the Pipe Nebula’s neighborhood, is a prominent example of such structures and offers an ideal environment to study the interplay between magnetic fields and gas dynamics in the early stages of star formation.

Aims. We investigated how magnetic fields influence the structure and dynamics of the Snake filament using both polarization data and molecular line observations. Our goal is to understand the role of magnetic fields in shaping the filamentary structure and explore the kinematics within the filament.

Methods. We conducted polarization observations in the optical and near-infrared bands using the 1.6 m and 60 cm telescopes at the Observatório do Pico dos Dias/Laboratório Nacional de Astrofísica (OPD/LNA). Molecular line observations of the ¹³CO (1-0) and C¹⁸O (1-0) lines were obtained using the IRAM 30 m telescope. We analyzed the data to characterize polarization and gas properties within the filament, with a focus on understanding the magnetic field orientation and its relationship with the filament’s structure.

Results. Our findings reveal that the polarization vectors align with the filament’s spine, indicating a magnetic field structure that is predominantly parallel to the filament in lower-density regions. A velocity gradient along the filament is observed in both ¹³CO (1-0) and C¹⁸O (1-0) lines, with C¹⁸O (1-0) tracing the denser regions of the gas. The polarization efficiency decreases with increasing visual extinction, which is consistent with reduced grain alignment in higher-density regions. The filament’s mass-to-length ratio is below the critical value required for gravitational collapse, indicating stability.