# Helical Magnetic Fields in Molecular Clouds? A New Method to Determine the Line-of-Sight Magnetic Field Structure in Molecular Clouds

## TL;DR

This paper introduces a novel method using Faraday rotation data to map the line-of-sight magnetic field structure in molecular clouds, revealing evidence of helical magnetic fields in star-forming regions.

## Contribution

The study develops and tests a new approach to determine the magnetic field's line-of-sight component in molecular clouds using rotation measures and chemical modeling.

## Key findings

- Evidence of helical magnetic field morphology in Orion A and California.
- Magnetic field strengths consistent with Zeeman measurements.
- Method successfully estimates magnetic field direction and magnitude in star-forming regions.

## Abstract

Magnetic fields pervade in the interstellar medium (ISM) and are believed to be important in the process of star formation, yet probing magnetic fields in star formation regions is challenging. We propose a new method to use Faraday rotation measurements in small scale star forming regions to find the direction and magnitude of the component of magnetic field along the line-of-sight. We test the proposed method in four relatively nearby regions of Orion A, Orion B, Perseus, and California. We use rotation measure data from the literature. We adopt a simple approach based on relative measurements to estimate the rotation measure due to the molecular clouds over the Galactic contribution. We then use a chemical evolution code along with extinction maps of each cloud to find the electron column density of the molecular cloud at the position of each rotation measure data point. Combining the rotation measures produced by the molecular clouds and the electron column density, we calculate the line-of-sight magnetic field strength and direction. In California and Orion A, we find clear evidence that the magnetic fields at one side of these filamentary structures are pointing towards us and are pointing away from us at the other side. Even though the magnetic fields in Perseus might seem to suggest the same behavior, not enough data points are available to draw such conclusions. In Orion B, as well, there are not enough data points available to detect such behavior. This behavior is consistent with a helical magnetic field morphology. In the vicinity of available Zeeman measurements in OMC-1, OMC-B, and the dark cloud Barnard 1, we find magnetic field values of $-23\pm38~\mu$G, $-129\pm28~\mu$G, and $32\pm101~\mu$G, respectively, which are in agreement with the Zeeman Measurements.