Two-component Magnetic Field along the Line of Sight to the Perseus Molecular Cloud: Contribution of the Foreground Taurus Molecular Cloud

TL;DR

This study uses Gaia distances and optical polarimetry to distinguish two dust clouds along the line of sight to Perseus, revealing a foreground Taurus cloud and a background Perseus cloud with different magnetic field orientations.

Contribution

It identifies the separate contributions of the Taurus and Perseus molecular clouds to polarization signals using Gaia distances and characterizes a dust cavity associated with the Per OB2 region.

Findings

The foreground cloud is part of the Taurus molecular cloud at 150 pc.

The Perseus cloud is at 300 pc with a distinct magnetic field orientation.

A dust cavity at 240 pc is associated with the Per OB2 region.

Abstract

Optical stellar polarimetry in the Perseus molecular cloud direction is known to show a fully mixed bi-modal distribution of position angles across the cloud (Goodman et al. 1990). We study the Gaia trigonometric distances to each of these stars and reveal that the two components in position angles trace two different dust clouds along the line of sight. One component, which shows a polarization angle of -37.6 deg +/- 35.2 deg and a higher polarization fraction of 2.0 +/- 1.7%, primarily traces the Perseus molecular cloud at a distance of 300 pc. The other component, which shows a polarization angle of +66.8 deg +/- 19.1 deg and a lower polarization fraction of 0.8 +/- 0.6%, traces a foreground cloud at a distance of 150 pc. The foreground cloud is faint, with a maximum visual extinction of < 1 mag. We identify that foreground cloud as the outer edge of the Taurus molecular cloud. Between the Perseus and Taurus molecular clouds, we identify a lower-density ellipsoidal dust cavity with a size of 100 -- 160 pc. This dust cavity locates at l = 170 deg, b = -20 deg, and d = 240 pc, which corresponds to an HI shell generally associated with the Per OB2 association. The two-component polarization signature observed toward the Perseus molecular cloud can therefore be explained by a combination of the plane-of-sky orientations of the magnetic field both at the front and at the back of this dust cavity.