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