Magnetic Field Strengths and Variations in Grain Alignment in the Local Bubble Wall

Abstract

Abstract Optical and infrared continuum polarization from the interstellar medium is known to generally be due to irregular dust grains aligned with the magnetic field. This provides an important tool to probe the geometry and strength of those fields, particularly if the variations in the grain alignment efficiencies can be understood. Here, we examine polarization variations observed throughout the wall of the Local Bubble, using a large polarization survey of the North Galactic cap (b > 30°) from Berdyugin et al. These data are analyzed together with archival photometric and spectroscopic data along with the mapping of the Local Bubble by Lallement et al. We can model the observational data by assuming that the mechanism driving alignment is due to the radiation from the surrounding star field. In particular we find that the fractional polarization is dominated by the light from the OB associations within 200 pc of the Sun, but is largely insensitive to the radiation field from red field stars. This behavior is consistent with the expected wavelength dependence of radiative grain alignment theory. We also probe the relative strength of the magnetic field in the wall of the Local Bubble using the Davis–Chandrasekhar–Fermi method. We find evidence for a systematically varying field strength distribution, where the variations in the field are correlated with the variations in grain alignment efficiency, indicating that the regions of relatively higher field strength might represent a compression of the wall by the interaction of the outflow in the Local Bubble and the opposing flows from the surrounding OB associations.