Interstellar Polarization Survey. IV. Characterizing the magnetic field strength and turbulent dispersion using optical starlight polarization in the diffuse interstellar medium
arxiv(2024)
摘要
Angular dispersion functions are typically used to estimate the fluctuations
in polarization angle around the mean magnetic field orientation in dense
regions, such as molecular clouds. The technique provides accurate turbulent to
regular magnetic field ratios, ⟨ B_t^2⟩^1/2/B_pos, which are
often underestimated by the classic Davis-Chandrasekhar-Fermi method. We assess
the technique's suitability to characterize the turbulent and regular
plane-of-sky magnetic field in low-density structures of the nearby
interstellar medium (ISM), particularly when the turbulence outer scale,
δ, is smaller than the smallest scale observed, ℓ_min. We use
optical polarization maps of three intermediate-latitude fields (|b| ≳
7.^∘5) with dimensions of 0.^∘3 × 0.^∘3,
sourced from the Interstellar Polarization Survey–General ISM (IPS-GI)
catalog. We decomposed the HI emission detected by the Galactic All-Sky Survey
(GASS) within our fields to estimate the multiphase ISM properties associated
with the structure coupled to the magnetic field. We produced maps of the
plane-of-sky magnetic field strength (B_pos), mass density (ρ), and
turbulent velocity dispersion (σ_v,turb). In the regions with
well-defined structures at d<400 pc, the average B_pos ranges from
∼3 μG to ∼9 μG, depending on the method and physical
properties. In the region where structures extend up to 1000 pc, B_pos
varies from ∼1 μG to ∼3 μG. The results agree with previous
estimations in the local, diffuse ISM. Finally, optical starlight polarization
and thermal dust polarization at 353 GHz consistently reveal a highly regular
plane-of-sky magnetic field orientation unfazed by diffuse dust structures
observed at 12 μm.
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