Abstract
Abstract In the present study, the magnetic field scaling on density, , was revealed in a single starless core for the first time. The κ index of 0.78 ± 0.10 was obtained toward the starless dense core FeSt 1-457 based on the analysis of the radial distribution of the polarization angle dispersion of background stars measured at the near-infrared wavelengths. The result prefers κ = 2/3 for the case of isotropic contraction, and the difference of the observed value from κ = 1/2 is 2.8 sigma. The distribution of the ratio of mass-to-magnetic flux was evaluated. FeSt 1-457 was found to be magnetically supercritical near the center (λ ≈ 2), whereas nearly critical or slightly subcritical at the core boundary (λ ≈ 0.98). Ambipolar diffusion-regulated star formation models for the case of moderate magnetic field strength may explain the physical status of FeSt 1-457. The mass-to-flux ratio distribution for typical dense cores (critical Bonnor–Ebert sphere with central λ = 2 and κ = 1/2–2/3) was calculated, and found to be magnetically critical/subcritical at the core edge, which indicates that typical dense cores are embedded in and evolve from magnetically critical/subcritical diffuse surrounding medium.
Highlights
Magnetic fields are believed to play an important role in controlling the formation and contraction of dense cores in molecular clouds
The determination of the relationships between the magnetic field strength, ∣B∣, and the gas volume density, ρ, usually expressed in a power-law form as ∣B∣ μ r k, is important because they are related to the accumulation history of both the magnetic flux and the cloud material (e.g., Crutcher 1999)
If an initially uniform magnetic field pervading a diffuse medium is assumed as a starting condition of the mass accumulation to form dense cores, the ∣B∣–ρ relationship of the core depends on (1) the shape of the progenitor cloud, (2) the magnetic field geometry, and (3) the direction of contraction
Summary
Magnetic fields are believed to play an important role in controlling the formation and contraction of dense cores in molecular clouds. Note that the mass-to-flux ratio depends on cloud geometries, and (M F)critical = [3p G 5 ]-1 can be obtained for a uniform sphere under virial equilibrium between gravity and the magnetic field, 3GM 2 5R = B2R3 3 33.7 ± 18.0 μG with a ratio of the observed mass-to-magnetic flux to a critical value of λ = 1.41 ± 0.38 (magnetically supercritical, Paper II) These analyses seem reliable, because observed NIR polarizations of stars show linear relationship with respect to the dust extinction, indicating that magnetic fields inside FeSt 1-457 are traced by the NIR polarimetry (Kandori et al 2018, Paper III). The value of δθint at each radius was obtained based on the Bpos–ρlos relationship (right-hand row of panels) and the Chandrasekhar–Fermi formula dqint = Ccorr (4pr los) sturb Bpos The accuracy of the κ value depends on the number of stars
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