2002/03/14 by Takenori Nakano, Ryoichi Nishi, Toyoharu Umebayashi · 2 citations
Physics and Astronomy · #Astrophysics and Star Formation Studies #Dust and Plasma Wave Phenomena #Optical properties and cooling technologies in crystalline materials #astro-ph
paper · pdf · doi:10.1086/340587
34 pages including 9 postscript figures, LaTex, accepted by Astrophysical Journal (vol.573, No.1, July 1, 2002)
arxiv created 2002/03/14 · openalex publication_date 2002/07/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/31
We investigate the detailed processes at work in the drift of magnetic fields in molecular clouds. To the frictional force, whereby the magnetic force is transmitted to neutral molecules, ions contribute more than half only at cloud densities n H ≲ 10 4 cm -3 , and charged grains contribute more than about 90% at n H ≳ 10 6 cm -3 . Thus, grains play a decisive role in the process of magnetic flux loss. Approximating the flux loss time t B by a power law t B ∝ B -γ , where B is the mean field strength in the cloud, we find γ ≈ 2, characteristic of ambipolar diffusion, only at n H ≲ 10 7 cm -3 , at which ions and the smallest grains are pretty well frozen to the magnetic fields. At n H > 10 7 cm -3 , γ decreases steeply with n H , and finally at n H ≈ n dec ≈ a few × 10 11 cm -3 , at which the magnetic fields effectively decouple from the gas, γ ≪ 1 is attained, reminiscent of Ohmic dissipation, although flux loss occurs about 10 times faster than by pure Ohmic dissipation. Because even ions are not very well frozen at n H > 10 7 cm -3 , ions and grains drift slower than the magnetic fields. This insufficient freezing makes t B more and more insensitive to B as n H increases. Ohmic dissipation is dominant only at n H ≳ 1 × 10 12 cm -3 . While ions and electrons drift in the direction of the magnetic force at all densities, grains of opposite charges drift in opposite directions at high densities, at which grains are major contributors to the frictional force. Although magnetic flux loss occurs significantly faster than by Ohmic dissipation even at very high densities, such as n H ≈ n dec , the process going on at high densities is quite different from ambipolar diffusion, in which particles of opposite charges are supposed to drift as one unit.