Chandrasekhar–Fermi method

{{Short description|Astrophysics equation}}

Chandrasekhar–Fermi method or CF method or Davis–Chandrasekhar–Fermi method is a method that is used to calculate the mean strength of the interstellar magnetic field that is projected on the plane of the sky. The method was described by Leverett Davis Jr in 1951{{cite journal |last1=Davis |first1=Leverett |title=The Strength of Interstellar Magnetic Fields |journal=Physical Review |date=1 March 1951 |volume=81 |issue=5 |pages=890–891 |doi=10.1103/PhysRev.81.890.2}}{{cite journal |last1=Davis |first1=Leverett Jr. |last2=Greenstein |first2=Jesse L. |title=The Polarization of Starlight by Aligned Dust Grains. |journal=The Astrophysical Journal |date=September 1951 |volume=114 |page=206 |doi=10.1086/145464|doi-access=free }} and independently by Subrahmanyan Chandrasekhar and Enrico Fermi in 1953.{{cite journal |last1=Chandrasekhar |first1=S. |last2=Fermi |first2=E. |title=Magnetic Fields in Spiral Arms. |journal=The Astrophysical Journal |date=July 1953 |volume=118 |page=113 |doi=10.1086/145731|doi-access=free }} According to this method, the magnetic field $B$ in the plane of the sky is given by

:$B\; =\; Q\backslash sqrt\{4\backslash pi\backslash rho\}\; \backslash frac\{\backslash delta\; v\}\{\backslash delta\; \backslash phi\}$

where $\backslash rho$ is the mass density, $\backslash delta\; v$ is the line-of-sight velocity dispersion and $\backslash delta\; \backslash phi$ is the dispersion of polarization angles and $Q$ is an order unity factor, which is typically taken it to be $Q\backslash approx\; 0.5$.{{cite journal |last1=Heitsch |first1=Fabian |last2=Zweibel |first2=Ellen G. |last3=Mac Low |first3=Mordecai-Mark |last4=Li |first4=Pakshing |last5=Norman |first5=Michael L. |title=Magnetic Field Diagnostics Based on Far-Infrared Polarimetry: Tests Using Numerical Simulations |journal=The Astrophysical Journal |date=10 November 2001 |volume=561 |issue=2 |pages=800–814 |doi=10.1086/323489|arxiv=astro-ph/0103286 }}{{cite journal |last1=Padoan |first1=Paolo |last2=Goodman |first2=Alyssa |last3=Draine |first3=B. T. |last4=Juvela |first4=Mika |last5=Nordlund |first5=Ake |last6=Rognvaldsson |first6=Ornolfur Einar |title=Theoretical Models of Polarized Dust Emission from Protostellar Cores |journal=The Astrophysical Journal |date=October 2001 |volume=559 |issue=2 |pages=1005–1018 |doi=10.1086/322504|doi-access=free |arxiv=astro-ph/0104231 }}{{cite journal |last1=Ostriker |first1=Eve C. |last2=Stone |first2=James M. |last3=Gammie |first3=Charles F. |title=Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models |journal=The Astrophysical Journal |date=10 January 2001 |volume=546 |issue=2 |pages=980–1005 |doi=10.1086/318290|arxiv=astro-ph/0008454 }} The method is also employed for prestellar molecular clouds.{{cite journal |last1=Crutcher |first1=Richard M. |last2=Nutter |first2=D. J. |last3=Ward-Thompson |first3=D. |last4=Kirk |first4=J. M. |title=SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores |journal=The Astrophysical Journal |date=January 2004 |volume=600 |issue=1 |pages=279–285 |doi=10.1086/379705|doi-access=free }}