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Let us consider an accretion disc around a star. We denote the aspect ratio of the disc by , which is also the ratio between the radial and Keplerian velocities . As the gas is moving inward, it has to overcome the magnetic pressure from the host star. We denote the magntic dipole moment of the star by , and the critical distance , then the magnetic field there is and the magnetic pressure is . The density of the gas at the critical radius is , and the ram pressure is . Equating the magnetic and ram pressure yields the critical radius

where is the Keplerian velocity, is the constant of gravity and is the mass of the host star. The disc cannot extend to radii smaller than .

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