Magnetohydrodynamic Dissipation

Extract

Before entering the subject of magnetohydrodynamic dissipation we comment briefly on the question of the supply of kinetic energy to the interstellar gas. This topic has been considered in the 1953 Symposium by Schlüter and myself, and also by Oort. As had been pointed out already by Spitzer in Paris, 1949, the visible HII regions, owing to their excess pressure as compared with the HI regions and the dilute HII regions, must be assumed to expand with a velocity of the order of 10 or 20 km/sec. By this expansion part of the radiation energy of the star is converted into kinetic energy. It was estimated (p. 153 of the proceedings) that a typical HII region around a BO star feeds 10³⁵ ergs/sec to the instellar gas, and that the number of these regions is such, that each region has to provide kinetic energy on the average to 10³⁶−10³⁷ g of interstellar material. Thus a value of 10⁻²−10⁻¹ erg g⁻¹ sec⁻¹ was found (which corresponds to 10⁻²⁶−10⁻²⁵ erg cm⁻³ sec⁻¹, assuming 10⁻²⁴ g/cm³ for the mean density of the interstellar material in the disk).