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# h3 - n z = n exp-z/z h Suppose you do a survey of G2 stars...

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Homework # 3, due 24 Feb 1. Calculate the size of a Stromgren sphere in a gas of density 10 3 cm - 3 sur- rounding an O3 star. This is the volume of gas nearly completely ionized by the star. Assume the recombination rate of protons and electrons to form neutral hydrogen is dn e dt = - 2 × 10 - 13 n e n p ± T 10 4 K ² - 3 / 4 cm - 3 s - 1 . If the star suddenly turned on, estimate how long it would take for the Stromgren sphere to come into radiative equilibrium. Also calculate the size of such a sphere surrounding a solar G2 star. 2. Suppose the number density of stars normal to the Galactic plane varies as
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Unformatted text preview: n ( z ) = n exp(-z/z h ). Suppose you do a survey of G2 stars in the direction of the North Galactic Pole complete to a magnitude m v = 30. What value would you obtain for V / V max ? 3. Compute expressions for the potential and kinetic energies of the singular isothermal sphere. Assume the temperature is 20 K. Suppose the sphere’s boundary is where it’s pressure is the same as that of the warm phase in the ISM. What is the radius of the sphere if its mass is 10 M ± ? What will happen if the sphere cools uniformly?...
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