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set6-f7 - formation of the galactic disk also required the...

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Astro 405/505, fall semester 2007 Homework, 6th set, return before Friday, October 19, 4pm. Don’t forget to give your name. Problem 11: Accretion disks In class we studied the properties of accretion disks and their radiation spectra. In the following we want to investigate accretion disks around a forming star ( M = 1 M = 2 · 10 33 gr . , R = R = 7 · 10 10 cm), a neutron star ( M = 1 M , R = R = 10 6 cm), and a massive black hole ( M = 10 8 M , R = R s = 3 · 10 13 cm) a) Calculate the mass accretion rate based on the assumption that the objects in question have obtained all their mass via steady-state accretion over a period of 10 9 years. b) Assume a Keplerian disk with an inner radius of ten times the radius of the central object. Calculate the frequency of peak emission and make an estimate for the total emitted power for face-on conditions ( cos θ = 1 ). Problem 12: Galaxy formation Spiral galaxies like the Milky Way have the same basic geometry as an accretion disk, and the
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Unformatted text preview: formation of the galactic disk also required the outward transport of angular momentum by, e.g., anomalous viscosity, even though galaxies are not gravitationally dominated by a central object. So early in the evolution of the Milky Way disk material must have Fowed in, i.e. the radial mass Fux ˙ M r 6 = 0 . a) Estimate the average radial Fow velocity required to transport material from about 200 kpc radius to the inner Galaxy within the lifetime of the Universe ( ∼ 4 · 10 17 s). b) In the Milky Way we do not observe a signi±cant radial Fow, on average v r . 1 km/s. Compare with your answer to a) and discuss what processes may cause the small value of the radial Fow velocity measured today....
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