Computer Science
Scientific paper
Jun 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........20m&link_type=abstract
Proquest Dissertations And Theses 2008. Section 0188, Part 0606 129 pages; [Ph.D. dissertation].United States -- New York: Univ
Computer Science
Hydrodynamics, Accretion Disks, Stellar Winds, Planet Structure, Solar Neighborhood, Oort Constants
Scientific paper
Using particle integrations of more than a million particles we explore the effect of spiral arms and the Galactic bar on the velocity distribution in the Milky Way disk. In this thesis we explore four topics:
(1) Heating due to multiple spiral patterns: We find that stochastic heating (increased random motions with time) is induced by interactions between two steady state spiral density waves moving at different pattern speeds. We examine a range of spiral strengths and spiral speeds and show that stars in this time dependent gravitational field can be heated. This is a new mechanism for increasing the stellar velocity dispersion in galactic disks. If multiple spiral patterns are present in the Galaxy we predict that there should be large variations in the stellar velocity dispersion as a function of radius.
(2) The effect of spiral structure perturbations on Oort constant measurements: We find that a two-armed spiral density wave affects measurements of the Oort constants introducing systematic errors in A and B of order 5 km s -1 kpc -1 . Oort's | C |, on average, is larger for lower stellar velocity dispersions, contrary to recent measurements (Olling & Dehnen 2003). We conclude that spiral structure alone cannot account for this observation. This discrepancy is resolved by considering the effect of the Galactic bar on the local stellar velocity distribution (see (3)).
(3) The effect of the Galactic bar on Oort's C -value: Previous work has related the Galactic bar to structure in the local stellar velocity distribution. We show that the bar also influences the spatial gradients of the velocity vector via the Oort constants. By numerical integration of test- particles we simulate measurements of the Oort C -value in a gravitational potential including the Galactic bar. We account for the observed trend that C is increasingly negative for stars with higher velocity dispersion. By comparing measurements of C with our simulations we improve on previous models of the bar, estimating that its pattern speed is O b /O 0 = 1.87 ± 0.04, where O 0 is the local circular frequency, and the bar angle lies within 20 ° [Special characters omitted.] [straight phi] 0 [Special characters omitted.] 45°. We find that the Galactic bar causes biases in measurements of the Oort constants A and B less than ~ 2 km s -1 kpc -1 for the hot stars.
(4) Signatures of spiral structure in Galactic pencil-beam and large-scale radial velocity surveys: By simulating pencil-beam and all-sky Galactic radial velocity surveys we show how the statistical kinematic properties of stars can be used to infer spiral structure parameters. Spiral pattern speeds can be estimated from the location of rings of increased velocity dispersion caused by Lindblad resonances. This information, however, must be combined with information related to the velocities and stellar number density in order to distinguish from a four-armed structure. The mean line-of-sight velocity and velocity dispersion are affected by up to ~ 35 km s -1 which is well within the detectable limit for forthcoming radial velocity surveys. If the pattern speed is such that the 2:1 inner Lindblad resonance is hidden inside the Galactic bulge, the 2:1 outer Lindblad resonance will be present in the outer Galaxy and thus can equivalently be used to estimate the pattern speed. Once the pattern speed is known, the solar angle with respect to spiral structure can be estimated from the line-of-sight velocities and the number density. Forthcoming radial velocity surveys are likely to provide powerful constraints on the structure of the Milky Way disk.
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