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Session 97 - Galactic Structure, Galactic Center.
Oral session, Friday, January 09

[97.02] The case for a leaner Milky Way

R. P. Olling, M. R. Merrifield (Southampton)

We present two new and entirely independent methods to determine our distance to the Galactic center (R_0), and the rotation speed at the Solar circle (\Theta_0). Both methods indicate that the Galaxy rotates more slowly and is smaller than currently accepted: R_0 \sim7.1 kpc and \Theta_0 \sim184 km s^-1. As a bonus we find that Milky Way's dark matter (DM) distribution is rather round, with an axial ratio of q= 0.75 \pm 0.25. Declining rotation curves (RC) are consistent with kinematical constraints for small R_0's and \Theta_0's. Likewise are rising RCs, which have large dark matter densities (\rho_DM), R_0's, and \Theta_0's. Further, flattened halos have larger \rho_DM's. We use two methods which are sensitive to \rho_DM, each a different dependence on the mass of the stellar disk (\Sigma_*). The thickness of the H\footnotesizeI\ \normalsize layer is smaller for flat halos, but is independent of the stellar mass at large radii (Olling 1995). In the Solar neighborhood, the total amount of matter within 1.1 kpc from the plane is well determined (Kuijken amp; Gilmore 1989,1991; KG), so that the amount of DM --and hence q-- depends strongly upon the value of \Sigma_*. Self-consistency requires that there is a value of \Sigma_* where both methods yield identical flattening. We can thus simultaneously determine q and \Sigma_* for a large number of Galaxy models. Assuming that the halo is oblate, we find an upper limit for \Theta_0 (\le 191 - 5.8\times(R_0-7)). The observed upper bound for \Sigma_* (KG) implies that R_0 \le 7.6 kpc. For these models, we also calculate the radial variation of the Oort constants A and B. Since the Oort functions vary substantially with radius, model and data have to be compared in the same regions. Hanson (1987) found ``small'' values from stars within \sim1 kpc, while on a \sim4 kpc scale Feast amp; Whitelock (1997) find substantially larger A and B values. These results can be reconciled when including the gravitational effects of a realistic H\footnotesizeI\ \normalsize distribution: it's peaks and troughs cause localized gradients in the RC, and hence A and B. However, only small Galactic constants do the job: R_0 = 7.1 \pm 0.4 kpc and \Theta_0 = 184 \pm 4 km s^-1.

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