AAS 195th Meeting, January 2000
Session 7. Galaxy Structures and Dynamics: Models and Observations
Display, Wednesday, January 12, 2000, 9:20am-6:30pm, Grand Hall

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[7.09] Oort's Constants Measured from the Tycho/ACT Catalogues

R. P. Olling (Rutgers University), W. Dehnen (Max-Planck Institut f. Astronomie, Heidelberg)

The Oort constants describe the local variations of the stellar streaming field. We discuss various problems complicating their determination. A hitherto apparently overlooked, source of potential error arises from longitudinal variations of the mean stellar parallax. Together with the Solar reflex motion, these mode mixing variations contribute to the proper motions that are indistinguishable from Oort's constants at \la20% of their amplitude. We estimate Oort's constants from the ACT catalog (106 stars with median proper motion error of 3\muarcsec/yr). We correct for the mode mixing using the latitudinal proper motions. We find significant variation of the inferred Oort constants with B-V color. We can ascribe these differences to several factors: 1) differing sample depths, or 2) ages, 3) changes in the asymmetric drift, 4) small-scale perturbations in the potential, and 5) the Galactic bar. Without additional information on the spectral type and luminosity class and age of the target stars, we can only guesstimate the ``true'' values of the Oort constants, in most cases. The most reliable tracer stars might be red giants. For these stars we find the following apparent values, in km/s/kpc, A~4.2, B~12.7, A-B~6.9, and C~.7 with internal (external) errors of about 1 (2). Applying a correction due to the asymmetric drift raises A and A-B by about 2.7 km/s/kpc

Our values for A and A-B are somewhat larger than previously found, a fact that we attribute to systematic errors in most previous studies. Moreover, these numbers are consistent with other knowledge of the Milky Way (e.g.\ a locally declining rotation curve and the proper motion of Sgr~A*).

The author(s) of this abstract have provided an email address for comments about the abstract: olling@physics.rugters.edu

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