4. Statistical parallax
Using the data listed in Table 1 (proper motions, V magnitudes, reddenings and radial velocities) and the program described by Hawley et al. (1986) we obtained the solutions shown in Table 2. It is important in the Statistical Parallax method to isolate a dynamically homogeneous sample of stars. In the present context this means separating the Halo and Old Disk components and we have done this by making a cut in metallicity. Based on previous work (eg Layden et al. 1996, Fig 4) it is clear that below [Fe/H] = -1.3 the stars are almost entirely Halo and above [Fe/H] = -0.8 they are almost entirely Old Disk. Unfortunately there are insufficient stars with [Fe/H] -0.8 to obtain a useful solution and so we have run instead a metal-rich solution which contains all stars with [Fe/H] -1.3. This sample is therefore not dynamically homogeneous in that it contains a mixture of both Old Disk and Halo stars. Similarly the first sample in Table 2, all RR Lyraes, and hence our preferred solution is that for the Halo RR Lyraes.
Table 2. Absolute magnitudes from statistical parallaxes
These results are very close to those from previous studies which used ground based proper motions (Hawley et al. 1986, Layden et al. 1996, Strugnell et al. 1986). We show in Fig.1 a comparison of the HIPPARCOS and ground-based proper motions, where the latter are taken from the paper by Layden et al. (1996) and are primarily from the Lick Northern Proper Motion Survey (Klemola et al. 1987). Whilst the HIPPARCOS errors are smaller, typically 2.2 mas/yr as compared to 5.6 mas/yr for the ground-based data, it can be seen that the overall agreement is good.
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998