New Analysis RR Lyrae Kinematics in the Solar Neighborhood

GOOD NEWS June 17, 1998 the Astronomical Journal has accepted the paper detailing this research for publication.

For those with a more scientific interest in my work, check out the preprint available.

What's it all about?

Purpose

We plan to compute full space velocities for a sample of RR Lyrae variables within about 6000 light-years of the Sun. By looking at the average motions and velocity dispersions of our sample we can learn about the structure and motion of stars in our galaxy as a whole. We expect to learn about the motion of stars in the disk and local halo of our galaxy from analysis of our sample. We are also going to combine this with data on the metallicity (chemical composition) of the RR Lyraes to learn about the way the galaxy has evolved over time.

To learn more about RR Lyrae stars click here.

To learn more about how we compute the velocity of stars click here.

To learn more about the general structure of our galaxy, the Milky Way, click here.

To learn what "kinematics" are click here.

To learn more about how metallicity (chemical composition) tells us about the past history of the galaxy click here.

Improvements over past work

The largest errors involved in the calculation of the space velocities of RR Lyraes are associated with the proper motions used. We have improved the errors in proper motions by using data from high precision astrometric catalogs and by averaging independent determinations of the proper motion from two or more catalogs. In this manner we will reduce the errors in the computed space velocities by a factor of two or better in many cases.

Progress

Improving Proper Motion Errors

Here is a table showing the improvements to the proper motion errors and errors in the over all space velocity after calculating the mean weighted by the inverse variance of the independent measures.

By this method we have reduced the errors in the computed space velocities by about 50% on average. These reduced errors will yield more accurate kinematic results.

Other Results

• The kinematics of the halo and thick disk samples of RR Lyraes are consistent with those found in previous studies
• The kinematics of the thick disk are a bit "squished" (too small) along the z-axis. We think that this means there are thin disk RR Lyraes in this sample because the thin disk population has a smaller velocity dispersion in the Z direction than the thick disk population. We are however unable to isolate a sample of thin disk RR Lyraes for study.
• We have isolated a sample of 21 RR Lyraes that have disk-like kinematics but halo-like metallicities. This sample is part of the metal-weak tail of the thick disk.
• The halo, instead of appearing kinematically smooth appears to have some lumpiness to it. This is an unexpected result that is seen in our data because we have improved the velocity errors and now have the "resolution" to see the lumps. This has implications for the formation of our galaxy. We feel that further study of this is needed before any conclusions can be drawn.
• If we re-reduce our data using the results from other work that used the Hipparcos satellite data to redefine the brightness of RR Lyraes, then we find that all the kinematics we calculate are outside the range of those accepted by other independent studies. This leads us to believe that fainter brightness of RR Lyraes such as those determined by statistical parallax is more consistent with our understanding of the structure of the Milky Way galaxy.

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