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Project 10: "Focus on RR Lyrae Stars--The Anatomy of the H-R Diagram"
by Barrett Duff
Level: Advanced
Requirements: Filter Wheel, Photometry Software, Light Curves
Objective
Measure the periods of as many RR Lyrae variables as possible in a given cluster and plot their light curves. M3, M5 and M15 are good examples of clusters containing RR Lyrae variables. Compare the maxima of the periods measured in Julian time with those reported in the literature for prior epochs for the same variable. Data obtained from this study is publishable!
Background
Astronomers study RR Lyrae variables to understand the evolution of stars from the Main Sequence of the Hertzsprung-Russell diagram to the red giant branch, then along the horizontal branch in the blue direction. Many observations over a long period of time are required, thus schools interested in this project should schedule two consecutive all-night sessions on the telescope, taking images of the selected cluster every 30 minutes. The data can be reduced over some period of time in the classroom for comparison with historical data. A second observing cycle can be repeated a year later.
Discussion of Work
The cluster to be studied should be close to the meridian at midnight to obtain the best results. In September-October, for example, M15 is in a good position. M3 and M5 are in good position in late Spring. Images through the V filter should be taken every 30 minutes to ensure good data for the light curve. RR Lyre variables are about 16th magnitude and vary about 1 magnitude over a period of 7 to 15 hours. Exposure times of 3 minutes or longer will be required to obtain a good signal to noise ratio required for accurate photometry. Flat field correction of the images is essential.
The RR Lyrae-rich clusters typically contain 50 or more identifiable variables that can be measured and listed from the images by a suitable software program. Plotting the light curve for each variable will determine the time of maximum intensity. The hours between maxima is the period of the variable. Time intervals between maxima on different nights divided by the number of intervening cycles gives an accurate value of the period which can be compared to literature values dating back to the early part of the century. It is the change in this period, or lack of change, that is of great interest to astronomers in the study of stellar evolution.
It would also be interesting to take two or three images each night through the I filter so that the color indexes of the variables can be calculated. See Projects 6 and 7. The I filter images would be taken in the interval between two V images and would require a flat field image in the same time interval. Thus, a nights' imaging schedule would include about 12 V images, 3 I images, 4 V flat fields and 3 I flat fields. Several classroom or extra-curriculum hours would be required to reduce the data. The data work could be distributed among the students with each one or two students being assigned an image to process. A couple of classroom sessions could be dedicated to orienting the students on the project and to present or review the basics of the Hertzsprung-Russell diagram.
References
This project was suggested by Allan Sandage of the Carnegie Observatories.
Arp,
H., Color-Magnitude diagrams for seven globular clusters, AJ 60, 317-37(1955)
Reid, Neill, The M5 RR Lyre population, Mon. Not. R. Astron. Soc. 278, 367-394(1996)
Sandage, Allan, Globular Clusters M3, M13, M15 and M92, ApJ 162, 841(1970)
Sandage A., Katem B. and Sandage M., ApJ Sup 46, 41-74(1981)
Silbermann N., and Smith H., Period changes of RR Lyre stars in M15, AJ 109,
1119(1995)
Smith H. and Sandage A., Study of period changes of 38 variables in M15, AJ
86,1870(1981)
Note: these references cover M15 and provide some data on other variables. Additional references can be found in the papers themselves.
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