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Session 11 - Instrumentation & Techniques.
Oral session, Monday, June 09
North Main Hall A,

[11.03] Variability of Yellow Supergiants

R. S. Patterson (SW Missouri St. U.)

During the past three years 58 of 131 yellow supergiant stars selected from the Sky Catalogue 2000.0, Volume 1, Stars to magnitude 8.0 (1982), edited by Hershfeld and Sinnott, have been monitored at the SW Missouri State University Baker Observatory using a Photometrics PM512 liquid-nitrogen cooled CCD detector on a 0.4m Cassegrain reflector. These stars are being studied in order to determine what fraction of yellow supergiants are variable and whether low amplitude variations occur. The technique of CCD differential photometry was used, which required that a comparison star be within five arc minutes of a program star in order to be imaged simultaneously. Each program star was imaged on several nights for a total of at least 10 times with a signal-to-noise ratio of at least 100. Nightly bias and flat images were obtained in order to calibrate the program images. The aperture photometry package in IRAF was used for image analysis. Also, concurrent observations were made of several control pairs of main sequence A and G spectral class binaries not known to be variable. At the 3 \sigma level of the standard deviation of the means of the differences in magnitude for the control pairs: six known variable stars have been recovered, three new suspected variables have been found, and 40 yellow supergiants have been found non-variable within the precision of the measurements. (Nine stars were later determined not to be supergiants.) These 40 non-variable yellow supergiants are distributed liberally across the Cepheid instability strip on the H-R diagram. The three suspected variables have V amplitudes of 0.02 to 0.06 magnitude.

This work is supported by NSF grant AST-9315061.

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