RR Lyrae Variables in the Globular Cluster M15

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Session 123 -- Intrinsic Variables and Active Stars
Oral presentation, Saturday, January 15, 2:15-3:45, Salon V Room (Crystal Gateway)

[123.01] RR Lyrae Variables in the Globular Cluster M15

N.A. Silbermann (Michigan State University)

The RR Lyrae variable stars in the globular cluster M15 afford us our best opportunity for confronting stellar pulsation and evolutionary theory with observations for Oosterhoff type II systems. Photometry spanning almost 100 years now exists for many of the RR Lyrae variables in M15. New 1991-1993 CCD data for M15 have been obtained and are used to construct new B, V and R lightcurves for the variables. Phase diagrams for 40 of the RR Lyrae have been contructed to study the evolution of the variables over the past 100 years. We find an overabundance of positive period changes for this globular cluster. This suggests that most RR Lyrae in M15 are evolving to the red during the final days of their horizontal branch lives. The new CCD data are used to construct a color-magnitude diagram of M15, including the RR Lyrae stars. This color-magnitude diagram will be used to address current controversies regarding the structure of the instability strip; in particular, the location of the division between RRab and RRc variables and whether or not these regions overlap.

Saturday program listing