PERIOD LUMINOSITY RELATIONS FOR RR LYRAE STARS Horace Smith (Michigan State University) Marcio Catelan (Pontificia Universidad Catolica de Chile) Barton Pritzl (Macalester College)

RR Lyrae stars Traditional population II distance indicators Traditional population II distance indicators They are on the “horizontal branch” so all RR Lyrae should have the same absolute magnitude, shouldn’t they? They are on the “horizontal branch” so all RR Lyrae should have the same absolute magnitude, shouldn’t they?

RR Lyrae luminosities What do theoretical period- luminosity relations look like for RR Lyrae stars? What do theoretical period- luminosity relations look like for RR Lyrae stars? How do the relations depend upon the filter used? How do the relations depend upon the filter used? How do the theoretical relations compare with observed period- luminosity relations? How do the theoretical relations compare with observed period- luminosity relations?

Why the difference? Period increases with increasing luminosity or decreasing temperature At B, the effects of the HB slope in T eff and L are nearly orthogonal At I, the effects of the HB slope in T eff and L are in the same sense

Results in more filters

How do the P-L relations depend upon HB morphology? Use the parameter L to characterize Use the parameter L to characterize HB morphology L = (B – R)/(B + V + R)

HB morphology and the B band

V band

I band

K band

Comparison with observations (I band)

Two more clusters

Still to do Quantitative comparison of observed P-L relations with theory Extension to systems beyond our own Galaxy