PERIOD LUMINOSITY PERIOD LUMINOSITY RELATIONS FOR RR LYRAE RELATIONS FOR RR LYRAE

STARSSTARSHorace SmithHorace Smith

(Michigan State University)(Michigan State University)Marcio CatelanMarcio Catelan

(Pontificia Universidad Catolica de Chile)(Pontificia Universidad Catolica de Chile)Barton PritzlBarton Pritzl

(Macalester College)(Macalester College)

RR Lyrae starsRR Lyrae stars

Traditional population II distance Traditional population II distance indicatorsindicators

They are on the “horizontal branch” They are on the “horizontal branch” so all RR Lyrae should have the same so all RR Lyrae should have the same absolute magnitude, shouldn’t they?absolute magnitude, shouldn’t they?

RR Lyrae luminositiesRR Lyrae luminosities

What do theoretical period-luminosity What do theoretical period-luminosity relations look like for RR Lyrae stars? relations look like for RR Lyrae stars?

How do the relations depend upon How do the relations depend upon the filter used?the filter used?

How do the theoretical relations How do the theoretical relations compare with observed period-compare with observed period-luminosity relations?luminosity relations?

Why the difference?

• Period increases with increasing luminosity or decreasing temperature

• At B, the effects of the HB slope in Teff and L are nearly orthogonal

• At I, the effects of the HB slope in Teff and L are in the same sense

Results in more filters

How do the P-L relations depend How do the P-L relations depend upon HB morphology?upon HB morphology?

Use the parameter Use the parameter L L to characterizeto characterize

HB morphologyHB morphology

L L = (B – R)/(B + V + R)= (B – R)/(B + V + R)

HB morphology and the B band

V band

I band

K band

Comparison with observations (I band)

Two more clustersTwo more clusters

Still to doStill to do

• Quantitative comparison of observed P-L relations with theory

• Extension to systems beyond our own Galaxy