The MACHO Project Large Magellanic Cloud Variable Star Inventory. III. Multimode RR Lyrae Stars, Distance to the Large Magellanic Cloud, and Age of the Oldest Stars
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abstract
We report the discovery of 73 double-mode RR Lyrae (RRd) stars in fields near
the bar of the LMC. The stars are detected among the MACHO database of
short-period variables that currently contains about 7900 RR Lyrae stars.
Fundamental periods (P_0) for these stars are found in the range 0.46-0.55 days
and first overtone-to-fundamental period ratios are found to be in the range
0.742 < P_1/P_0 < 0.748. A significant fraction of our current sample have
period ratios smaller than any previously discovered RRd variables. We present
mean magnitudes, colors, and lightcurve properties for all LMC RRd stars
detected to date. The range in period ratios is unexpectedly large.
We present a determination of absolute magnitudes for these stars based
primarily on pulsation theory and the assumption that all observed stars are at
the fundamental blue edge (FBE) of the instability strip. Comparison of the
calibrated MACHO V and R_KC photometry with these derived absolute magnitudes
yields an absorption-corrected distance modulus to the LMC of 18.57 +/- 0.19
mag which is in good agreement with that found (18.5) through comparison of
galactic and LMC Cepheids.
Adopting this luminosity calibration, we derive an increase in the distance
modulus, and thus a reduction in the age found via isochrone fitting for M15 of
about 33% and discuss the implications for cosmology.
