The MACHO Project Large Magellanic Cloud variable star inventory .3. Multimode RR Lyrae stars, distance to the large magellanic cloud, and age of the oldest stars

We report the discovery of 73 double-mode RR Lyrae (RRd) stars in fields near the bar of the Large Magellanic Cloud (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 0.45-0.55 days, and first-overtone-to-fundamental period ratios are found to be in the range 0.742 < P-1/P-0 < 0.748. The range in period ratios is unexpectedly large, and a significant fraction of our current sample has period ratios smaller than any previously discovered RRd variables. We present mean magnitudes, colors, and light-curve properties for all LMC RRd stars detected to date. We undertake a determination of the absolute magnitudes for these stars based primarily on pulsation theory and on the assumption that all observed stars are at the fundamental blue edge 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 18.48 +/- 0.19 mag that is in good agreement with that found (18.5) through comparison of Galactic and LMC Cepheids. Exploring 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 30% and discuss some implications for cosmology.