The Distance of the First Overtone RR Lyrae Variables in the MACHO LMC Database: A New Method to Correct for the Effects of Crowding

Previous studies have indicated that many of the RR Lyrae variables in the LMC have properties similar to the ones in the Galactic globular cluster M3. Assuming that the M3 RR Lyrae variables follow the same relationships among period, temperature, amplitude and Fourier phase parameter φ31 as their LMC counterparts, we have used the M3 φ31−log P relation to identify the M3-like unevolved first overtone RR Lyrae variables in 16 MACHO fields near the LMC bar. The temperatures of these variables were calculated from the M3 logP − log Teff relation so that the extinction could be derived for each star separately. Since blended stars have lower amplitudes for a given period, the period-amplitude relation should be a useful tool for determining which stars are affected by crowding. We find that the low amplitude LMC RR1 stars are brighter than the stars that fit the M3 period-amplitude relation and we estimate that at least 40% of the stars are blended. Simulated data for three of the crowded stars illustrate that an unresolved companion with V ∼ 20.5 could account for the observed amplitude and magnitude. We derive a corrected mean apparent magnitude 〈V0〉 = 19.01 ± 0.10 (extinction) ±0.02 (calibration) for the 51 uncrowded unevolved M3-like RR1 variables. Assuming that the unevolved RR1 variables in M3 have a mean absolute magnitude MV = 0.52±0.02 leads to an LMC distance modulus μ = 18.49± 0.11 Subject headings: galaxies: Magellanic clouds distances and redshifts stars: variables: other