Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8

The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained simultaneous near-ultraviolet (NUV) and far-ultraviolet (FUV) light curves of the ROTSE-I Catalog RR Lyrae type ab variable star J143753.84+345924.8. A series of 38 GALEX Deep Imaging Survey observations well distributed in phase within the star's 0.56432 day period shows an AB = 4.9 mag variation in the FUV (1350-1750 Å) band and an AB = 1.8 mag variation in the NUV (1750-2750 Å) band, compared with only a 0.8 mag variation in the broad, unfiltered ROTSE-I (≈4500-10,000 Å) band. These GALEX UV observations are the first to reveal a large RR Lyrae amplitude variation at wavelengths below 1800 Å. We compare the GALEX and ROTSE-I observations to predictions made by recent Kurucz stellar atmosphere models. We use published physical parameters for the comparable period (0.57433 days), well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree with the Kurucz predictions for [Fe/H] = -1.25 to within AB = 0.2 mag in the GALEX NUV and ROTSE-I bands and to within 0.5 mag in the FUV. At all metallicities between solar and 1/100 solar, the Kurucz models predict 6-8 mag of variation at wavelengths between 1000 and 1700 Å. Other variable stars with similar temperature variations, such as Cepheids, should also have large-amplitude FUV light curves, observable during the ongoing GALEX imaging surveys.