RR Lyrae Variables in the Ultraviolet: The View from GALEX

We present well-sampled ultraviolet (UV) light curves of seven RR Lyrae stars (SW Aqr, HL Her, SDSS J100133.3 + 014328.1, SDSS J104844.1 + 581539.4, SDSS J105513.7 + 564747.0, SDSS J105622.2 + 570520.6, and USNO-B1.0 1024-0010686) serendipitously obtained with the NASA GALEX satellite that show changes of up to AB = 6.5 mag in the far-ultraviolet (FUV; 1350-1800 angstrom) wavelength band. For the test case SW Aqr, we combine the GALEX FUV and NUV (1800-2800 angstrom) observations with visible U, B, and V photometry to determine the variation of physical parameters such as temperature, surface gravity, and radius during the RR Lyrae pulsation cycle of 0.459299 days. Assuming that SW Aqr changes temperature from 6100 K to 8400 K over this cycle, we find that the FUV and NUV light curves can be well matched using the 2004 stellar atmosphere models of Castelli and Kurucz (CK) and a best-fit metallicity of [Fe/H] = -1.5. Pulsation periods are also found for the remaining six stars from their UV light curves, and we have shown that the UV data are consistent with the predictions of CK models using an assumed temperature variation of 6100 K to 8000 K and a log g variation of 2.15 to 3.75 over the pulsation cycle for assumed metallicity values of -2.0 < [Fe/H] < 0.0. Our main finding from the present analysis is that CK models can be successfully used to predict the light-curve behavior of RR Lyrae stars at UV wavelengths if the minimum and maximum temperatures are known from (preferably) visible light-curve observations, such that the stellar metallicity values can be derived. Finally, we note that the model fits to the GALEX data are quite sensitive to the light curves around minimum light, such that the best-fit metallicity is well constrained over this region of the pulsation cycle.