Far-Ultraviolet Observations of RR Lyrae Stars in the Core of NGC 1851

There are extraordinarily few far-ultraviolet (FUV) observations of RR Lyrae stars in the literature. We present Hubble Space Telescope (HST) FUV (~1600 Å) imaging of the core of the globular cluster NGC 1851. Eleven new variables with light curves consistent with those of RR Lyrae stars are discovered, increasing the total number of RR Lyrae known in this cluster by a substantial amount. In agreement with basic physical theory, the observed amplitude of the variables in the FUV is enormous compared with the century of past optical observations, ranging up to 4 mag. HST Space Telescope Imaging Spectrograph FUV observations of cluster cores may prove an especially effective means of obtaining a near-complete census of RR Lyrae stars, combining high angular resolution, suppression of luminous red stars, and data in which the stellar pulsation amplitudes are greatly enhanced. Attention is also drawn to a peculiar blue object in the cluster, which is most probably a low-mass X-ray binary system in quiescence.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.