Cataclysmic and Close Binaries in Star Clusters. IV. The Unexpectedly Low Number of Erupting Dwarf Novae Detected by the Hubble Space Telescope in the Core of 47 Tucanae

Abstract

The standard model of tidal capture predicts that ˜45 moderately bright cataclysmic binaries should exist today in 47 Tuc. To test this prediction, we have conducted a search for erupting dwarf novae in the center of 47 Tuc with 12 separate epochs of Hubble Space Telescope (HST) images in visual through near-UV bands. We have found a second eruption of the dwarf nova V2 discovered by Paresce and De Marchi. In addition, we have discovered a faint blue variable with period ˜4.7 hr and amplitude 1.5 mag, which may be a cataclysmic variable. Detailed simulations demonstrate that our areal and temporal coverage and detection sensitivity is sufficient to have detected one-third of all dwarf novae (via their eruptions) in the center of 47 Tuc; even the very faint U Gem-type objects. We therefore claim that there are probably no more than three dwarf novae in the core of 47 Tuc, in significant disagreement with a key prediction of the standard model of tidal capture, unless the properties of dwarf novae in globulars differ (e.g., in outburst frequency) from those in the field. In addition, the cluster color-magnitude diagram reveals zero novalike variable candidates.