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3.1 Cataclysmic variables

Cataclysmic variables (CVs) are white dwarfs accreting matter from a companion that is usually a dwarf star or another white dwarf. They have been detected in globular clusters through identification of the white dwarf itself or through evidence of the accretion process. White dwarfs managed to avoid detection until observations with the Hubble Space Telescope revealed photometric sequences in several globular clusters [3837Jump To The Next Citation Point172199201200224Jump To The Next Citation Point95]. Spectral identification of white dwarfs in globular clusters has begun both from the ground with the VLT [161162] and in space with the Hubble Space Telescope [37Jump To The Next Citation Point58Jump To The Next Citation Point224Jump To The Next Citation Point163]. With spectral identification, it will be possible to identify those white dwarfs in hard binaries through Doppler shifts in the Hb line. This approach has promise for detecting a large number of the expected double white dwarf binaries in globular clusters. Photometry has also begun to reveal orbital periods [16557Jump To The Next Citation Point129] of CVs in globular clusters.

Accretion onto the white dwarf may eventually lead to a dwarf nova outburst. Identifications of globular cluster CVs have been made through such outbursts in the cores of M5 [152Jump To The Next Citation Point], 47 Tuc [171], NGC 6624 [214Jump To The Next Citation Point], M15 [212Jump To The Next Citation Point], and M22 [5Jump To The Next Citation Point25]. With the exception of V101 in M5 [152], original searches for dwarf novae performed with ground based telescopes proved unsuccessful. This is primarily due to the fact that crowding obscured potential dwarf novae up to several core radii outside the center of the cluster [209211]. Since binaries tend to settle into the core, it is not surprising that none were found significantly outside of the core. Subsequent searches using the improved resolution of the Hubble Space Telescope eventually revealed a few dwarf novae close to the cores of selected globular clusters [2082102142125].

A more productive approach has been to look for direct evidence of the accretion around the white dwarf. This can be in the form of excess UV emission and strong Ha emission [61Jump To The Next Citation Point89134Jump To The Next Citation Point13552Jump To The Next Citation Point] from the accretion disk. This technique has resulted in the discovery of candidate CVs in 47 Tuc [61134], M92 [63], NGC 2808 [52], NGC 6397 [37Jump To The Next Citation Point58Jump To The Next Citation Point224], and NGC 6712 [62]. The accretion disk can also be discerned by very soft X-ray emissions. These low luminosity X-ray binaries are characterized by a luminosity LX < 1034.5 erg/s, which distinguishes them from the low-mass X-ray binaries with 36 LX > 10 erg/s. Initial explanations of these objects focused on accreting white dwarfs [10], and a significant fraction of them are probably CVs [231Jump To The Next Citation Point234Jump To The Next Citation Point]. There have been 10 identified candidate CVs in 6752 [180Jump To The Next Citation Point], 19 in 6440 [181Jump To The Next Citation Point], 2 in w Cen [77Jump To The Next Citation Point], 5 in Terzan 5 [102Jump To The Next Citation Point], 22 in 47 Tuc [56Jump To The Next Citation Point], 5 in M80 [103Jump To The Next Citation Point], and 1 in M4 [14Jump To The Next Citation Point]. However, some of the more energetic sources may be LMXBs in quiescence [231Jump To The Next Citation Point], or even candidate QSO sources [14].

The state of the field at this time is one of rapid change as Chandra results come in and optical counterparts are found for the new X-ray sources. A living catalog of CVs has been created by Downes et al. [53] and may be the best source for confirmed CVs in globular clusters.


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