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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 [3736Jump To The Next Citation Point174199202201228Jump To The Next Citation Point93]. Spectral identification of white dwarfs in globular clusters has begun both from the ground with the VLT [162163] and in space with the Hubble Space Telescope [36Jump To The Next Citation Point56Jump To The Next Citation Point228Jump To The Next Citation Point164]. With spectral identification, it will be possible to identify those white dwarfs in hard binaries through Doppler shifts in the H β 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 [16655Jump To The Next Citation Point128] 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 [173], NGC 6624 [216Jump To The Next Citation Point], M15 [214Jump 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 [211213]. 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 [2102122162145].

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 H α emission [59Jump To The Next Citation Point87133Jump To The Next Citation Point13450Jump To The Next Citation Point] from the accretion disk. This technique has resulted in the discovery of candidate CVs in 47 Tuc [59133], M92 [61], NGC 2808 [50], NGC 6397 [36Jump To The Next Citation Point56Jump To The Next Citation Point228], and NGC 6712 [60]. The accretion disk can also be discerned by very soft X-ray emissions. These low luminosity X-ray binaries are characterized by a luminosity L < 1034.5 erg∕s X, 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 [9], and a significant fraction of them are probably CVs [234Jump To The Next Citation Point237Jump To The Next Citation Point]. There have been 10 identified candidate CVs in 6752 [182Jump To The Next Citation Point], 19 in 6440 [183Jump To The Next Citation Point], 2 in ω Cen [75Jump To The Next Citation Point], 5 in Terzan 5 [100Jump To The Next Citation Point], 22 in 47 Tuc [54Jump To The Next Citation Point], 5 in M80 [101Jump To The Next Citation Point], 7 in M54 [193], 2–5 in NGC 288 [136], 4 in M30 [146], 4 in NGC 2808 [209], and 1 in M4 [14Jump To The Next Citation Point].UpdateJump To The Next Update Information However, some of the more energetic sources may be LMXBs in quiescence [234Jump 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. [52] and may be the best source for confirmed CVs in globular clusters.

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