Properties of Cataclysmic Variables in Globular Clusters

Abstract

The study of star clusters plays an important role in our understanding of the Universe since these systems are natural laboratories for testing theories of stellar dynamics and evolution. Particularly, globular clusters (GCs) are one of the most important objects for studying the formation and the physical nature of exotic systems which in turn provide basic information and tools that can help us to understand the formation and evolution processes of star clusters themselves, galaxies and, in general, the young Universe. Among the most interesting objects in GCs are the cataclysmic variables (CVs), which are interacting binaries harbouring a white dwarf accreting from a lowmass companion. Since GC densities are sufficiently high that dynamical encounters involving binaries should be common, CV progenitors are expected to be affected by dynamics in some way in the early stages. In this article we review the formation channels and the influence of dynamics on the CV population in GCs. In particular, we review recent progress in numerical simulations. Furthermore, we discuss observational properties of CVs in GCs and the techniques used to identify and study them. We focus the discussion on the multi-wavelength observations carried out with HST and Chandra on the best-studied GCs NGC 6397, NGC 6752, 47 Tucanae and ω Centauri; on the recent spectroscopic findings with MUSE, and on updates regarding the correlation between the number of faint X-ray sources and the cluster stellar encounter rate. Finally, we discuss some observational prospects that might potentially help future investigations.

Reevaluating the Origin of Detectable Cataclysmic Variables in Globular Clusters: Testing the Importance of Dynamics

“…measurements that are less precise due to some distant clusters or clusters with a limited number of X-ray sources included in their sample (see Heinke et al 2020;Belloni & Rivera Sandoval 2021, for a deeper discussion on this), it is important to note that in Pooley & Hut (2006), there might be a bias due to small number statistics, which is important to consider when interpreting their correlation with γ.…”

“…Due to crowding, the identification of CVs in globular clusters (GCs) is challenging. Yet, they have been confirmed spectroscopically and mainly studied photometrically in several Galactic GCs (for a review of CVs in GCs, see Belloni & Rivera Sandoval 2021).…”

Discovery of Astrometric Accelerations by Dark Companions in the Globular Cluster ω Centauri

“…None of them are close to the detected astrometric binaries. In globular clusters, the typical X-ray sources are cataclysmic variables (Belloni & Rivera 2021). They are relatively tight binaries composed of a white dwarf or a neutron star and a main-sequence star as donor.…”

Rubin Observatory LSST Transients and Variable Stars Roadmap

“…Due to stellar interactions and their primordial formation, several of them are thought to be in cataclysmic variables. While many cataclysmic variables are expected to be in the cores of clusters due to mass segregation, around 50% of the predicted detectable cataclysmic variables are believed to be outside the half-light radus, particularly in those globular clusters with half-mass relaxation times longer than a few Gyr (see Belloni & Rivera Sandoval 2021, for a review on cataclysmic variables in globular clusters). The wide-field coverage and 10 yr duration of Rubin LSST will allow to us identify dwarf novae in the outskirts of globular clusters through their outbursts.…”

“…Photometry in multiple filters will be helpful to corroborate variability detection and to place the identified systems in the color-magnitude diagram of the host cluster. This will be useful to further investigate the two-population problem of cataclysmic variables in globular clusters (e.g., Cohn et al 2010;Belloni & Rivera Sandoval 2021) (c) Characterization of exotic binaries and pulsating stars Recent studies have shown the presence of several X-ray sources in the outer parts of globular clusters, which indicates binary interactions. Therefore, besides detecting accreting white dwarfs, the 10 yr duration and multicolor photometry of Rubin LSST will help to characterize binaries with neutron stars or even black holes.…”