Cluster AgeS Experiment (CASE): deficiency of observed dwarf novae in globular clusters

Abstract

We present the results of a search for dwarf novae (DNe) in globular clusters (GCs). It is based on the largest available homogeneous sample of observations, in terms of the time‐span, number of observations and number of clusters. It includes 16 Galactic GCs and yielded two new certain DNe: M55‐CV1 and M22‐CV2. All previously known systems located in our fields were recovered, too. We surveyed M4, M5, M10, M12, M22, M30, M55, NGC 288, NGC 362, NGC 2808, NGC 3201, NGC 4372, NGC 6362, NGC 6752, ω Centauri (NGC 5139) and 47 Tucanae (NGC 104). The discovery of two DNe, namely M55‐CV1 and M22‐CV2, was already reported by Kaluzny et al. and Pietrukowicz et al., respectively. In the remaining 14 GCs, we found no certain new DNe. Our result raises the total number of known DNe in the Galactic GCs to 12 DNe, distributed among seven clusters. Our survey recovered all three already known erupting cataclysmic variables (CVs) located in our fields, namely M5‐V101, M22‐CV1, and V4 in the foreground of M30. To assess the efficiency of the survey, we analysed images with inserted artificial stars mimicking outbursts of the prototype DNe SS Cygni and U Geminorum. Depending on the conditions, we recovered between 16–100 per cent of these artificial stars. The efficiency seems to be predominantly affected by duty cycle/time‐sampling and much less by distance/magnitude. Except for saturated tiny collapsed cores of M30, NGC 362 and NGC 6752 (and also the dense core of NGC 2808), crowding effects in the V band were avoided by our image subtraction technique augmented with auxiliary unsaturated B‐band images. Our results clearly demonstrate that in GCs common types of DNe are very rare indeed. However, great care must be taken before these conclusions can be extended to the CV population in GCs.

Relativistic Binaries in Globular Clusters

“…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 [424, 426, 431, 428, 16]. To date, there have been 14 found [374, 423], using the Hubble Space Telescope and Las Campanas Observatory (CASE).…”

Ground- and space-based study of two globular cluster cataclysmic variables: M22 CV1 and M5 V101

“…So far, relatively few DN outbursts of GC CVs have been observed (e.g. Shara et al 1996; Ó Tuairisg et al 2003; Pietrukowicz et al 2008) compared to the predicted populations of such systems. Ivanova et al (2006) predict from numerical simulations that the number of CVs per unit mass should be 2–3 times higher in GC cores than in the field.…”

A Dwarf Nova in the Globular Cluster M13

“…There have been few detections of DNe in GCs, with only 13 DNe reported so far in the 157 Galactic GCs (e.g., Pietrukowicz et al 2008;Kaluzny & Thompson 2009). Based on our knowledge of field CVs, which is extremely biased, it is tempting to try to estimate the expected number of DN outbursts we should have detected in M13, for example following Shara et al (1996).…”

“…(2) The lack of DN outbursts from CVs in GCs could be due to detection biases. Due to the crowding in optical, DN outbursts are indeed more difficult to catch in GCs (Pietrukowicz et al 2008). These questions can only be solved with further observations and comparisons with more complete populations of CVs, both in the field and in GCs.…”