All Quiet in Globular Clusters

Abstract

Cataclysmic variables (CVs) should be present in large numbers in globular clusters (GCs). Numerous low-luminosity X-ray sources identified over the past few years as candidate CVs in GCs support this notion. Yet, very few "cataclysms," the characteristic feature of this class of objects in the field, have been observed in GCs. We address this discrepancy here, within the framework of the standard disk instability model for CV outbursts. We argue that the paucity of outbursts in GCs is probably not a direct consequence of the donors' low metallicities. We present diagnostics based on outburst properties allowing tests of the hypothesis that rare cataclysms are entirely due to lower mass transfer rates in GCs relative to the field, and we argue against this explanation. Instead, we propose that a combination of low mass transfer rates (≳1014-1015 g s-1) and moderately strong white dwarf magnetic moments (≳1030 G cm3) stabilize CV disks in GCs and thus prevent most of them from experiencing frequent outbursts. If it is so, rare cataclysms in GCs would signal important evolutionary differences between field and cluster CVs.