Contact binaries at the short period cut-off – I. Statistics and the first photometric investigations of 10 totally eclipsing systems

Abstract

The period distribution of contact binaries exhibits a very sharp short\nperiod cut-off at 0.22 days. In order to provide valuable information on this\nshort period limit, we observed ten totally eclipsing contact binaries with\norbital periods near this cut-off. By detailed analysis using W-D code, we\ndetermined that two of these systems are A-subtype contact binaries while the\nothers are W-subtype contact binaries and all the targets show shallow contact\nconfigurations. Half of the targets exhibit stellar spot activity and four of\nthem have third light. A statistical work on well studied USPCBs was carried\nout, and physical parameters of fifty-five USPCBs were obtained. Some common\nproperties for these systems were derived. Due to the study of the period-color\ndiagram of USPCBs, we found that the period-color relation of USPCBs is\ndifferent from other W UMa type contact binaries and USPCBs are metal poor old\nstars. In addition, the evolutionary states of these systems were discussed by\nconstructing the color-density diagram. We derived that the evolutionary states\nof the two components of USPCBs show identical characteristics of other contact\nbinaries despite slower evolutionary status caused by smaller mass. We\nsuggested that both the fully convective limit claimed by Rucinski (1992) and\nthe AML theory recommended by Stepien (2006) can produce the short period\ncut-off and a tertiary companion is very important during the formation of the\nshort period contact binaries by removing angular momentum from the host\neclipsing pairs under certain circumstances.\n