Determination method of binary fractions by the integrated spectrum

Abstract: We need to resolve the individual stars for binary fraction determinations of stellar systems. Therefore, it is not possible to obtain the binary fractions for dense or distant stellar systems. % We proposed a method to determine the binary fraction of a dense or distant stellar system. The method is to first determine the binary fraction variation for any two adjacent regions and then add up those binary fraction variations along the radial direction to obtain the binary fraction for a stellar system. Binary fraction variation is derived by using ten binary fraction-sensitive spectral absorption feature indices (SAFIs) and the binary fraction variation calibrations in terms of these SAFIs. Using this method, we first presented the binary fraction variations for twenty-one Galactic globular clusters (GCs). By comparisons, we find that they agree well with the binary fractions based on the main-sequence fiducial line method by previous studies. This verifies that the above mentioned method is feasible. Next, we presented the binary fraction variations of thirteen Galactic GCs. We gave the relationships between binary fraction and various parameters, and found that binary fraction is negatively correlated with NHB and NRR, binary fraction of some studies is not strongly correlated with NBS, and the number of GCs with large binary fraction is greater at extreme blue horizontal branch population ratio. At last, if we want to obtain more accurate binary fraction, we suggest that the spectroscopic and photometric observations are conducted at an appropriate area interval for a stellar system.