A SURVEY OF THE HIGH ORDER MULTIPLICITY OF NEARBY SOLAR-TYPE BINARY STARS WITH Robo-AO

Abstract

We conducted a survey of nearby binary systems composed of main sequence\nstars of spectral types F and G in order to improve our understanding of the\nhierarchical nature of multiple star systems. Using Robo-AO, the first robotic\nadaptive optics instrument, we collected high angular resolution images with\ndeep and well-defined detection limits in the SDSS $i'$ band. A total of 695\ncomponents belonging to 595 systems were observed. We prioritized observations\nof faint secondary components with separations over $10''$ to quantify the\nstill poorly constrained frequency of their sub-systems. Of the 214 secondaries\nobserved, 39 contain such subsystems; 19 of those were discovered with Robo-AO.\nThe selection-corrected frequency of secondary sub-systems with periods from\n$10^{3.5}$ to $10^5$ days is 0.12$\\pm$0.03, the same as the frequency of such\ncompanions to the primary. Half of the secondary pairs belong to quadruple\nsystems where the primary is also a close pair, showing that the presence of\nsub-systems in both components of the outer binary is correlated. The\nrelatively large abundance of 2+2 quadruple systems is a new finding, and will\nrequire more exploration of the formation mechanism of multiple star systems.\nWe also targeted close binaries with periods less than 100~yr, searching for\ntheir distant tertiary components, and discovered 17 certain and 2 potential\nnew triples. In a sub-sample of 241 close binaries, 71 have additional outer\ncompanions. The overall frequency of tertiary components is not enhanced,\ncompared to all (non-binary) targets, but in the range of outer periods from\n$10^6$ to $10^{7.5}$ days (separations on the order of 500~AU), the frequency\nof tertiary components is 0.16$\\pm$0.03, exceeding by almost a factor of two\nthe frequency of similar systems among all targets (0.09).\n