The Brown Dwarf Kinematics Project (BDKP). II. Details on Nine Wide\n Common Proper Motion Very Low-Mass Companions to Nearby Stars

Abstract

We report on nine wide common proper motion systems containing late-type M,\nL, or T companions. We confirm six previously reported companions, and identify\nthree new systems. The ages of these systems are determined using diagnostics\nfor both stellar primaries and low--mass secondaries and masses for the\nsecondaries are inferred using evolutionary models. Of our three new\ndiscoveries, the M3+T6.5 pair G 204-39 and SDSS J1758+4633 has an age\nconstrained to 0.5-1.5 Gyr making the secondary a potentially useful brown\ndwarf benchmark. The G5+L4 pair G 200-28 and SDSS J1416+5006 has a projected\nseparation of ~25,000 AU making it one of the widest and lowest binding energy\nsystems known to date. The system containing NLTT 2274 and SDSS J0041+1341 is\nan older M4+L0 (>4.5 Gyr) pair which shows Halpha activity in the secondary but\nnot the primary making it a useful tracer of age/mass/activity trends. We find\na resolved binary frequency for widely-separated (> 100 AU) low--mass\ncompanions (i.e. at least a triple system) which is at least twice the\nfrequency found for the field ultracool dwarf population. The ratio of triples\nto binaries and quadruples to binaries is also high for this sample: 3:5 and\n1:4, respectively, compared to 8-parsec sample values of 1:4 and 1:26. The\nadditional components in these wide companion systems indicates a formation\nmechanism that requires a third or fourth component to maintain gravitational\nstability or facilitate the exchange of angular momentum. The binding energies\nfor the nine multiples discussed in this text are among the lowest known for\nwide low-mass systems, suggesting that weakly bound, low--to--intermediate mass\n(0.2M_sun < M_tot <1.0M_sun) multiples can form and survive to exist in the\nfield (1-8 Gyr).\n