On the nature of low-mass companions to solar-like stars

Low-mass companions (mass <70 Jupiter masses) to solar-like stars are compared statistically to stellar-mass secondaries in binaries of similar primary spectral types and orbital scales, based largely on the survey of Duquennoy & Mayor. To within the limits imposed by observational constraints, the orbital properties of these low-mass companions (LMCs) are statistically indistinguishable from those of binary systems. In both populations, orbital periods (P), semimajor axes (a), angular momenta (L), and binding energies (U) are all distributed approximately as f(x) proportional to x(-1) for x = P, a, L, U. In both populations, eccentricities are broadly distributed approximately as f(e) proportional to e(-0.5), with no significant correlation with other orbital elements, apart from a marked circularization of close orbits. The distribution of LMC masses is approximately a power law with index between -1 and -2; there is ambiguous evidence in the data for a mass spectrum bimodality about approximately 10 Jupiter masses. In both populations the joint distributions of mass with all orbital properties are largely scattergrams, with no statistically significant correlations. The overall statistical properties of LMCs are suggestive of a common formation mechanism with binary star systems. The similar form of the distributions of all orbital dynamic quantities in both populations may result from postformation dissipative orbital decay.