Search for sub-solar mass ultracompact binaries in Advanced LIGO's
  second observing run
  We present an Advanced LIGO and Advanced Virgo search for sub-solar mass
ultracompact objects in data obtained during Advanced LIGO's second observing
run. In contrast to a previous search of Advanced LIGO data from the first
observing run, this search includes the effects of component spin on the
gravitational waveform. We identify no viable gravitational wave candidates
consistent with sub-solar mass ultracompact binaries with at least one
component between 0.2 - 1.0 solar masses. We use the null result to constrain
the binary merger rate of (0.2 solar mass, 0.2 solar mass) binaries to be less
than 3.7 x 10^5 Gpc^-3 yr^-1 and the binary merger rate of (1.0 solar mass, 1.0
solar mass) binaries to be less than 5.2 x 10^3 Gpc^-3 yr^-1. Sub-solar mass
ultracompact objects are not expected to form via known stellar evolution
channels, though it has been suggested that primordial density fluctuations or
particle dark matter with cooling mechanisms and/or nuclear interactions could
form black holes with sub-solar masses. Assuming a particular primordial black
hole formation model, we constrain a population of merging 0.2 solar mass black
holes to account for less than 16% of the dark matter density and a population
of merging 1.0 solar mass black holes to account for less than 2% of the dark
matter density. We discuss how constraints on the merger rate and dark matter
fraction may be extended to arbitrary black hole population models that predict
sub-solar mass binaries.
