Exploring the Variable Sky with LINEAR. II. Halo Structure and
  Substructure Traced by RR Lyrae Stars to 30 kpc
  We present a sample of ~5,000 RR Lyrae stars selected from the recalibrated
LINEAR dataset and detected at heliocentric distances between 5 kpc and 30 kpc
over ~8,000 deg^2 of sky. The coordinates and light curve properties, such as
period and Oosterhoff type, are made publicly available. We find evidence for
the Oosterhoff dichotomy among field RR Lyrae stars, with the ratio of the type
II and I subsamples of about 1:4. The number density distribution of halo RRab
stars as a function of galactocentric distance can be described as an oblate
ellipsoid with the axis ratio q=0.63 and with either a single or a double power
law with a power-law index in the range -2 to -3. Using a group-finding
algorithm EnLink, we detected seven candidate halo groups, only one of which is
statistically spurious. Three of these groups are near globular clusters
(M53/NGC 5053, M3, M13), and one is near a known halo substructure (Virgo
Stellar Stream); the remaining three groups do not seem to be near any known
halo substructures or globular clusters, and seem to have a higher ratio of
Oosterhoff type II to Oosterhoff type I RRab stars than what is found in the
halo. The extended morphology and the position (outside the tidal radius) of
some of the groups near globular clusters is suggestive of tidal streams
possibly originating from globular clusters. Spectroscopic followup of detected
halo groups is encouraged.
