Machine-Learned Identification of RR Lyrae Stars from Sparse, Multi-band
  Data: the PS1 Sample
  RR Lyrae stars may be the best practical tracers of Galactic halo
(sub-)structure and kinematics. The PanSTARRS1 (PS1) $3\pi$ survey offers
multi-band, multi-epoch, precise photometry across much of the sky, but a
robust identification of RR Lyrae stars in this data set poses a challenge,
given PS1's sparse, asynchronous multi-band light curves ($\lesssim 12$ epochs
in each of five bands, taken over a 4.5-year period). We present a novel
template fitting technique that uses well-defined and physically motivated
multi-band light curves of RR Lyrae stars, and demonstrate that we get accurate
period estimates, precise to 2~sec in $>80\%$ of cases. We augment these light
curve fits with other {\em features} from photometric time-series and provide
them to progressively more detailed machine-learned classification models. From
these models we are able to select the widest ($3/4$ of the sky) and deepest
(reaching 120 kpc) sample of RR Lyrae stars to date. The PS1 sample of $\sim
45,000$ RRab stars is pure (90\%), and complete (80\% at 80 kpc) at high
galactic latitudes. It also provides distances precise to 3\%, measured with
newly derived period-luminosity relations for optical/near-infrared PS1 bands.
With the addition of proper motions from {\em Gaia} and radial velocity
measurements from multi-object spectroscopic surveys, we expect the PS1 sample
of RR Lyrae stars to become the premier source for studying the structure,
kinematics, and the gravitational potential of the Galactic halo. The
techniques presented in this study should translate well to other sparse,
multi-band data sets, such as those produced by the Dark Energy Survey and the
upcoming Large Synoptic Survey Telescope Galactic plane sub-survey.
