Augmenting WFIRST Microlensing with a Ground-based Telescope Network
  Augmenting the \emph{WFIRST} microlensing campaigns with intensive observations from a ground-based network of wide-field survey telescopes would have several major advantages. First, it would enable full two-dimensional (2-D) vector microlens parallax measurements for a substantial fraction of low-mass lenses as well as planetary and binary events that show caustic crossing features. For a significant fraction of the free-floating planet (FFP) events and all caustic-crossing planetary/binary events, these 2-D parallax measurements directly lead to complete solutions (mass, distance, transverse velocity) of the lens object (or lens system). For even more events, the complementary ground-based observations will yield 1-D parallax measurements. Together with the 1-D parallaxes from \emph{WFIRST} alone, they can probe the entire mass range $M\gtrsim M_\oplus$. For luminous lenses, such 1-D parallax measurements can be promoted to complete solutions (mass, distance, transverse velocity) by high-resolution imaging. This would provide crucial information not only about the hosts of planets and other lenses, but also enable a much more precise Galactic model. Other benefits of such a survey include improved understanding of binaries (particularly with low mass primaries), and sensitivity to distant ice-giant and gas-giant companions of \emph{WFIRST} lenses that cannot be detected by \emph{WFIRST} itself due to its restricted observing windows. Existing ground-based microlensing surveys can be employed if \emph{WFIRST} is pointed at lower-extinction fields than is currently envisaged. This would come at some cost to the event rate. Therefore the benefits of improved characterization of lenses must be weighed against these costs. 
Wei Zhu (OSU/MPIA), Andrew Gould (KASI/OSU/MPIA)
astro-ph.EP astro-ph.IM astro-ph.SR