'Auxiliary' Science with the WFIRST Microlensing Survey: Measurement of the Compact Object Mass Function over Ten Orders of Magnitude; Detection of ~10⁵ Transiting Planets; Astroseismology of ~10⁶ Bulge Giants; Detection of ~5x10³ Trans-Neptunian Objects; and Parallaxes and Proper Motions of ~6x10⁶ Bulge and Disk Stars

The Wide Field Infrared Survey Telescope (WFIRST) will monitor ∼2 deg² toward the Galactic bulge in a wide (∼1−2 μm) W149 filter at 15-minute cadence with exposure times of ∼50s for 6 seasons of 72 days each, for a total ∼41,000 exposures taken over ∼432 days, spread over the 5-year prime mission. This will be one of the deepest exposures of the sky ever taken, reaching a photon-noise photometric precision of 0.01 mag per exposure and collecting a total of ∼10⁹ photons over the course of the survey for a W149_(AB) ∼ 21 star. Of order 4×10⁷ stars will be monitored with W149_(AB) < 21, and 10⁸ stars with W145_(AB) < 23. The WFIRST microlensing survey will detect ∼54,000 microlensing events, of which roughly 1% (∼500) will be due to isolated black holes, and ∼3% (∼1600) will be due to isolated neutron stars. It will be sensitive to (effectively) isolated compact objects with masses as low as the mass of Pluto, thereby enabling a measurement of the compact object mass function over 10 orders of magnitude. Assuming photon-noise limited precision, it will detect ∼10⁵ transiting planets with sizes as small as ∼2 R⊕, perform asteroseismology of ∼10⁶ giant stars, measure the proper motions to ∼0.3% and parallaxes to ∼10% for the ∼6×10⁶ disk and bulge stars in the survey area, and directly detect ∼5×10³ Trans-Neptunian objects (TNOs) with diameters down to ∼10 km, as well as detect ∼10³ occulations of stars by TNOs during the survey. All of this science will completely serendipitous, i.e., it will not require modifications of the WFIRST optimal microlensing survey design. Allowing for some minor deviation from the optimal design, such as monitoring the Galactic center, would enable an even broader range of transformational science.