OGLE-2017-BLG-1434Lb: Eighth q<1×10-4mass-ratio microlens planet confirms turnover in planet mass-ratio function

We report the discovery of a cold Super-Earth planet (mp= 4.4 ± 0.5 M) orbiting a lowmass (M = 0.23 ± 0.03 M) M dwarf at projected separation a = 1.18 ± 0.10 a.u., i.e., about 1.9 times the distance the snow line. The system is quite nearby for a microlensing planet, DL= 0.86 ± 0.09 kpc. Indeed, it was the large lens-source relative parallax πrel= 1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, "microlens parallax πE∞ (πrel/M)1/2that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q < 1×10-4. We apply a new planet-detection sensitivity method, which is a variant of "V/Vmax, to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/dlnq ∞qp, with p = 1.05+0.78-0.68which confirms the "turnover" in the mass function found by Suzuki et al. relative to the power law of opposite sign n =-0.93 ± 0.13 at higher mass ratios q > 2× 10-4.We combine our result with that of Suzuki et al. to obtain p = 0.73+0.42-0.34.