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

Abstract

We report the discovery of a cold Super-Earth planet (m(p) = 4.4 +/- 0.5 M⊙) orbiting a low-mass (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)1/2 that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q <1 x 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 / d lnq ∝ qP , with p =1.05(-0.68)(+0.78), which 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 greater than or similar to 2 x 10-4. We combine our result with that of Suzuki et al. to obtain p = 0.73(-0.34)(+0.42).