OGLE-2017-BLG-1434Lb: Eighth q<1×10(-4) Mass-Ratio Microlens Planet Confirms Turnover in Planet Mass-Ratio Function

We report the discovery of a cold Super-Earth planet (m<SUB>p</SUB>= 4.4±0.5 M<SUB>⊕</SUB>) orbiting a low-mass (M=0.23±0.03 M<SUB>⊙</SUB>) dwarf at projected separation a<SUB>⊥</SUB>= 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, D<SUB>L</SUB>= 0.86±0.09 kpc. Indeed, it was the large lens-source relative parallax π<SUB>rel</SUB>=1.0 mas (combined with the low mass M) that gave rise to the large, and thus well-measured, microlens parallax π<SUB>E</SUB>∝(π<SUB>rel</SUB>/M)<SUP>1/2</SUP> that enabled these precise measurements. OGLE-2017-BLG-1434Lb is the eighth microlensing planet with planet-host mass ratio q<1×10<SUP>-4</SUP>. We apply a new planet-detection sensitivity method, which is a variant of V/V<SUB>max</SUB>, to seven of these eight planets to derive the mass-ratio function in this regime. We find dN/d lnq ∝ q<SUP>p</SUP>, with p=1.05<SUP>+0.78</SUP><SUB>-0.68</SUB>, 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≥2×10<SUP>-4</SUP>. We combine our result with that of Suzuki et al. to obtain p=0.73<SUP>+0.42</SUP><SUB>-0.34</SUB>.