2017AJ....154...59B

High-resolution Hubble Space Telescope (HST) image analysis of the MOA-2008-BLG-310 microlens system indicates that the excess flux at the location of the source found in the discovery paper cannot primarily be due to the lens star because it does not match the lens-source relative proper motion, µ_rel, predicted by the microlens models. This excess flux is most likely to be due to an unrelated star that happens to be located in close proximity to the source star. Two epochs of HST observations indicate proper motion for this blend star that is typical of a random bulge star but is not consistent with a companion to the source or lens stars if the flux is dominated by only one star, aside from the lens. We consider models in which the excess flux is due to a combination of an unrelated star and the lens star, and this yields a 95% confidence level upper limit on the lens star brightness of I_L 22.44 and V_L 23.62. A Bayesian analysis using a standard Galactic model and these magnitude limits yields a host star mass of M_h=0.21_–0.09^+0.21 M_☉ and a planet mass of m_p=23.4_–9.9^+23.9 M_⊕ at a projected separation of a_⊥=1.12_–0.17^+0.16 au. This result illustrates that excess flux in a high-resolution image of a microlens-source system need not be due to the lens. It is important to check that the lens-source relative proper motion is consistent with the microlensing prediction. The high-resolution image analysis techniques developed in this paper can be used to verify the WFIRST exoplanet microlensing survey mass measurements.