We present Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of the source and lens stars for planetary microlensing event OGLE-2005-BLG-169, which confirm the relative proper motion prediction due to the planetary light curve signal observed for this event. This (and the companion Keck result) provide the first confirmation of a planetary microlensing signal, for which the deviation was only 2%. The follow-up observations determine the flux of the planetary host star in multiple passbands and remove light curve model ambiguity caused by sparse sampling of part of the light curve. This leads to a precise determination of the properties of the OGLE-2005-BLG-169Lb planetary system. Combining the constraints from the microlensing light curve with the photometry and astrometry of the HST/WFC3 data, we find star and planet masses of M * = 0.69 ± 0.02M and m p = 14.1 ± 0.9M ⊕ . The planetary microlens system is located toward the Galactic bulge at a distance of D L = 4.1 ± 0.4 kpc and the projected star-planet separation is a ⊥ = 3.5 ± 0.3 AU, corresponding to a semimajor axis of a = 4.0 +2.2 −0.6 AU.

We present adaptive optics imaging from the NIRC2 instrument on the Keck-2 telescope that resolves the exoplanet host (and lens) star as it separates from the brighter source star. These observations yield the K-band brightness of the lens and planetary host star, as well as the lens-source relative proper motion, µ rel,H . in the heliocentric reference frame. The µ rel,H measurement allows determination of the microlensing parallax vector, π E , which had only a single component determined by the microlensing light curve. The combined measurements of µ rel,H and K L provide the masses of the host stat, M host = 0.426 ± 0.037M , and planet, m p = 3.27 ± 0.32M Jupiter with a projected separation of 3.4 ± 0.5 AU. This confirms the tentative conclusion of a previous paper (Dong et al. 2009b) that this super-Jupiter mass planet, OGLE-2005-BLG-071Lb, arXiv:1909.04740v1 [astro-ph.EP] 10 Sep 2019 -2orbits an M-dwarf. Such planets are predicted to be rare by the core accretion theory and have been difficult to find with other methods, but there are two such planets with firm mass measurements from microlensing, and an additional 11 planetary microlens events with host mass estimates < 0.5M and planet mass estimates > 2 Jupiter masses that could be confirmed by high angular follow-up observations. We also point out that OGLE-2005-BLG-071L has separated far enough from its host star that it should be possible to measure the host star metalicity with spectra from a high angular resolution telescope such as Keck, the VLT, the Hubble Space Telescope or the James Webb Space Telescope.

We present the analysis of five black hole candidates identified from gravitational microlensing surveys. Hubble Space Telescope astrometric data and densely sampled light curves from ground-based microlensing surveys are fit with a single-source, single-lens microlensing model in order to measure the mass and luminosity of each lens and determine if it is a black hole. One of the five targets (OGLE-2011-BLG-0462/MOA-2011-BLG-191 or OB110462 for short) shows a significant >1 mas coherent astrometric shift, little to no lens flux, and has an inferred lens mass of 1.6–4.4 M ⊙. This makes OB110462 the first definitive discovery of a compact object through astrometric microlensing and it is most likely either a neutron star or a low-mass black hole. This compact-object lens is relatively nearby (0.70–1.92 kpc) and has a slow transverse motion of <30 km s−1. OB110462 shows significant tension between models well fit to photometry versus astrometry, making it currently difficult to distinguish between a neutron star and a black hole. Additional observations and modeling with more complex system geometries, such as binary sources, are needed to resolve the puzzling nature of this object. For the remaining four candidates, the lens masses are <2M ⊙, and they are unlikely to be black holes; two of the four are likely white dwarfs or neutron stars. We compare the full sample of five candidates to theoretical expectations on the number of black holes in the Milky Way (∼108) and find reasonable agreement given the small sample size.

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