Browsing by Author "Lu JR"

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    Adaptive Optics Imaging Can Break the Central Caustic Cusp Approach Degeneracy in High-magnification Microlensing Events
    (IOP Publishing on behalf of the American Astronomical Society, 2022-11-01) Terry SK; Bennett DP; Bhattacharya A; Koshimoto N; Beaulieu J-P; Blackman JW; Bond IA; Cole AA; Lu JR; Marquette JB; Ranc C; Rektsini N; Vandorou A
    We report new results for the gravitational microlensing target OGLE-2011-BLG-0950 from adaptive optics images using the Keck Observatory. The original analysis by Choi et al. and reanalysis by Suzuki et al. report degenerate solutions between planetary and stellar binary lens systems. This particular case is the most important type of degeneracy for exoplanet demographics because the distinction between a planetary mass or stellar binary companion has direct consequences for microlensing exoplanet statistics. The 8 and 10 yr baselines allow us to directly measure a relative proper motion of 4.20 ± 0.21 mas yr−1, confirming the detection of the lens star system and ruling out the planetary companion models that predict a ∼4× smaller relative proper motion. The Keck data also rule out the wide stellar binary solution unless one of the components is a stellar remnant. The combination of the lens brightness and close stellar binary light-curve parameters yields primary and secondary star masses of M A = 1.12 − 0.09 + 0.11 and M B = 0.47 − 0.10 + 0.13 M ☉ at a distance of D L = 6.70 − 0.30 + 0.55 kpc and a projected separation of 0.39 − 0.04 + 0.05 au. Assuming that the predicted proper motions are measurably different, the high-resolution imaging method described here can be used to disentangle this degeneracy for events observed by the Roman exoplanet microlensing survey using Roman images taken near the beginning or end of the survey.
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    An Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing
    (IOP Publishing, 2022-07-06) Lam CY; Lu JR; Udalski A; Bond I; Bennett DP; Skowron J; Mróz P; Poleski R; Sumi T; Szymański MK; Kozłowski S; Pietrukowicz P; Soszyński I; Ulaczyk K; Wyrzykowski Ł; Miyazaki S; Suzuki D; Koshimoto N; Rattenbury NJ; Hosek Jr MW; Abe F; Barry R; Bhattacharya A; Fukui A; Fujii H; Hirao Y; Itow Y; Kirikawa R; Kondo I; Matsubara Y; Matsumoto S; Muraki Y; Olmschenk G; Ranc C; Okamura A; Satoh Y; Silva SI; Toda T; Tristram PJ; Vandorou A; Yama H; Abrams NS; Agarwal S; Rose S; Terry SK
    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 (1/4108) and find reasonable agreement given the small sample size.
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    Confirmation of Color-dependent Centroid Shift Measured After 1.8 Years with HST
    (American Astronomical Society, 2023-04-19) Bhattacharya A; Bennett DP; Beaulieu JP; Bond IA; Koshimoto N; Lu JR; Blackman JW; Ranc C; Vandorou A; Terry SK; Marquette JB; Cole AA; Fukui A
    We measured the precise masses of the host and planet in the OGLE-2003-BLG-235 system, when the lens and source were resolving, with 2018 Keck high resolution images. This measurement is in agreement with the observation taken in 2005 with the Hubble Space Telescope (HST). In the 2005 data, the lens and sources were not resolved and the measurement was made using color-dependent centroid shift only. The Nancy Grace Roman Space Telescope will measure masses using data typically taken within 3-4 yr of the peak of the event, which is a much shorter baseline when compared to most of the mass measurements to date. Hence, the color-dependent centroid shift will be one of the primary methods of mass measurements for the Roman telescope. Yet, mass measurements of only two events (OGLE-2003-BLG-235 and OGLE-2005-BLG-071) have been done using the color-dependent centroid shift method so far. The accuracy of the measurements using this method are neither completely known nor well studied. The agreement of the Keck and HST results, as shown in this paper, is very important because this agreement confirms the accuracy of the mass measurements determined at a small lens-source separation using the color-dependent centroid shift method. It also shows that with >100 high resolution images, the Roman telescope will be able to use color-dependent centroid shift at a 3-4 yr time baseline and produce mass measurements. We find that OGLE-2003-BLG-235 is a planetary system that consists of a 2.34 ± 0.43M Jup planet orbiting a 0.56 ± 0.06M ⊙ K-dwarf host star at a distance of 5.26 ± 0.71 kpc from the Sun.
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    Supplement: "an Isolated Mass-gap Black Hole or Neutron Star Detected with Astrometric Microlensing" (2022, ApJL, 933, L23)
    (IOP Publishing on behalf of the American Astronomical Society, 2022-07-06) Lam CY; Lu JR; Udalski A; Bond I; Bennett DP; Skowron J; Mróz P; Poleski R; Sumi T; Szymański MK; Kozłowski S; Pietrukowicz P; Soszyński I; Ulaczyk K; Wyrzykowski Ł; Miyazaki S; Suzuki D; Koshimoto N; Rattenbury NJ; Hosek Jr MW; Abe F; Barry R; Bhattacharya A; Fukui A; Fujii H; Hirao Y; Itow Y; Kirikawa R; Kondo I; Matsubara Y; Matsumoto S; Muraki Y; Olmschenk G; Ranc C; Okamura A; Satoh Y; Silva SI; Toda T; Tristram PJ; Vandorou A; Yama H; Abrams NS; Agarwal S; Rose S; Terry SK
    This supplement provides supporting material for Lam et al. We briefly summarize past gravitational microlensing searches for black holes (BHs) and present details of the observations, analysis, and modeling of five BH candidates observed with both ground-based photometric microlensing surveys and Hubble Space Telescope astrometry and photometry. We present detailed results for four of the five candidates that show no or low probability for the lens to be a BH. In these cases, the lens masses are <2 M ⊙, and two of the four are likely white dwarfs or neutron stars. We also present detailed methods for comparing the full sample of five candidates to theoretical expectations of the number of BHs in the Milky Way ( 1/4108).
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    Unveiling MOA-2007-BLG-192: An M Dwarf Hosting a Likely Super-Earth
    (American Astronomical Society, 2024-07-15) Terry SK; Beaulieu J-P; Bennett DP; Hamdorf E; Bhattacharya A; Chaudhry V; Cole AA; Koshimoto N; Anderson J; Bachelet E; Blackman JW; Bond IA; Lu JR; Marquette JB; Ranc C; Rektsini NE; Sahu K; Vandorou A
    We present an analysis of high-angular-resolution images of the microlensing target MOA-2007-BLG-192 using Keck adaptive optics and the Hubble Space Telescope. The planetary host star is robustly detected as it separates from the background source star in nearly all of the Keck and Hubble data. The amplitude and direction of the lens-source separation allows us to break a degeneracy related to the microlensing parallax and source radius crossing time. Thus, we are able to reduce the number of possible binary-lens solutions by a factor of ∼2, demonstrating the power of high-angular-resolution follow-up imaging for events with sparse light-curve coverage. Following Bennett et al., we apply constraints from the high-resolution imaging on the light-curve modeling to find host star and planet masses of M host = 0.28 ± 0.04 M ☉ and m p = 12.49 − 8.03 + 65.47 M ⊕ at a distance from Earth of D L = 2.16 ± 0.30 kpc. This work illustrates the necessity for the Nancy Grace Roman Galactic Exoplanet Survey to use its own high-resolution imaging to inform light-curve modeling for microlensing planets that the mission discovers.