arXiv subjects

Joshua W. Blackman

Publications and source records attributed to Joshua W. Blackman.

16 recordsLinked to original sources

arXiv2026

First Direct Identification of a Multi-Star Microlens System Hosting a Planet

Nearly 25 years have passed since gravitational microlensing delivered its first exoplanet discoveries. The relative proper motion between lens and source systems allows them to be resolved as they separate on the sky over many years. This enables scientists to study light coming directly from the lenses and place tight constraints on the physical properties of these planetary systems. In this paper we present an analysis of high angular resolution images of the microlensing target OGLE-2006-BLG-284 using Keck adaptive optics and HST data taken more than 15 years after the event. We clearly detect the lens system and measure its motion across the Keck and HST epochs. We measure a lens-source relative proper motion of $5.80 \pm 0.15$ mas yr$^{-1}$. A combination of the light curve parameters and the host star(s) brightnesses in $V,\, I$, and $K$ passbands gives primary and secondary host star masses of $M_{\rm 1} = 0.43 \pm 0.05\,M_{\odot}$ and $M_{\rm 2} = 0.12 \pm 0.02\,M_{\odot}$, and a planetary companion mass of $m_p = 165^{+40}_{-33}\,M_{\oplus}$. Although we are unable to determine if the planet orbits one or both stars in the system, our analysis motivates next-generation high-precision RV measurements that can distinguish these two possibilities. This is the first direct detection of flux coming from a two-star microlensing system which hosts a planet.

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arXiv2026

Characterizing Microlensing Planetary System OGLE-2014-BLG-0676L with High-Resolution Image Constrained Light Curve Modeling

We present an analysis that incorporates high-resolution Keck adaptive optics (AO) imaging into microlensing light-curve modeling for the planetary microlensing event OGLE-2014-BLG-0676. Using Keck AO observations obtained 6.3 years after the event, we directly resolved the lens and source. The Keck images reveal a tension, in that the $K$-band source flux is $0.52 \pm 0.22$ magnitudes brighter than predicted by previously reported light-curve models. By incorporating the Keck imaging constraints into the light-curve modeling, we find a host star mass of $M_{\rm host} = 0.60^{+0.17}_{-0.14}\,M_{\odot}$, a lens distance of $D_{\rm L} = 1.88^{+0.63}_{-0.35}$ kpc, a planet mass of $m_{\rm p} = 3.11^{+1.11}_{-0.63}\,M_{\rm J}$, and a projected separation of $a_{\perp} = 2.04^{+0.44}_{-0.35}$ au and $a_{\perp} = 3.72^{+0.92}_{-0.72}$ au for the close and wide solution, respectively. These results demonstrate the power of combining high-angular-resolution imaging with microlensing light-curve modeling to mitigate potential systematic effects and modeling degeneracies, enabling robust determinations of the physical properties of microlensing planetary systems. The results presented here can be confirmed by future observations from the \textit{Roman}'s Galactic Plane Survey.

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arXiv2025

MOA-2010-BLG-328: Keck and HST Expose the Limits of Occams Razor in Microlensing

We present high resolution follow-up data of the planetary microlensing event MOA-2010-BLG-328, using Keck and the Hubble. Keck data, taken 8 years after the event, reveal a strong lens detection enabling a direct measurement of lens flux and source-lens relative proper motion. We find the relative source-lens proper motion to be $μ_{\rm rel, Hel} = 4.07 \pm 0.34\ \rm mas\ yr^{-1}$, with the lens being $\sim10$ times fainter than the source. The lens was very faint in the Hubble passbands, and the small lens-source separation of $\sim$35 mas made its detection difficult. However, we obtained estimates of the lens magnitudes in Hubble bands by constraining its location to match the Keck K-band detection. The original analysis by \citet{Furusawa2013} reports a degenerate light curve, with several viable models depending on higher-order effects. We attempt to break the degeneracy by remodeling the event using constraints from follow-up data. Our best fit model includes parallax, orbital motion, xallarap and the magnification of a source companion. Models omitting any of these are excluded. However, even with a lens detection the solution remains unclear, as the degeneracy between a nearby late M dwarf and a distant early M dwarf in the disk persists, and cannot be broken with NIR data alone. We conclude the lens is either a $\sim0.2\ M_{\odot}$ star at $2-3$kpc, or a $\sim0.5\ M_{\odot}$ star at $4-5$kpc. This study highlights the importance of multi-band data and comprehensive modeling to resolve microlensing degeneracies.

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arXiv2025

OGLE-2016-BLG-1195Lb: A Sub-Neptune Beyond the Snow Line of an M-dwarf Confirmed by Keck AO

We present the analysis of high resolution follow-up observations of OGLE-2016-BLG-1195 using Laser Guide Star Adaptive Optics with Keck, seven years after the event's peak. We resolve the lens, measuring its flux and the relative source-lens proper motion, thus finding the system to be a $M_{\rm p} = 10.08\pm 1.18\ M_{\rm \oplus}$ planet orbiting an M-dwarf, $M_{\rm L} = 0.62\pm 0.05\ M_{\odot}$, beyond the snow line, with a projected separation of $r_\perp=2.24\pm 0.21$ AU at $D_{\rm L} = 7.45\pm 0.55$ kpc. Our results are consistent with the discovery paper, which reports values with 1-sigma uncertainties based on a single mass-distance constraint from finite source effects. However, both the discovery paper and our follow-up results disagree with the analysis of a different group that also present the planetary signal detection. The latter utilizes \textit{Spitzer} photometry to measure a parallax signal claiming the system is an Earth-mass planet orbiting an ultracool dwarf. Their parallax signal though is improbable since it suggests a lens star in the disk moving perpendicular to Galactic rotation. Moreover, microlensing parallaxes can be impacted by systematic errors in the photometry. Therefore, we reanalyze the \textit{Spitzer} photometry using a Pixel Level Decorrelation (PLD) model to detrend detector systematics. We find that we can not confidently recover the same detrended light curve that is likely dominated by systematic errors in the photometric data. The results of this paper act as a cautionary tale that a careful understanding of detector systematics and how they influence astrophysical constraints is crucial.

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arXiv2025

OGLE-2014-BLG-1760: A Jupiter-Sun analogue residing in the Galactic Bulge

We present the analysis of OGLE-2014-BLG-1760, a planetary system in the galactic bulge. We combine Keck Adaptive Optics follow-up observations in $K$-band with re-reduced light curve data to confirm the source and lens star identifications and stellar types. The re-reduced MOA dataset had an important impact on the light curve model. We find the Einstein ring crossing time of the event to be $\sim$ 2.5 days shorter than previous fits, which increases the planetary mass-ratio and decreases the source angular size by a factor of 0.25. Our OSIRIS images obtained 6 years after the peak of the event show a source-lens separation of 54.20 $\pm$ 0.23 mas, which leads to a relative proper motion of $μ_{\rm rel}$ = 9.14 $\pm$ 0.05 mas/yr, larger than the previous light curve-only models. Our analysis shows that the event consists of a Jupiter-mass planet of $M_{\rm p}$ = 0.931 $\pm$ 0.117 $M_{\rm Jup}$ orbiting a K-dwarf star of $M_*$ = 0.803 $\pm$ 0.097 $M_{\odot}$ with a $K$-magnitude of $K_{\rm L}$ = 18.30 $\pm$ 0.05, located in the galactic bulge or bar. We also attempt to constrain the source properties using the source angular size $θ_*$ and $K$-magnitude. Our results favor the scenario of the source being a younger star in the galactic disk, behind the galactic center, but future multicolor observations are needed to constrain the source and thus the lens properties.

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arXiv2024

Image-Constrained Modeling with Hubble and Keck Images Reveals that OGLE-2012-BLG-0563Lb is a Jupiter-Mass planet Orbiting a K Dwarf

We present high angular resolution imaging from the {\sl Hubble Space Telescope} combined with adaptive optics imaging results from the {\sl Keck}-II telescope to determine the mass of the OGLE-2012-BLG-0563L host star and planet to be $M_{\rm host} = 0.801\pm 0.033M_\odot$ and $M_{\rm planet} = 1.116 \pm 0.087 M_{\rm Jupiter}$, respectively, located at a distance of $D_L = 5.46\pm 0.56\,$kpc. There is a close-wide degeneracy in the light curve models that indicates star-planet projected separation of $1.50\pm 0.16\,$AU for the close model and $8.41\pm 0.87\,$AU for the wide model. We used the image-constrained modeling method to analyze the light curve data with constraints from this high angular resolution image analysis. This revealed systematic errors in some of the ground-based light curve photometry that led to an estimate of the angular Einstein Radius, $θ_E$, that was too large by a factor of $\sim 2$. The host star mass is a factor of 2.4 larger than the value presented in the \citet{fukui15} discovery paper. Although most systematic photometry errors seen in ground-based microlensing light curve photometry will not be repeated in data from the {\sl Roman Space Telescope}'s Galactic Bulge Time Domain Survey, we argue that image constrained modeling will be a valuable method to identify possible systematic errors in {\sl Roman} photometry.

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arXiv2024

A Candidate High-Velocity Exoplanet System in the Galactic Bulge

We present an analysis of adaptive optics (AO) images from the Keck-I telescope of the microlensing event MOA-2011-BLG-262. The original discovery paper by Bennett et al. 2014 reports two distinct possibilities for the lens system; a nearby gas giant lens with an exomoon companion or a very low mass star with a planetary companion in the galactic bulge. The $\sim$10 year baseline between the microlensing event and the Keck follow-up observations allows us to detect the faint candidate lens host (star) at $K = 22.3$ mag and confirm the distant lens system interpretation. The combination of the host star brightness and light curve parameters yields host star and planet masses of $M_{\rm host} = 0.19 \pm 0.03M_{\odot}$ and $m_p = 28.92 \pm 4.75M_{\oplus}$ at a distance of $D_L = 7.49 \pm 0.91\,$kpc. We perform a multi-epoch cross reference to \textit{Gaia} DR3 and measure a transverse velocity for the candidate lens system of $v_L = 541.31 \pm 65.75$ km s$^{-1}$. We conclude this event consists of the highest velocity exoplanet system detected to date, and also the lowest mass microlensing host star with a confirmed mass measurement. The high-velocity nature of the lens system can be definitively confirmed with an additional epoch of high-resolution imaging at any time now. The methods outlined in this work demonstrate that the \textit{Roman} Galactic Exoplanet Survey (RGES) will be able to securely measure low-mass host stars in the bulge.

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arXiv2024

Unveiling MOA-2007-BLG-192: An M Dwarf Hosting a Likely Super-Earth

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 solutions by a factor of ${\sim}2$, demonstrating the power of high angular resolution follow-up imaging for events with sparse light curve coverage. Following Bennett et al. 2023, we apply constraints from the high resolution imaging on the light curve modeling to find host star and planet masses of $M_{\textrm{host}} = 0.28 \pm 0.04M_{\odot}$ and $m_p = 12.49^{+65.47}_{-8.03}M_{\oplus}$ at a distance from Earth of $D_L = 2.16 \pm 0.30\,$kpc. This work illustrates the necessity for the Nancy Grace Roman Galactic Exoplanet Survey (RGES) to use its own high resolution imaging to inform light curve modeling for microlensing planets that the mission discovers.

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arXiv2024

Keck and Hubble Observations Show That MOA-2008-BLG-379Lb Is a Super-Jupiter Orbiting an M Dwarf

We present high angular resolution imaging that detects the MOA-2008-BLG-379L exoplanet host star using Keck adaptive optics and the Hubble Space Telescope. These observations reveal host star and planet masses of $M_{\rm host}=0.434\pm0.065 M_\odot$, and $m_p=2.44 \pm 0.49 M_{\rm Jupiter}$. They are located at a distance of $D_L=3.44\pm0.53\,$kpc, with a projected separation of $2.70\pm 0.42\,$AU. These results contribute to our determination of exoplanet host star masses for the Suzuki et al. (2016) statistical sample, which will determine the dependence of the planet occurrence rate on the mass and distance of the host stars. We also present a detailed discussion of the image constrained modeling version of the eesunhong light curve modeling code that applies high angular resolution image constraints to the light curve modeling process. This code increases modeling efficiency by a large factor by excluding models that are inconsistent with the high angular resolution images. The analysis of this and other events from the Suzuki et al. (2016) statistical sample reveals the importance of including higher order effects, such as microlensing parallax and planetary orbital motion even when these features are not required to fit the light curve data. The inclusion of these effects may be needed to obtain accurate estimates of the uncertainty of other microlensing parameters that affect the inferred properties of exoplanet microlens systems. This will be important for the exoplanet microlensing survey of the Roman Space Telescope, which will use both light curve photometry and high angular resolution imaging to characterize planetary microlens systems.

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arXiv2024

Precise mass measurement of OGLE-2013-BLG-0132/MOA-2013-BLG-148: a Saturn mass planet orbiting an M-dwarf

We revisit the planetary microlensing event OGLE-2013-BLG-0132/MOA-2013-BLG-148 using Keck adaptive optics imaging in 2013 with NIRC2 and in 2020, 7.4 years after the event, with OSIRIS. The 2020 observations yield a source and lens separation of $ 56.91 \pm 0.29$ mas, which provides us with a precise measurement of the heliocentric proper motion of the event $μ_{rel,hel} = 7.695 \pm 0.039$ mas $yr^{-1}$. We measured the magnitude of the lens in K-band as $K_{lens} = 18.69 \pm 0.04 $. Using these constraints, we refit the microlensing light curve and undertake a full reanalysis of the event parameters including the microlensing parallax $π_{E}$ and the distance to the source D$_S$. We confirm the results obtained in the initial study by \cite{Mroz_2017} and improve significantly upon the accuracy of the physical parameters. The system is an M dwarf of $0.495 \pm 0.054$ $M_\odot$ orbited by a cold, Saturn-mass planet of $0.26 \pm 0.028$ $M_{Jup}$ at projected separation $r_{\perp}$ = 3.14 $\pm$ 0.28 AU. This work confirms that the planetary system is at a distance of 3.48 $\pm$ 0.36 kpc, which places it in the Galactic disk and not the Galactic bulge.

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arXiv2022

Adaptive Optics Imaging Breaks the Central Caustic Cusp Approach Degeneracy in High Magnification Microlensing Events

We report new results for the gravitational microlensing target OGLE-2011-BLG-0950 from adaptive optics (AO) images using the Keck observatory. The original analysis by Choi et al. 2012 reports degenerate solutions between planetary and stellar binary lens systems. This is due to a degeneracy in high magnification events where the shape of the light curve peak can be explained by a source approach to two different cusp geometries with different source radius crossing times. 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-year baselines between the event and the Keck observations allow us to directly measure a relative proper motion of $4.20\pm 0.21\,$mas/yr, which confirms the detection of the lens star system and directly rules out the planetary companion models that predict a ${\sim}4 \times$ smaller relative proper motion. The combination of the lens brightness and close stellar binary light curve parameters yield primary and secondary star masses of $M_{A} = 1.12^{+0.06}_{-0.04}M_\odot$ and $M_{B} = 0.47^{+0.04}_{-0.03}M_\odot$ at a distance of $D_L = 6.70^{+0.55}_{-0.30}\,$kpc, and a primary-secondary projected separation of $0.39^{+0.05}_{-0.04}\,$AU. Since this degeneracy is likely to be common, the high resolution imaging method described here will be used to disentangle the central caustic cusp approach degeneracy for events observed by the \textit{Roman} exoplanet microlensing survey using the \textit{Roman} images taken near the beginning or end of the survey.

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arXiv2021

MOA-2009-BLG-319Lb: A Sub-Saturn Planet Inside the Predicted Mass Desert

We present an adaptive optics (AO) analysis of images from the Keck-II telescope NIRC2 instrument of the planetary microlensing event MOA-2009-BLG-319. The $\sim$10 year baseline between the event and the Keck observations allows the planetary host star to be detected at a separation of $66.5\pm 1.7\,$mas from the source star, consistent with the light curve model prediction. The combination of the host star brightness and light curve parameters yield host star and planet masses of M_host = 0.514 $\pm$ 0.063M_Sun and m_p = 66.0 $\pm$ 8.1M_Earth at a distance of $D_L = 7.0 \pm 0.7\,$kpc. The star-planet projected separation is $2.03 \pm 0.21\,$AU. The planet-star mass ratio of this system, $q = (3.857 \pm 0.029)\times 10^{-4}$, places it in the predicted "planet desert" at $10^{-4} < q < 4\times 10^{-4}$ according to the runaway gas accretion scenario of the core accretion theory. Seven of the 30 planets in the Suzuki et al. (2016) sample fall in this mass ratio range, and this is the third with a measured host mass. All three of these host stars have masses of 0.5 $\leq$ M_host/M_Sun $\leq$ 0.7, which implies that this predicted mass ratio gap is filled with planets that have host stars within a factor of two of 1M_Sun. This suggests that runaway gas accretion does not play a major role in determining giant planet masses for stars somewhat less massive than the Sun. Our analysis has been accomplished with a modified DAOPHOT code that has been designed to measure the brightness and positions of closely blended stars. This will aid in the development of the primary method that the Nancy Grace Roman Space Telescope mission will use to determine the masses of microlens planets and their hosts.

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arXiv2020

MOA-2007-BLG-400 A Super-Jupiter Mass Planet Orbiting a Galactic BulgeK-dwarf Revealed by Keck Adaptive Optics Imaging

We present Keck/NIRC2 adaptive optics imaging of planetary microlensing event MOA-2007-BLG-400 that resolves the lens star system from the source. We find that the MOA-2007-BLG-400L planetary system consists of a $1.71\pm 0.27 M_{\rm Jup}$ planet orbiting a $0.69\pm 0.04M_{\odot}$ K-dwarf host star at a distance of $6.89\pm 0.77\,$kpc from the Sun. So, this planetary system probably resides in the Galactic bulge. The planet-host star projected separation is only weakly constrained due to the close-wide light curve degeneracy; the 2$σ$ projected separation range is 0.6--$7.2\,$AU. This host mass is at the top end of the range of masses predicted by a standard Bayesian analysis that assumes that all stars have an equal chance of hosting a star of the observed mass ratio. This and the similar result for event MOA-2013-BLG-220 suggests that more massive stars may be more likely to host planets with a mass ratio in the $0.002 < q < 0.004$ range that orbit beyond the snow line. These results also indicate the importance of host star mass measurements for exoplanets found by microlensing. The microlensing survey imaging data from NASA's Nancy Grace Roman Space Telescope (formerly WFIRST) mission will be doing mass measurements like this for a huge number of planetary events. This host lens is the highest contrast lens-source detected in microlensing mass measurement analysis (the lens being 10$\times$ fainter than the source). We present an improved method of calculating photometry and astrometry uncertainties based on the Jackknife method, which produces more accurate errors that are $\sim$$2.5 \times$ larger than previous estimates.

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arXiv2020

Revisiting MOA 2013 BLG-220L: A Solar-type star with a Cold super-Jupiter Companion

We present the analysis of high-resolution images of MOA-2013-BLG-220, taken with the Keck adaptive optics system 6 years after the initial observation, identifying the lens as a solar-type star hosting a super-Jupiter mass planet. The masses of planets and host-stars discovered by microlensing are often not determined from light curve data, while the star-planet mass-ratio and projected separation in units of Einstein ring radius are well measured. High-resolution follow-up observations after the lensing event is complete can resolve the source and lens. This allows direct measurements of flux, and the amplitude and direction of proper motion, giving strong constraints on the system parameters. Due to the high relative proper motion, $μ_{\rm rel,Geo} = 12.62\pm0.11$ mas/yr, the source and lens were resolved in 2019, with a separation of $77.1\pm0.5$ mas. Thus, we constrain the lens flux to $K_{\rm Keck,lens}= 17.92\pm0.02$. By combining constraints from the model and Keck flux, we find the lens mass to be $M_L = 0.88\pm0.05\ M_\odot$ at $D_L = 6.72\pm0.59$ kpc. With a mass-ratio of $q=(3.00\pm0.03)\times10^{-3}$ the planet's mass is determined to be $M_P = 2.74\pm0.17\ M_{J}$ at a separation of $r_\perp = 3.03\pm0.27$ AU. The lens mass is much higher than the prediction made by the Bayesian analysis that assumes all stars have an equal probability to host a planet of the measured mass ratio, and suggests that planets with mass ratios of a few 10$^{-3}$ are more common orbiting massive stars. This demonstrates the importance of high-resolution follow-up observations for testing theories like these.

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arXiv2020

Confirmation of the Stellar Binary Microlensing Event, Macho 97-BLG-28

The high-magnification microlensing event MACHO-97-BLG-28 was previously determined to be a binary system composed either of two M dwarfs, or an M dwarf and a brown dwarf. We present a revised light-curve model using additional data from the Mt. Stromlo 74" telescope, model estimates of stellar limb darkening and fitting the blend separately for each telescope and passband. We find a lensing system with a larger mass ratio, $q = 0.28 \pm 0.01$, and smaller projected separation, $s = 0.61 \pm 0.01 $, than that presented in the original study. We revise the estimate of the lens-source relative proper motion to $μ_{rel}=2.8 \pm 0.5 \; \mathrm{mas\: yr^{-1}}$, which indicates that 16.07 years after the event maximum the lens and source should have separated by $46 \pm 8$ mas. We revise the source star radius using more recent reddening maps and angular diameter-color relations to $R_*=(10.3 \pm 1.9) R_\odot$. K and J-band adaptive optics images of the field taken at this epoch using the NIRC2 imager on the Keck telescope show that the source and lens are still blended, consistent with our light-curve model. With no statistically significant excess flux detection we constrain the mass, $M_L= 0.24^{+0.28}_{-0.12}M_\odot$, and distance, $D_L = 7.0 \pm 1.0$ kpc, of the lensing system. This supports the interpretation of this event being a stellar binary in the galactic bulge. This lens mass gives a companion mass of $M=0.07^{+0.08}_{-0.04}M_\odot$, close to the boundary between being a star and a brown dwarf.

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arXiv2020

Keck Observations Confirm a Super-Jupiter Planet Orbiting M-dwarf OGLE-2005-BLG-071L

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, $μ_{\rm rel,H}$. in the heliocentric reference frame. The $μ_{\rm rel,H}$ 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 $μ_{\rm rel,H}$ and $K_L$ provide the masses of the host stat, $M_{\rm host} = 0.426\pm 0.037 M_\odot$, and planet, $m_p = 3.27 \pm 0.32 M_{\rm Jup}$ with a projected separation of $3.4\pm 0.5\,$AU. This confirms the tentative conclusion of a previous paper (Dong et al. 2009) that this super-Jupiter mass planet, OGLE-2005-BLG-071Lb, orbits 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 $< 0.5M_\odot$ 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 metallicity withspectra from a high angular resolution telescope such as Keck, the VLT, the Hubble Space Telescope or the James Webb Space Telescope.

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