NIRC2, the Near Infrared Camera 2 on the Keck II telescope, was recently upgraded with a new suite of polarimetric observing modes. The new polarimetry modes (referred to as NIRC2-Pol) open up a wide range of new studies, including investigations of exoplanets, the Galactic center, active galactic nuclei, and solar system objects. The new modes enabled by the upgrade span the 1.1 to 4.1 micron range (i.e. J through L' bands) and include imaging polarimetry, coronagraphic imaging polarimetry, and spectropolarimetry. NIRC2-Pol is unique, as Keck II is the largest telescope (10 m) on which AO-fed infrared polarimetry capabilities are available, one of few with L' polarimetric imaging, and the only one where there is both a polarimetric mode and a vortex coronagraph. Here, we introduce the design of NIRC2-Pol, its capabilities, and its current operational status. We also present its first on-sky results: the first L' polarimetric images of the AB Aurigae circumstellar disk. These images more clearly reveal the disk's iconic spiral arms than previous L' total intensity imaging.

Stellar-mass black holes (BHs) above 30M circle dot are predicted to form from low-metallicity progenitors, but direct detections of such systems in the Milky Way remain scarce. Motivated by the recent discovery of Gaia BH3, a 33M circle dot BH with a very metal-poor giant companion, we conduct a systematic search for additional systems. Approximately 900 candidates are identified with Gaia as having significant deviations from single-star astrometric motion, evidence of radial velocity (RV) variability, and low metallicities inferred from Gaia XP spectra. We obtain single epoch high-resolution spectra for over 600 of these sources with Magellan/Magellan Inamori Kyocera Echelle and Lick/Automated Planet Finder and measure independent RVs with approximate to 1 km s-1 precision. After removing contaminants such as hot stars, pulsators, eclipsing binaries, and hierarchical triples, we identify about 15 promising candidates with large RV amplitudes or offsets from the Gaia reported values. This program establishes a well-characterized sample of BH candidates for detailed orbital modeling once Gaia DR4 epoch astrometry and RVs are released in late 2026; multiepoch RV follow-up is ongoing. Together, the Gaia and ground-based data will place new constraints on the demographics of BHs with metal-poor companions and test theoretical predictions linking low metallicity to the formation of the most massive stellar remnants.

As part of the Galactic Bulge Time Domain Survey (GBTDS), the Nancy Grace Roman Galactic Exoplanet Survey (RGES) will use microlensing to discover cold outer planets and free-floating planets not bound to stars. NASA has established several science requirements for the GBTDS to ensure RGES success. A key advantage of RGES is Roman's high angular resolution, which will allow detection of flux from many host stars. One requirement specifies that Roman must measure the masses and distances of 40% of detected planet hosts with 20% precision or better. To test this, we simulated microlensing events toward the GBTDS fields and used Fisher matrix analysis to estimate light curve parameter uncertainties. Combining these with Roman imaging observables (lens flux and relative lens-source proper motion), we estimated the achievable precision of lens mass and distance measurements. Using pyLIMASS, a publicly available code for estimating lens properties, we applied this analysis to 3000 simulated events. Assuming the A. Cassan et al. exoplanet mass function, we find that >= 40% of host stars meet the required 20% precision threshold, confirming that the GBTDS can satisfy the mission requirement. We validated our approach by comparing our inferred lens masses and distances to empirical measurements from detailed image-constrained light curve modeling of historical microlensing events with Hubble and Keck follow-up imaging. Our results agree within roughly 1 sigma, demonstrating that both approaches yield consistent and reliable mass and distance estimates, and confirming the robustness of our simulations for Roman-era microlensing science.

The Keck All Sky Precision Adaptive optics (KAPA) project upgrades the Keck I adaptive optics system to enable laser tomography using a four laser guide star (LGS) asterism. KAPA is now in operation in both narrow field and wide field modes to optimize correction on-axis or over the science field of view of the camera. The use of four LGSs, in conjunction with a tomographic reconstructor and pseudo open-loop control, leads to a significant reduction in wavefront error. We describe the overall architecture, development of the tomographic algorithm, real-time implementation and preliminary on-sky results here. By comparing the on-sky image quality with that obtained using a single LGS (sLGS) we clearly demonstrate the benefits of laser tomography, a technology which is crucial to the success of the next generation of extremely large telescopes.

We present PANTERA (Project for Astrophysical Nucleosynthesis and Targeted Exploration of metal-Rich Abundances), a high-resolution spectroscopic survey of the most metal-rich stars in the solar neighborhood. In this first paper, we report iron abundances for 56 metal-rich stars, selected from Gaia DR3 XP spectrophotometric metallicities. These targets were observed with the PEPSI spectrograph on the Large Binocular Telescope (LBT), the Levy spectrograph on the Automated Planet Finder (APF) telescope, and the HIRES spectrograph on the Keck telescope. We measure [Fe/H] from an equivalent-width analysis of iron lines, with the effective temperature taken from photometry and the surface gravity from the Gaia parallax. We verify our measurement using Gaia benchmark stars, observing some sources with more than one spectrograph, using a second synthesis code, and an independent equivalent-width measurement. We find that the Gaia-XP metallicities over-predict [Fe/H] for part of our sample: they are consistent for the dwarfs, though a selection bias limits what the dwarf agreement can show, and reach 0.16±0.02 dex for the cool giants. We attribute the over-prediction in part to the strong blue line blanketing of the cool metal-rich giants and to the survey labels on which the XP metallicities were trained. We also found 25 of the 56 stars to be ultra-metal-rich ([Fe/H] > 0.4), with the most iron-rich stars reaching [Fe/H]=+0.58. We discuss the implications of our result on studying metal-rich populations.

The Keck/NIRC2 infrared imager was upgraded in 2025 with dual-beam polarimetric observing modes spanning approximately 1.1–4.1 microns (JHKL' bands). We present a preliminary JHK calibration of NIRC2 Polarimetry using a wavelength-dependent Mueller matrix model of the Keck tertiary mirror (M3), half-wave plate (HWP), image rotator (IMR), downstream optics, and Wollaston prism. We constrain the model downstream of M3 using dome flat sequences spanning ten HWP and nine IMR angles in each band. Although the model reproduces the dominant modulation, the residuals show structure dependent on HWP and IMR angle. Measurement matrix inversion of unpolarized standard star observations gives M3 diattenuations of 0.0119+/-0.0009, 0.0098+/-0.0004, and 0.0068+/-0.0005 in J, H, and Kp, substantially closer to Fresnel predictions for aluminum than the values derived from dome flats. The larger dome flat modulation may indicate polarization in the incident dome illumination or Mueller matrix model inaccuracies. These results establish an initial calibration framework while motivating improved input polarization constraints, fixed HWP parameters from previous laboratory measurements, model validation with polarized standard stars, and extension to L'.

The Vera C. Rubin Legacy Survey of Space and Time will discover thousands of microlensing events across the Milky Way Galaxy, allowing for the study of populations of exoplanets, stars, and compact objects. It will reach deeper limiting magnitudes over a wider area than any previous survey. We evaluate numerous survey strategies simulated in the Rubin Operation Simulations (OpSims) to assess the discovery and characterization efficiencies of microlensing events. We have implemented three metrics in the Rubin Metric Analysis Framework: a discovery metric and two characterization metrics, where one estimates how well the lightcurve is covered and the other quantifies how precisely event parameters can be determined. We also assess the characterizability of microlensing parallax, critical for detection of free-floating black hole lenses, in a representative bulge and disk field. We find that, given Rubin's baseline cadence, the discovery and characterization efficiency will be higher for longer duration and larger parallax events. Microlensing discovery efficiency is dominated by observing footprint, where more time spent looking at regions of high stellar density including the Galactic bulge, Galactic plane, and Magellanic clouds, leads to higher discovery and characterization rates. However, if the observations are stretched over too wide an area, including low-priority areas of the Galactic plane with fewer stars and higher extinction, event characterization suffers by > 10%, which could impact exoplanet, binary star, and compact object events alike. We find that some rolling strategies (where Rubin focuses on a fraction of the sky in alternating years) in the Galactic bulge can lead to a 15-20% decrease in microlensing parallax characterization, so rolling strategies should be chosen carefully to minimize losses.

We present the first high-precision proper-motion catalog, tied to the International Celestial Reference System (ICRS), of infrared astrometric reference stars within R ≤ 25″ (1 pc) of the central supermassive black hole at the Galactic center (GC). This catalog contains ∼2900 sources in a highly extinguished region that is inaccessible via Gaia. New astrometric measurements are extracted from Hubble Space Telescope (HST) observations (14 epochs, 2010–2023) and transformed into the ICRS using 40 stars in common with Gaia-DR3. We implement a new method for modeling proper motions via Gaussian processes that accounts for systematic errors, greatly improving measurement accuracy. Proper-motion and position measurements reach precisions of ∼0.03 mas yr ^−1 and ∼0.11 mas, respectively, representing a factor of ∼20 improvement over previous ICRS proper-motion catalogs in the region. These measurements define a novel HST–Gaia reference frame that is consistent with Gaia-CRF3 to within 0.025 mas yr ^−1 in proper motion and 0.044 mas in position, making it the first ICRS-based reference frame precise enough to probe the distribution of extended mass within the orbits of stars near SgrA*. In addition, HST-Gaia provides an independent test of the radio measurements of stellar masers that form the basis of current GC reference frames. We find that the HST–Gaia and radio measurements are consistent to within 0.041 mas yr ^−1 in proper motion and 0.54 mas in position at 99.7% confidence. Gaia-DR4 is expected to reduce the HST–Gaia reference-frame uncertainties by another factor of ∼2, further improving the reference frame for dynamical studies.

Precise and accurate mass and radius measurements of evolved stars are crucial to calibrating stellar models. Stars in detached eclipsing binaries (EBs) are excellent potential calibrators because their stellar parameters can be measured with fractional uncertainties of a few percent, independent of stellar models. The All-Sky Automated Survey for Supernovae (ASAS-SN) has identified tens of thousands of EBs, >35,000 of which were included in the ASAS-SN eclipsing binaries catalog. Here, we select eight EBs from this sample that contain giants based on their Gaia colors and absolute magnitudes. We use LBT/PEPSI, APF, and CHIRON to obtain multi-epoch spectra of these binaries and measure their radial velocities using two-dimensional cross-correlation methods. We simultaneously fit the ASAS-SN light curves and the radial velocities with PHOEBE to derive accurate and precise masses and radii with fractional uncertainties of ≲3%. For four systems, we also include Transiting Exoplanet Survey Satellite (TESS) light curves in our PHOEBE models, which significantly improves the radius determinations. In seven of our systems, both components have evolved off of the main sequence, and one system has a giant star component with a main sequence, Sun-like companion. Finally, we compare our mass and radius measurements to single-star evolutionary tracks and distinguish between systems that are first ascent red giant branch stars and those that are likely core helium-burning stars.

KMT-2018-BLG-0029Lb and OGLE-2019-BLG-0960Lb were the lowest mass-ratio microlensing planets at the time of discovery. For both events, microlensing parallax measurements from the Spitzer Space Telescope implied lens systems that were more distant and massive than those inferred from the ground-based parallax. Here, we report on the detection of excess flux aligned to the event locations using Keck Adaptive Optics imaging, which is consistent with the expected brightness of main-sequence hosts under the ground-based parallax, but inconsistent with that predicted by Spitzer. Based on the excess flux, ground-based parallax, and angular Einstein radius, we determine KMT-2018-BLG-0029Lb to be a 4.2±0.5 M_⊕ planet orbiting a 0.70±0.07 M_⊙ host at a projected separation of 3.1±0.3 au, and OGLE-2019-BLG-0960Lb to be a 2.0±0.2 M_⊕ planet orbiting a 0.40±0.03 M_⊙ host at a projected separation of 1.7±0.1 au. We report on additional light-curve models for KMT-2018-BLG-0029 under the generalized inner-outer (offset) degeneracy, which were not reported in the original analysis. We point out inconsistencies in the inner/outer labeling of the degenerate models in the lens and source planes, and advocate for the lens-plane convention, which refers to the planet being closer or further to the host star compared to the image it perturbs. Lastly, we discuss the possibility of breaking this degeneracy via ground concurrent observations with the Roman Space Telescope.

The dark and dynamic parts of the Galaxy, including the bulk shape and movement of the Galactic Bulge and characteristics of dark compact object populations, such as a hypothetical population of primordial black holes (PBHs), are difficult to study directly by their very nature, but are critical to our understanding of the universe. Fortunately, all of these mysteries can be uniquely studied via gravitational microlensing, a method of astronomical detection that traces mass and dynamics as opposed to light. Using the OGLE-IV microlensing survey bulge fields, we apply a Bayesian hierarchical model to jointly infer properties of the Galaxy, the characteristics of compact objects, and and test PBHs with an extended mass distribution as a test PBHs as a viable explanation of dark matter, extending work focused on the Small and Large Magellanic Clouds, both with much lower event-rates. We infer a preference within the data for a lower patternspeed in the galactic model and a wider mass spectrum for compact objects. When adding a PBH component to the favored astrophysical model from our initial investigations, we find a Bayes factor of lnℬ = 20.23 preferring the PBH model. Upon further investigation of these results, we find the critical feature in the PBH model to be the velocity distribution, which is fundamentally different than the velocity distribution of astrophysical objects and uniquely able to explain a large number of low parallax, low timescale microlensing events. Noting that this effect is not unique to PBHs, we consider the implications of these results as applied to a hypothetical population of PBHs and discuss alternative explanations, including a variety of other possible astrophysical and survey or analysis systematics.

We present a structural and dynamical analysis of the young massive star cluster Westerlund 1 (Wd1). Using multiepoch Hubble Space Telescope observations, we measure the proper motions of 10,346 stars and present an unprecedented determination of cluster membership probability. We then determine the color membership after correcting for extinction. The stellar density map weighted by the membership probability and completeness correction displays a spatial elongation aligned with the Galactic plane with a high eccentricity of 0.71. The radial stellar density profile shows a decreasing core radius with increasing mass, indicative of weak but detectable mass segregation. Quantitatively, the measured mass segregation ratio is Λ _MSR = 1.11 ± 0.11, only above unity by 1 σ . Building on the structural modeling along with a suite of stellar evolutionary and atmospheric models, we fit the cluster age and distance in two color–magnitude diagrams, concluding that Wd1 is approximately 7.45 ± 0.53 Myr old and at a distance of 3.7 ± 0.1 kpc, with a degeneracy in the posterior which is compatible with the literature values. We measure a 1D velocity dispersion of 4.13 ± 0.13 km s ^−1 , indicating a subvirialized state. The crossing time is 0.30 Myr and the relaxation time is 0.26 Gyr. Given the age of Wd1, we expect dynamical mass segregation for stars more massive than 12 M _⊙ , which accounts for the minor mass segregation observed in the mass range of 1.12–15.21 M _⊙ in this work. This suggests that the mass segregation in Wd1 is more likely a dynamical effect.

The discovery of the massive black hole (BH) system Gaia BH3 in pre-release Gaia DR4 data suggests that wide BH binaries with luminous companions may be significantly overrepresented at low metallicities. Motivated by this finding, we have initiated a spectroscopic survey of low-metallicity stars exhibiting elevated RUWE values in Gaia DR3, using the FEROS and Automated Planet Finder spectrographs. We identify promising BH binary candidates as objects with instantaneously measured radial velocities (RVs) that are very different from their mean RVs reported in Gaia DR3. Thus far, we have observed over 500 targets, including a nearly complete sample of stars with [Fe/H] < -1.5, RUWE > 2, and G < 15. Our search has yielded one promising target exhibiting slow acceleration and an RV more than 98 km s(-1) different from its DR3 mean RV, as well as dozens of other candidates with smaller RV discrepancies. We quantify the sensitivity of our search using simulations, demonstrating that it recovers at least half of the BH companions within our selection criteria. We make all the spectra and RVs from our survey publicly available and encourage further follow-up.

Gravitational microlensing provides a unique opportunity to probe the mass distribution of stars, black holes, and other objects in the Milky Way. Population simulations are necessary to interpret results from microlensing surveys. The contribution from binary objects is often neglected or minimized in analysis of observations and simulations despite the high percentage of binary systems and microlensing's ability to probe binaries. To simulate the population effects, we added multiple systems to Stellar Population Interface for Stellar Evolution and Atmospheres ( SPISEA ), which simulates stellar clusters. We then inject these multiples into Population Synthesis for Compact-object Lensing Events ( PopSyCLE ), which simulates Milky Way microlensing surveys. When making OGLE observational selection criteria, we find that 55% of observed microlensing events involve a binary system. Specifically, 14.5% of events have a multiple lens and a single source, 31.7% have a single lens and a multiple source, and 8.8% have a multiple lens and a multiple source. The majority of these events have photometric light curves that appear single and are fit well by a single-lens, single-source model. This suggests that binary source and binary lens−binary source models should be included more frequently in event analysis. The mean Einstein crossing time shifts from 19.1 days for single events only to 21.3 days for single and multiple events, after cutting binary events with multiple peaks. The Einstein crossing time distribution of single and single-peaked multiple events is better aligned with observed distributions from OGLE than singles alone, indicating that multiple systems are a significant missing piece between simulations and reality.

We present the La Silla Schmidt Southern Survey (LS4), a new wide-field, time-domain survey to be conducted with the 1 m ESO Schmidt telescope. The 268 megapixel LS4 camera mosaics 32 2k x 4k fully depleted CCDs, providing a similar to 20 deg2 field of view with 1 '' pixel-1 resolution. The LS4 camera will have excellent performance at longer wavelengths: in a standard 45 s exposure the expected 5 sigma limiting magnitudes in g, i, z are similar to 21.5, similar to 20.9, and similar to 20.3 mag (AB), respectively. The telescope design requires a novel filter holder that fixes different bandpasses over each quadrant of the detector. Two quadrants will have i band, while the other two will be g and z band with color information obtained by dithering targets across the different quadrants. The majority (90%) of the observing time will be used to conduct a public survey that monitors the extragalactic sky at both moderate (3 days) and high (1 day) cadence, as well as focused observations within the Galactic plane and bulge. Alerts from the public survey will be broadcast to the community via established alert brokers. LS4 will run concurrently with the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST). The combination of LS4+LSST will enable detailed holistic monitoring of many nearby transients: high-cadence LS4 observations will resolve the initial rise and peak of the light curve while less-frequent but deeper observations by LSST will characterize the years before and after explosion. Here, we summarize the primary science objectives of LS4 including microlensing events in the Galaxy, extragalactic transients powered by massive black holes or stellar explosions, the search for electromagnetic counterparts to multi-messenger events, and supernova cosmology.

There are expected to be millions of isolated black holes in the galaxy resulting from the deaths of massive stars. Measuring the abundance and properties of this remnant population would shed light on the end stages of stellar evolution and the evolution paths of black hole systems. Detecting isolated black holes is currently only possible via gravitational microlensing, which has so far yielded one definitive detection. The difficulty in finding microlensing black holes lies in having to choose a small subset of events, based on characteristics of their light curves, to allocate expensive and scarce follow-up resources to confirm the identity of the lens. Current methods either rely on simple cuts in parameter space without using the full distribution information or are only effective on small subsets of events. In this paper, we present a new lens classification method. The classifier takes in posterior constraints on light-curve parameters and combines them with a Galactic simulation to estimate the lens class probability. This method is flexible and can be used with any set of microlensing light-curve parameters, making it applicable to large samples of events. We make this classification framework available via the popclass Python package. We apply the classifier to ∼10,000 microlensing events from the Optical Gravitational Lensing Experiment survey and find 23 high-probability black hole candidates. Our classifier also suggests that the only known isolated black hole is an observational outlier, according to current Galactic models, and the allocation of astrometric follow-up on this event was a high-risk strategy.

We present an analysis of adaptive optics images from the Keck I telescope of the microlensing event MOA-2011-BLG-262. The original discovery paper by Bennett et al. reports two 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 ∼10 yr 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 host = 0.19 ± 0.03 M ⊙ and m p = 28.92 ± 4.75 M ⊕ at a distance of D L = 7.49 ± 0.91 kpc. We perform a multiepoch cross reference to Gaia Data Release 3 and measure a transverse velocity for the candidate lens system of v L = 541.31 ± 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 Roman Galactic Exoplanet Survey will be able to securely measure low-mass host stars in the bulge.