Richard W. Pogge

To search for very wide-orbit binary-lens events, we crossmatch microlensing events detected by the Optical Gravitational Lensing Experiment (OGLE)-IV and KMTNet between 2011 and 2024. After removing contaminants from variable stars, we identify OGLE-2016-BLG-0523 as a microlensing event showing two short-lived bumps in its light curve, with the two separated by 8.36 yr. We investigate three scenarios to explain the two bumps, including a binary-lens single-source (2L1S), a single-lens binary-source (1L2S), and two unrelated single-lens single-source events (2×1L1S). The 1L2S interpretation is excluded due to its poorer fit to the data, while the 2×1L1S scenario is disfavored by the low event rate, unphysical total source flux, and unphysical color for one of the two putative sources. For the surviving 2L1S interpretation, the Einstein timescales of the two bumps are 8.343 ± 0.076 days and 3.231 ± 0.047 days, respectively, implying a binary mass ratio of q = 0.15. A Bayesian analysis indicates that the primary lens is most likely a low-mass M dwarf, while the companion is probably a low-mass brown dwarf or super-Jupiter. The inferred projected separation is about 450 au, making the companion the widest low-mass object discovered via microlensing to date. Our discovery highlights both the promise and challenges of identifying very wide, low-mass companions through microlensing. Although upcoming facilities such as Roman and Earth 2.0 space telescopes will improve sensitivity to such events, overcoming degeneracies and distinguishing them from variable phenomena will remain essential for building a robust statistical sample.

The primordial abundance of ⁴ He plays a central role in Big Bang nucleosynthesis (BBN) and in the cosmic microwave background (CMB). The LBT Y _p Project’s new measurement of the primordial ⁴ He mass fraction, Y _p = 0.2458 ± 0.0013, is the most precise determination to date. In this paper, we combine our new Y _p value with the latest primordial deuterium measurement and assess the consequences for cosmology. For standard BBN, where the number of light neutrino species is fixed at N _ν = 3, the single free parameter is the cosmic baryon density; the CMB measures this independently, with results consistent with each other. Combining Y _p , D/H, BBN, and the CMB gives the cosmic baryon-to-photon ratio η = (6.120 ± 0.038) × 10 ⁻¹⁰ , corresponding to a baryon density parameter Ω _B h ² = 0.02236 ± 0.00014. We then allow N _ν to vary and thus measure relativistic species present during nucleosynthesis. We find η = (6.101 ± 0.044) × 10 ⁻¹⁰ or Ω _B h ² = 0.02229 ± 0.00016, and N _ν = 2.925 ± 0.082, and for N _ν ≥ 3, Δ N _ν = N _ν − 3 ≤ 0.125 (95% confidence limit) during BBN and the CMB. Our results demonstrate consistency with the Standard Model of particle physics and the standard cosmology that links BBN at ∼ 1 s and the CMB at ∼400,000 yr.

We present a new determination of the primordial helium abundance based on new, high-quality Large Binocular Telescope (LBT) observations of 54 metal-poor H ii regions. These regions have been observed and analyzed uniformly. We also describe a number of updates to our methodology, including updated helium emissivities. Enabled by the large, high-quality dataset, we examine our sample targets for potential systematic errors, which could bias their results. We perform a standard 95% confidence level (CL) χ ² cut and find that a significantly larger fraction (47/54 = 87%) of our sample qualifies than for previous datasets. We also screen for quality and reliability, flagging targets that may introduce significant systematic errors, producing a dataset of 41 targets. In a significant breakthrough for the field, that dataset includes 15 high signal-to-noise ratio (SNR) targets with low metallicity (O/H ≤ 4 × 10 ⁻⁵ ). Due to this low-metallicity dataset, for the first time, a weighted average for determining the primordial helium abundance ( y _p ) is well justified and produces a robust result. By weighted average of our 15 low-metallicity targets, we determine y _p = 0.08146 ± 0.00058, with the corresponding mass fraction Y _p = 0.2458 ± 0.0013. This result achieves an unprecedented precision of 0.5%, and it is in good agreement with the Big Bang nucleosynthesis (BBN) result, Y _p = 0.2467 ± 0.0002, based on the Planck determination of the baryon density.

Accurately determining the elemental abundances of a low-metallicity nebula strongly depends on measuring the density ( n _e ) and temperature ( T _e ) of the gas. Because these two parameters are inherently degenerate when derived solely from H and He recombination lines, we rely on the density-sensitive He I λ 10830 line to assist in resolving this issue, especially for accurate He abundances. To facilitate this, we present near-IR (NIR) LBT Utility Camera in the Infrared (LUCI) spectra of 48 low-metallicity targets and homogeneously reduce them using Pypeit as part of the LBT Y _p Project. IR spectra require special care, and we wavelength calibrate by hand using the bright OH emission lines, carefully apply proper telluric corrections, and coadd the spectra of LUCI1 and LUCI2 on a resampled grid to ensure accurate results. We use a Gaussian profile to fit the emission lines and measure the fluxes relative to Paschen-gamma (P γ ), resulting in He I λ 10830 to P γ ratios consistent with previous studies. As a result, this work significantly expands the available dataset of NIR He I λ 10830 fluxes in low-metallicity galaxies. These high-quality measurements, where we find a median flux ratio uncertainty of σ ˜ = 0.08 , reduce the overall uncertainties in helium abundance estimates for individual targets. The increased size of the high-quality sample enables searching for systematic uncertainties and improves the reliability of the helium abundance determinations used to infer the primordial helium abundance.

Empirically measuring the primordial He mass fraction, Y _p , requires a significant number of low-metallicity nebulae with direct constraints on He/H and O/H abundances. This technique requires high-fidelity measurements of the gas-phase physical conditions, namely the electron temperature ( T _e ) and density ( n _e ). To this end, we present deep rest-optical spectroscopy for a sample of 62 low-metallicity (≲20% solar O/H) galaxies acquired using the Multi-Object Double Spectrographs on the LBT as part of the LBT Y _p Project. We discuss new fitting methods that recover the intensity of up to 61 H and He recombination lines, of which, up to 26 will be used to determine gas-phase He abundances, and we examine the emission line properties of the LBT Y _p Project sample. We assess different scaling relations in the low-metallicity interstellar medium (ISM), finding that n _e [Ar iv ] measured in 31 targets is systematically larger than n _e [S ii ] or n _e [O ii ]. The larger densities are insufficient to significantly bias T _e [O iii ] or the O/H abundance. T _e [S iii ] and T _e [O iii ] are strongly correlated over a range of ∼10 ⁴ K with very low scatter, and we calibrate new T _e [S iii ]– T _e [O iii ] scaling relations for use in other low-metallicity environments. We examine different T _e measured in the low-ionization gas, finding significant scatter compared to T _e [O iii ]. The precision direct O/H derived in this analysis (median uncertainty ∼4%) are consistent with prior literature measurements, albeit with relatively large scatter. These data provide a key component necessary to empirically measure Y _p and the abundance patterns of other elements in the ISM.

Extremely low-metallicity H ii regions have been observed with the goal of determining the primordial helium abundance ( Y _p ). Y _p , combined with standard Big Bang nucleosynthesis and the half-life of the neutron, provides a direct measurement of the number of neutrino families, but Y _p must be measured very precisely to provide meaningful constraints on physics beyond the Standard Model. Here we describe a program to combine new Large Binocular Telescope (LBT) observations with a new analysis methodology to significantly improve the determination of Y _p . The LBT, with its MODS and LUCI instruments, produces spectra, which, when combined with our new analysis methodology, are capable of delivering He abundances in individual H ii regions with uncertainties of approximately 2% or less. Archival LBT/MODS spectra of standard stars over a 4 yr period enable the determination of a wavelength-dependent uncertainty in the MODS spectral response, resulting in improved relative emission line uncertainties. An optimized sample of low-metallicity galaxies has been selected with the goal of producing a determination of Y _p with a precision of ∼0.5%, sufficient to provide an independent constraint on the effective number of neutrino families of ∼3%.

The Systematic KMTNet Planetary Anomaly Search series was conducted using the KMTNet data archived from 2016–2019. From this first phase of the series, we reported a total of 50 planetary systems hidden in the data archive, which represent about 35% of the total microlensing planets discovered from 2016–2019, demonstrating that this semi-machine-based search is a crucial channel for building a complete microlensing planet sample. We continue this series for 2021 and beyond to expand the microlensing planet sample. In this work for the 2021 KMTNet high-cadence fields (Prime fields), we find seven hidden planetary systems and three planet candidates. These new planets represent about 33% of the total microlensing planets discovered within the Prime fields observed during the 2021 bulge season. While the by-eye search is the primary channel for detecting microlensing planets (i.e., two-thirds of microlensing planet discoveries), this work clearly shows that a systematic search series is still necessary for constructing a complete microlensing planet sample. Such a sample is essential for conducting unbiased statistical studies of planet demographics in our Galaxy. Datasets for all of the events used for analyses in this work are publicly available: doi: https://zenodo.org/records/21472225 .

To complete the analysis of the 2023 KMTNet subprime-field microlensing planetary events identified by its AlertFinder system, we present the analysis of six events, KMT-2023-BLG-(1810, 0084, 1118, 0584, 1697, 2218). We find that the first three events are securely confirmed as planetary, with inferred mass ratios of log q ∼ − 1.9 , −2.0, and −2.6, respectively. The remaining three events exhibit the well-known degeneracy between binary-lens/single-source (2L1S) and single-lens/binary-source (1L2S) models, and two of these also admit viable stellar binary solutions. A Bayesian analysis indicates that the companions in the confirmed planetary events are likely either super-Jupiters orbiting beyond the snow line of M- or K-dwarf hosts or, for two degenerate solutions of KMT-2023-BLG-1118, Saturn-mass planets orbiting late-type M dwarfs. To date, the 2023 KMTNet sample contains 25 unambiguous planetary events, and its mass-ratio distribution is consistent with that of the KMTNet planetary sample from 2016 to 2019.

We report the discovery of four cold giant planets identified through the analysis of microlensing events detected by high-cadence surveys: OGLE-2016-BLG-0261, KMT-2025-BLG-0026, KMT-2025-BLG-0030, and KMT-2025-BLG-2272. The planetary signals appear as short-duration anomalies in the light curves and are well described by binary-lens single-source models with mass ratios between the lens components of order q ∼ 10 ⁻³ . Finite-source effects are securely measured in three out of four events, enabling determinations of the angular Einstein radius. A Bayesian analysis incorporating the measured event timescale and angular Einstein radius yields host masses of ∼0.07–0.6 M _⊙ and companion masses of ∼0.2–2.5 M _Jup , confirming that all companions lie in the giant-planet regime. The projected separations are ∼0.7–6 au, placing all planets at or beyond the snow lines of their host stars. The inferred lens distances span ∼6.6–7.9 kpc, with all systems consistent with bulge lenses. These detections expand the sample of cold giant planets from homogeneous high-cadence surveys and highlight the sensitivity of microlensing to planetary systems beyond the snow line, providing further constraints on the occurrence and properties of giant planets around low-mass stars.

WD J005311 is the peculiar stellar remnant of the Galactic supernova from 1181, and appears to have been the merger of two white dwarfs. We present time-resolved spectroscopy of WD J005311 showing emission line variability on a wide range of timescales. The strong O VI emission feature displays line profile variations (LPVs) on two distinct velocity scales. Broad variations with amplitudes of ±10% of the line flux are seen over the entire O VI line. These broad LPVs likely arise from instabilities in the line-driven wind produced in many Wolf–Rayet stars. There is a hint of coherent structure in the broad LPVs that is consistent with rotation over roughly two hours, although the features survive for less than a full cycle. Low-amplitude, narrow LPVs are also detected within the central ± 5000 km s ⁻¹ of the O VI line. We associate these features with an unstable disk formed from the rigidly rotating magnetosphere (RRM) of the remnant white dwarf. We also analyze archival near-UV photometry of WD J005311 and find a pseudo-periodic oscillation with an hour-long timescale that may be associated with the breakout instability of the RRM disk.