Pranav Nagarajan

Interaction in massive binary stars is expected to produce a large population of intermediate-mass (2 − 8 M _⊙ ) envelope-stripped stars, yet such objects have remained elusive in the Milky Way. We report the identification of an unambiguous Galactic example in a short-period ( P = 5.94 d), double-lined spectroscopic binary, discovered in the SDSS-V Milky Way Mapper survey. The system consists of a rapidly rotating O-type star and a hotter, lower-mass companion, which shows He II and N IV emission lines with large radial velocity variations, revealing its binary nature. Combined orbital constraints and joint spectroscopic and photometric modelling show that the companion is a hot ( T _* ≈ 60 kK), helium-rich star with a mass of 3.2 − 5.8 M _⊙ , placing it squarely in the intermediate-mass regime and below values typically inferred for classical Wolf–Rayet (WR) stars. Its spectral properties are inconsistent with accretion-powered emission and instead indicate a stripped star that formed through binary mass transfer. The system’s short period, negligible eccentricity, and rapidly rotating O-star point to a post-interaction configuration following efficient mass transfer and spin-up of the accretor. Comparison with binary evolution models suggests that the stripped star is observed in a brief inflated phase following mass transfer, which increases its optical flux contribution and facilitates its detection. The inferred mass-loss rate log Ṁ /(M _⊙ yr ⁻¹ ) = −6.3 ± 0.1 is in line with mass-loss rates observed for classical WR stars in the Milky Way and exceeds those measured for intermediate-mass stripped stars in the Magellanic Clouds, with the caveat that our target selection is biased towards systems with stronger emission features. As an unambiguous and well-characterised intermediate-mass stripped star, this system provides a key benchmark for models of binary evolution at solar metallicity, stripped-envelope supernova progenitors, and the formation of compact-object binaries.

We present the first model of the Milky Way’s detectable compact object–luminous star binary population from disrupted dense star clusters. We bridge large-scale cosmological star cluster formation with high-resolution dynamical evolution of compact object binaries by mapping the predicted star clusters from the EMP- Pathfinder simulations to N -body Cluster Monte Carlo models. We predict that approximately 3 × 10 ⁵ white dwarfs (WDs), 1.5 × 10 ⁵ black holes (BHs), and 1 × 10 ³ neutron stars (NSs) in binaries with luminous companions are released to the Galaxy from now-disrupted dense star clusters throughout the history of the Milky Way. Synthetic observations modeled with the gaiamock pipeline reveal that the modeled Gaia DR3 yields are sparse (≈2 WDs, 0 NS, 0 BHs at 90% credibility), with the majority lying beyond the detection horizon. Gaia DR4 is expected to increase the observational yield of these systems only marginally, as the benefits of an expanded search volume are largely offset by the diminished astrometric and photometric precision of more distant sources (≈14 WDs, 0 NS, 0 BHs). While the underlying BH binary population is similar to that of WDs, they are detected far less frequently; they tend to pair with lower-mass, dimmer companions and have less temporal coverage of their long orbital periods. For NSs, we suggest that the observed overrepresentation of metal-poor, halo systems is inconsistent with an origin in disrupted dense star clusters. Instead, the observed Gaia NS population could reflect the accretion history of metal-poor, dwarf galaxies into the Milky Way, isolated binary star evolution, or supernova physics.

The angle between stellar spin axes and planetary orbits—stellar obliquity—probes the dynamics of planetary migration and evolution. The obliquities of giant planets have been extensively studied because they are the most easily measured. Smaller planets, while more difficult to measure, have the advantage of better reflecting the dynamics of planetary systems because they trigger negligible backreactions onto the host star. This paper introduces a new observational campaign called the Small, Low-mass Oblique Planets Experiment (SLOPE) survey with the Keck Planet Finder (KPF) spectrograph and presents four new obliquity measurements. The SLOPE survey focuses on planets smaller than Saturn across a variety of system architectures. The sky-projected obliquities of the four planets measured—TOI-1386 b, TOI-480 b, TOI-4596 b, and TOI-1823 b—are all consistent with spin–orbit alignment. We validate the planetary nature of TOI-4596 b with a significant detection of the Rossiter–McLaughlin effect. Including these measurements, we conducted a statistical analysis of the obliquities of sub-Saturn-mass planets in different planetary system architectures. Compared to other architectures, those in compact multiplanet systems reside in orbits that appear preferentially aligned with the stellar equator with 6 σ confidence.

We calibrate the Next Generation Palomar Spectrograph (NGPS) for precision stellar radial-velocity (RV) measurements using repeated observations of 24 Gaia-ESO RV standards spanning G = 11–18. Because NGPS is mounted at the Hale telescope’s Cassegrain focus, changes in instrumental flexure produce wavelength-solution shifts of tens of km s ⁻¹ , with substantial time and wavelength dependence. We develop a pipeline that measures these shifts directly from each science exposure, using telluric absorption features in the R and I channels and the [O I ] 5577 Å sky-emission line in the G channel. This approach eliminates the need for frequent on-sky arc calibrations. The resulting wavelength-dependent flexure corrections reduce the median absolute discrepancy relative to Gaia-ESO RVs from tens of km s ⁻¹ to 1.6 km s ⁻¹ . Telluric absorption provides substantially more reliable flexure calibration than sky emission lines, which do not track point-source illumination of the slit. The I channel, which contains both many telluric lines and strong photospheric lines across a range of spectral types, generally yields the most stable RVs. The achieved precision remains above the photon-noise limit, indicating that NGPS RV performance is currently limited by calibration and modeling systematics. These results demonstrate that NGPS is well-suited for accurate and precise stellar RV measurements across a wide range of astrophysical studies.

Mass transfer is arguably the most defining interaction in binary stellar systems, yet many aspects of its physics remain poorly understood, from stability to endpoints and observable products. Comparing theory and observations is challenging because post-mass-transfer systems are studied across largely independent communities with different methods, nomenclature, and evolutionary frameworks. We present a unified review and catalog of ongoing and post-mass-transfer binaries spanning the full stellar mass range, but restricted to systems likely to have experienced only a single episode of mass transfer (i.e., only one component is evolved). We review 16 observational classes of binary interaction products and compile a curated sample of 5464 systems into a publicly available, community-driven catalog at https://binary-observations.github.io/post_mt_catalog/ . Using this catalog, we investigate global trends in orbital periods, eccentricities, masses, and mass ratios across post-mass-transfer binaries. We find (i) nonzero eccentricities are common at all periods and system classes, with both median values and scatter increasing with period, (ii) the e – log P relation depends on donor progenitor mass, with neutron star (NS) and black hole (BH) binaries showing the highest median eccentricities, likely reflecting effects of natal kicks, (iii) period distributions are broad and overlapping across evolutionary channels, and (iv) the Gaia BH and NS systems are extreme in mass ratio but otherwise consistent with the general post-mass-transfer population. Together, these results support a unified empirical view of post-mass-transfer binaries that highlights several tensions between theory and observations.

RR Lyrae stars (RRLs) are classical tracers of old stellar populations, yet growing evidence suggests the presence of a metal-rich (mathrm Fe/H ≳-0.5), intermediate-age (2--7 Gyrs) sub-population in the Milky Way disc. Binary evolution, particularly stable mass transfer, has been proposed as a viable formation channel, which predicts that most metal-rich (mathrm Fe/H ≳-0.5), intermediate-age (t_ age łesssim9 Gyr) RRLs should reside in binaries with orbital periods of ∼900–2000 days. However, no genuine RRL binaries have been robustly identified, including in the Data Release 3 (DR3) astrometric binary catalogues, despite being sensitive to the predicted orbital-period range. We investigate whether the lack of detections in DR3 reflects an intrinsically low binary fraction or instead arises from observational biases. We analysed a carefully selected sample of 100 DR3 RRLs designed to trace the metal-rich population with thin-disc kinematics and compared them with predictions from binary evolution models. We generated realistic observation mocks, including variability-induced astrometric biases, and assessed the detectability of binaries and the posterior constraints on the hidden binary fraction using astrometric quality indicators, such as ůwe, and a robust Bayesian inference. While current uncertainties prevent a definitive rejection of a high fraction of hidden binaries, our results reveal tensions between existing binary evolution predictions and the DR3 non-detections. This suggests either the presence of unaccounted systematics in the modelling of observations or the need to revise assumptions in binary evolution models. We predict that DR4 will significantly improve the binary detectability and provide powerful new constraints on the post-interaction binary populations. Gaia Gaia Gaia Gaia Gaia Gaia Gaia Gaia

Black holes (BHs) and neutron stars (NSs) with low-mass stellar companions challenge traditional isolated binary evolution models, motivating hierarchical triple evolution as a promising alternative. To search for tertiaries, we perform deep, adaptive optics-assisted, near-infrared imaging of five quiescent BH low-mass X-ray binaries (LMXBs), Gaia BH1, and twelve Gaia NSs. We detect several faint stars previously unresolved in survey imaging, but none are close enough to robustly rule out a chance alignment. To achieve high contrast sensitivity at close separations, we use the reference star differential imaging strategy with the Karhunen–Loéve Image Processing algorithm to model and subtract the point-spread function of each target. We identify tertiary candidates in the speckle-dominated regime, but injection-recovery tests suggest most 5 σ detections are likely artifacts. We derive 5 σ contrast curves and convert these to limits on the mass of main sequence (MS) tertiaries and the effective temperature of white dwarf (WD) tertiaries consistent with a non-detection. We rule out plausible MS tertiaries and young, hot WD tertiaries at projected separations ≳500 au for the Gaia compact object binaries and ≳2000 au for the more distant BH LMXBs. While the recent discovery of a 1.2 M _⊙ tertiary to V404 Cygni supports triple formation scenarios for BH LMXBs, our results suggest such companions are relatively rare. Our observations remain consistent with intermediate-mass tertiaries that have since evolved into cool WDs, detectable with deeper JWST imaging. Follow-up observations are required to measure proper motions and confirm or rule out physical association of tertiary candidates.

PSR J1928+1815 is a 10.55 ms millisecond pulsar in a 3.6 hr orbit with a massive (1.0–1.6 M _⊙ ) companion that produces extended radio eclipses. The companion, proposed to be a stripped helium star, is undetected in optical and infrared surveys. We present deep near-infrared imaging using Keck/NIRC2 with laser guide star adaptive optics. No source is detected at the pulsar position down to a 5 σ limit of K _s ≈ 21.3. Using stripped-star atmosphere models and conservative extinction estimates, we show that any plausible helium star companion would have been detected, ruling out this interpretation. A massive white dwarf (WD) companion remains consistent with the non-detection. We consider two possible origins for the eclipses: (1) absorption in a wind driven by a young, hot WD, and (2) material ablated from the WD by the pulsar. The former can naturally arise following Case BB mass transfer, which produces ∼1.2 M _⊙ WDs capable of sustaining winds of M ̇ ≳ 1 0 − 12 –10 ⁻¹³ M _⊙ yr ⁻¹ for ∼10 ⁴ –10 ⁵ yr, sufficient to obscure the pulsar at GHz frequencies. The latter requires efficient coupling of the pulsar’s spin-down luminosity to the companion to drive the needed mass loss, which may be difficult to achieve. If the eclipse is powered by a WD wind, the system is likely observed in a short-lived phase; alternatively, if the companion is an older WD, the origin of the eclipsing material remains unclear. The apparent uniqueness of PSR J1928+1815 is consistent with a short detectability lifetime, though formation rate estimates remain uncertain.

When low-mass (≲2 M _⊙ ) red giants lose their envelopes to a companion just before the helium flash, the resulting mass transfer can produce binaries hosting hot subdwarfs, horizontal branch stars, and undermassive red clump stars. Recent work predicts a continuum of such products, from fully stripped hot subdwarfs to partially stripped horizontal branch and red clump stars, and suggests that young, metal-rich RR Lyrae can form when partial stripping leaves a helium-burning star in the instability strip. To enable direct comparison with observations, we model these binaries in a simulated Milky Way–like galaxy with a realistic metallicity-dependent star formation history and 3D dust map, generate epoch astrometry using Gaia’s scanning law, and fit it with the cascade of astrometric models applied in Gaia DR3. We compare the simulated population to DR3 observations of hot subdwarfs, RR Lyrae, and red giants with high astrometric mass functions. The model significantly overpredicts the number of hot subdwarfs with astrometric binary solutions, partly because the predicted flux ratios are more unequal than observed. It also predicts ≳100 RR Lyrae with DR3 astrometric orbital solutions, while none are observed. We conclude that RR Lyrae in au-scale binaries may be substantially rarer than predicted. In contrast, the model plausibly explains the population of red clump stars with high astrometric mass functions, which we interpret as potential black hole impostors. We predict that ∼10 × more stripped-star binaries will be detectable in DR4, whose sensitivity to longer periods will more strongly test wide-orbit systems.

BE Lyncis (BE Lyn) is a well-studied high-amplitude δ Scuti variable star. Recently, Niu et al. analyzed a 39 yr baseline of times of maximum light of BE Lyn, reporting that it is the most eccentric binary known ( e ≈ 0.9989) and hosts the nearest black hole (BH) known to date. We analyze Hipparcos and Gaia astrometry of BE Lyn, predicting what the observed proper motion anomaly (PMA) over the 25 yr baseline between the two missions would be were the companion really a ≳17.5 M _⊙ BH. We find that the predicted PMA is at least an order of magnitude larger than the observed value of ≈1.7 ± 0.8 mas yr ⁻¹ , regardless of the assumed orientation of the orbit. We predict the expected Gaia DR3 RUWE for different orientations of the putative BH binary, finding that it ranges from ≈2.5 to 4.0, much larger than the reported value of 1.073. The observed value is instead consistent with a low-mass secondary or a single star. We find that BE Lyncis would have received a 7-parameter acceleration solution if it were a BH binary, in contradiction with its absence from the Gaia DR3 non-single star catalogs. Finally, we show that the reported orbit is impossible because the luminous star would overflow its Roche lobe at periastron, irrespective of inclination. We recommend caution in interpreting light-travel time effect models that require very high eccentricities, face-on inclinations, or large companion masses. The observed pulsation timing variations are most likely simply a result of red noise or pulsation phase evolution.