When a Bar Tricks the Eye: How Streaming Gas Motions Imitate a Bulge in the Milky Way
Junichi Baba’s study shows that gas motions in the Milky Way’s inner regions are strongly influenced by the central bar, creating non-circular streaming that mimics a massive bulge. Using simulations, he demonstrates that the steep rise in the inner rotation curve can be explained without extra mass, cautioning against overestimating the Galaxy’s central mass from gas-based methods alone.
A Cool Ocean World Beyond Earth? JWST Reveals K2-18 b’s Watery Interior
Renyu Hu et al. used JWST to study the atmosphere of K2-18 b, a temperate sub-Neptune. They detected methane and carbon dioxide but no water vapor, suggesting a water-rich interior beneath a thin hydrogen atmosphere. The findings hint at a possible liquid-water ocean, though alternative models remain plausible.
Unveiling the Faint Edges of Star Clusters in the Milky Way
Chiti et al. used DELVE survey data to search for faint, extra-tidal features around 19 Milky Way globular clusters. They discovered a clear stellar envelope around NGC 5897 and tentative evidence around NGC 7492. Simulations suggest deeper surveys like LSST can improve detections, though additional techniques may still be needed.
Tracing Ancient Mergers in the Heart of the Milky Way: RR Lyrae Stars and the Gaia-Enceladus/Sausage in the Inner Stellar Halo
Kunder et al. study RR Lyrae stars in the Milky Way’s inner halo to trace remnants of the ancient Gaia-Enceladus/Sausage merger. They find about 6-9% of these stars show motions and metallicities consistent with GES, less than in the solar neighborhood. Their results match simulations showing GES debris is less concentrated near the Galactic center.
Mapping the Milky Way’s Metal: Chemical Clues from Open Star Clusters
Jonah Otto and colleagues use data from 164 open star clusters to map how elements are distributed across the Milky Way. They find that metallicity generally decreases with distance from the center and detect, for the first time with OCCAM data, variations depending on direction (azimuth). Their high-quality sample confirms known trends and reveals new clues about Galactic evolution.
A White Dwarf in Waiting: A Precursor Star with a Mysterious Massive Companion
Gautham Adamane Pallathadka and colleagues discovered SDSS J0229+7130, a proto-white dwarf orbiting an unseen, massive companion, likely a heavy white dwarf or neutron star. Using spectra, lightcurves, and models, they determined the system’s 36-hour orbit and ruled out a main-sequence companion. This rare binary likely formed through stable mass transfer and may evolve into a long-lived, compact pair emitting gravitational waves.
Catching the Streams: Dynamical Moving Groups in the Milky Way’s Halo
This paper finds two inward-moving streams of halo stars, “Iphicles” and “the Beret”, near the Sun, likely caused by resonances with the Milky Way’s bar rather than past mergers. Using Gaia data and simulations, the authors show these streams trace the bar’s influence throughout the halo, helping measure the Galaxy’s mass and bar pattern speed.
Tracing the Milky Way’s Past: How Globular Clusters Reveal the History of the Gaia-Sausage-Enceladus Merger
Fernando Aguado-Agelet and colleagues studied 13 globular clusters linked to the Gaia-Sausage-Enceladus (GSE) merger to trace the Milky Way’s history. They found most clusters follow a clear age-metallicity pattern, with two distinct star-formation bursts about 2 billion years apart, likely triggered by GSE’s interaction with the Milky Way. Two clusters probably formed in the Milky Way, and two others may belong to different mergers.
Unwrapping the Milky Way’s Warp: Insights from Classical Cepheids
Zhou et al. used Cepheids from Gaia to model the Milky Way’s warp, finding it starts closer to the center than thought, rises smoothly outward, and twists into a leading spiral. Their best-fit model also measured a slow, nearly uniform precession rate of about 4.86 km/s/kpc, offering insights into the warp’s structure and evolution.
When Stars Collide: Evidence for a Stellar Flyby in the Solar System’s Past
Susanne Pfalzner and colleagues propose that a close stellar flyby early in the Solar System’s history explains both the orbits and colors of trans-Neptunian objects. Their simulations show that such a flyby naturally produces the observed patterns, with red TNOs remaining in low-inclination orbits and grey ones populating higher inclinations and eccentricities. Upcoming observations could test these predictions.
How Did the Milky Way’s Halo Form? Designing the HALO7D-X Survey
The paper presents the HALO7D-X survey, designed to study the Milky Way’s stellar halo by combining Hubble, Gaia, and Keck data. Using simulations, the authors show the survey can distinguish whether the halo formed through many small early mergers or a few large later ones. HALO7D-X will reveal how the Galaxy’s halo assembled over time.
A First Glimpse of the Interstellar Comet 3I/ATLAS
Cyrielle Opitom and collaborators present early VLT/MUSE observations of the interstellar comet 3I/ATLAS, revealing a red, dusty coma but no detectable gas at 4.47 au from the Sun. Its color is redder than typical comets, resembling distant Solar System bodies. Future observations will track its activity and test predictions about its composition.
A New Dance Partner for Uranus: Discovering a Minor Planet in Outer Resonance
Daniel Bamberger and colleagues discovered that minor planet 2015 OU194 is in a rare, stable 3:4 outer mean motion resonance with Uranus, the first known example. Using extended observations and simulations, they showed its orbit remains resonant for over a million years. Two other candidates were examined, but none matched 2015 OU194’s stability and near-circular orbit.
How Do Ultra-Faint Dwarf Galaxies Get Their Metals?
Wheeler et al. investigate what determines the metallicity of ultra-faint dwarf galaxies. They find that the metallicity of the intergalactic gas these galaxies accrete is too low to explain observations. Instead, the metallicity is set mainly by internal processes: how much metal the galaxy produces and how much it loses through outflows. Their models match observed metallicities when outflow strength varies appropriately.
Unearthing the Dark Side: What Three Tiny Galaxies Reveal About Dark Matter
Hao Yang and colleagues studied the dark matter in three Milky Way dwarf galaxies using DESI data. They compared single- and two-population models, finding diverse inner dark matter profiles: Draco showed a cusp-like center, while Sextans and Ursa Minor leaned toward cores. Their results align with previous findings but also highlight uncertainties from data and modeling choices.
Turning Off the Lights: What Earth Hour Teaches Us About Light Pollution in Cities
Chu Wing So and colleagues used Hong Kong’s Earth Hour as a natural experiment to study light pollution. They found that turning off about 120 decorative and advertising lights reduced sky brightness by up to 50%. Most urban light pollution came from a few highly visible buildings and billboards, suggesting targeted lighting policies could significantly improve urban night skies.
How Galactic Collisions Sculpt the Cosmos: The Impact of Merger Orbits on Galaxy Structure
Wu et al. study how the paths of galaxy mergers affect the resulting structure using simulations. They find that spiral-in orbits tend to preserve disks, while head-on collisions destroy them and build bulges and halos. The hot inner stellar halo grows in nearly all mergers, making it a strong indicator of merger history.
A New Visitor From Beyond: Discovery and First Look at 3I/ATLAS
Darryl Seligman and colleagues report the discovery of 3I/ATLAS, the third known interstellar object. Unlike ‘Oumuamua or Borisov, it shows weak activity, a flat light curve, and a moderately red color. Estimated to be ~10 km wide, it’s larger than Borisov and offers a new glimpse into interstellar diversity. The authors call for more observations to understand its nature before and after perihelion.
How Titan’s Soil Controls Its Skies: Thermal Inertia and the Boundary Layer on Saturn’s Largest Moon
This study explores how Titan’s surface thermal inertia influences its near-surface atmosphere. Using simulations, Han and Lora show that daily temperature swings depend strongly on surface properties, while seasonal changes are mostly shaped by the atmosphere. Their results explain past Huygens probe data and predict stable conditions for the upcoming Dragonfly mission.
Tracking Solar Supergranulation: How the Sun’s Surface Patterns Evolve Over Its Magnetic Cycle
This study tracks how the Sun’s supergranulation, a surface flow pattern, varies over its magnetic cycle and impacts exoplanet detection. Using HARPS-N data and two correction methods, the authors found that supergranulation timescales are longest during solar minimum. They also show that observing strategies must account for this variability to improve planet-hunting precision, both for the Sun and other stars.