Yang Huang

Supermassive neutron stars ( M ≳ 2   M ⊙ ) provide stringent constraints on the equation of state of dense matter. We report the discovery of a wide binary system composed of a red giant star and an invisible companion with an orbital period P ≈ 1218 ± 3   days and a comparatively low eccentricity e ≈ 0.09 ± 0.01 (with 1σ uncertainty). By combining radial velocity measurements from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the Apache Point Observatory Galactic Evolution Experiment (APOGEE) with astrometric data from Gaia DR2 and DR3 catalogs, we derive a Keplerian orbit with an inclination angle i ≈ 73 ∘ ± 11 ∘ . The unseen companion has a mass in the range of 1.8   to   2.8   M ⊙ (spanning the full spread of all estimates), with a best-fit value of 2.20 − 0.13 + 0.14   M ⊙ . This potentially places the unseen companion within the mass gap between neutron stars and black holes. Spectral disentangling using high-resolution near-infrared APOGEE spectra shows no absorption features from an additional component, suggesting the presence of a compact object: either a supermassive neutron star or a low-mass black hole. The wide, low-eccentricity orbit is difficult to explain through standard binary evolution channels, pointing to the need for improved theoretical models to account for such systems.

Dwarf galaxies serve as fossils for understanding the early star formation history (SFH) of the local Universe. In this work, we use stellar abundances for individual stars derived from the first data release (DR1) of the Dark Energy Spectroscopic Instrument (DESI) to investigate the SFH of the dwarf spheroidal galaxy Draco. We simultaneously fit the stellar metallicity distribution function (MDF) and the [Mg/Fe]–[Fe/H] trend using the galactic chemical evolution (GCE) model flexCE . Our best-fit models imply a low star formation efficiency (SFE) of 0.003–0.007 Gyr ⁻¹ , which is about 10 times lower than values reported in literature, but consistent with recent SFE estimates for ultrafaint dwarf galaxies. By combining recent literature measurements, we find that the SFEs of Milky Way dwarf galaxies scale with stellar mass as SFE ∝ M ⋆ 0.24 . A low SFE is associated with a strong outflow efficiency; we estimate a large wind mass-loading factor η _wind of ≃172–260. This is well in line with the observed outflow–stellar mass relation and with the Feedback in Realistic Environment (FIRE) cosmological simulations. Our results demonstrate the power of large-scale, low- to medium-resolution stellar spectroscopic surveys to deliver robust constraints on the early formation history of galaxies in the local Universe.

The mass distribution of the Andromeda galaxy (M31) is fundamental for understanding its dark matter content, assembly history, and dynamical role within the Local Group. However, estimates of M31’s total mass remain quite uncertain, partly because of limited dynamical constraints at large galactocentric radii. Moreover, M31’s recent merger history has produced abundant tidal substructures and may have left its outer halo out of dynamical equilibrium, complicating equilibrium-based mass estimates. We use over 8500 tracers with line-of-sight velocities to constrain the rotation curve (RC) of M31 over R = 2–167 kpc. In the disk, we derive the RC from a clean tracer sample using the Jeans equation and find good agreement with previous H i measurements. In the halo, we use Gaussian mixture modeling to account for tidal substructures and Galactic foreground contamination and constrain the kinematics of the residual dynamically hot halo. Assuming this halo population can be treated as an equilibrium tracer, we infer the outer RC and constrain M31’s mass distribution for different velocity anisotropies β and dark matter halo profiles. The Navarro–Frenk–White (NFW) and Dekel–Zhao (DZ) models yield broadly consistent total masses ranging from 1.1 2 − 0.10 + 0.11 × 1 0 12 M ⊙ to 1.6 3 − 0.17 + 0.21 × 1 0 12 M ⊙ for β = 0.3–0.7. In contrast, the Einasto profile favors substantially lower masses of 0.4 1 − 0.06 + 0.09 to 0.8 6 − 0.16 + 0.21 × 1 0 12 M ⊙ , highlighting significant dependence on the adopted halo profile, although Einasto solutions are more in tension with independent mass estimates based on M31 satellite galaxies than the NFW and DZ models at fixed β . Overall, our analysis provides a comprehensive measurement of M31’s mass distribution under the assumption of dynamical equilibrium, while also quantifying the systematic uncertainties associated with halo velocity anisotropy and the choice of dark matter density profile.

Extremely metal-poor (EMP; [Fe/H] < −3.0) stars preserve key information about the earliest stages of Galactic chemical evolution. Based on a training set of spectroscopic metallicities, we constructed catalogs of more than 120,000 EMP candidates from large-scale narrowband photometric surveys. To validate this approach, we obtained high-resolution CFHT/ESPaDOnS spectra for 13 candidates selected from the SkyMapper-based catalog. The follow-up observation confirms that the narrowband photometric selection is effective in identifying very metal-poor (VMP) stars and retaining a substantial EMP fraction: four targets are confirmed as EMP stars, and all targets remain VMP with [Fe/H] < −2.4. The photometric metallicities are systematically lower than the spectroscopic values by 0.44 dex, with a relatively small scatter of 0.19 dex, indicating a systematic offset in the photometric metallicity scale that could be reduced through improved calibration. Abundances for more than 20 chemical species are derived, leading to the identification of a new potassium-enhanced r -II star, J2001-1215. By combining chemical abundances with orbital properties, five stars are found to be dynamically consistent with known Galactic substructures, including Gaia–Sausage–Enceladus (GES), Thamnos, and Sequoia. These stars provide high-resolution chemical measurements for the EMP regime of these Galactic substructures. These results underscore the role of narrowband photometry in efficiently selecting EMP candidates for targeted high-resolution spectroscopic follow-up, thereby enabling detailed chemical and dynamical studies of the early Milky Way.

Forward shocks produced by interactions between a single ejected blob and the circumnuclear medium (CNM) around a supermassive black hole are widely invoked to explain radio emission from tidal disruption events (TDEs). However, recent observational evidence such as rapidly rising radio emission, double-peaked broadband spectral energy distributions, and declining shock energies at late times poses challenges to this picture. Alternative scenarios have therefore been proposed, including bow shocks arising from outflows colliding with dense clouds, the production of new ejecta at late times, and variations in microphysical parameters. To investigate the roles of these factors, we analyze the long-lived radio flare of the TDE candidate AT 2019avd, for which dust has been identified through mid-infrared observations, using a radio dataset spanning more than 5 yr. We find that a bow-shock scenario can account for the majority of the observed radio emission, while a forward shock, or an additional bow shock involving different clouds, may begin to dominate at late times. However, a forward-shock scenario remains viable when allowing for evolving microphysical parameters and more complex CNM structures. We discuss the remaining tensions in both the outflow–CNM and outflow–cloud interaction scenarios.

Context. The unstable mass accretion process in young stellar objects (YSOs) often triggers observable outbursts. These episodic accretion events play a critical role in stellar mass assembly during the pre-main-sequence phase. In this paper, we present observations of an eruptive young star in the Rosette Nebula, identified by the Gaia Science Alerts system using Gaia time-series data. Aims. We aim to investigate the evolution of the brightness and mass accretion rate of V557 Mon throughout its outburst and subsequent decline. In addition, we trace the evolution of the inner accretion disk during the outburst by monitoring molecular emission features. Methods. We compiled multi-band photometric time-series from Gaia, ZTF, and several 1 m-class ground-based telescopes and obtained optical and near-infrared spectra at multiple epochs covering the outburst and fading phases. Stellar parameters were derived from quiescent colour, spectra and spectral energy distribution (SED) fitting. We also measured the mass accretion rate and fit models to molecular emission bands. Results. Since late 2024, V557 Mon has undergone a year-long outburst consistent with EXor variability. Based on quiescent photometry, V557 Mon has a spectral type of M1 with an extinction of A _V = 1.8 ± 0.3 mag, consistent with a 0.4–0.5 M _⊙ star at an age of 2 Myr. Our multi-epoch spectra and u -band photometry indicate a peak accretion rate of 6.3 × 10 ⁻⁷ M _⊙ yr ⁻¹ during the outburst, roughly 70 times higher than in quiescence. We report the detection of hot water vapour emission bands, together with TiO, VO, and CO emission features. Using ExoMol models, we measured that the inner-disk temperature changed from 3000 K to 2000 K during the fading phase of the outburst. Conclusions. We report a recent EXor outburst in a low-mass Class II YSO. Our observations reveal the transient formation of a hot molecular inner disk, traced by variable water vapour emission during the EXor event. A positive correlation is found between the molecular excitation temperature and the overall stellar brightness.

Stellar X-ray emission serves as a direct diagnostic of coronal activity, which is fundamentally linked to coronal heating processes. It also strongly influences the atmospheres and long-term habitability of orbiting exoplanets. Investigating how this high-energy emission evolves is therefore essential for understanding the evolution of stellar magnetic dynamos and planetary atmospheres and habitability. In this work, we investigate the evolution of X-ray activity and X-ray and UV (XUV) irradiation for a sample of F–M dwarf stars based on Chandra X-ray Observatory and XMM-Newton Observatory observations. We find that F- and G-type stars broadly follow the traditional evolutionary picture of an early saturated (or weakly declining) phase followed by a modest decline, whereas K- and M-type stars exhibit a clear three-phase evolution of a saturated phase, an intermediate phase of rapid decay, and a final modest decline phase. By combining X-ray, UV, and Ca II H and K bands, we show that coronal emission becomes increasingly dominant toward lower-mass stars. Based on the cumulative XUV emission calculated from our fitted relation, planets around F- and G-type stars experience relatively moderate XUV environments, while those around K- and M-type stars may exceed the empirical cosmic shoreline shortly after reaching the main sequence, though this conclusion depends on the adopted shoreline value.

Astronomical image interpretation requires not only visual perception, but also familiarity with domain-specific representations, observational conventions, and astrophysical reasoning. Although recent multimodal large language models (MLLMs) have shown strong general visual–language capabilities, their performance on specialized astronomical figures remains insufficiently evaluated. To address this gap, we introduce AstroMMBench, an astronomy-specific benchmark for evaluating MLLMs on figure-associated astronomical multiple-choice questions. AstroMMBench contains 592 expert-screened multiple-choice questions covering six major astrophysical subfields: Astrophysics of Galaxies, Cosmology and Extragalactic Astrophysics, Earth and Planetary Astrophysics, High Energy Astrophysical Phenomena, Instrumentation and Methods for Astrophysics, and Solar and Stellar Astrophysics. The questions were generated from astronomical figures and associated textual context through an automated pipeline, followed by multi-stage filtering and expert review to assess scientific correctness, image–question alignment, and answer uniqueness. Using AstroMMBench, we evaluated 25 selected MLLMs, including 22 open-source and 3 closed-source models. The results show substantial performance differences across models and astrophysical subfields. Ovis2-34B achieved the highest point-estimate overall accuracy of 71.3%, with performance comparable to strong closed-source models such as ChatGPT-4o and Doubao-1.5-vision-pro. Subfield-level analysis shows the lowest mean point-estimate performance in cosmology, whereas high-energy astrophysics exhibits substantial between-model variability rather than uniformly low performance. Question-only results on the 592 benchmark questions yielded 27.53% for Qwen2.5-VL-7B and 24.83% for InternVL3-38B, compared with image-present accuracies of 58.11% and 68.24%, respectively. These descriptive comparisons suggest a substantial contribution from visual input in the two models. AstroMMBench provides a structured and extensible resource for comparing model behavior on figure-associated astronomical questions.

Blue horizontal-branch (BHB) stars are standard candles for tracing the structure and assembly history of the Galactic halo. We present color–absolute-magnitude relations for BHB stars in multiple optical and near-infrared bands. Using a calibration sample of 164 BHB stars with precise Gaia Early Data Release 3 (EDR3) parallaxes, we derive linear relations in the observed Gaia ( BP , G , RP ) and Two Micron All Sky Survey ( J , H , K _s ) systems. We also provide supplementary calibrations from Gaia DR3 synthetic photometry for commonly used optical bands, including UBVI , griz , y , and Hubble Space Telescope F606W and F814W. The M _G –( G − RP ) ₀ relation, with an intrinsic scatter of 0.143 mag, is applied to 8767 BHB stars from LAMOST and Sloan Digital Sky Survey (SDSS). The distances are up to ∼101 kpc, with a median uncertainty of 6.7%. We validate our distance scale through comparison with Gaia EDR3 geometric distances for 1143 high-quality stars, finding a median difference of 6.4%. Additional tests using globular clusters and two high-precision reference catalogs get median offsets of −1.1% and −5.8%, respectively. These results show that our calibrations provide reliable BHB distances and offer a useful complement to Gaia parallaxes in the distant Galactic halo.

The Palomar 5 (Pal 5) globular cluster and its tidal tails provide a sensitive probe of globular-cluster evolution in the time-dependent Milky Way potential. We study the past 3 Gyr of evolution of Pal 5 using collisional direct N -body simulations with PeTar , adopting Galactic potential models that include spiral arms, the Galactic bar, halo flattening, the Large Magellanic Cloud (LMC), and bar deceleration. We find that halo shape strongly influences the projected stream track, reflecting Pal 5’s sensitivity to Galactic force-field flattening. The LMC causes only modest direct changes to the present-day projected stream morphology, but can alter the pericentric distance and, hence, the progenitor’s mass evolution. The Galactic bar strongly affects stream length and debris redistribution along the tails, producing model-dependent density structures and leading–trailing asymmetries. Comparison with observations from D. Erkal et al. and Y.-A. Xiao et al. shows that no single model simultaneously reproduces all observed properties of Pal 5, including cluster evolution, stream length, track, width, and line-density profile. Although our simulations capture several global properties, the remaining discrepancies indicate that a more precise match likely requires better constraints on the initial properties of the Pal 5 progenitor and a more complex Galactic potential, including perturbations from small-scale perturbers such as dark matter subhalos and giant molecular clouds. Future work may combine self-consistent direct N -body simulations with particle-spray methods to investigate these discrepancies more efficiently.