arXiv subjects

Er-gang Zhao

Publications and source records attributed to Er-gang Zhao.

6 recordsLinked to original sources

arXiv2026

Long Photometric Cycles in Double Periodic Variables from Nodal Precession of a Tilted Accretion Disk

We investigate whether the long photometric cycles observed in double-periodic variables (DPVs) can arise from nodal precession of a tilted accretion disk driven by the tidal torque of the companion. Within a simple analytical framework, we derive testable relations linking the long-to-orbital period ratio to the binary mass ratio, the normalized disk size, and the disk tilt angle $β$, which itself can be inferred from the long-cycle amplitude, orbital inclination $i$, and disk luminosity fraction. The model naturally reproduces the two observed long-cycle light-curve morphologies -- sinusoidal and double-hump -- distinguished by the geometric criterion $i+β\le 90^\circ$ versus $i+β>90^\circ$. Applying these relations to a sample of DPVs, we find that the inferred disk sizes are physically reasonable and consistent with independent light-curve modeling for a non-negligible subset of systems. Our results show that tidal nodal precession represents a viable and potentially important contributor to the long-period variability of DPVs and provide a quantitative framework for future observational and theoretical studies.

astro-ph.SR↗

arXiv2026

A unified framework for hot accretion flows with finite angular momentum: from Bondi-like to disc-like regimes

Observations of X-ray luminous elliptical galaxies suggest that the accretion rate onto the central supermassive black hole can reach a substantial fraction of the Bondi rate. However, classical accretion theory applicable to such hot accretion flows treats spherically symmetric Bondi accretion and disc-like advection-dominated accretion flows (ADAFs) as two distinct limiting cases, lacking a unified framework for flows with finite angular momentum. In this work, we develop such a framework that continuously connects these two regimes. Our model naturally recovers the Bondi solution in the limit of vanishing angular momentum and approaches the properties of classical ADAFs at high angular momentum, while providing a physically well-defined description of the intermediate regime where neither limiting case is strictly applicable. We further demonstrate that the accretion rate is jointly regulated by the angular momentum of the ambient gas and the gas viscosity. For sufficiently large but physically reasonable viscosity, the accretion rate can remain at a significant fraction of the Bondi rate even in the presence of substantial gas rotation. These results offer a natural explanation for how such accretion rates can be sustained despite finite angular momentum in realistic galactic environments.

astro-ph.HE↗

arXiv2025

Observational Properties of $β$ Cephei Stars: 88 new samples discovered Based on TESS and Gaia Data

We present a systematic investigation of $β$ Cephei (BCEP) stars by integrating photometric data from the Transiting Exoplanet Survey Satellite (TESS) with astrometric parameters from Gaia Data Release 3. Utilizing TESS's short-cadence (SC) and full-frame image (FFI) photometry, along with Gaia parallaxes and temperatures derived from the Extended Stellar Parametrizer for Hot Stars (ESP-HS) pipeline, we identify 88 new BCEP stars and candidates--85 from SC data and 3 from SPOC-processed FFI observations. These targets exhibit visual magnitudes ranging from 8.0 to 12.0 mag, parallaxes between 0.11 and 1.74 mas, effective temperatures of 18,000 to 30,000 K, and luminosities from 1,500--38,000 $L_\odot$, consistent with previously cataloged BCEP populations, thereby demonstrating the robustness of our classification criteria. Key findings include: (1) a significant detection disparity between SC and FFI datasets, with 30\% of SC targets exceeding 18,000 K compared to only 0.7\% in FFI, reflecting observational biases toward high-luminosity, hotter stars in SC data; (2) four samples near the red edge of the theoretical instability strip, exhibiting sparse pulsation modes that are important samples for testing pulsation models under low-mass, low-temperature conditions; and (3) spatial clustering within the Galactic disk ($|b| < 20^\circ$), with two high-latitude outliers likely representing runaway stars ejected from disk environments. Our analysis underscores the critical role of space-based photometry in detecting low-amplitude pulsators and the transformative potential of multi-survey integration in the era of time-domain astronomy. These results provide new samples to constrain stellar pulsation theories of massive stars and to study Galactic dynamics.

astro-ph.SR↗

arXiv2025

Accretion rates of stellar-mass compact objects embedded in AGN discs

Stellar-mass compact objects (COs) embedded in active galactic nucleus (AGN) discs are commonly assumed to accrete via Bondi or Bondi-Hoyle-Lyttleton (BHL) prescriptions, neglecting gas angular momentum. We show that differential rotation in AGN discs can impart non-negligible angular momentum, in which case accretion proceeds through a viscous disc rather than Bondi/BHL flow. Our model provides a new framework estimating the CO accretion rate as $\dot{M}_\mathrm{CO} = \min\{\dot{M}_\mathrm{vis}, \dot{M}_\mathrm{BHL}\}$, where the viscous rate $\dot{M}_\mathrm{vis}$ accounts for gas--CO relative motion decomposed into a local gradient term (due to differential rotation) and bulk motion (from differing orbital parameters). This rate can be expressed as $\dot{M}_\mathrm{vis} = αξ(r_\mathrm{H}/r_\mathrm{BHL})^3\dot{M}_\mathrm{BHL}$, where $ξ$ is a coefficient of order unity. It can also be approximately scaled to the global AGN accretion rate as $\dot{M}_\mathrm{vis} \propto \dot{M}_1$, with the scaling coefficients in both forms determined by the specific dynamical configuration. The accretion is viscosity-limited when $q > [αξ(1+\mathcal{M}^2)^{3}/3]^{1/2} h^3$, where $q$ is the mass ratio between the CO and the supermassive black hole, $α$ the viscosity parameter, $\mathcal{M}$ the Mach number of the bulk relative motion, and $h$ the aspect ratio of the AGN disc. In thin AGN discs this condition is satisfied for most stellar-mass or more massive COs. Our framework also naturally allows for the inclusion of established outflow corrections, thereby enabling a more realistic treatment of super-Eddington flows. Our formulation thus improves upon Bondi/BHL prescriptions and offers a more physically motivated basis for studying CO evolution in AGN environments.

astro-ph.HE↗

arXiv2025

Spherically Symmetric Accretion with Self-Gravity: Analytical Formulae and Numerical Validation

Spherically symmetric accretion incorporating self-gravity constitutes a three-point boundary value problem (TPBVP) governed by constraints at the outer boundary, sonic point, and accretor surface. Previous studies have two limitations: either employing an incorrect formula for self-gravity potential in analytical treatments, or introducing additional input parameters in numerical implementations to circumvent solving the full TPBVP. To address these issues, we present a self-consistent TPBVP formulation, solved using the relaxation method. We also derive approximate analytical formulae that enable rapid estimates of self-gravity effects. Our analysis identifies a dimensionless parameter $β\equiv 2G \barρ r_\mathrm{out}^2/a_\mathrm{out}^2$ that characterizes the strength of self-gravity, where $\barρ$ and $r_\mathrm{out}$ are the mean density and outer radius of the flow, respectively, and $a_\mathrm{out}$ is the adiabatic sound speed of the external medium. For practical estimation, $\barρ$ may be approximated by the external medium density $ρ_\mathrm{out}$. We identify an upper limit for $β$, beyond which steady accretion becomes unsustainable -- a behavior consistent with classical gravitational instability that previous studies failed to capture. The accretion rate enhancement decreases monotonically as the adiabatic index $γ$ increases. For $γ=5/3$, self-gravity ceases to augment the accretion rate. These theoretical predictions are validated by our numerical solutions. We further apply our results to two astrophysical scenarios: hyper-Eddington accretion onto supermassive black hole seeds in the early Universe, where self-gravity is significant; and accretion onto stellar-mass objects embedded in active galactic nuclei (AGN) disks, where self-gravity is non-negligible under certain conditions and should be evaluated using $β$.

astro-ph.HE↗

arXiv2024

Observational Properties of 155 $β$ Cephei pulsating variable stars

$β$ Cephei pulsating variable (BCEP) stars are the most massive pulsating variable stars in the main sequence, exhibiting both p- and g-mode pulsations. In this study, we identified 155 BCEP stars or candidates using data from TESS and Gaia, of which 83 were first confirmed as BCEP stars. They have visual magnitudes ranging from 8 to 12 mag and effective temperatures between approximately 20,000 and 30,000 K, while the parallaxes of most targets are between 0.2 and 0.6 mas. The study indicates that these BCEP stars have pulsation periods ranging from 0.06 to 0.31 days, with amplitudes ranging from 0.1 to 55.8 mmag in the TESS band. Additionally, the number of BCEP stars increases as the pulsation amplitude decreases. These targets align with the distribution region of BCEP stars in the luminosity-period (L-P) and temperature-period (T-P) diagrams. We have updated the L-P relation of BCEP stars. The Hertzsprung-Russell (H-R) diagram indicates that these targets are in the main-sequence evolutionary phase, with masses ranging from 7 to 20 $M_{\odot}$ and luminosities between 2800 and 71,000 $L_{\odot}$. They are almost in the theoretical instability region of BCEP stars but as previously reported, this region at the low-mass end (red) is not filled. The distribution of the pulsation constant indicates that the dominant pulsation periods of BCEP stars consist mainly of low-order p-mode pulsations with a high proportion of radial fundamental modes. These BCEP stars are excellent objects for enhancing our understanding of the structure and evolution of massive stars through asteroseismology.

astro-ph.SR↗