arXiv subjects

Xiaozhen Yang

Publications and source records attributed to Xiaozhen Yang.

12 recordsLinked to original sources

arXiv2026

Discovery and Characterization of White Dwarf-FGK Main-Sequence Binaries within the Optical Main-Sequence Locus

White dwarf main-sequence (WDMS) binaries provide important laboratories for studying binary evolution and the formation of low-mass white dwarfs. In this work, we identify 654 reliable WDMS candidates with FGK-type companions from an initial set of 772 ultraviolet-excess sources, selected using stellar atmospheric parameters from LAMOST spectroscopy and subsequently refined with \textit{Gaia} DR3 astrometry and photometry together with ultraviolet data from \textit{GALEX}. Candidates were selected based on ultraviolet excess relative to the \textit{Gaia} main-sequence locus and refined using isochrone constraints to exclude systems inconsistent with MS companions. Binary spectral energy distribution fitting yields effective temperatures and radii for both components, as well as distance and extinction estimates. The MS companions are dominated by G-type stars ($\sim52\%$), with comparable fractions of F- and K-type companions, and no A-type primaries. Using white-dwarf evolutionary cooling models, we find that the WD components are predominantly low-mass ($M_{\rm WD}\,\sim\,0.2\text{--}0.4\,M_\odot$), including a substantial population of extremely low-mass ($<0.3\,M_\odot$) WDs likely produced through binary interaction. The WDs are generally hot ($\sim1.5\times10^4\,\mathrm{K}$), consistent with the ultraviolet selection bias favoring luminous, large-radius WDs. Multi-epoch LAMOST radial velocities show larger amplitudes than those of a comparison sample of MS stars, supporting the close-binary nature of these systems. Although subject to strong selection effects, the catalog offers a clean and well-characterized sample of FGK+WD binaries.

astro-ph.SR↗

arXiv2026

Revisiting Ca II Activity Indices in FGK Stars: Systematic Biases in Infrared Triplet Measurements

Synthetic-template subtraction is widely used to measure chromospheric activity in large spectroscopic surveys. However, many solar-like FGK stars show systematically negative Ca II infrared triplet (IRT) residual indices, implying that the observed line cores are deeper than those predicted by parameter-matched templates. We investigate this effect using solar-like stars from LAMOST DR9, MaStar, and XSL DR3, measuring activity indices (R+) for both the Ca II H&K and IRT lines in a uniform framework. We find that observational effects, including atmospheric-parameter offsets, treatment of the instrumental line-spread function, and propagated measurement uncertainties, contribute to scatter but do not explain the systematic negative bias in R+_IRT. The results instead suggest that the negative bias most likely arises because photospheric templates underestimate the depth of the IRT cores, likely owing to missing chromospheric structure and, to a lesser extent, NLTE effects. An empirical increase in the adopted microturbulent velocity deepens the synthetic IRT cores and partially mitigates the negative offset. In addition, R+ values derived from different synthesis configurations show systematic offsets but generally preserve strong linear correlations, indicating that they can be cross-calibrated. These results clarify the origin of negative Ca II IRT residual indices and help interpret template-dependent systematics in chromospheric activity measurements based on synthetic-template subtraction.

astro-ph.SR↗

arXiv2026

Characterizing the Instrumental Profile of LAMOST

The instrumental profile (IP) of a telescope is of great significance for spectroscopic analyses, especially for wavelength calibration and stellar parameter measurements. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) employs arc lamps for wavelength calibration. These lamps produce sharp emission lines with known wavelengths, and the observed arc lamp spectra can well characterize the IP. However, IPs are influenced by multiple factors, making them difficult to model accurately with traditional methods. Neural networks, which can automatically capture complex patterns and nonlinear features in data, provide a promising approach for high-precision IP measurement. We therefore construct a multi-layer perceptron (MLP) based on The Payne neural network to derive IPs for LAMOST. After training, the model can retrieve the IP for any fiber, at any wavelength, and at any time. We then apply the derived IP to stellar radial velocity (RV) measurements and analyze the impact of different IP center localization methods on the results. Finally, the dispersion of the measured RVs is reduced by approximately 3 km/s. This improvement will facilitate the search for long-period binary stars via RV variations.

astro-ph.IM↗

arXiv2025

The survey of DA double white dwarf candidates based on DESI EDR

Mergers of double white dwarfs are considered significant potential progenitors of type Ia supernovae. Although there is no direct observational evidence to definitively determine the formation pathways of SNe Ia, studying the physical properties of DWDs provides valuable insights into their evolutionary processes, interaction modes, and merger mechanisms, which are essential for understanding the explosion mechanisms of SNe Ia. This study aims to identify DWD candidates through spectroscopic radial velocity measurements and analyze their physical properties based on DESI EDR. We crossmatched DESI EDR with the Gaia EDR3 to select DA. We measured the spectroscopic RV using the cross-correlation function and assessed the significance of RV variability using a chi-squared-based variability method. Spectroscopic Teff and log g were derived by fitting the hydrogen Balmer lines, with 3D convection corrections applied. Orbital periods and semi-amplitudes were obtained through a Lomb-Scargle analysis of the RV time series. We interpolated WD cooling models and applied Monte Carlo simulations to calculate masses, cooling ages, radii, and their associated uncertainties. We also analyzed their photometric and spectral energy distribution properties to derive photometric temperatures and radii, which were then compared with the corresponding spectroscopic parameters. We identified 33 DA DWD candidates with significant RV variability, including 28 new discoveries. Among them, we found an extremely low-mass DWD candidate and a potential triple system. For these candidates, we measured key physical parameters including Teff, log g, mass, and radius, and estimated the orbital periods based on the available data. Of these, 17 candidates exhibit relatively clear periodic RV variability in the current data, and we report their best-fitting periods and RV semi-amplitudes.

astro-ph.SR↗

arXiv2025

LAMOST J101356.33+272410.7: A Detached White Dwarf-Main-sequence Binary with a Massive White Dwarf Within the Period Gap

We report the analysis of the detached eclipsing spectroscopic binary system LAMOST J101356.33+272410.7, which features a massive white dwarf. Using LAMOST and SDSS spectra, we determined the stellar parameters and radial velocities of both components. SED fitting of photometric data from GALEX, 2MASS, and Pan-STARRS1 yielded the effective temperatures and photometric radii. Eclipsing analysis of high-speed photometric data from the Liverpool Telescope provided orbital inclination, masses, radii, and related physical parameters. The white dwarf in this system has a mass of $1.05 \pm 0.09 \, M_\odot$ and a radius of $0.0090 \pm 0.0008 \, R_\odot$, while the main-sequence star has a mass of $0.299 \pm 0.045 \, M_\odot$ and a radius of $0.286 \pm 0.018 \, R_\odot$. Emission lines observed in the spectra indicate the likely presence of stellar magnetic activity in this system. The relatively cool temperature of the white dwarf suggests that the system could be a post-common-envelope binary (PCEB) that has not undergone mass transfer, while the presence of a massive white dwarf indicates that the system might also represent a detached cataclysmic variable (dCV) crossing the period gap. We suggest that the system is more likely to be a PCEB, and it is predicted to evolve into a cataclysmic variable and begin mass transfer in approximately 0.27 Gyr.

astro-ph.SR↗

arXiv2023

Multifunctional Metasurface: Simultaneous Beam Steering, Polarization Conversion and Phase Offset

A varactor-based reconfigurable multifunctional metasurface capable of simultaneous beam steering, polarization conversion and phase offset is proposed in this paper. The unit cell is designed to naturally decompose the incident waves into two equal amplitude orthogonal linear components, and by integrating varactors, the reflection phase of the field components can be engineered from $-180^{\circ}$ to $180^{\circ}$.Taking advantage of the infinite states of the varactors, this design integrates a new function, the phase offset. After simulation validation of its capability, a four-layer $7$ by $6$ unit one-dimensional prototype is fabricated as a printed circuit board. It is experimentally demonstrated that it switches between X/Y and circular polarization with more than $10$ dB cross polarization isolation, while reaching $\pm45^{\circ}$ steering and $\pm180^{\circ}$ phase offset.

physics.app-ph↗

arXiv2023

Real-data-driven Real-time Reconfigurable Microwave Reflective Surface

Manipulating the electromagnetic (EM) reflection behavior from an arbitrary surface dynamically on arbitrary design goals is an ultimate ambition for many EM stealth and communication problems, yet it is nearly impossible to accomplish with conventional analysis and optimization techniques. In this paper we present a reconfigurable conformal metasurface prototype as well as a workflow that enables it to respond to multiple design targets on the reflection pattern with extremely low on-site computing power and time. The metasurface is driven by a sequential tandem neural network which is pre-trained using actual experimental data, avoiding any possible errors that may arise from calculation, simulation or manufacturing tolerances. This platform empowers the surface to operate accurately in a complex environment including varying incident angle and operating frequency, or even with other scatterers present close to the surface. The proposed data-driven approach requires minimum amount of prior knowledge and human effort yet provides maximized versatility on the reflection control, stepping towards the end form of an artificial-intelligence-based tunable EM surface.

physics.app-ph↗

arXiv2023

All-passive Microwave-Diode Nonreciprocal Metasurface

Breaking reciprocity in the microwave frequency range will have important implications for modern electronic systems. Since it usually involves bulky biasing magnets or complex spatial-temporal modulations, exploring a lightweight, all-passive approach becomes intriguing. Starting from a circuit model, we theoretically demonstrate the nonreciprocal behaviour on a transmission line building block creating a strong field asymmetry with a switchable matching stub to enable two distinct working states. After translating to an electromagnetic model, this concept is first proved by simulation and then experimentally verified on a microstrip-line-based diode-integrated metasurface showing nonreciprocal transmission. This printed circuit board design is expected to find various applications in electromagnetic protecting layers, communication systems, microwave isolators and circulators.

physics.app-ph↗

arXiv2022

Designing Topological Defect Lines Protected by Gauge-dependent Symmetry Indicators

Symmetry indicators are a modern tool for characterizing topological phases that require only minimal computational expense but provide an elegant means of designing practical devices. This paper demonstrates how a rotational symmetry indicator can be used to construct and characterize a topologically robust waveguide, which is then verified experimentally on a printed circuit board (PCB) platform. The design takes advantage of the real-space gauge-dependency of the symmetry indicators and adopts a $C_6$ lattice with simple shifts, forming a defect line supporting topological edge modes. It is shown that the modes can realize the same features as previous topological waveguides, but in addition possesses a greater degree of reconfigurability and the unique ability to form a one-way termination. Moreover, the design illustrates the critical role real space information plays in determining the topological properties of photonic crystals, enabling a wider range of possible realizations.

physics.app-ph↗

arXiv2021

Power-dependent Reflective Metasurface with Self-induced Bandgap

A metallic ring based, diode-integrated, low-profile, power-dependent, reflective metasurface working from 3 GHz to 3.6 GHz is proposed in this letter. Unlike the previous study which shifts a band up and down to change the impedance of the surface, the triggering of the diodes directly transforms the structure from a surface wave supportive state to a self-induced bandgap topology if exposed to high power RF illumination. We demonstrate the concept by conducting the EM-circuit co-simulation and measurements for a 6 by 8 unit 2D prototype. Near field scan experiments verify that the proposed topology works in two distinct states, the ON and OFF state, and high-power measurements prove that the reflection varies with the incident signal power. The highest 10 dB decrement in transmission occurs at 3.3 GHz with 52 dBm illumination. This structure can be used to protect sensitive devices from large signals while otherwise supporting a communication channel for small signals.

physics.app-ph↗

arXiv2021

Broadband time-modulated absorber beyond the Bode-Fano limit by energy trapping

Wide-band absorption is a popular topic in microwave engineering to protect sensitive devices against broadband sources. However, the Bode-Fano criterion defines the trade-off between bandwidth and efficiency for all passive, linear, time-invariant systems. In this letter, we propose a broadband absorber beyond the Bode-Fano limit by creating an energy trap using time-modulated switch/diodes. This work starts with an ideal circuit model to prove the concept, followed by two EM realizations - a freuqnecy selective surface (FSS) approach for general bandwidth broadening and a low-profile PCB design. The prototype of the latter is built and measured, demonstrating a Bode-Fano integral larger than one. This approach paves a way to many practical ultra-wide band absorber designs.

physics.app-ph↗

arXiv2010

Molecular Dynamics Simulation of Macromolecules Using Graphics Processing Unit

Molecular dynamics (MD) simulation is a powerful computational tool to study the behavior of macromolecular systems. But many simulations of this field are limited in spatial or temporal scale by the available computational resource. In recent years, graphics processing unit (GPU) provides unprecedented computational power for scientific applications. Many MD algorithms suit with the multithread nature of GPU. In this paper, MD algorithms for macromolecular systems that run entirely on GPU are presented. Compared to the MD simulation with free software GROMACS on a single CPU core, our codes achieve about 10 times speed-up on a single GPU. For validation, we have performed MD simulations of polymer crystallization on GPU, and the results observed perfectly agree with computations on CPU. Therefore, our single GPU codes have already provided an inexpensive alternative for macromolecular simulations on traditional CPU clusters and they can also be used as a basis to develop parallel GPU programs to further speedup the computations.

physics.comp-ph↗