arXiv subjects

Jifeng Liu

Publications and source records attributed to Jifeng Liu.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of Stellar Activity and Habitable Zone (EATEN): III. X-ray Activity of Dwarfs in Open Clusters and Field Stars

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 XUV irradiation for a sample of F-M dwarf stars based on Chandra and XMM-Newton 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, ultraviolet, and Ca II H&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.

astro-ph.SR↗

arXiv2026

Flare waiting time as a novel proxy of stellar magnetic activity

Stellar flares have long served as stellar magnetic activity tracers. The flare waiting time, defined as the interval between two consecutive flares, provides a valuable diagnostic for probing underlying mechanisms of energy storage and release in stellar atmospheres. In this work, utilizing flaring M dwarfs observed by the Kepler satellite, we establish a simple yet effective activity proxy, i.e., median flare waiting time ($t_{\rm{w, med}}$). Our results show that the $t_{\rm{w, med}}$ can trace long-term activity levels similar to the flare rate. However, $t_{\rm{w, med}}$ corresponding to different waiting time percentiles may encode richer physical insights than flare rate. In addition, for the first time we construct a clear relation between $t_{\rm{w, med}}$ and stellar rotation period, which is quite similar to the canonical activity--rotation relation. More intriguingly, this relation exhibits a more notable supersaturation effect (i.e., below a critical rotation period, $t_{\rm{w, med}}$ begins to increase instead of keeping constant) compared to other activity proxies. The filling factor--rotation period relation favors poleward migration of active regions as the explanation for supersaturation, rather than coronal stripping. With the dramatic increase in stellar flares detected by missions like TESS and the upcoming Earth 2.0 satellite, $t_{\rm{w, med}}$ will become a powerful diagnostic for probing stellar magnetic activity and underlying physics.

astro-ph.SR↗

arXiv2026

Pandora cluster Lensing, AGN, and Transient Exploration (PLATE) from JWST Multi-Epoch Imaging. I. Discovery of a type II supernova candidate in a spiral galaxy at $z=0.7$

We report the discovery and multi-wavelength analysis of a $z\sim0.7$ transient PLATE-23a in the Abell 2744 field, as the first results of the Pandora Lensing, AGN, and Transient Exploration (PLATE) project. Using multi-epoch JWST NIRCam imaging spanning from 2022 to 2025, we detect PLATE-23a in 12 filters. Difference-imaging analysis reveals its rising and declining phases. The host galaxy of PLATE-23a is a barred spiral at $z=0.688$ with a stellar mass of $\sim 10^{10.4}M_{\odot}$ and a star formation rate of $\sim5.1M_{\odot}~\rm yr^{-1}$, placing it on the star-forming main sequence. Bayesian light-curve classification strongly favors a type IIP supernova (SN) origin. It lies $\sim 13\rm kpc$ (after lensing correction) from the galaxy center in a region of low local star formation, suggesting the progenitor may have migrated from a distant star-forming clump. Physical properties derived from blackbody modeling indicate a temperature decreasing from $\sim8010\rm K$ to $\sim6100\rm K$ at around 65 days after the explosion; the late-time SED is consistent with entering the radioactive decay phase at about one rest-frame year. This work demonstrates the power of deep, multi-epoch JWST observations for studying transients at cosmological distances.

astro-ph.GA↗

arXiv2026

Investigation of GeSn aspect ratio trapping growth up to 8% Sn

Aspect ratio trapping (ART) growth of germanium-tin (GeSn) is a promising approach to target important objectives on the quest towards commercialization of complementary metal-oxide-semiconductor (CMOS)-compatible GeSn optoelectronics devices. Its local growth on patterned substrate allows for versatile device integration into photonics integrated circuit or for stand-alone structure like focal plane array imager. Additionally, high aspect ratio from nano-sized window can terminate early threading dislocation propagation on the oxide sidewalls, leaving subsequent growth defect-free and potentially improving the device performance. Knowledge remains missing regarding GeSn ART growth kinetics, morphology and how they evolve from thin film growth, with successful growth itself yet to be demonstrated. In this work, we report GeSn ART growth up to 8% Sn. Two configurations -- self-induced Ge core/GeSn shell for Sn content between 6% and 8%, and bulk GeSn ART for Sn content below 1% -- are observed. We present a comprehensive study on GeSn ART growth kinetics through different growth rounds and designs, showing a link between pyramid shape of ART island and successful Sn incorporation, as well as the role of growth selectivity and local heating.

cond-mat.mtrl-sci↗

arXiv2026

The XMM-Newton Line Emission Analysis Program (X-LEAP) III: Earth's Magnetospheric X-ray Emission Revealed by 22-Year XMM-Newton Observations

The magnetosphere, protecting the Earth from intense solar activity, is also shaped by the solar wind, while its structure is still uncertain in observation. In this study, we map the X-ray emission in the magnetosphere, which is induced by the charge exchange between the highly-ionized solar wind and the neutral gas around the Earth, known as the magnetospheric solar wind charge exchange (SWCX). In particular, we extract the magnetospheric SWCX in the O VII line emission data adopted from the XMM Line Emission Analysis Program (X-LEAP). The observed magnetospheric SWCX shows an enhanced emission of approximately $I_{\rm OVII}^{\rm mag}\approx 2$ photons $\rm cm^{-2}~ s^{-1}~sr^{-1}$ toward the Sun, showing a consistent shape predicted by numerical simulations. Furthermore, this magnetospheric SWCX exhibits a dependence on the XMM pointing direction, which traces the path length of SWCX emission in the Earth's magnetosphere. Building on this directional dependence, we model the 3D magnetosheath structure using soft X-ray observations for the first time, constraining the averaged boundary geometry and SWCX emissivity distribution over 22 years. Finally, utilizing the XMM data, we derive an empirical O VII emission efficiency of $\alpha_{\rm OVII}=(2.1\pm0.4) \times 10^{-16}\ {\rm eV\,cm^{2}}$.

astro-ph.HE↗

arXiv2026

Study of GeSn Selective Area Growth with Demonstration of SWIR Light Detection

As germanium-tin (GeSn) epitaxial growth quality continuously improves, the search for an efficient integration strategy of GeSn optoelectronics devices into complementary metal-oxide-semiconductor (CMOS) manufacturing line also accelerates. Selective area growth (SAG) on patterned substrate emerges as a promising approach for this quest, with locally controlled growth of GeSn laser/detector suitable for either co-integration with silicon-based waveguide structure or stand-alone module like focal plane array. In this work, we report successful GeSn SAG with Sn content ranging from 3.2% to 8.7% of good optical quality, with demonstration of tunable GeSn SAG photoluminescence and GeSn SAG photoconductor device, the latter with detection cutoff wavelength up to 2 um. In addition, we present a comprehensive study of GeSn SAG condition at different window sizes, from 2 um to 100 um, and shapes: circle, square, octagon, and rectangle. Presence of loading effect is revealed, where GeSn growth rate increases as pattern fill factor and window size shrink. It introduces a different growth condition compared to thin film growth, which can weaken or inhibit Sn incorporation at very small window size and induce Sn segregation in high Sn content SAG growth.

physics.app-ph↗

arXiv2026

Unveiling the nature of G6096: a likely hierarchical triple system

G6096 (Gaia DR3 609651611028044544) was recently reported as a wide ($P\sim 450$ days) and eccentric ($e\sim0.18$) binary possibly hosting a massive white dwarf or neutron star. In this work, through analyses of the projected rotational velocity between the blue and red bands, spectral disentangling, joint radial velocity and astrometric fitting, and X-ray emission, we suggest that the system contains additional visible component(s) rather than a compact object. We develop a new approach to reveal the nature of G6096 by jointly modeling the spectral energy distribution, rotational velocity, and astrometric measurements. Finally, we speculate that G6096 is a hierarchical triple main-sequence star system, comprising a primary with a mass of $\sim 0.75\,M_\odot$ orbited by an inner binary consisting of two dwarfs with masses of $\sim 0.62\,M_\odot$ and $\sim 0.40\,M_\odot$, respectively. This method may help reveal a population of triple systems when applied to {\it Gaia} astrometric data, particularly the upcoming DR4.

astro-ph.SR↗

arXiv2026

Bolometric correction factor and radiative efficiency for the super-Eddington accretion flow in tidal disruption events

The estimate of the bolometric luminosity and the radiative efficiency are two key aspects for understanding the properties of the accretion flow around a supermassive black hole (BH). In this paper, we focus on the estimate of the bolometric luminosity and the radiative efficiency of the early super-Eddington accretion flow in tidal disruption events (TDEs). Specifically, we first perform radiation hydrodynamic simulations of super-Eddington accretion flow in TDE environment, and then calculate the corresponding emergent spectra with the method of post processing for the simulation data. Based on the emergent spectra, we calculate the isotropic-equivalent X-ray bolometric correction factor $k_\mathrm{bol}$ and the radiative efficiency $\eta$ of the super-Eddington accretion flow. We find that both $k_\mathrm{bol}$ and $\eta$ are BH mass and viewing-angle dependent. $k_\mathrm{bol}$ is in the range of about a few tens to a few thousands, and $\eta$ is in the range of $\sim 10^{-3}-10^{-1}$ for BH mass in the range of $10^{6-7}M_\odot$ and the viewing angle in the range of $0^{\rm o}-90^{\rm o}$. Finally, we apply the derived $k_\mathrm{bol}$ and $\eta$ to some specific TDEs to estimate the accreted mass during an event, which can significantly alleviate the so-called missing energy problem in TDEs.

astro-ph.HE↗

arXiv2026

Characterizing Orbital Parameters of Hot Subdwarf Binaries with Multiple Spectroscopic Surveys

Hot subdwarfs (HSDs) provide critical insights into the physical mechanisms governing binary evolution. In this work, we conduct a systematic analysis of 157 HSDs, selected from Gaia EDR3 and characterized using multi-survey spectroscopic data. Atmospheric parameters of these HSDs are derived via a convolutional neural network (CNN) method and template-matching method. Based on the atmospheric parameters from CNN method, these HSDs exhibit a median mass of $0.45^{+0.19}_{-0.17} M_{\odot}$ and radius of $0.18^{+0.04}_{-0.05} R_{\odot}$, consistent with earlier work. Orbital parameters of 23 systems are determined through the fitting of radial velocity data and light curves, with 11 of them being new solutions. We find that reflection-dominated binaries typically have periods longer than 0.1 d and host low-mass main-sequence companions ($\sim$ 0.2 $M_{\odot}$) with rotation-inflated radii. In contrast, binaries including an HSD and a white dwarf show very short periods ($P < 0.2$ d), with the closest systems hosting more massive white dwarfs. Most of these systems share a similar mass--period distribution with that of post-common-envelope binaries, supporting a common-envelope origin.

astro-ph.SR↗

arXiv2026

Revisiting Approaches to Stellar White-Light Flare Energy Based on Spatiotemporally Resolved Solar Observations

Accurately estimating the bolometric energy of solar and stellar white-light flares (WLFs) is crucial for understanding their physical nature and impact on surrounding planets. However, the lack of spatial resolution in stellar observations forced pioneering stellar WLF studies to adopt simplified energy estimation methods, typically assuming either a constant flare temperature or a fixed radiating area. To assess the physical plausibility of these assumptions, we utilize high-spatiotemporal-resolution solar observations to analyze the true evolution of source region's radiating area and temperature of 70 solar WLFs. It is revealed that both area and temperature of most solar WLFs undergo significant temporal evolution, and the flare area strongly correlates with the flare's peak optical continuum flux. Therefore, we propose a new energy estimation method that permits both flare area and temperature to evolve. Compared with existing methods, our dynamic approach yields systematically lower flare energies, which then prompts us to revisit classical macroscopic scaling laws related to the flare energy. It is further revealed that different energy estimation approaches can systematically alter these scaling relations, calling for a re-examination of these established statistical results and their targeted testing or revision in future work.

astro-ph.SR↗

arXiv2026

Nature of HD 251108: an RS CVn binary with a long-term evolving spot

Recently, the Lobster Eye Imager for Astronomy (LEIA) detected the longest-lasting and most energetic stellar X-ray flare event from HD 251108. In this work, we re-determined the atmospheric parameters of HD 251108 using three spectroscopic observations obtained with the 2.4 m Lijiang Telescope. Combined with the stellar radius derived from spectral energy distribution fitting, we found that HD 251108 contains a K-type giant with a mass of approximately 1.3 $M_{\odot}$. Long-term photometric monitoring over 12 years reveals a modulation suggestive of a stellar activity cycle, but inconclusive given the limited time span to date. Light curve fitting indicates that the variations in both amplitude and shape are primarily driven by the evolution of a large spot. The fitting further indicates that the spot migrated from low latitudes toward the pole between 2014 and 2020, and began to recede from the pole after 2022. Using spot parameters from light curve fitting, we found that the observed radial velocity variations arise from both the spot-induced distortions and the Keplerian orbital motion of the giant star. Additionally, we detect a possible M-dwarf companion with a mass of approximately 0.25 $M_{\odot}$. Our finding suggests a notable effect on the radial velocity caused by stellar magnetic activity.

astro-ph.SR↗

arXiv2026

SN 2023fyq: direct detection of a Type Ibn supernova progenitor and its multi-wavelength environmental constraints

Context. Type Ibn supernovae (SNe) are characterized by narrow helium emission lines arising from ejecta-circumstellar medium interaction, yet their progenitors remain debated, with both massive Wolf-Rayet stars and low-mass helium stars in binaries proposed. Aims. We aim to directly identify the progenitor of the Type Ibn SN 2023fyq and to characterize its environment in order to constrain the progenitor's nature and evolutionary channel. Methods. We search for the SN progenitor based on pre-explosion and late-time HST and JWST images and derive its properties by fitting the spectral energy distribution. We investigate the SN environment by probing the stars, dust, ionized gas and molecular gas with a multi-wavelength dataset including HST and JWST imaging, VLT/MUSE integral-field-unit spectroscopy and ALMA CO (2--1) radio interferometry. Results. We discover a pre-explosion source at the SN position, which is consistent with a hot ($T>$15000 K) and luminous (log($L$/$L_\odot$) $\gtrsim$ 5.5) SN progenitor and a possible host star cluster. The progenitor is confirmed to have disappeared after explosion. Analysis of the SN environment implies that the progenitor likely has an age of log($t$/yr) = 7.1--7.2. These phenomena disfavor a very massive single-star progenitor and instead support a binary scenario involving a low-mass helium star and a compact object; the observed progenitor emission likely arises from binary interaction that began at least $\sim$12 yr before the explosion. Conclusions. SN 2023fyq is the first Type Ibn SN with a directly detected progenitor and a possible host star cluster. It adds to the diversity of Type Ibn SNe in terms of their progenitor channels and mass-loss mechanisms.

astro-ph.SR↗

arXiv2026

Not All Who Wander Are Lost: Early Excess Demographics in the Volume-limited ZTF DR2 SN Ia Sample

Early-time flux excesses in Type Ia supernovae (SNe~Ia) offer a unique insight into their progenitor systems and explosion mechanisms. Although individual early-excess events and larger searches have been reported, demographic studies remain limited by sample size. We present a systematic search for early-time excess emission in a volume-limited sample ($z<0.06$) of SNe~Ia based on the Zwicky Transient Facility Data Release 2 (ZTF DR2). Using ZTF $g$- and $r$-band light curves, we identify candidates showing early-excesses shortly after the explosion time, and we apply conservative coverage and quality requirements to build reliable ``excess'' and ``no-excess'' bump and no-bump catalogs. From an initial sample of 1547 SNe~Ia, our final catalogs contain 42 early-excess and 110 no-excess events. We compare the two populations using SN and host environment parameters from ZTF DR2 and quantify the differences using two-sample statistical tests. We find the strongest differences are in SN light-curve properties: early-excess events have larger SALT2 stretch $x_1$ ($7.91\sigma$) and larger $r$-band secondary-maximum flux $\mathcal{F}_{r_2}$ ($6.25\sigma$), while differences in SALT2 color $c$ are weak ($0.57\sigma$). Early-excess events also favor bluer $(g-z)_{\rm local}$ ($3.41\sigma$) and lower $\log_{10} (M_*/M_\odot)_{\rm local}$ ($2.73\sigma$). Our results connect early excesses with SNe~Ia diversity, and motivate further analyses of upcoming larger samples.

astro-ph.HE↗

arXiv2026

A Search for Variability of Ultracool Dwarfs with the Zwicky Transient Facility

Rotationally modulated photometric variability of ultracool dwarfs encodes key information about cloud structure and temperature contrasts. Large homogeneous optical datasets are crucial for linking atmospheric heterogeneity to fundamental parameters such as rotation, mass, and age. We present a search for rotation periods in ultracool dwarfs using Zwicky Transient Facility (ZTF) optical light curves. By propagating the coordinates to the ZTF epoch and applying Lomb-Scargle analysis, we identified 226 periodic variables, including 32 robust detections and 194 tentative cases. Among the robust detections, 12 have no previously published periods, while 20 have literature counterparts, most of which are consistent with the published values. Most robust detections are M dwarfs, reflecting the optical sensitivity limits of ZTF. We find a trend of decreasing periods toward later spectral types in relatively old dwarfs (> 100 Myr), suggesting faster rotation for late-M types than for mid-M types. The age-period relation of our sample is broadly consistent with angular-momentum-conservation models at higher-mass regime of brown dwarfs, consistent with the M-dwarf bias of our catalog. Many additional candidates remain to be confirmed due to sparse sampling or low S/N. Future high-cadence, multi-wavelength monitoring and systematic mining of ZTF and upcoming surveys will be crucial for validating these periods, extend sensitivity to later (L/T) types, and better connect rotation with cloud physics across the stellar-substellar boundary.

astro-ph.SR↗

arXiv2026

SPHEREx Ultracool Dwarf spectrophotometric Atlas (SUDA): Atmospheric and Fundamental Parameters of Ultracool Dwarfs

We present the SPHEREx Ultracool Dwarf spectrophotometric Atlas (SUDA), a homogeneous sample of 1675 field ultracool dwarfs with continuous 0.75--5~$\mu$m spectrophotometry from SPHEREx Quick Release 2 (QR2). Using the SAND and ATMO2020++ atmospheric grids, we derive atmospheric parameters, compute bolometric luminosities ($L_{\rm bol}$), and combine $T_{\mathrm{eff}}$ and radii with evolutionary tracks to estimate masses, ages, and evolutionary $\log g$. For sources at 1700--2500~K, spectroscopic $\log g$ is systematically lower than evolutionary $\log g$, with a median offset of $\sim$1.05~dex, likely driven primarily by cloud-related model uncertainties. We further construct an empirical atlas by binning the measurements in $T_{\mathrm{eff}}$ and evolutionary $\log g$, producing 57 spectrophotometric templates spanning $T_{\mathrm{eff}}\simeq700$--3000~K. Molecular indices trace a coherent atmospheric sequence: H$_2$O and CH$_4$ indices strengthen toward lower $T_{\mathrm{eff}}$, while CO and CO$_2$ indices increase below $\sim$1500~K and turn over near $\sim$1000~K. Model comparisons indicate that CO$_2$ index is more sensitive to metallicity than gravity at $T_{\mathrm{eff}}\sim800$--1200~K. SUDA provides a reference sample linking 0.75--5~$\mu$m spectrophotometric morphology to atmospheric and evolutionary trends in ultracool dwarfs.

astro-ph.SR↗

arXiv2026

Thin film synthesis of SrZn2P2 with SrI2 post-annealing for enhanced crystallinity and optoelectronic quality

Ternary Zintl phosphides are promising light-absorbing semiconductors for thin-film optoelectronic applications, but strategies for controlling their microstructure and optoelectronic quality remain underexplored. Here, we report the synthesis of phase-pure SrZn2P2 thin films using radio-frequency co-sputtering in a PH3 + Ar atmosphere and investigate the impact of post-growth processing on their structural and optical properties. Grazing-incidence X-ray scattering and Raman spectroscopy confirm the formation of crystalline SrZn2P2 films over a finite compositional window. Optical measurements reveal strong absorption near the direct-band-gap energy (~1.8 eV) and near-band-edge photoluminescence. Further, we have studied the effects of chemically compatible halide-assisted annealing. It is found that SrI2 treatments lead to pronounced grain growth and reduced diffraction peak broadening while preserving phase purity, in contrast to rapid thermal or forming-gas annealing. Notably, annealing with SrI2 at 450 {\deg}C significantly enhances both the intensity and spatial uniformity of the photoluminescence, thus connecting the observed microstructural consolidation with improved radiative recombination. Our study demonstrates that halide-assisted annealing provides an effective pathway for microstructural control in SrZn2P2 thin films and highlights a generalizable processing strategy for advancing Zintl phosphide semiconductors toward optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2026

StarCLR: Contrastive Learning Representation for Astronomical Light Curves

With the rapid development of time-domain surveys, the availability of massive light curve data offers new opportunities for studying stellar evolution and variable star classification, while simultaneously posing challenges for feature extraction and modeling. We present StarCLR, a contrastive pretraining framework for large-scale light curves. By constructing positive pairs from partially overlapping sub-sequences, StarCLR encourages the model to learn temporal representations. We pretrain StarCLR on the TESS dataset and fine-tune it for variable star classification on three surveys with distinct observational characteristics, namely TESS (18 types), ZTF (11 types), and Gaia (24 types). StarCLR achieves macro-F1 scores of 84.35%, 87.82%, and 92.73%, and micro-F1 scores of 94.46%, 92.83%, and 99.49%, respectively. Compared with LSTM and Transformer trained from scratch, StarCLR performs better on TESS and ZTF, with the largest gains on sparsely sampled ZTF light curves, demonstrating promising generalization. For Gaia, which involves a broader class space, the evaluation is not directly comparable, and performance is likely influenced by astrophysical features, resulting in a more limited contribution from the pretrained backbone. Systematic ablations on embedding design, pooling strategy, and pretraining settings further indicate that the pretrained representations provide performance gains by capturing informative temporal characteristics of light curves. Looking ahead, with standardized datasets and more diverse labeling schemes, the generalization ability of StarCLR can be further enhanced.

astro-ph.SR↗

arXiv2026

Germanium-tin (GeSn) avalanche photodiode with up to 2.7 micro cutoff wavelength for extended SWIR detection

Separate absorption charge multiplication germanium tin on silicon avalanche photodiode offers a viable solution to achieve CMOS compatible, high sensitivity detection technology in SWIR or extended SWIR range, leveraging the excellent k-factor of Si as multiplication layer and SWIR or e-SWIR band absorption of GeSn. However, unlike well-established growth of GeSn on Si with thick Ge buffer in-between to reduce threading dislocation density due to lattice mismatch, GeSn on Si APD design requires relatively thin Ge buffer to limit electric field drop through the background p-doped buffer and efficiently transporting photocarrier from GeSn absorber to Si multiplication layer, therefore making growth of high Sn content APD for e-SWIR coverage very challenging. In this work, we experimentally demonstrate GeSn on Si APD up to 12.7 percent Sn, monolithically grown on Si substrate with 122-nm-thick Ge buffer in between, which is considerably thinner than widely used 700-900 nm thick Ge buffer. Stronger relaxation of GeSn absorber via thin Ge buffer favors Sn incorporation, leading to higher Sn content than the nominal target of 8 percent Sn. Device detection range is significantly improved compared to previous work - with cutoff wavelength increased up to 2.7 micro at 300 K, in parallel with high avalanche gain at 77 K up to 21 at 1.55 micro and up to 52 at 2 micro, and good responsivity in SWIR or e-SWIR range, up to 1.45 AW-1 at 1.55 micro and 0.66 AW-1 at 2 micro.

physics.app-ph↗