arXiv subjects

Zhao-Yu Zuo

Publications and source records attributed to Zhao-Yu Zuo.

14 recordsLinked to original sources

arXiv2026

Time-Domain Axion Searches with Magnetic White Dwarfs

Magnetic white dwarfs can convert photons into axions in their strong magnetic fields, with the conversion probability modulating the light curve as the star rotates. However, this observable is degenerate with intrinsic stellar variability if the background is modeled too simplistically. We develop a controlled background-degeneracy framework that computes the axion-induced modulation from reconstructed stellar magnetic fields while fitting it simultaneously with a flexible Fourier model of the intrinsic light curve. Applying this framework to TESS observations of PG~1015+014 using two independent magnetic field reconstructions, we find that a sinusoidal stellar background can produce an apparent preference for nonzero axion-photon conversion. This preference is absorbed once the background light curve includes the second harmonic, indicating that higher-harmonic stellar variability is a leading degeneracy for precise photometric axion searches in magnetic white dwarfs. Interpreting the two-harmonic fit as a conservative baseline, we obtain competitive constraints for sub-$\mu\mathrm{eV}$ axions. We further derive an analytic target-ranking estimate for other TESS magnetic white dwarfs, identifying systems where phase-resolved magnetic modeling would be most valuable for competitive axion probes.

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arXiv2026

KIC 3868420: A high-amplitude $δ$ Scuti-$γ$ Dor hybrid star crossing the Hertzsprung gap

We report a photometric and asteroseismic analysis of KIC 3868420, a newly identified high-amplitude $δ$ Scuti-$γ$ Doradus hybrid star located in the Hertzsprung gap - a short-lived and rarely observed post-main-sequence phase. Using four years of Kepler long-cadence photometry, we detect 36 significant frequencies, including 11 independent modes spanning both low- and high-frequency regimes. Grid-based modeling with MESA and GYRE, including rotation, shows that five independent frequencies match a combination of radial p-modes and non-radial g-modes, supporting its hybrid nature. The best-fit models yield an evolved post-main-sequence star ($M \sim 2.26 - 2.30 M_\odot$, $R \sim 4.41 - 4.43 R_\odot$, $τ\sim 5.4 \times 10^{8}$ yr), although degeneracies from rotation and unknown inclination preclude a unique solution. KIC 3868420 thus represents a rare example of a high-amplitude hybrid pulsator in the Hertzsprung gap, demonstrating that high-amplitude pulsation can coexist with mixed p/g-mode behavior in a rapidly evolving star. This finding highlights the value of space-based photometry for identifying such rare objects.

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arXiv2025

KIC 5623923: A Faint Eclipsing Binary Consisting of $δ$ Scuti pulsations

In this paper, we present a detailed analysis of the light variation of KIC 5623923 using high-precision time-series data from the $Kepler$ mission. The analysis reveals this target is an eclipsing binary system with $δ$ Scuti type pulsations from the primary component, rather than from the secondary as previously reported. The frequency analysis of three short-cadence data reveals 41 significant frequencies, including the orbital frequency ($f_{orb}$ = 0.827198 d$^{-1}$) due to orbital motion from binary system and the pulsational frequencies. Most of the pulsational signal lies in the frequency range of 20 - 32 d$^{-1}$, with amplitude between 0.3 and 8.8 mmag, in which seven peaks are identified as `independent' modes. The strongest one ($f_{3}$ = 28.499399 d$^{-1}$) likely corresponds to a high-order radial mode. In other peaks ($f_{7}$, $f_{10}$, and $f_{18}$), several pairs of multiplet structures centered on them are found. The fitting of spectral energy distribution (SED) using the collected photometry measurement of multiple bands reveals the effective temperatures of the primary and secondary components as $8348^{+230}_{-225}$~K and $4753^{+237}_{-229}$~K, respectively, which place the primary star in the classical pulsating instability zone. The characteristic light curve morphology and short orbital period are consistent with a tidally locked system. Based on the characteristics of amplitude spectra of pulsating stars in close binaries, the analysis of the multiplet structures reveals that three independent frequencies (i.e. $f_{7}$, $f_{10}$, and $f_{18}$) correspond to non-radial modes with $l = 2$, while the associated sidelobes are produced by the orbital motion. We highlight the potential of this method in future studies of pulsating binary stars.

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arXiv2025

Can the Anomalous Magnetic Braking of Ap/Bp Stars Explain the Orbital Decay of Algol-type Binaries?

Several Algol-type binaries were detected to be experiencing rapid orbital decay, which is in contradiction with the conservative mass transfer model.} In this work, we investigate whether anomalous magnetic braking (MB) of intermediate-mass Ap/Bp stars, characterized by surface magnetic fields of $\sim10^2 \mathendash 10^4~\rm G$, could drive orbital decay through magnetic wind-field coupling. Using the stellar evolution code {\ttfamily MESA}, we simulate the evolution of several \add{main-sequence binaries containing Ap/Bp stars}, with typical initial parameters \add{of Algol binaries}. Our models indicate that the anomalous MB mechanism could induce orbital decay in long timescales (hundreds of Myr to several Gyr), \add{reproducing several basic Algol parameters such as the effective temperatures and luminosities of donor stars. However, the predicted orbital period decay rates are much lower than those observed in several Algol systems. We analyze the limitations of the anomalous MB model and discuss alternative mechanisms that could account for the long- or short-term orbital period variations observed in Algol systems, including a surrounding circumbinary disk, stellar expansion, the Applegate mechanism, and the light travel-time effect. Long-term observations are still required to distinguish between these mechanisms in the future.

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arXiv2023

KIC 9845907: A $δ$ Scuti star with the first overtone as the dominant frequency and with many equidistant structures in its spectrum

In this paper, we present an analysis of the pulsating behavior of Kepler target KIC 9845907. Using the data from Kepler, we detected 85 significant frequencies, including the first overtone $f_{1}$ = 17.597 day$^{-1}$ as the dominant frequency, the non-radial independent frequency $f_{3}$ = 31.428 day$^{-1}$ ($\ell$=1), as well as two modulation terms $f_{m1}$ = 0.065 day$^{-1}$ and $f_{m2}$ = 1.693 day$^{-1}$. We found fourteen pairs of triplet structures with $f_{m1}$ or $f_{m2}$, four pairs of which can further form quintuplet structures. We note these are the most intriguing features discovered in this study and they were recognized for the first time in $δ$ Scuti stars. We discussed several possible explanations, i.e., beating, the Blazhko effect, combination mode hypothesis, nonlinear mode coupling, large separation, and stellar rotational splitting for these equidistant structures. Our asteroseismic models indicate this modulation with $f_{m1}$ might be related to the rotational splitting. The study of more $δ$ Scuti stars with triplet and/or quintuplet structures using high-precision space photometry would be helpful to further explore its origin.

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arXiv2021

A catalogue of double-mode high amplitude $δ$ Scuti stars in the Galaxy and their statistical properties

We present the first catalogue of double-mode and multi-mode high amplitude $δ$ Scuti star (HADS) in the Galaxy. The catalogue contains source name, coordinates, radial modes such as the fundamental (F, period P0), first-overtone (1O, period P1), second-overtone (2O, period P2), and the third-overtone (3O, period P3) if available, period ratios, magnitude, and the relevant literature. Totally, 155 sources were collected until March 2021, in which 142 HADS with double-mode (132 with F and 1O, and 10 with 1O and 2O), 11 triple-mode, and 2 quadruple-mode. Statistical analysis shows clear features: P0 lies in a range of 0.05 days to 0.175 days (sample: 132 double-mode HADS pulsating in F and 1O); P1/P0 lies in a range of 0.761 $-$ 0.787 (sample: 142 with P0 and P1), in which about 90\% in 0.765 $-$ 0.785, which is wider than previous studies. The Petersen diagram was created with a much larger sample (144 HADS with P0 and P1) and we find that stars with periods in 0.05 $-$ 0.1 days scatter largely from the updated linear relation (i.e., Eq.1), the reason of which however needs further investigation. Particularly, we discover that the ratio P2/P1 (sample: 21 HADS with P1 and P2) equals 0.802$\pm$0.004, which could be viewed as a possible indicator to identify the modes 1O and 2O for multi-mode HADS. In addition, several unusual stars are pointed out, which may need more attention to their pulsations and stellar parameters in the future.

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arXiv2021

TIC 308396022: a $δ$ Scuti--$γ$ Doradus hybrid with large-amplitude radial fundamental mode and regular g-mode period spacing

We analyse the pulsating behaviour of TIC 308396022 observed by the TESS mission. The star is a high-amplitude $δ$ Sct star (HADS) that shows a very rich amplitude spectrum using the 3-yr light curve. Among these frequencies, the strongest peak of $f_{1}= 13.20362567(12) \rm{d^{-1}}$ is identified as the radial fundamental mode, and we also find the first and second overtones ($f_2$ and $f_3$). In the low frequency range (< 2.5 $\rm{d^{-1}}$), 22 peaks are identified to be gravity modes, which show a regular period spacing of about 2460\,s and have the angular degree $l = 1$. The period spacing pattern does not show a significant downward trend, suggesting the star rotates slowly. We note that this is a $δ$ Sct--$γ$ Dor hybrid star containing a high-amplitude radial fundamental mode and a regular g-mode period spacing pattern. With O-C analysis, we find the star shows a significant time delay, implying that the star has a companion which is likely to be a white dwarf. The history of possible mass transfer provides a great opportunity to test the current theories of binary evolution, mass transfer, and pulsation.

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arXiv2021

KIC 8840638: A new eclipsing binary consisting of $δ$ Scuti-type oscillations with an extremely cold companion star

In this paper, we analyze the light variation of KIC 8840638 using high-precision time-series data from $Kepler$ mission. The analysis reveals this target is a new Algol-type eclipsing binary system with a $δ$ Scuti component, not a pure single $δ$ Scuti star previously known. The frequency analysis of the short-cadence light curve reveals 95 significant frequencies, most of which lie in a frequency range of 23$-$32 d$^{-1}$. Among them, seven independent frequencies are detected in the typical frequency range of $δ$ Scuti stars and they are identified as pressure modes. Besides, the orbital frequency $f_{orb}$ (=0.320008 d$^{-1}$) and its harmonics are also detected directly in the frequency spectrum. The binary modellings derived from the Wilson-Devinney code indicate the binary parameters are not affected by the pulsations, and the binary system is in semi-detached configure with a mass ratio of $q$ = 0.454, an inclination angle of 58 degrees, and a temperature difference of larger than 4000 K between the two components. The derived parameters suggest the primary of this system is a main-sequence star with spectral type about A7V, while the secondary seems to be the coolest companion star among the known semi-detached Algol systems, implying it may have stepped into a highly evolved stage.

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arXiv2020

KIC 10975348: A double-mode or triple-mode high-amplitude $δ$ Scuti star?

In this paper, we analyze the light variations of KIC 10975348 using photometric data delivered from $Kepler$ mission. This star is exceptionally faint ($K_{p}$ = 18.6 mag), compared to most well-studied $δ$ Scuti stars. The Fourier analysis of the short cadence data (i.e. Q14, Q15 and Q16, spanning 220 days) reveals the variations are dominated by the strongest mode with frequency F0 = 10.231899 $\rm{d^{-1}}$, which is compatible with that obtained from $RATS-Kepler$. The other two independent modes with F1 (= 13.4988 $\rm{d^{-1}}$) and F2 (= 19.0002 $\rm{d^{-1}}$) are newly detected and have amplitudes two orders of magnitude smaller than F0. We note that, for the first time, this star is identified to be a high-amplitude $δ$ Sct (HADS) star with amplitude of about 0.7 mag, and the lower ratio of F0/F1 = 0.758 suggests it might be a metal-rich variable star. The frequency F2 may be a third overtone mode, suggesting this target might be a new radial triple-mode HADS star. We perform $O - C$ analysis using 1018 newly determined times of maximum light and derive an ephemeris formula: $T_{max}$ = 2456170.241912(0)+0.097734(1) $\times$ $E$. The $O - C$ diagram shows that the pulsation period of KIC 10975348 seems to show no obvious change, which is in contrast to that of the majority of HADS stars. The possible cause of that may be due to the current short time span of observations. To verify its possible period variations, regular observation from space with a longer time span in future is needed.

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arXiv2014

Common Envelope Mechanisms: Constraints from the X-ray Luminosity Function of High Mass X-ray Binaries

We use the measured X-ray luminosity function (XLF) of high-mass X-ray binaries (HMXBs) in nearby star-forming galaxies to constrain the common envelope (CE) mechanisms, which play a key role in governing the binary evolution. We find that the XLF can be reproduced quite closely under both CE mechanisms usually adopted, i.e., the $α_{\rm CE}$ formalism and the $γ$ algorithm, with a reasonable range of parameters considered. Provided that the parameter combination is the same, the $γ$ algorithm is likely to produce more HMXBs than the $α_{\rm CE}$ formalism, by a factor of up to $\sim$ 10. In the framework of the $α_{\rm CE}$ formalism, a high value of $α_{\rm CE}$ is required to fit the observed XLF, though it does not significantly affect the global number of the HMXB populations. We present the detailed components of the HMXB populations under the $γ$ algorithm and compare them with those in Zuo et al. and observations. We suggest the distinct observational properties, as well as period distributions of HMXBs, may provide further clues to discriminate between these two types of CE mechanisms.

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arXiv2014

On the Common Envelope Efficiency

In this work, we try to use the apparent luminosity versus displacement (i.e., $L_{\rm X}$ vs. $R$) correlation of high mass X-ray binaries (HMXBs) to constrain the common envelope (CE) efficiency $α_{\rm CE}$, which is a key parameter affecting the evolution of the binary orbit during the CE phase. The major updates that crucial for the CE evolution include a variable $λ$ parameter and a new CE criterion for Hertzsprung gap donor stars, both of which are recently developed. We find that, within the framework of the standard energy formula for CE and core definition at mass $X=10$\%, a high value of $α_{\rm CE}$, i.e., around 0.8-1.0, is more preferable, while $α_{\rm CE}< \sim 0.4$ likely can not reconstruct the observed $L_{\rm X}$ vs. $R$ distribution. However due to an ambiguous definition for the core boundary in the literature, the used $λ$ here still carries almost two order of magnitude uncertainty, which may translate directly to the expected value of $α_{\rm CE}$. We present the detailed components of current HMXBs and their spatial offsets from star clusters, which may be further testified by future observations of HMXB populations in nearby star-forming galaxies.

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arXiv2013

Population synthesis on high-mass X-ray binaries: prospects and constraints from the universal X-ray luminosity function

Using an updated population synthesis code initially developed by Hurley et al. we modeled the synthetic X-ray binary (XRB) populations for direct comparison with the universal, featureless X-ray luminosity function (XLF) of high mass X-ray binaries (HMXBs) in star-forming galaxies. Our main goal is to use the universal XLF to constrain the model parameters, given the current knowledge of binary evolution. We find that the one-dimensional (1D) Maxwellian velocity dispersion of the natal kick can be constrained to be of the order of ~150km/s, supporting earlier findings that neutron stars (NSs) formed in binaries seem to receive significantly smaller natal kicks than the velocities of Galactic single pulsars would indicate. The super-Eddington accretion factor is further confirmed in the framework of stellar mass black holes (BHs), revealing the true origin of the most of the ultraluminous X-ray sources (ULXs) may indeed be the high-luminosity extension of ordinary HMXBs which harbor stellar-mass BHs rather than exotic intermediate-mass BHs or ones. We present the detail properties of the model-predicted present-day HMXBs, which may be investigated by future high-resolution X-ray and optical observations of sources in nearby star-forming galaxies.

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arXiv2011

The contribution of X-ray binaries to the evolution of late-type galaxies: Evolutionary population synthesis simulations

X-ray studies of normal late-type galaxies have shown that non-nuclear X-ray emission is typically dominated by X-ray binaries, and provides a useful measure of star formation activity. We have modeled the X-ray evolution of late-type galaxies over the $\sim$ 14 Gyr of cosmic history, with an evolutionary population synthesis code developed by Hurley et al. Our calculations reveal a decrease of the X-ray luminosity-to-mass ratio $L_{\rm X}/M$ with time, in agreement with observations (Fig.~7$a$). We show that this decrease is a natural consequence of stellar and binary evolution and mass accumulating process in galaxies. The X-ray-to-optical luminosity ratio $L_{\rm X}/L_{\rm B}$ is found to be fairly constant (around $\sim 10^{30}$ erg\,s$^{-1}$$L_{\rm B,\odot}^{-1}$, Fig.~7$b$), and insensitive to the star formation history in the galaxies. The nearly constant value of $L_{\rm X}/L_{\rm B}$ is in conflict with the observed increase in $L_{\rm X}/L_{\rm B}$ from $z=0$ to 1.4. The discrepancy may be caused by intense obscured star formation activity that leads to nonlinear relationship between X-ray and B-band emission.

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arXiv2010

On the displacement of X-ray binaries from star clusters in star-burst galaxies

We have modeled the displacement of luminous X-ray binaries from star clusters in star-burst galaxies with an evolutionary population synthesis code developed by Hurley et al. (2000, 2002). In agreement with Kaaret et al. (2004), we find significant spatial offset of X-ray sources from their parent clusters, and the apparent X-ray luminosity vs. displacement correlation can be roughly reconstructed. The correlation is not sensitive to the fundamental properties of the clusters (e.g., initial mass functions of the binary stars) and the kick velocity imparted to the newborn compact stars, except the common envelope parameter. We present the distributions of the main parameters of the current X-ray binaries, which may be used to constrain the models for the formation and evolution of X-ray binaries with future optical observations.

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