arXiv subjects

Zhibin Dai

Publications and source records attributed to Zhibin Dai.

17 recordsLinked to original sources

arXiv2026

A homogeneous analysis of archival and new low-resolution spectra of YZ Cancri

YZ Cancri is an SU UMa-type dwarf nova with sparse and heterogeneous optical spectroscopy. We present a homogeneous analysis of archival outburst-related spectra and seven new low-resolution quiescent spectra. We consistently characterize the Balmer, He I, Fe II, O I, and occasional He II and Bowen features, and assess which state-dependent differences are robustly supported. We applied a common continuum-normalization and line-measurement procedure, measuring equivalent widths, integrated line intensities, and selected profile parameters, and comparing line morphologies and intensity ratios between quiescent and outburst-related states. Because the spectra have modest and nonuniform resolving power and are not orbital-phase resolved, profile widths and centroid shifts are used only as relative diagnostics. Outburst-related spectra are dominated by broad Balmer absorption or composite profiles, whereas quiescent spectra show strong single-peaked Balmer and He I emission, recurrent Fe II λ5169, and weak O I λ7772 emission. He II λ4686 is detected in only two quiescent exposures, and the Bowen blend in one. The quiescent Balmer decrement is relatively flat, while the He I ratios show substantial exposure-to-exposure scatter. The measured Fe II ratios show no clear systematic departure from optically thin reference values, although blending limits their interpretation. These spectra establish an empirical state-dependent description of YZ Cnc and provide a reference dataset for future phase-resolved, higher-resolution spectroscopy.

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arXiv2026

Stage-dependent superhump waveform evolution and non-stationary positive-superhump timing in the near-period-gap dwarf nova YZ Cancri

We present a photometric and timing study of the near-period-gap SU~UMa-type dwarf nova YZ~Cancri, based on nearly continuous Transiting Exoplanet Survey Satellite (TESS) photometry and long-baseline ground-based observations. Our main observational result is that the positive-superhump (SH) waveform follows a repeatable, stage-dependent sequence during superoutbursts (SOs). In the two well-covered TESS SOs, and consistently in the long-baseline ground-based SO sample, the plateau waveform evolves from an early saw-tooth profile (ST), through a double-humped profile with unequal maxima (DHd), to a more symmetric double-humped profile with nearly similar maxima (DHs). The global and time-resolved Lomb--Scargle periodograms show that power near the positive-SH time scale and its harmonics is concentrated during SOs, whereas quiescent and normal-outburst intervals lack a comparably persistent SH-band signal. The dense TESS maxima--minima timing sequence shows different clock stability in different waveform stages: precursor modulations have slightly longer local periods, the middle-to-late DHs plateau is the most regular timing interval, and the post-plateau evolution is affected by phase switching and possible secondary/late-SH contamination. The TESS data also reveal profile-clock coupling, with SH amplitude and rise/decay durations evolving together with the timing residuals. As a secondary timing constraint, we obtain a common TESS-timing-based mean positive-SH period of $P'_{\rm sh}=0.09043(27)$~d, corresponding to a SH excess of $4.03(31)\%$ and an approximate mass ratio of $q=0.175(11)$. The repeatable ST--DHd--DHs sequence makes YZ~Cnc a useful system near the lower edge of the period gap, and may trace the growth, redistribution, stabilization, and decay of the light source associated with an eccentric, precessing accretion disk during SOs.

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arXiv2026

Can dwarf novae produce type Ia supernovae?

Accreting white dwarfs (WDs) are considered one of the most promising progenitor candidates for Type Ia supernovae (SNe Ia). Dwarf novae (DNe), a subclass of cataclysmic variables (CVs), consist of a carbon--oxygen (CO) WD accretor and a low-mass donor star, which may be either a main-sequence (MS) star or a slightly evolved subgiant. Previous studies have suggested that, under the thermal--viscous disk instability mechanism, the time-averaged accretion rate in long-period DNe may approach the regime of stable hydrogen burning, potentially allowing the WD to grow toward the Chandrasekhar mass, (M_{\rm Ch}). However, whether such periodic accretion can sustain stable hydrogen burning and lead to WD mass growth remains uncertain. In this work, we explore whether high accretion rates on short periodic timescales can maintain stable hydrogen burning on the WD surface and drive the WD toward (M_{\rm Ch}). Using Modules for Experiments in Stellar Astrophysics (MESA), we investigate the mass-retention efficiency of WDs undergoing intermittent DN-like accretion and examine its dependence on duty cycle, accretion rate, and initial WD properties. We find that periodic accretion fails to maintain stable hydrogen burning. During quiescent phases, the WD cools and becomes increasingly degenerate, leading to nova outbursts with progressively decreasing mass-retention efficiency and ultimately preventing further WD mass growth. We therefore suggest that DNe are unlikely to be progenitors of SNe Ia.

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arXiv2026

Evidence of Long-Lived Powerful Gyrosynchrotron Radio Emission in the Close Binary FF UMa

RS Canum Venaticorum (RS CVn) close binaries, characterized by tidal locking, rapid rotations, and strong magnetic fields, are ideal laboratories for high-resolution radio observations to probe emission processes, magnetic field configurations, and interaction activity. Despite their importance, only a few RS CVn sources have been explored by polarimetric observations of very long baseline interferometry (VLBI). To expand the effort, we have analyzed the existing Very Long Baseline Array (VLBA) astrometric data for the RS CVn binary FF Ursae Majoris (FF UMa). In the 5GHz VLBA experiments conducted between 2021 and 2024, both total intensity and circularly polarized emission were clearly detected at six of seven epochs. The consistently high brightness temperatures (10^7 K) and the moderate fractional circular polarization (10%-30%) over about three years indicate that the radio emission is mainly produced by gyrosynchrotron radiation from mildly relativistic electrons in the highly-ordered magnetic field. The radio luminosities are also comparable to those of previously studied powerful RS CVn binaries and show a significant anti-correlation with fractional circular polarization. A mean centroid offset of 13.4 +/- 3.1 solar radii between the Stokes I and V emission was found across multiple epochs, indicating a possible additional contribution from the secondary star via a magnetically active corona, a giant magnetic loop, or significant interaction activity with the primary star in the quiescent state.

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arXiv2025

A New Algol-type Binary with an Accretion disk

We present a comprehensive photometric and spectroscopic analysis of the Algol-type binary \textit{Gaia} DR3 1892576067672499328. We identified the system as a spectroscopic binary based on medium-resolution LAMOST spectra. Combined with \textit{TESS} photometry, we determine an orbital period of $ P = 2.47757 (1) $ days, a low mass ratio of $ q = 0.098 \pm 0.002 $, and an orbital inclination of $ i = 46.934^{+2.613}_{-1.11} $ degrees. The orbit is consistent with being circular ($ e = 0 $). The binary comprises a $ M_1 = 1.817 ^{ +0.106}_{-0.202} \,M_\odot $, $ R_1 = 1.265^{+0.121}_{-0.160}\,R_\odot $ A-type primary and a Roche-lobe-filling secondary of $ M_2 = 0.179 ^{ +0.011}_{-0.020} \,M_\odot $, $ R_2 = 1.994 ^{ +0.041}_{-0.077} \,R_\odot $. The double-peak H$α$ emission line indicates the possible existence of a Keplerian accretion disc. We established a simple standard accretion disc model and modeled the geometric and dynamical properties of the accretion disc. The obtained outer disc radius $R_{\mathrm{out}} \approx 3.36 \pm 0.43\,R_\odot$ is consistent with the values inferred from the emission velocity of H$α$. Systemic velocity variations observed over time suggest the possible presence of a tertiary companion, with a minimum mass of $M_3 > 0.369 \pm 0.024 \,M_\odot$. Given the low mass ratio, the secondary may evolve into a proto-helium white dwarf, forming an \text{EL CVn}-type system in the future. This system offers valuable insights into accretion dynamics and the formation of binaries.

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arXiv2024

Minute-Cadence Observations of the LAMOST Fields with the TMTS: IV -- Catalog of Cataclysmic Variables from the First 3-yr Survey

The Tsinghua University--Ma Huateng Telescopes for Survey (TMTS) started to monitor the LAMOST plates in 2020, leading to the discovery of numerous short-period eclipsing binaries, peculiar pulsators, flare stars, and other variable objects. Here, we present the uninterrupted light curves for a sample of 64 cataclysmic variables (CVs) observed/discovered using the TMTS during its first three-year observations, and we introduce new CVs and new light-variation periods (from known CVs) revealed through the TMTS observations. Thanks to the high-cadence observations of TMTS, diverse light variations, including superhumps, quasi-periodic oscillations, large-amplitude orbital modulations, and rotational modulations, are able to be detected in our CV samples, providing key observational clues for understanding the fast-developing physical processes in various CVs. All of these short-timescale light-curve features help further classify the subtypes of CV systems. We highlight the light-curve features observed in our CV sample and discuss further implications of minute-cadence light curves for CV identifications and classifications. Moreover, we examine the H$α$ emission lines in the spectra from our nonmagnetic CV samples (i.e., dwarf novae and nova-like subclasses) and find that the distribution of H$α$ emission strength shows significant differences between the sources with orbital periods above and below the period gap, which agrees with the trend seen from the SDSS nonmagnetic CV sample.

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arXiv2020

Evolution of the Quiescent Disk surrounding a Superoutburst of the Dwarf Nova TW Virginis

Portions of the Kepler K2 Short Cadence light curve of the dwarf nova (DN) TW Vir at quiescence are investigated using light curve modeling. The light curve was separated into 24 sections, each with a data length of $\sim\,$0.93\,d, comprising 4 sections before and 20 after a superoutburst (SO). Due to the morphological differences, the quiescent orbital modulation is classified into three types. Using a fixed disk radius and the two component stellar parameters, all 24 synthetic disk models from the sections show a consistent configuration, consisting of a disk and two hotspots: one at the vertical side of the edge of the disk and the other one on the surface of the disk. Before the SO, the disk and a ringlike surface-hotspot are suddenly enhanced, triggering a precursor and then SO. At the end of the quiescent period following the SO and before the first normal outburst, the edge-hotspot becomes hotter, while the surface-hotspot switches into a ``coolspot" with a coverage of nearly one-half of the disk surface. During quiescence, the surface-hotspot is always located at the outer part of the disk with a constant radial width. A flat radial temperature distribution of the disk is found and appears flatter when approaching the outburst. Like many U\,Gem-type DN with orbital periods of 3-5\,hr, the mass transfer rate is significantly lower than the predictions of the standard/revised models of CV evolution.

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arXiv2020

Followup ground-based observations of the dwarf nova KZ Gem

We present spectroscopy of stars in the immediate vicinity of the dwarf nova (DN) KZ Gem to confirm its identification, which had been ambiguous in the literature. Analysis of 73 radial velocities spanning from 2014 to 2019 provides a high-precision orbital period of 0.2224628(2)\,d ($\sim5.34$\,hr) and shows KZ\,Gem to be a double-lined DN. Time series photometry taken from 2016 to 2018 shows a variable double-hump modulation with a full amplitude of $\sim0.3$\,mag, along with five Gaussian-like transient events lasting $\sim30$\,min or more. Using the light curve code XRBinary and nonlinear fitting code NMfit, we obtain an optimized binary model of the dwarf nova (DN) KZ Gem, from time series photometry, consisting of a Roche-lobe-filling K type dwarf with a mass transfer rate of $2.7\,-\,7.9\times10^{-10}\,{\rm M}_{\odot}\,{\rm yr}^{-1}$ to a large, cool and thick disk surrounding a white dwarf, in an orbit with an inclination of $51^{\circ}.6(\pm1^{\circ}.4)$. Two hotspots on the disk are demonstrated to cause the observed variations in the ellipsoidal modulations from the secondary star. This physical model is compatible with the Gaia distance of KZ\,Gem.

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arXiv2018

A Phenomenological Model for the Light Curve of three Quiescent Low-inclination Dwarf Novae and one Pre-Cataclysmic Variable

We used the light curve code XRBinary to model the quiescent K2 light curves of three low-inclination cataclysmic variables (CVs): 1RXS\,J0632+2536 (J0632+2536), RZ\,Leo, TW\,Vir and the pre-CV WD\,1144+011. Optimized light curve models were obtained using a nonlinear fitting code NMfit and visualized by Phoebe 2.0. The disk model of J0632+2536 shows that one hotspot at the edge of the disk is enough to describe its light curve, while the other two dwarf nova (DN): RZ\,Leo and TW\,Vir require two hotspots. A typical pre-CV model with a weak irradiation effect for WD\,1144+011 can explain its single-hump modulation, and the newly observed spectrum confirms its previous classification. The synthetic analyses for the DN clearly indicate that phase zero of the double-hump modulations occurs around the secondary minimum and the primary hump is mainly caused by the hotspot at the edge of the disk. The quiescent disk has a flat temperature distribution with a power index of $\sim0.11$. The disk model of RZ\,Leo implies a truncated disk, supporting its previously speculated classification as an intermediate polar (IP). Except for the IP model of RZ\,Leo, which lacks a component related to the inferred accretion curtain, the models of J0632+2536, TW\,Vir and WD\,1144+011 are consistent with results from the Gaia mission. The derived masses and radii of the secondaries of the three DN are consistent with the semi-empirical relations for CV donor stars, while their effective temperatures are higher than the predictions. Irradiation of the donor stars is investigated to explain this discrepancy.

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arXiv2018

A Comprehensive K2 and Ground-Based Study of CRTS J035905.9+175034, an Eclipsing SU UMa System with a Large Mass Ratio

CRTS J035905.9+175034 is the first eclipsing SU UMa system for which a superoutburst has been observed by Kepler in the short-cadence mode. The light curve contains one superoutburst, eight normal outbursts (including a precursor to the superoutburst), and several minioutbursts that are present before -- but not after -- the superoutburst. The superoutburst began with a precursor normal outburst, and shortly after the peak of the precursor, the system developed large-amplitude superhumps that achieved their maximum amplitude after just three superhump cycles. The period excess of the initial superhump period relative to the orbital period implies a mass ratio of 0.281 +/- 0.015, placing it marginally above most theoretical predictions of the highest-possible mass ratio for superhump formation. In addition, our analysis of the variations in eclipse width and depth, as well as the hot spot amplitudes, generally provides substantiation of the thermal-tidal instability model. The K2 data, in conjunction with our ground-based time-resolved spectroscopy and photometry from 2014-2016, allows us to determine many of the fundamental parameters of this system.

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arXiv2017

Followup Observations of SDSS and CRTS Candidate Cataclysmic Variables II

Spectra of 38 candidate or known cataclysmic variables are presented. Most are candidate dwarf novae or systems containing possible highly magnetic white dwarfs, while a few (KR Aur, LS Peg, V380 Oph and V694 Mon) are previously known objects caught in unusual states. Individual spectra are used to confirm a dwarf nova nature or other classification while radial velocities of 15 systems provide orbital periods and velocity amplitudes that aid in determining the nature of the objects. Our results substantiate a polar nature for four objects, find an eclipsing SW Sex star below the period gap, another as a likely intermediate polar, as well as two dwarf novae with periods in the middle of the gap.

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arXiv2017

Quiescent photometric modulations of two low-inclination cataclysmic variables KZGem and TWVir

The quiescent periodic photometric modulations of two low-inclination cataclysmic variables observed in Kepler K2 Campaigns 0 and 1, KZ Gem and TW Vir, are investigated. A phase-correcting method was successfully used to detect the orbital modulations of KZ Gem and TW Vir and improve their orbital periods. The light curve morphologies of both CVs were further analyzed by defining flux ratios and creating colormaps. KZ Gem shows ellipsoidal modulations with an orbital period of 0.22242(1) day, twice the period listed in the updated RK catalogue (Edition 7.24). With this newly determined period, KZ Gem is no longer a CV in the period gap, but a long-period CV. A part of the quiescent light curve of TW Vir that had the highest stability was used to deduce its improved orbital period of 0.182682(3) day. The flat patterns shown in the colormaps of the flux ratios for KZ Gem demonstrate the stability of their orbital modulations, while TW Vir show variable orbital modulations during the K2 datasets. In TW Vir, the single versus double-peaked nature of the quiescent orbital variations before and after superoutburst may be related to the effect of the superoutburst on the accretion disk.

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arXiv2017

Hubble Space Telescope Ultraviolet Light Curves Reveal Interesting Properties of CC Sculptoris and RZ Leonis

Time-tag ultraviolet data obtained on the Hubble Space Telescope in 2013 reveal interesting variability related to the white dwarf spin in the two cataclysmic variables RZ Leo and CC Scl. RZ Leo shows a period at 220s and its harmonic at 110s, thus identifying it as a likely Intermediate Polar (IP). The spin signal is not visible in a short single night of ground based data in 2016, but the shorter exposures in that dataset indicate a possible partial eclipse. The much larger UV amplitude of the spin signal in the known IP CC Scl allows the spin of 389s, previously only seen at outburst, to be visible at quiescence. Spectra created from the peaks and troughs of the spin times indicate a hotter temperature of several thousand degrees during the peak phases, with multiple components contributing to the UV light.

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arXiv2016

Cataclysmic Variables observed during K2 Campaigns 0 and 1

There are 15 cataclysmic variables (CVs) observed in the first two campaigns of the K2 mission. In this paper, the eight CVs showing distinct features are analyzed in detail. Among these eight, modulations during quiescence are evident at the known orbital periods in the SU UMa stars QZ Vir and RZ Leo, and at our newly determined orbital periods of 1RXS J0632+2536 and WD 1144+011. The periodogram analysis for the quiescent light curve of QZ Vir reveals multi-period modulations and the coexistence of orbital and superhump periods. The phased orbital light curves for the other 3 CVs in quiescence display wide (about half cycle) and shallow (< 0.5 mag) eclipse features. Besides these modulations, their quiescent light curves reveal several transient events: a sudden decrease of system light in 1RXS J0632+2536, a low level flare-like event in QZ Vir, a short brightening event in RZ Leo and a temporary disappearance of the orbital modulation in WD 1144+011. The two known dwarf novae UV Gem and TW Vir and the CVs USNO-B1.01144-00115322 and CSS130516:111236.7+002807 show outbursts, including 1 complete and 3 incomplete normal outbursts and 2 complete superoutbursts. An incomplete but typical normal outburst confirms the dwarf nova identification of the USNO-B1.01144-00115322. The one complete normal outburst in UV Gem possibly provides the orbital period, since its modulations are shorter than the previously observed superhump period. The superoutbursts of TW Vir and CSS130516:111236.7+002807, along with their corresponding superhump periods, indicate that both objects are SU UMa stars. The derived superhump period of CSS130516:111236.7+002807 is 1.44 hr, implying that this new SU UMa star is close to the period minimum.

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arXiv2013

Updated photometry and orbital period analysis for the polar \objectname{am herculis} on the upper edge of the period gap

Twenty-one new optical light curves, including five curves obtained in 2009 and sixteen curves detected from the AAVSO International Database spanning from 1977 to 2011, demonstrate 16 new primary minimum light times in the high state. Furthermore, seven newly found low-state transient events from 2006 to 2009 were discovered, consisting of five Gaussian-shaped events and two events with an exponential form with decay timescales of $<$0.005 days; these timescales are one order of magnitude shorter than those of previous X-ray flare events. In the state transition, two special events were detected: a "disrupted event" with an amplitude of $\sim$ 2 mag and a duration of $\sim$ 72 minutes and continuing R-band twin events larger than all known R-band flares detected in M-type red dwarfs. All 45 available high-state data points spanning over 35 yr were used to construct an updated O-C diagram of \objectname{AM Herculis}, which clearly shows a significant sine-like variation with a period of 12-15 yr and an amplitude of 6-9 minutes. Using the inspected physical parameters of the donor star, the secular variation in the O-C diagram cannot be interpreted by any decided angular momentum loss mechanism, but can satisfy the condition $τ_{\dot{\rm M}_{2}}\simeqτ_{\rm KH}>>τ_{\dot{\rm R}_{\rm 2}}$, which is required by numerical calculations of the secular evolution of cataclysmic variables. In order to explain the prominent periodic modulation, three plausible mechanisms - spot motion, the light travel-time effect, and magnetic active cycles - are discussed in detail.

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arXiv2010

Orbital period analysis of eclipsing Z Cam-type Dwarf Nova EM Cygni: Evidence of magnetic braking and a third body

Combining with our newest CCD times of light minimum of EM Cygni, all 45 available times of light minimum including 7 data with large scatters are compiled and the updated O-C analysis is made. The bestfit for the O-C diagram of EM Cygni is a quadratic-plus-sinusoidal fit. The secular orbital period decrease rate -2.5(\pm 0.3)x10^{-11} s s^{-1} means that magnetic braking effect with a rate of mass loss via stellar wind, 2.3x10^{-10}Msunyr^{-1}, is needed for explaining the observed orbital period decrease. Moreover, for explaining the significant cyclical period change with a period of \sim 17.74(\pm 0.01)yr shown in the O-C diagram, magnetic activity cycles and light travel-time effect are discussed in detail. The O-C diagram of EM Cygni cannot totally rule the possibility of multi-periodic modulation out due to the gaps presented after 25000 cycles. Based on the hypothesis of a K-type third star in literature, light trave-time effect may be a more plausible explanation. However, the low orbital inclination of the third body (\sim 7.4 degree) suggests that the hypothetic K-type third star may be captured by EM Cygni. But assuming the spectral contamination from a block of circumbinary material instead of a K-type third star, the third star may be a brown dwarf in case of the coplanar orbit with parent binary.

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arXiv2010

A cyclical period variation detected in the updated orbital period analysis of TV Columbae

The two CCD photometries of the intermediate polar TV Columbae are made for obtaining the two updated eclipse timings with high precision. There is an interval time \sim 17yr since the last mid-eclipse time observed in 1991. Thus, the new mid-eclipse times can offer an opportunity to check the previous orbital ephemerides. A calculation indicates that the orbital ephemeris derived by Augusteijn et al. (1994) should be corrected. Based on the proper linear ephemeris (Hellier, 1993), the new orbital period analysis suggests a cyclical period variation in the O-C diagram of TV Columbae. Using Applegate's mechanism to explain the periodic oscillation in O-C diagram, the required energy is larger than that a M0-type star can afford over a complete variation period \sim 31.0(\pm 3.0)yr. Thus, the light travel-time effect indicates that the tertiary component in TV Columbae may be a dwarf with a low mass, which is near the mass lower limit \sim 0.08Msun as long as the inclination of the third body high enough.

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