Zhanwen Han

We present optical photometric and spectroscopic observations of the Type II supernova (SN) SN 2025abyc. During the optically thick phase, which occurs approximately 10 and 70 d after explosion, its light curves show strongly wavelength-dependent decline rates of approximately 2.7, 2.1, 0.9, and 0.8 mag (100 d) ⁻¹ in the g , c , r , and o bands, respectively. At approximately 70 d, the light curves begin to depart from their nearly linear plateau evolution and gradually transition towards the radioactive tail. A Fermi–Dirac fit to the well-sampled ATLAS o -band light curve yields a transition midpoint of t _PT ≈ 100.5 d. The interval between the end of the linear plateau and this transition midpoint is approximately 30 d, indicating a prolonged plateau-to-tail transition. This timescale is comparable to those measured for SN 2013by, SN 2013ej, and SN 2014G. Spectroscopically, at +13 d post-explosion, the H α profile appears weak and broad, whereas H β and H γ display clear P-Cygni profiles. This morphology can be explained by the normal early spectroscopic evolution of SNe II, although partial filling of the H α absorption trough by emission associated with circumstellar interaction cannot be excluded. SN 2025abyc otherwise follows the general photospheric velocity evolution of SNe II, while remaining towards the high-velocity side of the comparison distribution in H α , H α , and Fe II . Exploratory light-curve modelling suggests a synthesised ⁵⁶ Ni mass of approximately 0.03–0.04 M _⊙ . We suggest that the extended circumstellar environment, ⁵⁶ Ni distribution, and hydrogen-envelope structure could all play a role in shaping the observed light-curve evolution, particularly the prolonged plateau-to-tail transition.

Context. YZ Cancri is an SU UMa-type dwarf nova for which the available optical spectroscopy is sparse and heterogeneous. We present a homogeneous analysis of archival outburst-related spectra and seven new low-resolution spectra obtained during quiescence. Aims. We aim to consistently characterize the Balmer, He I , Fe II , O I , and occasional He II and Bowen features in YZ Cnc and to identify which state-dependent differences are robustly supported by the available data. Methods. We applied a common continuum-normalization and line-measurement procedure to the calibrated spectra. We measured equivalent widths, integrated line intensities, and selected profile parameters, and compared line morphologies and intensity ratios between quiescent and outburst-related states. Because the spectra have a modest and nonuniform resolving power and are not orbital-phase resolved, the profile widths and centroid shifts were used only as relative diagnostics. Results. The calibrated outburst-related spectra are dominated by broad Balmer absorption or composite profiles, whereas the quiescent spectra show strong single-peaked Balmer and He I emission together with recurrent Fe II λ 5169 and weak O I λ 7772 emission. He II λ 4686 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 the adopted optically thin reference values, although blending limits their interpretation. Conclusions. The spectra establish an empirical state-dependent description of YZ Cnc. The results are consistent with a nonuniform dwarf-nova accretion environment, but the present low-resolution, non-phase-resolved data do not uniquely determine the spatial origin or geometry of the line-forming gas. The homogeneous measurements provide a reference dataset for future phase-resolved, higher-resolution spectroscopy.

Binary star evolution plays a fundamental role in astrophysics. Approximately half of all stars are born in binary or multiple systems, and their interactions profoundly alter end products. Roche-lobe overflow (RLOF) and common-envelope (CE) evolution are the two central processes shaping binary populations. Yet, RLOF stability criteria and CE outcomes remain highly uncertain. In this talk we review classical theories, recent advances in adiabatic and thermal equilibrium mass-loss models, hydrodynamic simulations, and the emerging observational evidence that constrains these processes. We argue that mass transfer is more stable than previously thought, and that common-envelope ejection efficiency is not universal but depends on binary parameters. Observations of hot subdwarfs, double white dwarfs, luminous red novae, and circumstellar ejecta provide critical constraints. The implications extend to Type Ia supernovae, compact-object mergers, and gravitational-wave astronomy.

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 sawtooth 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 timescale 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 postplateau 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 sh ′ = 0.09043 ( 27 ) day, corresponding to an 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.

Context. A new class of supernovae (SNe) with prominent narrow emission features of silicon (Si), sulfur (S), and argon (Ar) has recently been reported, i.e., SN 2021yfj-like events (so-called SNe Ien). Their progenitor origin is currently unknown. It has been suggested that a massive CO white dwarf (WD) can evolve into a WD rich in both Si and S when accreting He-rich matter at a high mass-transfer rate. If the He companion subsequently evolves into another more massive WD, the merger of this double WD system can generate Si- and S-rich circumstellar material through tidal stripping. Following the SN explosion, the interaction between the SN ejecta and the Si- and S-rich circumstellar material could produce the Si, S, and Ar emission lines characteristic of SN 2021yfj-like events. Aims. We aim to determine the initial parameter space of WD+He star systems that can lead to SN 2021yfj-like events via the double WD merger scenario, and to quantify their theoretical Galactic birthrate and delay-time distributions. Methods. We performed detailed binary evolution simulations of a large number of semidetached WD+He star systems to obtain the parameter space that leads to the formation of Si- and S-rich WDs and subsequent double WD mergers. We then used binary population synthesis to calculate the Galactic birthrate and delay-time distribution of such events. Results. For the formation of SN 2021yfj-like events, the initial CO WD and He companion masses must lie in the ranges 1.0–1.2 M _⊙ and 2.2–2.5 M _⊙ , respectively. The derived merger rate for this scenario is ∼(2.6 − 4.4)×10 ⁻⁵ yr ⁻¹ with delay times longer than 450 Myr, which is about 1% of the observed SN Ia rate. Conclusions. The double WD merger scenario involving a Si- and S-rich WD originating from a CO WD+He star system represents a significant and competitive model for SN 2021yfj-like events, underscoring the need for further observations of similar events coupled with relevant theoretical investigations.

White dwarfs (WDs) in binaries can form through either the stable mass-transfer process or common envelope evolution (CEE). Compared to CEE, the stable mass-transfer process can lead to a distinct mass–orbital period ( M _WD − P _orb ) relation. Thus, this relation of WDs contains the information about the evolution channels. We can study the relation in WD binary systems to determine whether their progenitors undergo a CEE. We use the stellar evolution code MESA as our primary computational tool and adopt the quasi-adiabatic criterion to ensure that our models satisfy the conditions for stable mass transfer. Our study considers different mass-transfer schemes, varying metallicities, and the relation for both low-mass and intermediate-mass progenitors. Previous studies have focused on the relation for low-mass progenitors, which cannot explain some long-period, high-mass WD binaries. Our results show that the M _WD − P _orb distribution for intermediate-mass progenitors whose cores remain nondegenerate prior to central helium burning can account for the formation channels of long-period and massive WD binaries.

Context. The surface brightness–color relation (SBCR) links stellar color to angular diameter and is a key ingredient of geometric distance measurements on the first rung of the cosmic distance ladder. However, previous calibrations based on the red giant branch (RGB) were limited by small samples and did not observably constrain the role of metallicity. Aims. We aim to quantify the metallicity dependence of the SBCR for RGB stars and to test the robustness of the relation using asteroseismic radii, Gaia distances, and atmospheric parameters from APOGEE. Methods. We selected more than 2000 RGB stars from APOKASC-3 to calibrate and validate the SBCR. Johnson V magnitudes were synthesized from Gaia XP spectra and homogenized to widely used SBCR photometric systems, while K _s photometry was taken from 2MASS. Angular diameters derived from asteroseismic radii and Gaia distances (the ARD method) were used to construct the SBCR. We explored three fitting strategies: metallicity-free, metallicity-binned, and global metallicity-dependent relations. Results. Over the range ( V − K _s ) ≈ 2 − 3, the SBCR shows only a weak metallicity dependence. A change of 1 dex in [Fe/H] modifies the predicted angular diameter by less than 1%, well below the intrinsic scatter of the calibration (∼0.05 mag). This result is consistent with theoretical expectations. Comparison with the interferometric sample reveals a systematic offset of ∼1.5% toward smaller angular diameters in our SBCR predictions, with a mild color dependence. Conclusions. The metallicity effect on the SBCR is small in the color range explored here, but it becomes relevant for sub-percent distance measurements. Our results show that large RGB samples with asteroseismic radii and Gaia distances provide a powerful observational route for SBCR calibration, with clear potential for extensions to cooler and redder giants as the precision and parameter coverage of the input data improve.

Post-common-envelope binaries are the natural laboratories for constraining the physics of common-envelope evolution, which is one of the most uncertain phases in binary stellar evolution. Traditional binary population synthesis models, adopting mass-transfer stability criteria based on polytropic stellar models, systematically overpredict the number of post-common-envelope binaries with solar-type main-sequence companions. In this work, we present an updated binary population synthesis model using the rapid binary evolution code binary star evolution, incorporating a physically motivated mass-transfer stability criterion and a self-consistent envelope binding-energy prescription. We compile a comprehensive sample of classic white dwarf + main-sequence post-common-envelope binaries with well-measured parameters, hosting both M dwarf and A/F/G/K stars. We find that the enhanced mass-transfer stability is an additional mechanism responsible for the observed dearth of post-common-envelope binaries with solar-type main-sequence companions; neither magnetic braking nor selection effects alone can fully account for this deficit, and a combination of all three processes is most likely required. Models with inefficient common-envelope evolution ( α _CE = 0.25) provide the best overall match to the observed population. These results highlight the critical role of mass-transfer stability in shaping the observed post-common-envelope binary population and provide new constraints on common-envelope evolution.