Xuefei Chen

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.

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.

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.

Most massive stars reside in binary systems, and binary interactions can profoundly alter the properties of both stellar components. In young open clusters, such interactions can produce multiple main-sequence populations, despite the coeval nature of the stellar population. In this work, we extend our previous studies on the role of binary evolution in shaping main-sequence morphologies by performing a more detailed and systematic analysis. We investigate the progenitor properties of different main-sequence populations from 10 to 100 Myr using detailed binary models computed with MESA, assuming SMC-like metallicity, and initial primary masses of 3–100 M _⊙ . We provide physical explanations for the features identified in our earlier work. We focus especially on post–Case A mass-transfer systems, which produce a prominent “bridge-like” feature connecting the reddest Be stars and the bluest blue stragglers. We show that the positions of these stars are sensitive to surface helium enrichment and rotation. We further perform quantitative comparisons with observed clusters aged 30–90 Myr whose turn-off masses are from approximately 9 to 5 M _⊙ . We find that our models broadly reproduce the observed stellar distributions. However, we underpredict the fraction of Be stars, suggesting that a larger fraction of binaries may undergo stable mass transfer. We also overpredict the number of stars on the redder side of the unresolved-binary sequence, indicating that the initial mass-ratio distribution may deviate from a flat distribution. Overall, this work provides a comprehensive analysis of how binary evolution produces diverse main-sequence populations and lays the foundation for future population-synthesis studies.

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.

Erratum: “Adiabatic Mass Loss in Binary Stars. IV. Low- and Intermediate-mass Helium Binary Stars” (2024, ApJS, 274, 11), Zhang, Lifu, Ge, Hongwei, Chen, Xuefei, Han, Zhanwen Erratum: “Adiabatic Mass Loss in Binary Stars. IV. Low- and Intermediate-mass Helium Binary Stars” (2024, ApJS, 274, 11), Zhang, Lifu, Ge, Hongwei, Chen, Xuefei, Han, Zhanwen

Classical binary evolution models predict dynamically unstable mass transfer in symbiotic stars with high mass ratios (q_ > 1.6), leading to a common envelope (CE). However, a large fraction of observed S-type symbiotic systems maintain long-lived interactions, suggesting that an additional stabilizing mechanism may be at work. crit We investigated whether atmospheric Roche-lobe overflow (RLOF) can prolong the mass transfer phase in symbiotic binaries, and thereby reconcile theoretical predictions with observed system numbers. We implemented the rapid unified mass transfer (RUMT) framework within and computed a grid of white dwarf--giant binaries across a wide parameter space in donor mass, mass ratio, and orbital period. We then compared the resulting lifetimes and evolutionary tracks with a sample of well-constrained Galactic S-type symbiotic systems. MESA For convective giant donors, our models recover dynamically stable mass transfer up to q ,M_⊙, ^ consistent with previous studies, but atmospheric overflow strongly extends the symbiotic phase. RGB and early-AGB systems with q łesssim 1.5 can remain interacting for up to 10^6 yr at dot M ≳ 10^ -9 yr -1 , two to three orders of magnitude longer than commonly assumed in population studies sim10^3 yr pre-CE lifetimes. In these models the orbit shrinks mildly or re-expands after the mass ratio is less than 1.0. Systems with higher mass ratios still evolve towards a CE, but even for q the symbiotic phase lasts 10^4-10^5 yr, one to two orders of magnitude longer than usually assumed. The synthetic distribution in the P--q plane and individual evolutionary tracks are consistent with observed S-type symbiotic binaries, including recurrent novae. The RUMT framework, which incorporates atmospheric RLOF, provides an explanation for the long-term stability of many symbiotic binaries and may account for their high observed occurrence rate.

Type Ia supernovae (SNe Ia) serve as one of cosmic standard candles, but their exact progenitor channel is still an open question. SNe Ia commonly come from binary star evolution. Therefore, one of the major differences among the proposed progenitor channels is whether there is a more-or-less intact companion star remaining at the time of explosion, which causes the SN ejecta to be more asymmetrical. As the SN ejecta evolved into SN remnants (SNRs), the imprint formed by the companion interaction may affect the morphology of the SNR. In addition, the progenitor systems may have experienced different mass transfer histories and therefore led to the formation of different circumstellar material environments, which may also affect the early evolution of SNR. In this study, we use GADGET and RAMSES codes to simulate these physical effects and follow the evolution into the early phases of SNRs. In our simulations, we consider different ejecta models and track the element distribution. We compare our simulation with actual observations and conclude that despite some SNRs having morphology resemblance to our simulation results, their highly asymmetric expansion rates are hard to explain by interaction between SN ejecta and a companion star alone.