Neutrinos can experience fast flavor conversions (FFCs) in highly dense astrophysical environments, such as core-collapse supernovae and neutron star mergers, potentially affecting energy transport and other processes. The simulation of fast flavor conversions under realistic astrophysical conditions requires substantial computational resources and involves significant analytical challenges. While machine learning methods like Multilayer Perceptrons have been used to accurately predict the asymptotic outcomes of FFCs, their 'black-box' nature limits the extraction of direct physical insight. To mitigate this limitation, we employ two distinct interpretable machine learning frameworks-Kolmogorov-Arnold Networks (KANs) and Sparse Identification of Nonlinear Dynamics (SINDy)-to learn interpretable surrogates for the asymptotic input-output mapping from a FFC simulation dataset. Our analysis reveals a fundamental trade-off between predictive accuracy and model simplicity. The KANs demonstrates high fidelity in reconstructing post-conversion neutrino energy spectra, achieving accuracies of up to 90%. In contrast, SINDy yields a low rank, compact closed-form approximation of the input-output mapping, at the expense of some predictive accuracy. Critically, using these structured and sparse surrogates as diagnostic tools, we identify that the system's evolution is most sensitive to the initial number density of heavy-lepton neutrinos when FCCs are triggered, compared to other physical quantities. Ultimately, this work provides a methodological framework for interpretable machine learning that supports genuine data-driven scientific discovery in astronomy and astrophysics, going beyond prediction alone.

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 MT stability in shaping the observed post-common-envelope binaries population and provide new constraints on common envelope evolution.

Reliable estimates of stellar effective temperature (T-eff) are fundamental to stellar population studies and Galactic astrophysics. However, the majority of stars observed in modern large-scale photometric surveys lack spectroscopic measurements, making empirical colour-T-eff relations essential tools. In this work, we present updated empirical colour-T-eff calibrations based on Sloan Digital Sky Survey (SDSS) ugriz photometry combined with 2MASS JHKs data. Effective temperatures are determined on a homogeneous InfraRed Flux Method scale using a combined sample of 3902 GALAH and 2535 APOGEE stars with high-quality photometry and well-characterized atmospheric parameters. Using this data set, we establish empirical relations between T-eff and colour indices constructed from SDSS and 2MASS combinations. We provide both colour-metallicity-T-eff and colour-T-eff relations for dwarfs and giants. The calibrations are derived using low-order polynomial models with iterative 3cr clipping. Their performance depends on the adopted colour index, with long-baseline colours such as (g-K-s)0 and (g-z)(0) achieving internal precisions of similar to 30-50 K. Comparisons with previous calibrations show general agreement, with differences attributable to sample selection, photometric zero-points, and functional form. The resulting relations provide a homogeneous and internally consistent framework for estimating T-eff from SDSS and 2MASS photometry alone, and are well suited for application to large photometric surveys lacking spectroscopic information.

The massive binary common envelope (CE) phase plays a pivotal role in the formation of close black hole (BH)/neutron star binaries, yet significant uncertainties remain in our understanding of this process. In this study, we aim to constrain the massive binary CE phase by systematically reconstructing three observed BH X-ray binaries (BHXBs): GRO J1655-40, SAX J1819.3-2525, and 4U 1543-47. Through comprehensive binary evolution simulations and parametric supernova modeling, we establish lower limits for the CE efficiency parameters under different energy considerations within the standard energy formalism. Specifically, we derive minimum values for three cases: alpha 0.5U and alpha U, representing CE efficiencies with half and all of the internal energy contributing to the envelope ejection, respectively, and alpha H, accounting for the envelope's enthalpy. Our analysis reveals that the self-consistent formation of these three BHXBs requires CE efficiency parameters satisfying alpha 0.5U greater than or similar to 6.7, alpha U greater than or similar to 4.2, and alpha H greater than or similar to 1.7. Notably, we find no viable solutions with CE efficiency values below unity, even when considering the most extreme scenarios, in which the envelope binding energy is significantly reduced through enthalpy inclusion. Our results strongly imply that either additional energy sources are required or the formalism itself must be revised. Furthermore, we quantitatively assess the impact of BH natal kicks on our results. A key finding is that 4U 1543-47's formation requires substantial natal kicks (greater than or similar to 50 km s-1), as lower kick velocities are incompatible with isolated binary evolution.

Be stars are rapidly rotating main-sequence stars that play a crucial role in understanding stellar evolution and binary interactions. In this Letter, we propose a new formation scenario for black hole (BH) + Be star binaries (hereafter BHBe binaries), where the Be star is produced through the wind Roche lobe overflow (WRLOF) mechanism. Our analysis is based on numerical simulations of the WRLOF process in massive binaries, building on recent theoretical work. We demonstrate that the WRLOF model can efficiently form BHBe binaries under reasonable assumptions on stellar wind velocities. Using rapid binary population synthesis, we estimate the population of such systems in the Milky Way, predicting ∼1800−3200 currently existing BHBe binaries originating from the WRLOF channel. These systems are characterized by high eccentricities and exceptionally wide orbits, with typical orbital periods exceeding 1000 days and a peak distribution around ∼10,000 days. Due to their long orbital separations, these BHBe binaries are promising targets for future detection via astrometric and interferometric observations.

ATLAS J1138-5139 is a newly detected ultra-compact double white dwarf (DWD) system which is composed of a 1.02 M-circle dot carbon-oxygen white dwarf (CO WD) and a 0.24 M-circle dot helium (He) WD with an orbital period of about 27.68 minutes , making it one of the shortest-period DWD systems known. The future evolution and final fate of this system remain unexplored. In this work, we investigate the evolution of ATLAS J1138-5139 with the one-dimensional stellar evolution code Modules for Experiments in Stellar Astrophysics. We find that ATLAS J1138-5139 will evolve into an AM Canum Venaticorum system in about similar to 6.3 Myr. Afterwards, the transferred material from the He WD companion start to build up to form a He shell near the surface of the CO WD. This accumulated He-shell masses can be up to approximately 0.12 M-circle dot, which is likely to trigger a double-detonation (DDet) explosion of the CO WD. We therefore expect that ATLAS J1138-5139 will likely explode as a type Ia supernova eventually through the DDet explosion mechanism. Moreover, our calculations show that ATLAS J1138-5139 will be a promising target for gravitational-wave detection by future detectors like LISA, TianQin and Taiji.

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'_ 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.

Helium white dwarfs (WDs) with masses less than 0.3 M ⊙ are known as extremely low-mass WDs (ELM WDs), which cannot be produced by single stellar evolution in theory. Generally, these stars are believed to form through binary interactions. Recently, two ELM WDs in unusually wide orbits were reported, i.e., KIC 8145411 and HE 0430-2457. Their orbital separations are too wide to be produced by the binary evolution scenario. In this work, we study the formation of wide-orbit ELM WD binaries from hierarchical triple systems. In this scenario, an ELM WD is formed from the inner binary and subsequently forms a wide binary system with the third object. We find that the merger of an evolved star with a brown dwarf in the inner binary fails to produce single ELM WDs, but Type Ia supernovae (SNe Ia) explosions can successfully do so. Furthermore, we investigate the impact of the supernova explosion on the orbital distribution of the surviving binary and find that this channel may have a probability of reproducing the orbital parameters of HE 0430-2457, but fails to reproduce the observed features of KIC 8145411. This supports recent observational recalibrations suggesting that KIC 8145411 resides in a triple system rather than a binary.

Observationally, blue horizontal branch (BHB) stars are defined as hot stars occupying a characteristic region between the extreme blue horizontal branch and RR Lyrae variables in the Hertzsprung-Russell diagram. Most of them are interpreted as stripped core-helium-burning stars, but the role of binary interaction in their formation remains unclear. Here, we report the discovery of a metal-rich BHB star in a 0.82628-day binary system (64) comprising a 0.35±0.03 M_⊙ BHB star and a likely 1.26±0.17 M_⊙ white dwarf (WD). The BHB star has an effective temperature of 15,524±310 K and a luminosity of 39.7±4.1 L_⊙. Stellar evolution modelling indicates that it is a helium-shell-burning star produced through the common-envelope channel, retaining a hydrogen-rich envelope that is more massive than previously thought for low-mass stars. This finding provides direct evidence for binary interaction in the formation of BHB stars, offering a fresh perspective on interpreting this emerging population.

Dark matter heating in planets has been proposed as a potential probe for dark matter detection. Assuming near-equilibrium conditions, we find that the energy input from dark matter raises planetary temperatures and accelerates rotation. The distribution of energy between heating and rotational acceleration depends on both planetary properties and external inputs, suggesting that previous studies may have overestimated the heating contribution. At high dark matter densities, planetary rotation stabilizes earlier and becomes primarily governed by dark matter effects.

Hot subdwarf (sdB) stars in binary systems with main-sequence (MS) companions provide valuable insights into mass transfer and envelope ejection processes in binary evolution. Their mass ratios, orbital periods, and stellar properties encode key information about their evolutionary histories. In this work, we analyze a sample of 123 composite-spectrum sdB+MS binaries identified from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope Low-Resolution Survey (LAMOST-LRS) Data Release (DR) 8. We adopt atmospheric parameters from spectral decomposition and estimate stellar masses and radii using theoretical evolutionary tracks. Radial velocities for both the sdBs and cool companions are measured independently through cross correlation with synthetic templates. Orbital periods are statistically estimated using single-epoch radial velocity separations and a Monte Carlo method that accounts for random inclination and orbital phase. We find that sdB masses are narrowly distributed around 0.5 M circle dot, consistent with expectations for core-helium-burning stars, while MS companion masses span 0.6-1.9 M circle dot, with most falling between 1.0 and 1.4 M circle dot. The inferred orbital-period distribution shows a clear concentration toward long periods, broadly consistent with expectations for binaries formed through stable Roche-lobe overflow. Given that our sample consists of composite-spectrum sdB binaries, mainly sdB+FGK systems, the prevalence of long periods is largely driven by observational selection effects rather than the intrinsic period distribution of the sdB binary population. This study provides one of the largest uniform catalogs of composite spectrum sdB binaries to date, offering new observational constraints on their physical properties and formation channels.

In low-mass core-collapse supernova (SN) progenitors, nuclear burning beyond oxygen can become explosive under degenerate conditions, triggering eruptive mass loss before the final explosion. We investigate such pre-SN eruptions using SNEC hydrodynamic simulations and realistic stellar models, parameterizing the nuclear energy deposition as a fraction of the binding energy of the combined He layer and H-rich envelope. For the lowest-mass model (9 M _⊙ ), the ejecta mass ( M _ej ) scales with the energy gained by the H-rich envelope via a power law (index ∼ 3.5). Across 9–10 M _⊙ , this relation shows limited scatter within a factor of ∼2.6, enabling an estimation of the gained energy from M _ej . The shock passage also flattens the bound envelope, which can affect the SN light-curve morphology and provide another diagnostic for the eruption. Then, we compute the associated precursor light curves for the 9 M _⊙ model with the multigroup radiative-transfer code STELLA . These signals are typically faint, with bolometric luminosities of ∼10 ^39 erg lasting hundreds of days. Their cool blackbody spectra make them brighter in the infrared yet several magnitudes fainter than observed pre-SN precursors at the threshold for full envelope ejection. To aid future studies, we make our posteruption stellar profiles and precursor light curves publicly available.

White dwarf-main sequence (WDMS) binaries provide important laboratories for studying binary evolution and the formation of low-mass white dwarfs. In this work, we identify 654 reliable WDMS candidates with FGK-type companions from an initial set of 772 ultraviolet-excess sources, selected using stellar atmospheric parameters from LAMOST spectroscopy and subsequently refined with Gaia DR3 astrometry and photometry together with ultraviolet data from GALEX. Candidates were selected based on ultraviolet excess relative to the Gaia main-sequence (MS) locus and refined using isochrone constraints to exclude systems inconsistent with MS companions. Binary spectral energy distribution fitting yields effective temperatures and radii for both components, as well as distance and extinction estimates. The MS companions are dominated by G-type stars (similar to 52%), with comparable fractions of F- and K-type companions, and no A-type primaries. Using white-dwarf (WD) evolutionary cooling models, we find that the WD components are predominantly low-mass (M-WD similar to 0.2-0.4 M-circle dot), including a substantial population of extremely low-mass (<0.3 M-circle dot) WDs likely produced through binary interaction. The WDs are generally hot (similar to 1.5 & times; 10(4) K), consistent with the ultraviolet selection bias favoring luminous, large-radius WDs. Multiepoch LAMOST radial velocities show larger amplitudes than those of a comparison sample of MS stars, supporting the close-binary nature of these systems. Although subject to strong selection effects, the catalog offers a clean and well-characterized sample of FGK+WD binaries.

Hot subdwarf (sdB) stars in binary systems with main-sequence (MS) companions provide valuable insights into mass transfer and envelope ejection processes in binary evolution. Their mass ratios, orbital periods, and stellar properties encode key information about their evolutionary histories. In this work, we analyze a sample of 123 composite-spectrum sdB+MS binaries identified from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope Low-Resolution Survey (LAMOST-LRS) Data Release (DR) 8. We adopt atmospheric parameters from spectral decomposition and estimate stellar masses and radii using theoretical evolutionary tracks. Radial velocities for both the hot subdwarfs and cool companions are measured independently through cross-correlation with synthetic templates. Orbital periods are statistically estimated using single-epoch RV separations and a Monte Carlo method that accounts for random inclination and orbital phase. We find that sdB masses are narrowly distributed around 0.5 Msun, consistent with expectations for core helium-burning stars, while MS companion masses span 0.6-1.9 Msun, with most falling between 1.0 and 1.4 Msun. The inferred orbital-period distribution shows a clear concentration toward long periods, broadly consistent with expectations for binaries formed through stable Roche-lobe overflow. Given that our sample consists of composite-spectrum sdB binaries, mainly sdB+FGK systems, the prevalence of long periods is largely driven by observational selection effects rather than the intrinsic period distribution of the sdB binary population. This study provides one of the largest uniform catalogs of composite spectrum sdB binaries to date, offering new observational constraints on their physical properties and formation channels.

Extremely low-mass (ELM) white dwarfs (WDs) are helium WDs with masses below similar to 0.3 M circle dot, mainly formed through binary interaction. ELM WD binaries typically are formed from two channels, namely the stable Roche lobe overflow (RLOF) channel and the common envelope ejection channel. For ELM WD binaries produced from the RLOF channel, the ELM WD mass has a strong correlation with the orbital period, i.e., the so-called WD mass-orbital period relation. However, the observations in the ELM Survey show that the orbital periods of ELM WD binaries from the RLOF channel are typically shorter than the theoretically predicted values. Extra angular momentum loss (AML) may be needed to explain such a phenomenon. In this work, we assumed that part of the transferred mass from the donor is lost at the outer Lagrangian point, and simulated the formation of ELM WD binaries. Enhanced AML enables more mass to be lost during thermal-timescale mass transfer, thereby affecting nuclear burning in the transfer phase and producing ELM WDs with distinct internal structures. These structural differences alter the (pre-)He WD mass-radius relation at the end of mass transfer, which in turn shifts the WD mass-orbital period relation downward. These adjustments enable our model to successfully reproduce the majority of observed systems from the relevant survey projects.

Accurate photometric zero-points are essential for translating observed magnitudes into physical fluxes, from comparing with models to ensuring consistency across surveys. We determine the zero-points needed to place the Sloan Digital Sky Survey (SDSS) $ugriz$ system on its nominal AB magnitude system definition, by exploiting the sensitivity of the Infrared Flux Method (IRFM) to broad-band flux calibration. Using benchmark effective temperatures for over 6000 FGK-type stars, we invert the method to identify the zero-point corrections required for SDSS photometry to reproduce the adopted temperature scale. The r band is found to be very well standardized, while the i and z bands show offsets of a few hundredths of a magnitude, consistent with previous studies. We also find a small offset in the g band. The largest discrepancy occurs in the u band, where the derived offset depends strongly on the adopted filter transmission curves, in particular whether one uses the original definition commonly adopted in the literature or the updated measurements that account for the presence of a red leak. This effect introduces a colour-dependent zero-point offset that becomes apparent when using a sample of late-type stars. Independent comparisons with CALSPEC spectrophotometric standards and Gaia XP spectra broadly support the offsets derived from the IRFM analysis. Our results provide a revised set of SDSS zero-points anchored to the IRFM temperature scale and demonstrate that large stellar samples can be used to constrain photometric calibration. The methodology presented here offers a complementary approach to traditional spectrophotometric calibration and may prove useful for future large-scale surveys.

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 Ks 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 − Ks) ≈ 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.

Helium white dwarfs (He WDs) are end products of low-mass red giant donors in close binary systems via stable mass transfer or common envelope evolution. At the end of stable mass transfer, there is a well-known relation between the He WD mass and orbital period. Although this relation has been widely investigated, the influence of different types of opacity at low temperatures is ignored. In this work, we modeled the evolution of white dwarf (WD) binaries with the stellar evolution code MESA and two types of opacity at low temperatures from Ferguson et al. and Freedman et al. We investigated the relation between the WD mass and orbital period and compared these results with observations. We find that the relation derived from the opacity of Freedman et al. is below that from the opacity of Ferguson et al., and the relation derived from the opacity of Freedman et al. can better explain the observations. In addition, we provided fitting formulae for the relations derived from the opacity of Freedman et al. at different metallicities.

Be stars are rapid rotators generally produced by binary interactions. The single Be stars in the observations pose challenges to the Be star formation theory. In this paper, we propose a new pathway for the formation of single Be stars, in which the Be star is taken as the ejected companion star from a Type Ia supernova (SN Ia) explosion. Recent numerical simulations suggest that explosive oxygen burning, initiated via the convective Urca process in certain helium (He) stars near the Chandrasekhar mass limit, can set off an SN Ia. Based on this proposition, we further demonstrate that about 0.4% of He star + main-sequence (MS) star binaries may evolve into single Be stars, where the MS star is spun up due to the mass accretion from the He star, and then the He star explodes as an SN Ia. We employ detailed binary evolutionary simulations and find the parameter space that would produce single Be stars via the SN Ia channel. Around 22% of Be stars from the SN Ia progenitor channel exhibit peculiar tangential velocities exceeding 24 km s(-1), classifying them as runaway stars. This suggests that the SN Ia channel plays a meaningful role in forming single Be stars, particularly within the runaway star population.