Xiaobin Zhang

High-quality, short-cadence photometry from TESS enables the detection of rapid oscillators with unprecedented sensitivity. In this work, we conduct a homogeneous search for high-frequency variability using 20 s cadence light curves from TESS Cycle 7 (Sectors 84–96). From ∼3.9 × 10 ⁴ light curves, we compute Lomb–Scargle periodograms and select candidates exhibiting at least one significant signal with a false-alarm probability ≤10 ⁻⁴ at frequencies f ≥ 50 day ⁻¹ . After excluding previously reported objects and performing pixel-level and light-curve vetting to mitigate contamination, we identify 73 rapid oscillators, including 24 pulsating white dwarfs, 31 hot subdwarfs, and 18 A/F stars. Using an iterative prewhitening procedure, we carry out a detailed frequency analysis for each target and derive the oscillation frequencies and amplitudes. We further investigate the physical origins of the detected frequency content and present statistical characterizations of the rapid-oscillator sample. We highlight one white dwarf and one subdwarf that exhibit clear frequency multiplets consistent with rotational splitting. This work enlarges the sample of rapid oscillators accessible with TESS data and provides a uniformly measured frequency–amplitude catalog, establishing a consistent basis for future asteroseismic and population studies.

Theoretical investigations predict a minimum mass ratio of about 0.085–0.095 for contact binaries. If the mass ratio is below the critical value, a tidal instability occurs, forcing the stars to merge into a single object. Observationally, however, more and more W UMa-type binaries have been found to have mass ratios lower than the theoretical limit. In this study, we try to solve this discrepancy by considering the stellar structure and evolution probed through asteroseismology. We searched for and detected gravity ( g )- and Rossby ( r )-mode pulsations in four contact binaries with mass ratios less than 0.1. A seismic study of the pulsations led us to identify the pulsation modes and determine the near-core and envelope rotation rates as well as the asymptotic period spacings of the pulsating, primary stars. By asteroseismic modeling, the stellar parameters were constrained, and the structure models were constructed. This enables us to determine the gyration radii of the primaries and investigate in detail the orbital stability of the contact systems. We indicate that both the mass and age have independent effects on orbital stability, with massive and older primaries being more stable. Moreover, differential core-to-surface rotation would significantly reduce the gyration radius and hence the minimum mass ratio of a contact system. Adopting the median value of the gyration radius derived for the four stars, we estimate a critical mass ratio of 0.042–0.044, depending on the filling factor.

The radius inflation is predominantly observed in short-period, low-mass stellar systems, while being rarely detected in long-period binaries. Using Transiting Exoplanet Survey Satellite photometric and Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic survey data, we conducted a detailed photometric and spectroscopic analysis of six long-period (P > 7 days) detached eclipsing binaries: TIC 17303250, TIC 196817076, TIC 219488423, TIC 252991035, TIC 344753509, and TIC 72696540. Our results indicate that the first five systems are composed of low-mass stars (M ≤ 0.8M _⊙), while TIC 72696540 is likely composed of subgiant stars with M ₁ = 1.267 ± 0.011M _⊙, R ₁ = 2.365 ± 0.043R _⊙, and M ₂ = 1.281 ± 0.011M _⊙, R ₂ = 2.517 ± 0.035R _⊙. Their large radii are solely due to their evolutionary stage. Apart from TIC 72696540, none of the components of these systems displays a clear Li I λ6708 line. Under the assumption of binary rotational synchronization, the rotational velocities of these low-mass stars are found to be less than 8 km s⁻¹, which is the upper limit related to the LAMOST-MRS resolution. Notably, however, significant radius inflation is evident in the systems TIC 17303250, TIC 344753509, and TIC 252991035. Their radii exceed the predictions of the 12 Gyr isochrones. A positive correlation between radius inflation and metallicity is evident in four of the five low-mass stars, with TIC 196817076 being the sole exception. These results suggest that metallicity may play a significant role in causing radius inflation.

This study presents a thorough analysis of 10 detached eclipsing binaries that contain main-sequence stars. We have utilized a multifaceted approach, incorporating data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), TESS photometry, and broadband photometric spectral energy distributions, to determine the atmospheric parameters, radii, and masses of the component stars with high precision. The integration of these diverse data sets allows for the cross verification of results, thereby enhancing the accuracy and trustworthiness of our findings. Our investigation reveals consistent atmospheric parameters and stellar radii across the three methodologies, with exceptions like TIC 369605666 and 54759992, where the effective temperatures determined by the spectra differ from those calculated using the light-curve orbital solution. We explored possible reasons for the discrepancies in parameter estimation among the different methods for specific sources. Utilizing precise stellar parameters, determined and validated by multiple means, we compared them with PARSEC models to assess these samples’ evolutionary states and infer their ages. Regarding the determination of binary ages, the components of most sources are in agreement. However, for two targets with eccentric orbits, where the component stars’ ages diverge, we explored the potential causes.

Context. Surface brightness-color relations (SBCRs) are essential for estimating distances and stellar properties. Previously, SBCRs were based on a limited number of red clump giant (RCG) stars, and red giant branch (RGB) stars with infrared interferometric measurements or eclipsing binaries with high-precision radii measurements, resulting in discrepancies in precision and accuracy. Recently, the large number of RGB stars with asteroseismic parameters and precise Gaia distance measurements has enabled the development of more accurate and robust SBCRs. Aims. The asteroseismic radius and Gaia distance (ARD) method has been proposed to establish the SBCRs for late-type stars. Methods. We selected Kepler RGB stars with high-precision asteroseismic radii (uncertainties < 1%) and crossmatched them with 2MASS, APASS, and Gaia to obtain Johnson-B, Johnson-V, G, J, H, and K_s-band photometric data. After applying selection criteria, we obtained 626 RGB stars to build the SBCR. Among these, 100 RGBs were used as independent validation for the distance, and the remaining samples were used to fit the SBCR. Results. First, using 526 targets with asteroseismic radii and Gaia distances, nine SBCRs were proposed based on 2MASS (J, H, K_s), APASS (Johnson-B, Johnson-V), and Gaia (G) photometry. The average rms scatter in these relations is 0.075 mag, which corresponds to an uncertainty of approximately 3.5% in distance. These relations were further validated using 100 independent samples with Gaia distances, showing no bias, with a dispersion of approximately 3%. Compared to interferometric measurements, a systematic underestimation of 2.3% was observed, and the discrepancy decreases as the angular diameter increases. Additionally, the distances of eclipsing binaries in the Large Magellanic Cloud and Small Magellanic Cloud obtained using our SBCRs are generally consistent with the ones measured in the literature, with a dispersion of 1% and a slight overestimation of 1% to 2.5%. Conclusions. The ARD method capitalizes on two key advantages for precise stellar distance determination: a statistically robust sample of homogeneous RGB stars with low observational costs, and independent distance verification through Gaia data. Such SBCRs can be further calibrated and expanded more efficiently and effectively.

EL CVn–type systems represent a rare evolutionary stage in binary star evolution, providing ideal laboratories for investigating stable mass-transfer processes and the formation of extremely low-mass white dwarfs (ELM WDs). The Transiting Exoplanet Survey Satellite (TESS) has delivered an extensive collection of high-precision time-domain photometric data, which is invaluable for studying EL CVn binaries. In this study, we identified 29 EL CVn systems from the TESS eclipsing binary catalogs (sectors 1–65), 11 of which are newly discovered. These systems consist of smaller, hotter prehelium WDs and A/F main-sequence stars. The orbital periods of these binaries range from 0.64 to 2.5 days. Utilizing TESS light curves, Gaia distances, and multiband photometric data (e.g., Galaxy Evolution Explorer, Two Micron All Sky Survey, Wide-field Infrared Survey Explorer, SkyMapper), we modeled the light curves and spectral energy distributions to derive system parameters, including effective temperatures, masses, and radii. These systems were then compared with the WD mass–period relation and the evolutionary tracks of ELM WDs. The comparison reveals that these binaries are consistent with the expected mass–period relation for WDs and align well with the evolutionary tracks on the T _eff − log g diagram for ELM WDs. This result suggests that these EL CVn systems likely formed through stable mass-transfer processes. We provide a catalog of complete parameters for 29 EL CVn systems identified from the TESS survey. This catalog will serve as an essential resource for studying binary mass transfer, WD formation, and pulsation phenomena in EL CVn–type systems.

Triple star systems are critical for understanding stellar dynamics and compact objects in astrophysics, yet confirmed hierarchical triples identified via spectroscopy remain limited. In this study, we identified 23 triple systems by cross matching the Gaia DR3 non-single-star catalog with Large Sky Area Multi-Object Fiber Spectroscopic Telescope DR10 spectroscopic data, 18 of them are new discoveries. For two well-observed triples, we performed radial velocity curve fitting and light curve analysis to determine their orbital parameters, with inner and outer periods of 1.26 and 656 days for one triple, and 3.42 and 422 days for the other. And we compared the results with other studies. We also analyzed the radial velocities of these 23 tripls, revealing a range of ΔV from approximately 40–210 km s⁻¹. Due to spectral resolution and detection limitations, velocity differences below 45 km s⁻¹ in binaries and below 90 km s⁻¹ in the inner binaries of triple systems are challenging to detect. Consequently, our detection range for inner orbital periods is restricted to 0.2–20 days, with the highest efficiency for periods under 10 days. These findings underscore the advantage of spectroscopic observations for identifying triple systems with short inner orbital periods.

KIC 6362386 is an eclipsing binary system that exhibits both γ Doradus (γ Dor)–type pulsations and starspots. In this study, we investigated this binary system using the Kepler photometry and the spectroscopic data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope. After employing the PHOEBE program for light-curve and radial-velocity-curve synthesis, analyses reveal that the binary is a circle (e ∼ 0.0006), has a small mass ratio (q ∼ 0.311), and is a detached system consisting of an F-type primary star and an M-type secondary star with masses and radii of M ₁ = 1.43 ± 0.13 M _⊙, R ₁ = 1.68 ± 0.08 R _⊙ and M ₂ = 0.44 ± 0.18 M _⊙, R ₂ = 0.46 ± 0.06 R _⊙, respectively. Utilizing the Padova isochrone, we estimate the age of the binary system to be 1.58 − 0.13 + 0.15 Gyr. By analyzing the out-of-eclipse residuals, we identify variations in the residuals attributed to both starspots and stellar pulsations. The autocorrelation function analysis indicates the decay time of starspots is approximately 37 days with the rotation period aligning with the orbital period. Considering the masses, radii, and positions of the two components on the Hertzsprung–Russell diagram, we deduce that the γ Dor–type g-mode pulsations came from the primary star where the main frequency is 0.1642c/d. Consequently, KIC 6362386 becomes a valuable target for the investigation of γ Dor–type pulsations and asteroseismology in a binary system.

Utilizing data from the Transiting Exoplanet Survey Satellite (TESS) and LAMOST, we present a photometric and spectroscopic investigation of two new K-type low-mass eclipsing binaries, TIC 56913729 and TIC 97729372. Our analysis yields masses and radii for TIC 56913729, M ₁ = 0.7822 ± 0.0054M _⊙, R ₁ = 0.7891 ± 0.0021R _⊙, and M ₂ = 0.7532 ± 0.0052M _⊙, R ₂ = 0.7648 ± 0.0021R _⊙. For TIC 97729372, the results are M ₁ = 0.6410 ± 0.0058M _⊙, R ₁ = 0.6537 ± 0.0069R _⊙ and M ₂ = 0.6480 ± 0.0058M _⊙, R ₂ = 0.6418 ± 0.0062R _⊙. In addition, by analyzing the out-of-eclipse starspot light variations, the lower limit of starspot coverage varies in different TESS sectors from 2% to 12%. We observed a clear radius inflation in the mass–radius diagram for both stars, when plotted against PARSEC and SPOT isochrones with 1 Gyr. Yet, this apparent discrepancy disappears when the comparison is made with either an older PARSEC isochrone (12 Gyr) or a SPOT isochrone (250 Myr) with high starspot coverage (F _spot ∼ 85%), which is significantly higher than the typical starspot coverage deduced from light curves. Due to the lack of strong age constraints, we cannot firmly exclude that the observed radius inflation may be the result of a post-main-sequence evolutionary effect, although the spectral and kinematic properties of these stars are hardly compatible with 10–12 Myr old (Pop II) stars. It is more likely that the radius inflation is produced by the strong magnetic activity in these rapidly rotating stars, even if it is impossible to infer the actual total spot coverage, due to the age uncertainties.

We present a comprehensive photometric and spectroscopic analysis of the short-period (∼5.32 h) and low-mass eclipsing binary TMTSJ0803 discovered by Tsinghua-Ma Huateng Telescope for Survey (TMTS). By fitting the light curves and radial velocity data with the Wilson–Devinney code, we find that the binary is composed of two late spotted active M dwarfs below the fully convective boundary. This is supported by the discovery of a significant Balmer emission lines in the LAMOST spectrum and prominent coronal X-ray emission. In comparison with the typical luminosity of rapidly rotating fully convective stars, the much brighter X-ray luminosity (L_X/L_bol = 0.0159 ± 0.0059) suggests the stellar magnetic activity of fully convective stars could be enhanced in such a close binary system. Given the metallicity of [M/H] = − 0.35 dex as inferred from the LAMOST spectrum, we measure the masses and radii of both stars to be M₁ = 0.169 ± 0.010 M_⊙, M₂ = 0.162 ± 0.016 M_⊙, R₁ = 0.170 ± 0.006 R_⊙, and R₂ = 0.156 ± 0.006 R_⊙, respectively. Based on the luminosity ratio from the light-curve modelling, the effective temperatures of two components are also estimated. In comparison with the stellar evolution models, the radii, and effective temperatures of two components are all below the isochrones. The radius deflation might be mainly biased by a small radial velocity (RV) data or (and) a simple correction on RVs, while the discrepancy in effective temperature might be due to the enhanced magnetic activity in this binary.