Tao Wu

Blue large-amplitude pulsators (BLAPs) are rare, hot, short-period pulsating stars whose rapid oscillations provide a unique probe of stellar interiors and atmospheres. In this paper, we study two short-period BLAPs in the Galactic bulge, Optical Gravitational Lensing Experiment (OGLE)-BLAP-142 ( P ≈ 6.6 minutes) and OGLE-BLAP-148 ( P ≈ 7.5 minutes), by combining light curves from the OGLE, KMTNet, and DREAMS surveys, spanning 17 yr with cadences ranging from minutes to hours to days. We perform generalized Lomb–Scargle periodogram analyses of their light curves and identify, for each BLAP, three closely spaced pulsation modes separated by ∼1%. The dominant mode can switch within timescales of order a year, accompanied by a likely blueward color shift (∼0.1–0.2 mag). The yearly period excursions of the resolved mode families imply apparent ∣ P ̇ / P ∣ values of order 10 ⁻⁶ –10 ⁻⁴ yr ⁻¹ , large enough to mimic or exceed smooth evolutionary period changes. One mode family in OGLE-BLAP-148 shows a coherent decade-long drift at the ∼10 ⁻⁵ yr ⁻¹ level, but its failure to extrapolate to the earliest and latest epochs suggests a transient mode-family drift rather than secular evolution. Such close-spaced multiple pulsations and mode switching may be common among BLAPs with periods shorter than 10 minutes, and their physical origin remains unclear.

The TESS space mission has generated an extensive collection of light curves, providing a valuable opportunity to search for solar-like oscillations. Numerous studies have utilized TESS observations to search for solar-like oscillations and achieved excellent results. However, the number of oscillators found by the previous works may be incomplete and significantly below the predicted count. Therefore, we employed a novel method to search for solar-like oscillations. In this work, we utilized TESS 2 minute cadence data to identify solar-like oscillators in the southern ecliptic hemisphere, detecting the presence of oscillations by identifying an excess power hump in the spectrum and combining visual inspection. As a result, we identified 10,548 stars exhibiting solar-like oscillations including 2972 that had not been reported in previous studies. The ν _max and Δ ν of oscillators were determined using the fitting and autocorrelation, and the reliability of the Δ ν measurements was further evaluated using the Fourier spectrum of the autocorrelation function. Based on this assessment, 4775 oscillators were classified as having reliable Δ ν , including 1920 that have Δ ν measured for the first time.

Context. The synergy between close binary stars and asteroseismology enables constraints on mass-transfer episodes and their consequences for internal structure, rotation profiles, and oscillation modes. Aims. We investigate how mass accretion and donation in close binaries affects the internal structure and oscillation modes of main-sequence stars. Methods. Building on the established relation between the Brunt–Väisälä (buoyancy) glitch and the Fourier spectra of g-mode period spacings, we quantitatively explain the origins of the g-mode period-spacing differences between single-star and mass-accretion or donation models of intermediate-mass stars ( M = 2.0, 3.0, and 4.5 M _⊙ ). In particular, the hydrogen mass fraction profiles X of the donor model show two chemical gradient regions, which results in a double-peaked Brunt–Väisälä profile. The presence of additional buoyancy glitches gives rise to further periodic modulations in the g-mode period spacings. Results. Mass-accretion–induced changes in the chemical profile create sharp features in the buoyancy frequency, which modify both the amplitudes and frequencies of the g-mode period-spacing variations. This behaviour resembles that produced by multiple chemical transition zones in compact pulsators such as white dwarfs and sub-dwarf B stars. Similarly, for acoustic modes in the M = 1 M _⊙ solar-like models, we attribute the differences in frequency-separation ratios between single-star and mass-donor models to the variations in the internal sound-speed gradient (acoustic glitches). We discuss future prospects of using asteroseismology to discover the mass-transfer products and constrain the mass-transfer processes in binary star evolution.

Context. The synergy between close binary stars and asteroseismology enables constraints on mass-transfer episodes and their consequences for internal structure, rotation profiles, and oscillation modes. Aims. We investigate how mass accretion and donation in close binaries affects the internal structure and oscillation modes of main-sequence stars. Methods. Building on the established relation between the Brunt-Vaisala (buoyancy) glitch and the Fourier spectra of g-mode period spacings, we quantitatively explain the origins of the g-mode period-spacing differences between single-star and mass-accretion/donation models of intermediate-mass stars (M = 2.0, 3.0, and 4.5 Msun). In particular, the hydrogen mass fraction profiles X of the donor model show two chemical gradient regions, which results in a double-peaked Brunt-Vaisala profile. The presence of additional buoyancy glitches gives rise to further periodic modulations in the g-mode period spacings. Results. Mass-accretion induced changes in the chemical profile create sharp features in the buoyancy frequency, which modify both the amplitudes and frequencies of the g-mode period-spacing variations. This behavior resembles that produced by multiple chemical transition zones in compact pulsators such as white dwarfs and sub-dwarf B stars. Similarly, for acoustic modes in the M = 1 Msun solar-like models, we attribute the differences in frequency-separation ratios between single-star and mass-donor models to the variations in the internal sound-speed gradient (acoustic glitches). We discuss future prospects for using asteroseismology to discover the mass-transfer products and constrain the mass-transfer processes in binary star evolution.

Blue Large Amplitude Pulsators (BLAPs) are a type of variable star that has been identified relatively recently. They are characterized by their large amplitude, high gravity, and long periods (2–75 min). Some BLAPs exhibit a rich helium abundance on their surfaces, while some of the others show rich hydrogen atmospheres. Up to date, both the mode identification and formation pathways of BLAPs remain subjects of ongoing debate. In this work, we gave an non-adiabatic pulsation analyses for low-order radial and non-radial dipole modes on a theoretical model grid to explore the characteristics of BLAPs. The effects of element diffusion and radiative levitation are not taken into account our theoretical models. This is because these processes could lead to discrepancies in surface element abundance of the sdB or BLAP stars between theoretical models and observations. Based on the theoretical model grid, our analyses revealed that the radial fundamental modes align well with various observed properties of BLAPs, including the pulsation instability, the position of the excitable models on the HR diagram, surface element compositions, period ranges, and period changes. Our theoretical models also predicted a period–gravity ($\log g$) relation with a narrow distribution, while the observations exhibit a broader range. Furthermore, our findings indicate that the excitation of pulsation in BLAPs is primarily influenced by metal abundance. The thickness of the outer rich hydrogen envelope and the ratio of helium to hydrogen on the surface almost do not affect the pulsation instability.

Recent convincing evidence is found within asteroseismology that suggests the magnetic fields exist in three red giants. Research on small-scale magnetic fields in the Sun and HD 49385 has shown that they have a certain corrective effect on the systematic discrepancies between observed and theoretical frequencies. Here we apply a similar method applied for the Sun to a red giant, KIC 9145955, to explore the impact of small-scale magnetic fields in the photosphere on its frequencies. We find that the calculated frequencies of our best-fit model, which simulates the effect of the magnetic fields by artificially modifying the Eddington T−τ relation, perfectly match those of the observed l = 0, 1, and 2 modes, indicating the existence of small-scale magnetic fields with an upper strength limit of 65 G and concentrating at a height 13,100 km in the photosphere. Based on the best-fit model, we revise the stellar parameters of KIC 9145955 as M = 1.23 ± 0.04 M _⊙, R = 5.57 ± 0.06 R _⊙, L = 19.85 ± 0.5 L _⊙, Age = 3.83 ± 0.5 Gyr, M _He = 0.2108 ± 0.0005 M _⊙, and R _He = 0.0306 ± 0.0001 R _⊙.

Given their tendency to mix in with real pulsations, the reflection of super-Nyquist frequencies (SNFs) pose a threat to asteroseismic properties. Although SNFs have been studied in several pulsating stars, a systematic survey remains to be explored. Here, we propose a method for identifying SNFs from Kepler and TESS photometry by characterizing their periodic frequency modulations using a sliding Fourier transform. After analyzing long-cadence photometry in the Kepler legacy, we identified 304 SNFs in 56 stars from 45 607 frequencies in ∼600 γ Doradus stars, corresponding to a fraction of approximately 0.67% and 9.2%, respectively. Most SNFs were detected in the frequency range of pressure mode over 120 μHz and the fraction of SNF detection increases as frequency up to ∼7%. We found only two potential SNFs mixed with gravity modes in two γ Doradus stars. These findings indicate that SNFs have a negligible impact on global seismic properties, such as those derived from period spacing in γ Doradus stars. However, we stress that SNFs must be carefully and systematically examined by this method in other pulsating stars, particularly δ Scuti and hot B subdwarf stars, to establish a solid foundation for the precise asteroseismolgy of various types of pulsators.

Recent asteroseismic studies have shown convincing evidence that magnetic fields may exist in the interior of some pulsating red giants. Inspired by this breakthrough, we explored the effect of small-scale magnetic fields on the p-mode oscillations in an evolved star, HD 49385. We incorporate a modified Eddington T–τ equation that phenomenologically mimics the effect of the magnetic fields in the atmosphere of HD 49385, and calculate the frequencies of p-modes with l = 0, 1, and 2. By comparing the calculated frequencies with the observed ones, we select two best-fit models with either GS98 or A09 chemical composition. Our best-fit models not only fit satisfactorily the observed frequencies but also well reproduce some spectroscopically observed stellar parameters such as effective temperature and log g. Based on the two best-fit models, we have estimated that the small-scale magnetic fields possess a strength of approximately 80 G and spread concentratively at approximately a height of 1850 km in the atmosphere. By selecting the best-fit models with a special requirement on the avoided-crossing mode, we have confirmed that the frequency of the avoided-crossing mode is tightly related to the helium core of the star, and determined the size of the helium core as 0.117 M _⊙ in mass and 0.078 R _⊙ in radius. Based on the improvements of the previous two sides, we can accurately determine the mass of HD 49385 to be 1.25 ± 0.02 M _⊙ with an age of 4.1 Gyr for GS98 composition and 4.5 Gyr for A09 composition.

Stellar flares are critical phenomena on stellar surfaces, which are closely tied to stellar magnetism. While extensively studied in main-sequence (MS) stars, their occurrence in evolved compact stars, specifically hot subdwarfs and white dwarfs (WDs), remains scarcely explored. Based on Cycles 1–5 of TESS photometry, we conducted a pioneering survey of flare events in ∼12,000 compact stars, corresponding to ∼38,000 light curves with a 2 minute cadence. Through dedicated techniques for detrending light curves, identifying preliminary flare candidates, and validating them via machine learning, we established a catalog of 1016 flares from 193 compact stars, including 182 from 58 sdB/sdO stars and 834 from 135 WDs, respectively. However, all flaring compact stars showed signs of contamination from nearby objects or companion stars, preventing sole attribution of the detected flares. For WDs, it is highly probable that the flares originated from their cool MS companions. In contrast, the higher luminosities of sdB/sdO stars diminish companion contributions, suggesting that detected flares originated from sdB/sdO stars themselves or through close magnetic interactions with companions. Focusing on a refined sample of 23 flares from 13 sdB/sdO stars, we found their flare frequency distributions were slightly divergent from those of cool MS stars; instead, they resemble those of hot B/A-type MS stars having radiative envelopes. This similarity implies that the flares on sdB/sdO stars, if these flares did originate from them, may share underlying mechanisms with hot MS stars, which warrants further investigation.