Search Results (19)

The high-precision and long-duration photometry provided by the $Kepler$ mission has greatly advanced frequency analyses of a large number of pulsating stars, a fundamental step in asteroseismology. For $\delta$ Scuti stars, analyses are typically confined to frequencies below the Nyquist frequency. However, signals above this limit can be reflected into the sub-Nyquist range, especially in long-cadence data, where they may overlap with genuine pulsation modes and lead to misinterpretation. To address this issue, a recently proposed method—the sliding Lomb–Scargle periodogram (sLSP)—can effectively distinguish real frequencies from aliased ones. In this study, we compiled a sample of 68 $\delta$ Scuti stars whose frequency analyses were based on the $Kepler$ photometry. Using the sLSP method, we systematically examined the 1406 reported frequencies in the literature. As a result, we identified six previously unrecognized reflected super-Nyquist frequencies in four stars: KIC 3440495, KIC 5709664, KIC 7368103, and KIC 9204718. We have once again demonstrated the ability of the sLSP method to detect and correct such artifacts. This technique improves the reliability of frequency selection, thereby enhancing the accuracy of asteroseismic interpretation and stellar modeling for pulsating stars. Full article

Be stars are characterized by the presence of a circumstellar Keplerian disk formed from material ejected from the rapidly rotating stellar surface. This article presents recent observational and theoretical progress on two central aspects of this phenomenon: the mechanisms driving mass loss, and the fate of the ejected material. Using simultaneous TESS photometry and ground-based spectroscopy, we examine the short-term variability associated with discrete mass ejection events, or “flickers”, and review strong evidence linking them to pulsational activity near the stellar surface. Complementary 3D hydrodynamic simulations reproduce key observational signatures and establish that disk formation requires compact and asymmetric ejection sites with sufficient angular momentum to overcome re-accretion. In systems with binary companions, new high-resolution simulations resolve the outer disk for the first time and identify five dynamically distinct regions, including a circumsecondary disk and a circumbinary spiral outflow. Together, these results provide a coherent framework that traces the full life cycle of disk material from pulsation-driven ejection near the stellar surface to its final destination, whether re-accreted by the companion or lost from the system entirely. Full article

Determining the physical parameters of pulsating variable stars such as RR Lyrae is essential for understanding their internal structure, pulsation mechanisms, and evolutionary state. In this study, we present a machine learning framework that uses feedforward artificial neural networks (ANNs) to infer stellar parameters—mass (M), luminosity (log($L/L_{\odot }$)), effective temperature (log($T_{\mathrm{eff}}$)), and metallicity (Z)—directly from Transiting Exoplanet Survey Satellite (TESS) light curves. The network is trained on a synthetic grid of RRab light curves generated from hydrodynamical pulsation models spanning a broad range of physical parameters. We validate the model using synthetic self-inversion tests and demonstrate that the ANN accurately recovers the input parameters with minimal bias. We then apply the trained model to RRab stars observed by the TESS. The observed light curves are phase-folded, corrected for extinction, and passed through the ANN to derive physical parameters. Based on these results, we construct an empirical period–luminosity–metallicity (PLZ) relation: log($L/L_{\odot }$) = (1.458 ± 0.028) log(P/days) + (–0.068 ± 0.007) [Fe/H] + (2.040 ± 0.007). This work shows that ANN-based light-curve inversion offers an alternative method for extracting stellar parameters from single-band photometry. The approach can be extended to other classes of pulsators such as Cepheids and Miras. Full article

In this work, we present a study on the long time-scale period variations of four single-mode high-amplitude delta Scuti stars (HADS) via the classical $O-C$ analysis. The target HADS are (i) XX Cygni, (ii) YZ Bootis, (iii) GP Andromedae, and (iv) ZZ Microscopii. The newly determined times of maximum light came from the Transiting Exoplanet Survey Satellite (TESS), American Association of Variable Star Observers (AAVSO), and Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV) projects. Together with the times of maximum light obtained in the historical literature, the $O-C$ analysis was performed on these HADS, in which we obtained the linear period variation rates $\dot{P}/P$ as $(9.2\pm 0.2)\times {10}^{-9}{\mathrm{yr}}^{-1}$, $(3.2\pm 0.2)\times {10}^{-9}{\mathrm{yr}}^{-1}$, $(4.22\pm 0.03)\times {10}^{-8}{\mathrm{yr}}^{-1}$, and $(-2.06\pm 0.02)\times {10}^{-8}{\mathrm{yr}}^{-1}$, respectively. Based on these results and some earlier research, we also discuss the evolutionary stages and the mechanisms of the period variation of these four HADS. Full article

Be stars are rapidly rotating B-type stars surrounded by circumstellar disks formed from self-ejected material. Understanding the mechanisms driving mass ejection and disk formation, known as the Be phenomenon, requires a detailed investigation of their variability and underlying physical processes. In this study, we analyze the photometric, spectroscopic, and seismic characteristics of three Be stars—HD 212044, 28 Cyg, and HD 174237—using high-cadence data from the TESS mission and spectral data from the BeSS database. Photometric variability was analyzed through iterative prewhitening and wavelet techniques, revealing distinct frequency groups associated with non-radial pulsations (NRPs). Spectral data provided equivalent width measurements of the H$\alpha$ line, which correlated with photometric changes, reflecting dynamic interactions between the stars and their disks. Seismic analysis identified core rotation rates and buoyancy travel times for HD 212044 and 28 Cyg, offering insights into internal stellar processes and angular momentum distribution. HD 212044 exhibits a strong negative correlation between photometric brightness and H$\alpha$ equivalent width, whereas this correlation is weaker in the case of 28 Cyg. The findings for these two stars highlight the interplay between NRPs, rapid rotation, and circumstellar disk dynamics. In contrast, the case of HD 174237 presents an example of how a binary system with mass transfer and a B-type component is revealed when observed simultaneously with space-based photometry and ground-based spectroscopy, demonstrating the importance of distinguishing classical Be stars from interacting binaries. Full article

Using the atmospheric pulsation code written by George Bowen, we have performed a parameter study examining the effects of modifying various parameters of models of oxygen-rich AGB atmospheres pulsating in the fundamental and first-overtone modes. For each pulsation mode, we have examined the effects of adjusting the dust condensation temperature, dust condensation temperature range, pulsation amplitude, dust opacity, and metallicity. Our model grids are generated with the constraint that their luminosities are chosen to span the range of observed mass loss rates at a chosen mass. The dust condensation temperature, pulsation amplitude, and dust opacity have strong effects on the ultimate location and shape of the final model grids in the mass luminosity plane. The mass loss rate evolution of the fundamental and first-overtone mode models show a significant difference in behavior. While the fundamental mode models exhibit the typically assumed power–law relation with mass and luminosity, the first-overtone mode models show significant non-power law behavior at observed mass loss rates. Effectively, models in the first-overtone mode require somewhat higher luminosities to reach the same mass loss rates seen in fundamental mode models of the same mass, consistent with observed AGB stars. Full article

The conditions in Mira variable atmospheres make them wonderful laboratories to study a variety of stellar physics such as molecule–grain formation, dust production, shock chemistry, stellar winds, mass loss, opacity-driven pulsation, and shocks. We were awarded an NSF grant to analyze over a decade of synoptic observations from the Palomar Testbed Interferometer (PTI) of 106 Miras to curate a Mira Reference Dataset. The Miras included in this dataset include M-types, S-types, and C-types, and span a wide range of pulsation periods. PTI measured K-band angular sizes that when combined with a distance allow us to directly determine fundamental stellar parameters such as effective temperature, radial size, and bolometric flux. Supplementing observations with interferometric measurements of the stars opens the Mira laboratory to a wealth of different experiments. We provide two case studies to serve as examples of the power of the Mira Reference Dataset. The first case study describes combining PTI measurements with Spitzer IRS spectra of M-type Miras, which allowed us to fully characterize ${\mathrm{CO}}_2$ gas in their atmospheres. The second case study examines how PTI narrow-band data can be used to study phase-dependent pulsation effects on the stellar atmosphere. We provide a list of all the Miras (with coordinates) included in the set for anyone who would like to add them to their observing programs. All the data we produce and collate for this Mira Reference Dataset will be hosted and curated on a website open to the public so that other researchers and citizen scientists can participate in expanding the utility and body of knowledge on this set of “wonderful” stars. Full article

Heartbeat star systems have been reported to exhibit two distinct different orbital dynamic evolution processes: apsidal precession (e.g., KIC 4544587) and orbital decay (e.g., KIC 3766353). While experiencing similar dynamic tidal interactions, these binary systems display different dynamical behaviors, which is a puzzling phenomenon. In this work, we deduced a theoretical relation between the timescale of stellar pulsation $P_{\mathrm{pul}}$ and orbital periods $P_{\mathrm{orb}}$ of heartbeat stars based on the resonance criteria representing the orbital local low-energy configuration. The theoretical relation shows that when the ratio of $P_{\mathrm{orb}}$ to $P_{\mathrm{pul}}$ is an integer, the specific orbital period is captured in the resonance state, resulting in resonance locking. The resonance criteria are verified by periodograms of the pulsations and orbits of the two systems KIC 4544587 and KIC 3766353 from observations. KIC 4544587 is an apsidal precession heartbeat star with eight observed resonant frequencies available from observations and has an almost integer ratio of $P_{\mathrm{pul}}/P_{\mathrm{orb}}=67.968$. On the contrary, KIC 3766353 is undergoing the process of orbital shrinkage with only three weak pulsation–orbital resonance frequencies available and shows a non-integer ratio of $P_{\mathrm{pul}}/P_{\mathrm{orb}}=83.163$. Given the results, the theoretical relation is a potential proxy to distinguish between apsidal precession and orbital decay binary systems. Furthermore, we predict that the orbital period of KIC 3766353 will be reduced to 2.492 days, at which time it will be transformed into apsidal precession. Full article

We revisited the short-period (∼1.5 days) binary system KIC 5444392, which shows quasi-period modulated light variations. Previous studies indicated that these variations might be caused by stellar pulsations. In our work, we used the PHOEBE program, which revealed that this binary is an almost circular (e $\approx 0.007$) detached system with two G-type stars. The masses and radii of the primary and secondary stars were obtained as $M_1=1.21\pm 0.06{\mathrm{M}}_{\odot }$, $R_1=1.69\pm 0.09{\mathrm{R}}_{\odot }$ and $M_2=1.27\pm 0.06{\mathrm{M}}_{\odot }$, $R_2=1.69\pm 0.09{\mathrm{R}}_{\odot }$, respectively. Based on these parameters, the isochrone fitting showed that this system consists of a subgiant and a main-sequence star, whose ages are $3.{89}_{-0.34}^{+0.37}$ Gyr. Neither the primary nor the secondary star is in the mass range of Cepheid and Gamma Dor. Fourier analysis showed that the fitting residuals varied stochastically in a frequency around the orbital frequency, which means that the quasi-periodic signals resulted from starspots rather than stellar pulsation. Similar stellar parameters of both components of KIC 5444392 and the frequency analysis lead us to believe that starspots are in both stars. The autocorrelation analysis on the residuals indicates that the decay timescale of the starspots is about 53 days, and the rotational periods of both stars are very close to the orbital period of the binary. This result adheres to the trend that the decay timescale increases following the rotational frequency. Thus, studying this binary could increase our understanding of the light variations in the binary system. Full article

We aim to combine asteroseismology, spectroscopy, and evolutionary models to establish a comprehensive picture of the evolution of Galactic blue supergiant stars (BSG). To start such an investigation, we selected three BSG candidates for our analysis: HD 42087 (PU Gem), HD 52089 ($ϵ$ CMa), and HD 58350 ($\eta$ CMa). These stars show pulsations and were suspected to be in an evolutionary stage either preceding or succeding the red supergiant (RSG) stage. For our analysis, we utilized the 2-min cadence TESS data to study the photometric variability, and we obtained new spectroscopic observations at the CASLEO observatory. We used non-LTE radiative transfer models calculated with CMFGEN to derive their stellar and wind parameters. For the fitting procedure, we included CMFGEN models in the iterative spectral analysis pipeline XTgrid to determine their CNO abundances. The spectral modeling was limited to changing only the effective temperature, surface gravity, CNO abundances, and mass-loss rates. Finally, we compared the derived metal abundances with prediction from Geneva stellar evolution models. The frequency spectra of all three stars show stochastic oscillations and indications of one nonradial strange mode, $f_r=$ 0.09321 $d^{-1}$ in HD 42087 and a rotational splitting centred in $f_2=$ 0.36366 $d^{-1}$ in HD 52089. We conclude that the rather short sectoral observing windows of TESS prevent establishing a reliable mode identification of low frequencies connected to mass-loss variabilities. The spectral analysis confirmed gradual changes in the mass-loss rates, and the derived CNO abundances comply with the values reported in the literature. We were able to achieve a quantitative match with stellar evolution models for the stellar masses and luminosities. However, the spectroscopic surface abundances turned out to be inconsistent with the theoretical predictions. The stars show N enrichment, typical for CNO cycle processed material, but the abundance ratios did not reflect the associated levels of C and O depletion. We found HD 42087 to be the most consistent with a pre-RSG evolutionary stage, HD 58350 is most likely in a post-RSG evolution and HD 52089 shows stellar parameters compatible with a star at the TAMS. Full article

The analysis of fifty empirical period-radius relations and forty-three empirical period-luminosity relations is performed for the Cepheids. It is found that most of these relations have significant systematic errors. A new metrological method is suggested to exclude these systematic errors using the new empirical metrological relations and the empirical temperature scale of the various samples of the Cepheids. In this regard, the reliable relations between the mass, radius, effective surface temperature, luminosity, absolute magnitude on the one hand, and the pulsation period on the other hand, as well as the reliable dependence of the radius on the mass are determined for the Cepheids of types δ Cephei and δ Scuti from the Galaxy. These reliable relations permit us to accurately determine the empirical value of the pulsation constant for the Cepheids of both types for the first time. It is found that the pulsation constant very weakly depends on the pulsation period of the Cepheid, contrary to the known theoretical calculation. Hence, the Cepheids pulsate almost as a unified whole and homogeneous spherical body in wide ranges of a star’s mass and evolutionary state with an extremely inhomogeneous distribution of stellar substance over its volume. Therefore, it is first suggested that the pulsation of the Cepheid is, first of all, the pulsation of the almost unified whole and homogenous shell of its gravitational mass. This pulsation is triggered by well-known effects; for example, the local optical opacity of the stellar substance and overshooting, using the usual pulsation of the stellar substance. Full article

The investigation of the magnetic activity of different types of variable stars holds significant implications for our understanding of the physical processes and evolution of stars. This study’s International Variable Star Index (VSX) variable star catalog was cross-matched with Transiting Exoplanet Survey Satellite (TESS) data, resulting in 26,276 labeled targets from 76,187 light curves. A total of 25,327 stellar flare events were detected, including 245 eclipsing binaries, 2324 rotating stars, 111 pulsating stars, and 629 eruptive stars. The results showed that flares from eclipsing binaries, rotating stars, eruptive stars, and pulsating stars have durations such that 90% are less than 2 h, and 91% of their amplitudes are less than 0.3. Flare events mainly occurred in the temperature range of 2000 K to 3000 K. The power-law indices of different types of variable stars were $1.72\pm 0.025$ (eclipsing binaries), $1.82\pm 0.062$ (rotating stars), $1.80\pm 0.0116$ (eruptive stars), and $1.73\pm 0.060$ (pulsating stars). Among them, the flare energy of pulsating stars is more concentrated in the high-energy range. In all samples, flare energies were distributed from $3.99\times {10}^{31}$ erg to $6.18\times {10}^{38}$ erg. The LAMOST DR9 low-resolution spectral survey has provided H$\alpha$ equivalent widths for 398 variable stars. By utilizing these H$\alpha$ equivalent widths, we have determined the stellar activity of the variable stars and confirmed a positive correlation between the flare energy and H$\alpha$ equivalent width. Full article

Coarse Grain Spectral Analysis (CGSA) can explain the possible multiscaling nature of the thousands of low-amplitude peaks observed in the power spectra of some pulsating stars. Space-based observations allowed for the scientific community to find this kind of structure thanks to their long-duration, high-photometric precision and duty cycle compared to observations from the ground. Although these time series are far from perfect (outliers, trends, gaps, etc.), we used our own data preprocessing method, known as the 2K+1 stage interpolation method, to improve the background noise up to a factor 14, avoiding spurious effects. We applied both techniques, the 2K+1 stage method and the CGSA analysis, to shed some light on a real problem regarding stellar seismology: finding the physical nature of the low-amplitude signature for multiperiodic stellar pulsators. Full article

Globular clusters are both primary fossils of galactic evolution and formation and are ideal laboratories for constraining the evolution of low-mass and metal-poor stars. RR Lyrae and type II Cepheid variables are low-mass, radially pulsating stars that trace old-age stellar populations. These stellar standard candles in globular clusters are crucial for measuring their precise distances and, in turn, absolute ages, and for the calibration of the extragalactic distance scale. Herein, the evolutionary stages of RR Lyrae and type II Cepheids are discussed, and their pulsation properties, including the light curves, color–magnitude and period–amplitude diagrams, and period–luminosity relations in globular clusters at optical and infrared wavelengths are presented. The RR Lyrae visual magnitude–metallicity relation and the multiband period–luminosity–metallicity relations in globular clusters covering a wide metallicity range are also discussed in detail for their application to the RR Lyrae-based distance scale. Full article

Binary stars are crucial laboratories for stellar physics, so have been photometric targets for space missions beginning with the very first orbiting telescope (OAO-2) launched in 1968. This review traces the binary stars observed and the scientific results obtained from the early days of ultraviolet missions (OAO-2, Voyager, ANS, IUE), through a period of diversification (Hipparcos, WIRE, MOST, BRITE), to the current era of large planetary transit surveys (CoRoT, Kepler, TESS). In this time observations have been obtained of detached, semi-detached and contact binaries containing dwarfs, sub-giants, giants, supergiants, white dwarfs, planets, neutron stars and accretion discs. Recent missions have found a huge variety of objects such as pulsating stars in eclipsing binaries, multi-eclipsers, heartbeat stars and binaries hosting transiting planets. Particular attention is paid to eclipsing binaries, because they are staggeringly useful, and to the NASA Transiting Exoplanet Survey Satellite (TESS) because its huge sky coverage enables a wide range of scientific investigations with unprecedented ease. These results are placed into context, future missions are discussed, and a list of important science goals is presented. Full article