Pulsation Spectrum Research Articles

TIC 441725813: A new bright hybrid hot B subdwarf pulsator with differential core versus envelope rotation

Context. The Transiting Exoplanet Survey Satellite (TESS) performs high-precision photometry over almost the whole sky primarily in search of exoplanet transits. It also provides exquisite data to study stellar variability, in particular for pulsating hot B subdwarf (sdB) stars. Aims. We present a detailed analysis of a new hybrid (p- and g-mode) sdB pulsator, TIC 441725813 (TYC 4427-1021-1), discovered and monitored by TESS for 670 days. Methods. The TESS light curves available for this star were analysed using prewhitening techniques to extract mode frequencies accurately. The pulsation spectrum was then interpreted through methods that include asymptotic period spacing relationships and the identification of rotational multiplets. We also exploited a high signal-to-noise ratio (S/N), low-resolution spectrum of TIC 441725813 using grids of non-local thermodynamic equilibrium (NLTE) model atmospheres to derive its atmospheric parameters. Results. The light curve analysis reveals that frequencies are mostly found in the g-mode region, but several p-modes are also detected, indicating that TIC 441725813 is a hybrid sdB pulsator. We identify 25 frequencies that can be associated with ℓ = 1 g-modes, 15 frequencies corresponding to ℓ = 2 g-modes, and six frequencies characteristic of p-modes. Interestingly, several frequency multiplets interpreted as rotational splittings of deep-probing g-modes indicate a slow rotation period of at least 85.3 ± 3.6 day, while splittings of mostly envelope-probing p-modes suggest a significantly shorter rotation period of 17.9 ± 0.7 day, which implies the core (mainly the helium mantle with possibly the deeper partially mixed helium-burning core that it surrounds) rotates at least 4.7 times slower than the envelope. The radial velocity curves indicate that TIC 441725813 is in a close binary system with a low-luminosity companion, possibly a white dwarf. While elusive in the available TESS photometry, a low-frequency signal that would correspond to a period of ∼6.7 h is found, albeit at a low S/N. Furthermore, we estimate the inclination angle to be ∼60° by two independent means. Conclusions. TIC 441725813 is a particularly interesting sdB star whose envelope rotates faster than the core. We hypothesise that this might be caused by the effects of a tidal interaction with a companion, although in the present case, the presence of such a companion will have to be further investigated. This analysis paves the way towards a more detailed seismic probing of TIC 441725813 using optimisation techniques, which will be presented in a second paper.

Discovery of 226 [formula omitted] Scuti and [formula omitted] Doradus Stars near NGC 6871 with TESS

We present the discovery of 269 pulsating variable stars of δ Scuti, γ Doradus, and Maia types in the vicinity of the open cluster NGC 6871, using data from the Transiting Exoplanet Survey Satellite (TESS). Our small-scale regional survey centered on the δ Scuti star V1821 Cyg in the open cluster NGC 6871, covering a radius of one degree. The results include a remarkable total of 1512 newly classified variable stars, comprising the following categories: 105 δ Scuti stars, 121 γ Doradus stars, 50 Maia variables, 198 eclipsing binary systems, with 12 exhibiting pulsating or rotating components, 500+ rotating variable stars, and dozens of other types. Out of 1512 newly discovered variable stars, 108 are confirmed members of NGC 6871 with a membership probability exceeding 50%. Notably, dedicated Fourier analyses were applied to eight representative stars from the newly discovered variables. Among these, one star exhibits a rich and complex pulsation spectrum characterized by amplitude variations in dominant pulsations. To contextualize the new pulsators, we plotted them in the Hertzsprung–Russell diagrams alongside the largest known group of class member stars. Surprisingly, both δ Scuti and γ Doradus stars occupy nearly the same region in the diagrams, hinting at a potential unified pulsation mechanism. This study contributes valuable insights into the variability census of NGC 6871 and sheds light on the pulsation behavior of different stellar types. Further investigations into the physical properties and evolutionary status of these stars are warranted.

Constraints from Parallaxes and Average Period Spacings in the Asteroseismic Study of Eight Hydrogen-atmosphere Pulsating White Dwarfs

With space missions such as Kepler, TESS, and Gaia, we have a wealth of data on pulsating white dwarfs that can be leveraged in white dwarf asteroseismology. We address the question of the proportion of white dwarfs with thin hydrogen layers versus those with thick hydrogen layers. We also provide a mass–radius relation for carbon–oxygen-core, hydrogen-atmosphere white dwarfs. Such a relationship can be used in conjunction with magnitudes and distance measurements to constrain the mass and effective temperature of the white dwarfs. We select nine hydrogen-atmosphere pulsating white dwarfs for their rich pulsation spectra. From such pulsation spectra, we can derive the asymptotic period spacing, which in turn allows us to determine the thickness of the hydrogen and helium envelope of the models, without having to perform period-by-period fitting. We find that the majority of the white dwarfs have thicker hydrogen layers and we determine an upper limit of M r = 1–10−2.2 for the location of the base of the helium layer, in accordance with stellar evolution models. We confirm a finding from earlier studies that used a mass–radius relation and Gaia data to determine the effective temperatures of white dwarfs. The Gaia data systematically point to white dwarfs of lower effective temperature than indicated by the spectroscopy. Our results also support the hypothesis that white dwarfs with thicker hydrogen layers are more common than those with thinner layers.

A Model for Pair Production Limit Cycles in Pulsar Magnetospheres

It was recently proposed that the electric field oscillation as a result of self-consistent e ± pair production may be the source of coherent radio emission from pulsars. Direct particle-in-cell simulations of this process have shown that the screening of the parallel electric field by this pair cascade manifests as a limit cycle, as the parallel electric field is recurrently induced when pairs produced in the cascade escape from the gap region. In this work, we develop a simplified time-dependent kinetic model of e ± pair cascades in pulsar magnetospheres that can reproduce the limit-cycle behavior of pair production and electric field screening. This model includes the effects of a magnetospheric current, the escape of e ±, as well as the dynamic dependence of pair production rate on the plasma density and energy. Using this simple theoretical model, we show that the power spectrum of electric field oscillations averaged over many limit cycles is compatible with the observed pulsar radio spectrum.

Scattering model of scintillation arcs in pulsar secondary spectra

ABSTRACT The dynamic spectra of pulsars frequently exhibit diverse interference patterns, often associated with parabolic arcs in the Fourier-transformed (secondary) spectra. Our approach differs from previous ones in two ways: first, we extend beyond the traditional Fresnel–Kirchhoff method by using the Green’s function of the Helmholtz equation, i.e. we consider spherical waves originating from three-dimensional space, not from a two-dimensional screen. Secondly, the discrete structures observed in the secondary spectrum result from discrete scatterer configurations, namely plasma concentrations in the interstellar medium, and not from the selection of points by the stationary phase approximation. Through advanced numerical techniques, we model both the dynamic and secondary spectra, providing a comprehensive framework that describes all components of the latter spectra in terms of physical quantities. Additionally, we provide a thorough analytical explanation of the secondary spectrum.

Numerical Investigation of Rotor and Stator Matching Mode on the Complex Flow Field and Pressure Pulsation of a Vaned Centrifugal Pump

The match of rotor and stator blades significantly affects the flow field structure and flow-induced pressure pulsation characteristics inside the pump. In order to study the effects of the rotor and stator matching mode on the complex flow field and pressure pulsation of a centrifugal pump with a vaned diffuser, this paper designs three different vaned diffusers (DY5, DY8 and DY9) and uses the DDES (Delayed Detached Eddy Simulation) numerical method combined with structured grids to simulate the unsteady flow phenomena of the model pump under rated conditions. The results show that, under different rotor and stator matching modes, the pressure pulsation spectrum is dominated by the blade passing frequency and its harmonics. The matching mode of the rotor and stator significantly affects the time–frequency domain characteristics of the pressure pulsation inside the pump, and it is observed that the pressure pulsation energy of vaned diffusers with more blades is significantly smaller than that of fewer-blade vaned diffusers in comparison to the energy of the pressure pulsation at the blade passing frequency and within the 10–1500 Hz frequency band. Combined with the distribution characteristics of the complex flow field inside the pump, it can be found that increasing the number of vaned diffuser blades can reduce the energy of flow-induced pressure pulsation, improve the distribution of high-energy vortices in the interaction zone and stabilize the flow inside the centrifugal pump effectively.

Spatial Variations and Breaks in the Optical–Near-infrared Spectra of the Pulsar and Pulsar Wind Nebula in Supernova Remnant 0540–69.3

The supernova remnant (SNR) 0540–69.3, twin of the Crab Nebula, offers an excellent opportunity to study the continuum emission from a young pulsar and pulsar wind nebula (PWN). We present observations taken with the Very Large Telescope instruments MUSE and X-shooter in the wavelength range 3000–25000 Å, which allow us to study spatial variations of the optical spectra, along with the first near-infrared (NIR) spectrum of the source. We model the optical spectra with a power law (PL) F ν ∝ ν −α and find clear spatial variations (including a torus–jet structure) in the spectral index across the PWN. Generally, we find spectral hardening toward the outer parts, from α ∼ 1.1 to ∼0.1, which may indicate particle reacceleration by the PWN shock at the inner edge of the ejecta or alternatively time variability of the pulsar wind. The optical–NIR spectrum of the PWN is best described by a broken PL, confirming that several breaks are needed to model the full spectral energy distribution of the PWN, and suggesting the presence of more than one particle population. Finally, subtracting the PWN contribution from the pulsar spectrum we find that the spectrum is best described with a broken-PL model with a flat and a positive spectral index, in contrast to the Crab pulsar that has a negative spectral index and no break in the optical. This might imply that pulsar differences propagate to the PWN spectra.

Numerical simulation of a double helix vortex structure in a tangential chamber

The paper investigates the flow structure in a tangential vortex chamber based on numerical simulation of unsteady turbulent flow by the LES method. The swirl flow in a cylindrical chamber was organized using 12 nozzles, supplying the flow tangentially. Three regimes were considered with different design swirl number S = 5.78, 8.67, and 17.34, which was varied by closing one or two nozzle rows. The unsteady computation shows the formation of a rotating double helix vortex structure in the regimes with S equal to 8.67 and 17.34, which is also verified by experimental observations. Several recirculation zones were formed in the tangential chamber near its axis, as well as along the wall at the lower part, and on the axis near the top lid. A dominant frequency was observed in the pressure pulsation spectrum, which increased with increasing swirl parameter, and for the S = 8.67 regime, it was approximately twice the rotational frequency of the double helix structure. In the course of rotation of the double helix structure, the vorticity redistributed between its branches from an approximately symmetric pattern and up to approaching a single-vortex structure at certain points in time.

Synchro-curvature description of γ-ray light curves and spectra of pulsars: global properties

Abstract This work presents a methodological approach to generate realistic γ-ray light curves of pulsars, resembling reasonably well the observational ones observed by the Fermi-Large Area Telescope instrument, fitting at the same time their high-energy spectra. The theoretical light curves are obtained from a spectral and geometrical model of the synchro-curvature emission. Despite our model relies on a few effective physical parameters, the synthetic light curves present the same main features observed in the observational γ-ray light curve zoo, such as the different shapes, variety in the number of peaks, and a diversity of peak widths. The morphological features of the light curves allows us to statistically compare the observed properties. In particular, we find that the proportion on the number of peaks found in our synthetic light curves is in agreement with the observational one provided by the third Fermi-LAT pulsar catalog. We also found that the detection probability due to beaming is much higher for orthogonal rotators (approaching 100%) than for small inclination angles (less than 20 %). The small variation on the synthetic skymaps generated for different pulsars indicates that the geometry dominates over timing and spectral properties in shaping the gamma-ray light curves. This means that geometrical parameters like the inclination angle can be in principle constrained by gamma-ray data alone independently on the specific properties of a pulsar. At the same time, we find that γ-ray spectra seen by different observers can slightly differ, opening the door to constraining the viewing angle of a particular pulsar.

Radio spectra of pulsars fitted with the spectral distribution function of the emission from their current sheet

ABSTRACT In their catalogue of pulsars’ radio spectra, Swainston et al. distinguish between five different forms of these spectra: those that can be fitted with (i) a simple power law, (ii) a broken power law, (iii) a low-frequency turn-over, (iv) a high-frequency turn-over or (v) a double turn-over spectrum. Here, we choose two examples from each of these categories and fit them with the spectral distribution function of the caustics that are generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star. In contrast to the prevailing view that the curved features of pulsars’ radio spectra arise from the absorption of the observed radiation in high-density environments, our results imply that these features are intrinsic to the emission mechanism. We find that all observed features of pulsar spectra (including those that are normally fitted with simple or broken power laws) can be described by a single spectral distribution function and regarded as manifestations of a single emission mechanism. From the results of an earlier analysis of the emission from a pulsar’s current sheet and the values of the fit parameters for each spectrum, we also determine the physical characteristics of the central neutron star of each considered example and its magnetosphere.

The Energy-dependent Gamma-Ray Light Curves and Spectra of the Vela Pulsar in the Dissipative Magnetospheres

We study the pulsar energy-dependent γ-ray light curves and spectra from curvature radiation in the dissipative magnetospheres. The dissipative magnetospheres with the combined force-free (FFE) and Aristotelian are computed by a pseudo-spectral method with a high-resolution simulation in the rotating coordinate system, which produces a near-FFE field structure with the dissipative region only near the equatorial current sheet outside the light cylinder. We use the test-particle trajectory method to compute the energy-dependent γ-ray light curves and phase-average and phase-resolved spectra by including both the accelerating electric field and radiation reaction. The predicted energy-dependent γ-ray light curves and spectra are then compared with those of the Vela pulsar observed by Fermi. Our results can generally reproduce the observed trends of the energy-dependent γ-ray light curves and spectra for the Vela pulsar.

Influence of liquid layer height on pressure pulsations during evaporation/boiling under reduced pressure conditions

The paper presents experimental data on the influence of the height of the horizontal liquid layer on the amplitude of pressure pulsations obtained during evaporation/boiling under reduced pressure conditions. Also presented are the spectra of pressure pulsation power versus frequency obtained using fast Fourier transform for different heights of the horizontal liquid layer under reduced pressure conditions. It is obtained that the use of a thin film of working fluid during evaporation/boiling under reduced pressure conditions reduces the amplitude of pressure pulsations compared to thicker fluid layers. Analysis of the pressure pulsation spectra for explosive boiling regimes allows us to determine the characteristic frequency corresponding to the periodicity of explosive boiling. Also, this approach allows a quantitative comparison of the power of pressure pulsations obtained in different heat exchange systems and apparatuses.

Simulation of supersonic jet flow past a blunt body in a laboratory experiment using computer vision

The complex flow of a supersonic jet (Mach number 2.4–2.5) around the blunt aerodynamic body (consisting of a cylinder with a diameter of 16 mm and a truncated cone) was studied using the high-speed shadowgraph technique and computer vision-based visual data processing. The problem considered is directly related to the rocket and space systems stages safe separation problem in a sufficiently dense atmosphere. For systems with a sequential arrangement of stages, their separation often occurs under the rocket engine jet action. The occurrence of lateral forces in violation of axial symmetry can lead to overturning of the discarded stage and undesirable interaction with the outgoing stage. The goals of the paper were to study the pulsation characteristics of the incoming supersonic jet streamlining the body, as well as the pulsations of the bow shock wave in both axisymmetric and asymmetric flows. The recording frame rate in the experiments was up to 775 000 fps. Computer vision and various machine learning approaches were used to process a large number of shadowgraph images. Canny edge detection and Hough transform, as well as a convolutional neural network (CNN) based on YOLOv2 architecture, were used to automatically track the position of the bow shock relative to the body. Jet sound generation was studied - the amplitude dependence on the distance from the nozzle and the evolution in time. The position of the bow shock relative to the streamlined body and flow pulsations over time were analyzed. Bow shock pulsation spectra at zero angle of attack were compared to those at a small angle of attack (α = 2.5°–3°). The power spectral density was also estimated. The slope of the spectra for the zero angle of attack case was found to be horizontal and the spectra for the small angle of attack case had a slope close to −5/3. The strongest of the measured bow shock frequencies were found to be close to those of the jet mixing layer pulsations. The hardware signal spectrum was also measured and this signal can be considered as noise. The described approach makes it possible to obtain the physical characteristics of the flow at any point of interest. It was also shown that the automation of flow visualization experiments using computer vision techniques allows to increase the speed of data processing and obtaining new physical information, which may be important for engineers developing aircraft and spacecraft technologies.

Identification of pressure pulsation spectrum in a hydraulic system with a vibrating proportional valve

In this paper, attention was focused on identifying the components of the amplitude-frequency spectrum of pressure pulsations in a hydraulic system in which an external low-frequency mechanical vibration was subjected to a proportional hydraulic directional control valve. It was observed that an operating machine or device equipped with hydraulic valves is a source of mechanical vibrations with a wide frequency spectrum. The influence of these vibrations on the pressure pulsation spectrum was analyzed, identifying the components resulting from the forcing of the directional control valve spool. In addition, it was noted that the pressure pulsation spectrum includes components resulting from the pulsation of the displacement pump output feeding the hydraulic system. A description of these spectrum components was also made and, using the solution superposition method, the components of the pressure pulsation spectrum over a wide frequency range were identified.

Study of vortex-resolving method of throttling effect on pressure pulsation spectra in a paired supersonic air intake

С помощью вихреразрешающего RANS/ILES метода исследовано течение в спаренном модельном сверхзвуковом воздухозаборнике (ВЗ) смешанного сжатия с системой слива пограничного слоя. Проанализировано влияние типа системы слива пограничного слоя и дросселирования соседних ВЗ на особенности течения, уровень и спектры пульсаций давления в каждом из ВЗ. Рассмотрены варианты с одинаковым и различным дросселированием в каждом из ВЗ, общей и раздельной для соседних ВЗ системой слива. Дано краткое описание метода расчета и граничных условий. Представлены уровни OASPL в ВЗ в зависимости от степени дросселирования. Выявлено влияние дросселирования одного из ВЗ на течение и особенности 1/3-октавных спектров пульсаций давления в соседнем ВЗ.

High-resolution VLBI astrometry of pulsar scintillation screens with the θ - θ transform

ABSTRACT The recent development of $\theta - \theta$ techniques in pulsar scintillometry has opened the door for new high-resolution imaging techniques of the scattering medium. By solving the phase retrieval problem and recovering the wavefield from a pulsar dynamic spectrum, the Doppler shift, time delay, and phase offset of individual images can be determined. However, the results of phase retrieval from a single dish are only known up to a constant phase rotation, which introduces extra parameters when doing astrometry using very long baseline interferometry. We present an extension to previous $\theta - \theta$ methods using the interferometric visibilities between multiple stations to calibrate the wavefields. When applied to existing data for PSR B0834+06, we measure the effective screen distance and lens orientation with five times greater precision than was possible in previous work.

A Systematic Study of the Connection Between White Dwarf Period Spectra and Model Structure

To date, pulsational variability has been measured for nearly 70 pulsating helium-atmosphere white dwarfs (DBVs) and 500 pulsating hydrogen-atmosphere white dwarfs (DAVs), with only a fraction of these having been the subject of asteroseismic analysis. One way to approach white dwarf asteroseismology is forward modeling, where one assumes an interior structure and calculates the model’s periods. Many such models are calculated, in the search for the one that best matches the observed period spectrum. It is not computationally manageable, nor necessary, to vary every possible parameter for every object. We engage in a systematic study, based on a sample of 14 hydrogen-atmosphere white dwarfs, chosen to be representative of the types of pulsation spectra we encounter in white dwarf asteroseismology. These white dwarfs are modeled with carbon and oxygen cores. Our goal is to draw a connection between the period spectra and what parameters to which they are most sensitive. We find that the presence of longer-period modes generally muddies the mass and effective temperature determinations, unless continuous sequences of ℓ = 1 and ℓ = 2 modes are present. All period spectra are sensitive to the structure in the helium and hydrogen envelopes and most to at least some features of the oxygen abundance profile. Such sensitivity can be achieved either by the presence of specific low-radial-overtone modes, or by the presence of longer-period modes. Convective efficiency only matters when fitting periods greater than 800 s. The results of this study can be used to inform parameter selection and pave the way to pipeline asteroseismic fitting of white dwarfs.

Red horizontal branch stars: An asteroseismic perspective

Robust age estimates of red giant stars are now possible thanks to the precise inference of their mass based on asteroseismic constraints. However, there are cases where such age estimates can be highly precise yet very inaccurate. An example is giants that have undergone mass loss or mass transfer events that have significantly altered their mass. In this context, stars with ‘apparent’ ages significantly higher than the age of the Universe are candidates for stripped stars, or stars that have lost more mass than expected, most likely via interactions with a companion star or because of the poorly understood mass-loss mechanism along the red-giant branch. In this work we identify examples of such objects among red giants observed by Kepler, both at low ([Fe/H] ≲ −0.5) and solar metallicity. By modelling their structure and pulsation spectra, we find a consistent picture that confirms that they are indeed low-mass objects consisting of a He core of ≈0.5 M⊙ and an envelope of ≈0.1 − 0.2 M⊙. Moreover, we find that these stars are characterised by a rather extreme coupling (q ≳ 0.4) between the pressure-mode and gravity-mode cavities, one that is much higher than the typical value for red clump stars, thus providing a direct seismic signature of their peculiar structure. The complex pulsation spectra of these objects, if observed with sufficient frequency resolution, hold detailed information about the structural properties of likely products of mass stripping and can hence potentially shed light on their formation mechanism. On the other hand, our tests highlight the difficulties associated with reliably measuring the large frequency separation, especially in shorter datasets, which impacts the reliability of the inferred masses and ages of low-mass red clump stars with, for example, K2 or TESS data.

TESS Observations of the Pleiades Cluster: A Nursery for δ Scuti Stars

We studied 89 A- and F-type members of the Pleiades open cluster, including five escaped members. We measured projected rotational velocities () for 49 stars and confirmed that stellar rotation causes a broadening of the main sequence in the color–magnitude diagram. Using time-series photometry from NASA’s TESS Mission (plus one star observed by Kepler/K2), we detected δ Scuti pulsations in 36 stars. The fraction of Pleiades stars in the middle of the instability strip that pulsate is unusually high (over 80%), and their range of effective temperatures agrees well with theoretical models. On the other hand, the characteristics of the pulsation spectra are varied and do not correlate with stellar temperature, calling into question the existence of a useful relation for δ Scutis, at least for young main-sequence stars. By including δ Scuti stars observed in the Kepler field, we show that the instability strip is shifted to the red with increasing distance by interstellar reddening. Overall, this work demonstrates the power of combining observations with Gaia and TESS for studying pulsating stars in open clusters.

Influence of the Volute Tongue Shape on Cavitation Performance in Aviation Fuel Centrifugal Pump

During the operation of an aviation fuel centrifugal pump, the performance of the pump dropped sharply due to cavitation, which caused cavitation damage near the volute tongue, which seriously threatened the safe and stable operation of the aircraft. According to the characteristics that the cavitation in the volute tongue region occurs in large flow rate, the volute tongue shape of aviation fuel centrifugal pump was improved in this paper, and the influence of the volute tongue shape on the cavitation characteristics of aviation fuel centrifugal pump at 1.2 times of design flow rate was studied. The research shows that under 1.2 times of design flow rate, the critical cavitation number of the improved volute tongue shape is reduced by 7.4% compared with the original scheme. The improved volute tongue effectively suppresses the generation of cavitation in the volute tongue region. The vortex core distribution area is reduced compared to the original scheme. When the cavitation number is low, doubling the axial passing frequency on the pressure pulsation spectrum at the monitoring point of the volute tongue is obviously weakened.