Discovery, TESS Characterization, and Modeling of Pulsations in the Extremely Low-mass White Dwarf GD 278

We report the discovery of pulsations in the extremely low-mass (ELM), likely helium-core white dwarf GD 278 via ground- and space-based photometry. GD 278 was observed by the Transiting Exoplanet Survey Satellite (TESS) in Sector 18 at a 2 minute cadence for roughly 24 days. The TESS data reveal at least 19 significant periodicities between 2447 and 6729 s, one of which is the longest pulsation period ever detected in a white dwarf. Previous spectroscopy found that this white dwarf is in a 4.61 hr orbit with an unseen >0.4 M ⊙ companion and has T eff = 9230 ± 100 K and log g = 6.627 ± 0.056 , which corresponds to a mass of 0.191 ± 0.013 M ⊙. Patterns in the TESS pulsation frequencies from rotational splittings appear to reveal a stellar rotation period of roughly 10 hr, making GD 278 the first ELM white dwarf with a measured rotation rate. The patterns inform our mode identification for asteroseismic fits, which, unfortunately, do not reveal a global best-fit solution. Asteroseismology reveals two main solutions roughly consistent with the spectroscopic parameters of this ELM white dwarf, but with vastly different hydrogen-layer masses; future seismic fits could be further improved by using the stellar parallax. GD 278 is now the tenth known pulsating ELM white dwarf; it is only the fifth known to be in a short-period binary, but is the first with extended, space-based photometry.

Estimation on the Underwater Explosion Equivalent Based on the Threshold Monitoring Technique

Underwater nuclear explosions can be monitored in near real-time by the hydroacoustic network of the International Monitoring System (IMS) established by the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which could also be used to monitor underground and atmospheric nuclear explosions. The equivalent is an important parameter for the nuclear explosions’ monitoring. The traditional equivalent estimation method is to calculate the bubble pulsation period, which is difficult to obtain satisfactory results under the current conditions. In this paper, based on the passive sonar equation and the conversion process of acoustic energy parameters in the hydroacoustic station, the threshold monitoring technique used for underwater explosion equivalent estimation was studied, which was not limited to the measurement conditions and calculation results of the bubble pulsation period. Through the analysis of practical monitoring data, estimation on the underwater explosion equivalent based on the threshold monitoring technique was verified to be able to reach the accuracy upper boundary of current methods and expand the measurement range to further ocean space, along with the real-time monitoring capability of IMS hydroacoustic stations which could be estimated by this method.

Improved hydrodynamic pulsation models for the pulsating extreme helium star V652 Herculis

New non-linear hydrodynamic models have been constructed to simulate the radial pulsations observed in the extreme helium star V652 Her. These use a finer zoning to allow higher radial resolution than in previous simulations. Models incorporate updated OPAL and OP opacity tables and adopt a composition based on the best atmospheric analyses to date. Key pulsation properties including period, velocity amplitude and shock acceleration are examined as a function of the mean stellar parameters (mass, luminosity, and effective temperature). The new models confirm that, for large amplitude pulsations, a strong shock develops at minimum radius, and is associated with a large phase delay between maximum brightness and minimum radius. Using the observed pulsation period to constrain parameter space in one dimension, other pulsation properties are used to constrain the model space further, and to critically discuss observational measurements. Similar models may be useful for the interpretation of other blue large amplitude pulsators, which may also exhibit pulsation-driven shocks.

The rapidly oscillating Ap star γ Equ: linear polarization as an enhanced pulsation diagnostic?

ABSTRACT We present the first short time-scale observations of the rapidly oscillating Ap (roAp) star γ Equ in linear polarized light obtained with the Potsdam Echelle Polarimetric and Spectroscopic Instrument installed at the Large Binocular Telescope. These observations are used to search for pulsation variability in Stokes Q and U line profiles belonging to different elements. The atmospheres of roAp stars are significantly stratified with spectral lines of different elements probing different atmospheric depths. roAp stars with strong magnetic fields, such as γ Equ with a magnetic field modulus of 4 kG and a pulsation period of 12.21 min, are of special interest because the effect of the magnetic field on the structure of their atmospheres can be studied with greatest detail and accuracy. Our results show that we may detect changes in the transversal field component in Fe i and rare earth element lines possessing large second-order Landé factors. Such variability can be due to the impact of pulsation on the transverse magnetic field, causing changes in the obliquity angles of the magnetic force lines. Further studies of roAp stars in linear polarized light and subsequent detailed modelling are necessary to improve our understanding of the involved physics.

The ASAS-SN Catalog of Variable Stars IX: The Spectroscopic Properties of Galactic Variable Stars

The All-Sky Automated Survey for Supernovae (ASAS-SN) provides long baseline (∼4 yrs) V −band light curves for sources brighter than V≲ 17 mag across the whole sky. We produced V-band light curves for a total of ∼61.5 million sources and systematically searched these sources for variability. We identified ∼426, 000 variables, including ∼219, 000 new discoveries. Most (${\sim }74\%$) of our discoveries are in the Southern hemisphere. Here we use spectroscopic information from LAMOST, GALAH, RAVE, and APOGEE to study the physical and chemical properties of these variables. We find that metal-poor eclipsing binaries have orbital periods that are shorter than metal-rich systems at fixed temperature. We identified rotational variables on the main-sequence, red giant branch and the red clump. A substantial fraction (${\gtrsim }80\%$) of the rotating giants have large vrot or large NUV excesses also indicative of fast rotation. The rotational variables have unusual abundances suggestive of analysis problems. Semi-regular variables tend to be lower metallicity ($\rm [Fe/H]{\sim }-0.5$) than most giant stars. We find that the APOGEE DR16 temperatures of oxygen-rich semi-regular variables are strongly correlated with the WRP − WJK color index for $\rm T_{eff}\lesssim 3800$ K. Using abundance measurements from APOGEE DR16, we find evidence for Mg and N enrichment in the semi-regular variables. We find that the Aluminum abundances of the semi-regular variables are strongly correlated with the pulsation period, where the variables with $\rm P\gtrsim 60$ days are significantly depleted in Al.

Spatiotemporal Intermittency in Pulsatile Pipe Flow

Despite its importance in cardiovascular diseases and engineering applications, turbulence in pulsatile pipe flow remains little comprehended. Important advances have been made in the recent years in understanding the transition to turbulence in such flows, but the question remains of how turbulence behaves once triggered. In this paper, we explore the spatiotemporal intermittency of turbulence in pulsatile pipe flows at fixed Reynolds and Womersley numbers (Re=2400, Wo=8) and different pulsation amplitudes. Direct numerical simulations (DNS) were performed according to two strategies. First, we performed DNS starting from a statistically steady pipe flow. Second, we performed DNS starting from the laminar Sexl–Womersley flow and disturbed with the optimal helical perturbation according to a non-modal stability analysis. Our results show that the optimal perturbation is unable to sustain turbulence after the first pulsation period. Spatiotemporally intermittent turbulence only survives for multiple periods if puffs are triggered. We find that puffs in pulsatile pipe flow do not only take advantage of the self-sustaining lift-up mechanism, but also of the intermittent stability of the mean velocity profile.

Investigations of space magnetism at the Crimean Astrophysical Observatory. II. Direct spectropolarimetric measurements of stellar magnetic fields

A research on stellar magnetism in Crimea was initiated by pioneer works of A.B. Severny, V.E. Stepanov, and D.N. Rachkovsky. Today, the study of stellar magnetic fields is a key field of research at the Crimean Astrophysical Observatory (CrAO). The 2.6 m Shajn telescope equipped with the echelle spectrograph ESPL, CCD, and Stokesmeter (a circular polarization analyzer) allows us to study the magnetic field of bright stars up to 5m–6m. The Single Line (SL) technique is developed for measuring magnetic fields at CrAO. This technique is based on the calculation of the Zeeman effect in individual spectral lines. A key advantage of the SL technique is its ability to detect local magnetic fields on the surface of stars. Many results in the field of direct measurements of stellar magnetic fields were obtained at CrAO for the first time. In particular, the magnetic field on supergiants (ǫ Gem), as well as on a number of subgiants, giants, and bright giants was first detected. This, and investigations of other authors, confirmed the hypothesis that a magnetic field is generated at all the stages of evolution of late-type stars, including the stage of star formation. The emergence of large magnetic flux tubes at the surface of stars of V-IV-III luminosity classes (61 Cyg A, β Gem, β Aql) was first registered. In subgiants, the magnetic field behavior with the activity cycle was first established for β Aql. Using the long-term Crimean spectroscopic and spectropolarimetric observations of α Lyr, the 22-year variability cycle of the star, supposedly associated with meridional flows, is confirmed. Magnetic field variability with the pulsation period was first detected for different types of pulsating variables: the classical Cepheid β Aql, the low-amplitude β Cep-type variable γ Peg, and others. In this review we cover more than a half-century history of the formation of the Crimean scientific school for high-precision direct measurements of stellar magnetic fields.

BRITE photometry and STELLA spectroscopy of bright stars in Auriga: Rotation, pulsation, orbits, and eclipses

Context. Knowing rotational and pulsational periods across the Hertzsprung-Russell diagram is of top priority for understanding stellar activity as a function of time. Aims. We aim to determine periods for bright stars in the Auriga field that are otherwise not easily accessible for ground-based photometry. Methods. Continuous photometry with up to three BRITE satellites was obtained for 12 targets and subjected to a period search. Contemporaneous high-resolution optical spectroscopy with STELLA was used to obtain radial velocities through cross correlation with template spectra as well as to determine astrophysical parameters through a comparison with model spectra. Results. The Capella red light curve was found to be constant over 176 days with a root mean square of 1 mmag, but the blue light curve showed a period of 10.1 ± 0.6 d, which we interpret to be the rotation period of the G0 component. From STELLA we obtained an improved orbital solution based on 9600 spectra from the previous 12.9 yr. We derive masses precise to ≈0.3% but 1% smaller than previously published. The BRITE light curve of the F0 supergiant ε Aur suggests 152 d as its main pulsation period, while the STELLA radial velocities reveal a clear 68 d period. An ingress of an eclipse of the ζ Aur binary system was covered with BRITE and a precise timing for its eclipse onset derived. A possible 70 d period fits the proposed tidal-induced, nonradial pulsations of this ellipsoidal K4 supergiant. η Aur is identified as a slowly pulsating B (SPB) star with a main period of 1.29 d and is among the brightest SPB stars discovered so far. The rotation period of the magnetic Ap star θ Aur is detected from photometry and spectroscopy with a period of 3.6189 d and 3.6177 d, respectively, likely the same within the errors. The radial velocities of this star show a striking non-sinusoidal shape with a large amplitude of 7 km s−1. Photometric rotation periods are also confirmed for the magnetic Ap star IQ Aur of 2.463 d and for the solar-type star κ1 Cet of 9.065 d, and also for the B7 HgMn giant β Tau of 2.74 d. Revised orbital solutions are derived for the eclipsing SB2 binary β Aur, which replaces the initial orbit dating from 1948 for the 27-year eclipsing SB1 ε Aur, and for the RS CVn binary V711 Tau, for which a spot-corrected orbital solution was achieved. The two stars ν Aur and ι Aur are found to be long-term, low-amplitude RV and brightness variables, but provisional orbital elements based on a period of 20 yr and an eccentricity of 0.7 could only be extracted for ν Aur. The variations of ι Aur are due to oscillations with a period of ≈4 yr.

Instabilities and pulsations in models of the B-type supergiant κ Cassiopeiae (HD 2905)

ABSTRACT For the B-type supergiant κ Cassiopeiae (HD 2905), variabilities with periods between several hours and a few days have been observed both photometrically and spectroscopically. A recent study of this star by Simón-Díaz et al. has revealed variability with a dominant period of 2.7 d. To understand this variability, we present a linear non-adiabatic stability analysis with respect to radial perturbations for models of κ Cassiopeiae. Instabilities associated with the fundamental mode and the first overtone are identified for models with masses between 27 and 44 M⊙. For selected models, the instabilities are followed into the nonlinear regime by numerical simulations. As a result, finite amplitude pulsations with periods between 3 and 1.8 d are found. The model with a mass of 34.5 M⊙ exhibits a pulsation period of 2.7 d consistent with the observations. In the nonlinear regime, the instabilities may cause a substantial inflation of the envelope.