Stability of Nonradial Pulsations in the Be Star [MA93]1506

Period analysis of the 1993–2020 MACHO and OGLE photometry for Be star [MA93]1506 reveals non-radial pulsations are present in all observing seasons, with a mean period of 1.09942 ± 0.00018 days.

CCD Time-Series Photometry of White Dwarf Stars

<p>This thesis describes a practical programme that focused on CCD photometry of pulsating white dwarf (WD) stars. The first part of this thesis describes the development of two high-speed CCD photometer instruments and their data reduction pipeline, while the remainder describes the observation and analysis of several pulsating WDs and other targets. The two photometers (Puoko-nui North and South) share a common hardware design that is optimized for acquiring efficient photometry with integration periods of milliseconds through to minutes. The design integrates a commercial CCD (Charge-Coupled Device) camera and GPS (Global Positioning System) receiver with custom timing electronics and control software. The reduction and visualization software developed for these instruments provide detailed real-time information to the observer, and a streamlined data reduction pipeline. EC04207-4748 is a pulsating helium atmosphere WD that shows significant non-sinusoidal intensity variations. We show that the pulsation spectrum of this WD can be described by four independent pulsation eigenmodes plus linear combinations that arise from non-linear energy transport through a sub-surface convection layer. Our results are consistent with similar analyses that have been made for similar stars, and add an additional data point to the growing catalogue of these convection measurements. We argue that the convection layer depth may form a useful substitute for the effective temperatures of these WDs. GWLibrae is the class prototype of the accreting WD pulsators. These stars exist in cataclysmic variable (CV) systems, and show a mix of CV and pulsating WD-related phenomena. Our observations of GW Librae four - six years after its 2007 outburst show signs of quasi-stable intensity modulations that we believe may be caused by non-radial pulsations, but these are not convincingly explained by existing WD or CV models. L19-2 is a hydrogen atmosphere WD pulsator that shows extremely stable pulsation behaviour. We combine new observations with archival observations dating back to the mid 1970's, and derive a preliminary estimate of the period rate of change Ṗ for two of the pulsation modes in this target. We show a clear result for the main 192 s pulsation mode Ṗf₂ ≾ 10⁻¹⁴ s s-¹, and discuss the improvements that we plan to make in order to convincingly improve this constraint by an additional order of magnitude. Observations of other rapidly variable targets include two extremely low mass (ELM) WDs, which exhibit variability due to their orbital motion (J0751) as well as non-radial pulsations (J1518); the 33 ms optical period of the Crab Pulsar; the helium atmosphere WD pulsators EC05221-4725 and EC20058-5234; the stable hydrogen atmosphere pulsator G117–B15A; and the eclipsing sdB binary system PG1336-018.</p>

CCD Time-Series Photometry of White Dwarf Stars

<p>This thesis describes a practical programme that focused on CCD photometry of pulsating white dwarf (WD) stars. The first part of this thesis describes the development of two high-speed CCD photometer instruments and their data reduction pipeline, while the remainder describes the observation and analysis of several pulsating WDs and other targets. The two photometers (Puoko-nui North and South) share a common hardware design that is optimized for acquiring efficient photometry with integration periods of milliseconds through to minutes. The design integrates a commercial CCD (Charge-Coupled Device) camera and GPS (Global Positioning System) receiver with custom timing electronics and control software. The reduction and visualization software developed for these instruments provide detailed real-time information to the observer, and a streamlined data reduction pipeline. EC04207-4748 is a pulsating helium atmosphere WD that shows significant non-sinusoidal intensity variations. We show that the pulsation spectrum of this WD can be described by four independent pulsation eigenmodes plus linear combinations that arise from non-linear energy transport through a sub-surface convection layer. Our results are consistent with similar analyses that have been made for similar stars, and add an additional data point to the growing catalogue of these convection measurements. We argue that the convection layer depth may form a useful substitute for the effective temperatures of these WDs. GWLibrae is the class prototype of the accreting WD pulsators. These stars exist in cataclysmic variable (CV) systems, and show a mix of CV and pulsating WD-related phenomena. Our observations of GW Librae four - six years after its 2007 outburst show signs of quasi-stable intensity modulations that we believe may be caused by non-radial pulsations, but these are not convincingly explained by existing WD or CV models. L19-2 is a hydrogen atmosphere WD pulsator that shows extremely stable pulsation behaviour. We combine new observations with archival observations dating back to the mid 1970's, and derive a preliminary estimate of the period rate of change Ṗ for two of the pulsation modes in this target. We show a clear result for the main 192 s pulsation mode Ṗf₂ ≾ 10⁻¹⁴ s s-¹, and discuss the improvements that we plan to make in order to convincingly improve this constraint by an additional order of magnitude. Observations of other rapidly variable targets include two extremely low mass (ELM) WDs, which exhibit variability due to their orbital motion (J0751) as well as non-radial pulsations (J1518); the 33 ms optical period of the Crab Pulsar; the helium atmosphere WD pulsators EC05221-4725 and EC20058-5234; the stable hydrogen atmosphere pulsator G117–B15A; and the eclipsing sdB binary system PG1336-018.</p>

3D MHD simulation of a pulsationally-driven MRI decretion disc

We explore the pulsationally driven orbital mass ejection mechanism for Be star disc formation using isothermal, 3D magnetohydrodynamic (MHD) and hydrodynamic simulations. Non-radial pulsations are added to a star rotating at 95 per cent of critical as an inner boundary condition that feeds gas into the domain. In MHD, the initial magnetic field within the star is weak. The hydrodynamics simulation has limited angular momentum transport, resulting in repeating cycles of mass accumulation into a rotationally-supported disc at small radii followed by fall-back on to the star. The MHD simulation, conversely, has efficient (Maxwell αM ∼ 0.04) angular momentum transport provided by both of turbulent and coherent magnetic fields; a slowly decreting midplane driven by the magnetorotational instability and a supersonic wind on the surface of the disc driven by global magnetic torques. The angle and time-averaged properties near the midplane agree reasonably well with a 1D viscous decretion disc model with a modified $\tilde{\alpha }=0.5$, in which the gas transitions from a subsonic thin disc to a supersonic spherical wind at the critical point. 1D models, however, cannot capture the multi-phase decretion/angular structure seen in our simulations. Our results demonstrate that, at least under certain conditions, non-radial pulsations on the surface of a rapidly rotating, weakly magnetized star can drive a Keplerian disc with the basic properties of the viscous decretion disc paradigm, albeit coupled to a laminar wind away from the midplane. Future modeling of Be star discs should consider the possible existence of such a surface wind.

Non-Radial Pulsations of Distorted Stellar Structures: Effect of Mass Variation

Eigen-frequencies (EF) of non–radial modes (NRM) of pulsations of differentially rotating (D R) and tidally distorted (T D) stellar models by considering the effect of mass variation (MV) on its equi-potentials surfaces inside a star. The method utilizes an averaging proposal of Kippenhahn and Thomas (K and T) with conjunction of the concept of Roche-equipotential. The study accolades and corrects earlier studies of non-radial (NR) pulsations of DR and TD stellar structures of different natures such as radial and non-radial oscillations, X-ray, gamma ray and other electromagnetic disturbances. The reflection of the work comes from the requirements of the inclusion of non-uniform densities that yield Lane-Emden equation to have reliable results up to second order disturbances.

Similar but different: the varied landscape of Onfp/Oef stars variability

ABSTRACT The Oef category gathers rapidly rotating and evolved O-stars displaying a centrally reversed He ii λ 4686 emission line. The origin of the variability of their photospheric and wind spectral lines is debated, with rotational modulation or pulsations as the main contenders. To shed new light on this question, we analysed high-quality and high-cadence Transiting Exoplanet Survey Satellite photometric time series for five Oef stars. We also collected a new time series of spectra for one target (λ Cep) which had been the subject of specific debates in the last years. These observations reveal the variety of Oef behaviours. While space-based photometric data reveal substantial red noise components in all targets, only ζ Pup seems to display a long-lived periodicity. In our sample, stars exhibit a dominant signal at low frequencies but it appears relatively short-lived. This is reminiscent of rotational modulations by transient photospheric spots, though this scenario is challenged by the case of HD 14 442, whose 1.230 d−1 signal significantly exceeds the critical rotational frequency. In parallel, no evidence of persistent p mode non-radial pulsations is found in either photometry or spectroscopy of the stars, only temporary excitation of g mode pulsations could offer an alternative explanation for the dominant signals. Finally, the revised luminosities of the stars using GAIA-DR2 show that they are not all supergiants as ζ Pup. The question then arises whether the Oef peculiarity denotes a homogeneous class of objects after all.

Rotating convective core excites non-radial pulsations to cause rotational modulations in early-type stars

ABSTRACT We discuss low-frequency g modes excited by resonant couplings with weakly unstable oscillatory convective modes in the rotating convective core in early-type main-sequence stars. Our non-adiabatic pulsation analyses including the effect of Coriolis force for $2\, \mathrm{ M}_\odot$ main-sequence models show that if the convective core rotates slightly faster than the surrounding radiative layers, g modes in the radiative envelope are excited by a resonance coupling. The frequency of the excited g mode in the inertial frame is close to |mΩc| with m and Ωc being the azimuthal order of the g mode and the rotation frequency of the convective core, respectively. These g-mode frequencies are consistent with those of photometric rotational modulations and harmonics observed in many early-type main-sequence stars. In other words, these g modes provide a non-magnetic explanation for the rotational light modulations detected in many early-type main-sequence stars.