G-mode Oscillations Research Articles

Modeling the leech heartbeat elemental oscillator II. Exploring the parameter space

In the previous paper, we described a model of the elemental heartbeat oscillator in the leech. Here, the parameters of our model are explored around the baseline canonical model. The maximal conductances of the currents and the reversal potential of the leak current are varied to reveal the effects of individual currents and the interaction between synaptic and intrinsic currents in the model. The model produces two distinct modes of oscillation as the parameters are varied, S-mode and G-mode. These two modes are defined, their origin is identified, and the parameter space is mapped into S-mode and G-mode oscillation and no oscillation. Finally, we will make predictions for how the period can be modulated in heart interneurons.

Sensitivity of solar g-modes to varying G cosmologies

The sensitivity of the solar g-mode oscillation spectrum to variability in the universal gravitational constant G is described. Solar models in varying G cosmologies were constructed by evolving a zero-age main-sequence stellar model to the Sun's current age, while allowing the value of G to change according to the power law G(t) proportional to t(exp -beta), where Beta approximately equals delta G/GH and H is the Hubble constant. All solar models were constrained to the observed luminosity and radius at the current age of the Sun by adjusting the helium abundance and the mixing-length parameter of the models in the usual way for standard stellar models. Low-l g-mode oscillation periods were calculated for each of the models and compared to the claimed observation of the solar g-mode oscillation spectrum by Hill & Gu (1990). If one accepts Hill & Gu's claims, then within the uncertainties of the physics of the solar model calculation, our models rule out all but (delta G/GH) less than approximately 0.05. In other words, we conclude that G could not have varied by more than 2% over the past 4.5 Gyr, the lifetime of the present-day Sun. This result lends independent support to the validity of the standard solar model.

Resonant oscillations and tidal heating in coalescing binary neutron stars

Tidal interaction in a coalescing neutron star binary can resonantly excite the g-mode oscillations of the neutron star when the frequency of the tidal driving force equals the intrinsic g-mode frequencies. We study the g-mode oscillations of cold neutron stars using recent microscopic nuclear equations of state, where we determine self-consistently the sound speed and Brunt–Väisälä frequency in the nuclear liquid core. The properties of the g-modes associated with the stable stratification of the core depend sensitively on the pressure–density relation, as well as on the symmetry energy of the dense nuclear matter. The frequencies of the first ten g-modes lie approximately in the range 10–100 Hz. Resonant excitations of these g-modes during the last few minutes of the binary coalescence result in energy transfer and angular momentum transfer from the binary orbit to the neutron star. The angular momentum transfer is possible because a dynamical tidal lag develops even in the absence of fluid viscosity. Since the coupling between the g-mode and the tidal potential is weak, however, the amount of energy transfer during a resonance and the induced orbital phase error are very small. Resonant excitations of the g-modes play an important role in the tidal heating of binary neutron stars. Without the resonances, viscous dissipation is effective only when the stars are close to contact. The resonant oscillations result in dissipation at much larger orbital separation. The actual amount of tidal heating depends on the viscosity of the neutron star. Using the microscopic viscosity, we find that the binary neutron stars are heated to a temperature of ~ 108 K before they come into contact.

Dynamo generation of magnetic field in the white dwarf GD 358

On the basis of Whole Earth Telescope observations of the g-mode oscillation spectrum of the white dwarf GD 358, Winget et al. find evidence for significant differential rotation and for a time-varying magnetic field concentrated in the surface layers of this star. Here we argue on theoretical grounds that this magnetic field is produced by an alpha omega dynamo operating in the lower part of a surface convection zone in GD 358. Our argument is based on numerical solutions of the nonlinear, local dynamo equations of Robinson & Durney, with specific parameters based on our detailed models of white-dwarf convective envelopes, and universal constants determined by a calibration with the the Sun's dynamo. The calculations suggest a dynamo cycle period of about 6 years for the fundamental mode, and periods as short as 1 year for the higher-order modes that are expected to dominate in view of the large dynamo number we estimate for GD 358. These dynamo periods are consistent with the changes in the magnetic field of GD 358 over the span of 1 month inferred by Winget et. al. from their observations. Our calculations also suggest a peak dynamo magnetic field strength at the base of the surface convection zone of about 1800 G, which is consistent with the field strength inferred from the observations.

Density Discontinuities and the g-Mode Oscillation Spectra of Neutron Stars

view Abstract Citations (20) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Density Discontinuities and the g-Mode Oscillation Spectra of Neutron Stars Strohmayer, T. E. Abstract Density discontinuities resulting from sudden composition variations, as well as smooth variations in entropy, both provide sources of buoyancy which together determine the global g-mode oscillation spectra of neutron stars. Previous calculations by Finn and McDermott investigated the g-modes induced by density discontinuities at zero temperature. That is, they neglected the contributions to buoyancy due to smooth variations in entropy at finite temperature. They also investigated only purely fluid models. We have extended these previous calculations by computing the quadrupole, spheroidal oscillation spectra, both with and without a single density discontinuity of two finite-temperature, three-component (core, crust, ocean) neutron star models. We have inserted in each model a single density discontinuity associated with the 56Fe to 62Ni transition predicted by the BPS equation of state (Baym, Pethick, & Sutherland). For one model this discontinuity occurs in the solid crust of the star, while for the other, hotter (younger) model it resides in the fluid ocean overlying the crust. When the discontinuity resides in the solid crust we find no significant differences between the low-order, spheroidal oscillation spectra computed with and without the discontinuity. However, when the discontinuity lies in the fluid ocean the surface g-mode spectrum is substantially altered. In particular, for a 5% density jump, the period of the fundamental surface g-mode of this model drops from about 50 ms to 37 ms. The periods of the higher overtones are also perturbed away from their values without a discontinuity. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173309 Bibcode: 1993ApJ...417..273S Keywords: STARS: NEUTRON; STARS: OSCILLATIONS full text sources ADS |

Standard solar model. II - g-modes

The paper presents the g-mode oscillation for a set of modern solar models. Each solar model is based on a single modification or improvement to the physics of a reference solar model. Improvements were made to the nuclear reaction rates, the equation of state, the opacities, and the treatment of the atmosphere. The error in the predicted g-mode periods associated with the uncertainties in the model physics is predicted and the specific sensitivities of the g-mode periods and their period spacings to the different model structures are described. In addition, these models are compared to a sample of published observations. A remarkably good agreement is found between the 'best' solar model and the observations of Hill and Gu (1990).

The pulsations of ZZ Ceti stars - IV. The instability strip

DA models become unstable to the observable g-mode oscillations when the hydrogen ionization zone is sufficient to excite modes with periods ≥100 s. As the effective temperature decreases, modes of longer period are excited up to a maximum period of about 1000 s. Further cooling does not lead to the excitation of the high-order modes of still longer period (to a detectable amplitude) and the brightness changes associated with the excited modes decrease below the detection threshold. The position of the theoretical instability strip in the T e -log g plane is thus determined

Line profile variations caused by low-frequency nonradial pulsations of rapidly rotating stars

Line profile variations caused by low-frequency nonradial g-mode oscillations of a rotating star are presented. For a nonradial oscillation mode whose oscillation frequency in the corotating frame is comparable to or smaller than the rotation frequency, the latitudinal dependence of the oscillation amplitude deviates significantly from that for a nonrotating star. Rotation usually causes oscillations confined to a narrow equatorial belt as well as causing toroidal velocity fields. The concentration of the amplitude of the oscillation toward the equator leads to a reduction in the strength of the bumps in line profiles if the maximum velocity of nonradial pulsation is fixed. For the same normalization, the line profile variations due to the sectoral prograde mode are most visible, while those caused by retrograde waves are almost invisible due to a cancellation effect. The toroidal component generated by stellar rotation appreciably affects the line profile variation of some modes. 24 refs.

Second-Order Effects due to Rotation in Pulsating Da White Dwarfs

The ZZ Ceti star L 19-2 is a stable pulsator whose light curve has now been deciphered with the help of over 300 hours of white light, high-speed photometry (O’Donoghue and Vamer 1982, 1987, hereafter ODV). The analysis indeed reveals the presence in the light curve of five coherent oscillations, with periods ranging from 113s to 350s. Among those, the 192s oscillation possesses three components, almost equally separated in frequency. Most importantly, the slight, but statistically significant, inequality in the frequency spacing of the triplet has been interpreted by these authors as second-order splitting of rotationally-perturbed g-mode oscillations. And indeed, the measured splitting appears consistent with the theoretical predictions of Chlebowski (1978), which are based on somewhat archaic white dwarf models. As pointed out by ODV, it is clearly of great interest to investigate 1) to what extent theoretical predictions based on more realistic, current-generation white dwarf models agree with ODVs identification, and 2) to what extent such second order effects can, eventually, be used to identify individual pulsation modes or constrain the structural parameters of variable white dwarf stars. Motivated by these questions, we have initiated a study of second-order effects due to rotation in ZZ Ceti stars, and we report here the first results of this program.

The 152-day periodicity of the solar flare occurrence rate

The occurrence rate of solar flares exhibits a periodicity of about 152 days. This periodicity was discovered from analyses of flares observed with the Solar Maximum Mission (SMM), and confirmed by analysis of flares observed by other observatories. This periodicity was also discovered from the flare rate of solar cycle 20 (1965–75). The cause of the 152-day periodicity still remains a mystery. But answers to the following questions will enhance our understanding of it. (1) Is the periodicity a local or a global phenomenon? (2) Is the periodicity due to the interaction of 'hotspots' rotating at different rates such that they align with one another once every 152-day period? (3) Is the periodicity due to the interaction of rotating features originating from g-mode oscillations? Here we report our analysis of 'major flares' observed with the Hard X-ray Burst Spectrometer (HXRBS) aboard SMM and we conclude that the 152-day periodicity is a global phenomenon, and that the answers to questions (2) and (3) are negative.

Short-time variations of solar neutrinos and solar activity cycle

It is shown from the statistical analysis of the sunspot data and solar neutrino data that both the data exhibits 5, 10, 15, 20, 25, and 30 months period and these periods may be g-mode oscillation of the core associated with the solar activity.

Solar activity and the coupling of g-mode oscillations

The solar g-mode oscillations depend strongly on the difference between actual and adiabatic temperature gradients in the solar interior. Solar activity anchored at the base of the convection zone disturbs this difference and couples modes with similar eigenfrequencies. This coupling is computed, and the possible observational effects at the photosphere are discussed. These include a scatter in the frequencies deduced for weakly coupled modes and the apparent temporal behavior of strongly coupled modes.

The 160 minutes oscillations

We describe basic observational data regarding the 160-min oscillations of the Sun as well as the implications for helioseismology. The most acceptable theoretical interpretation seems to be a resonant interaction of gravity g-mode oscillations of the solar model with a slight modification to the equilibrium structure (with low heavy element abundance).

Astrophysics: Neutrinos and the Sun

Astrophysics: Neutrinos and the Sun

Solar irradiance changes caused by g-modes and large scale convection

Solar irradiance measurements from the ACRIM experiment show a clear response to the rotation periods of g-mode oscillations (l = 1, 2, and 3) and their first harmonics. Peaks in the ACRIM spectrum at 16.6, 18.3, 20.7, 36.5, and ≃- 71 days all lie within about 1% of periods arising from g-mode rotation. This means that the g-modes are a fundamental cause of irradiance fluctuations. On time scales of months and less they modulate the irradiance by means of transient flows of global scale which they stimulate in the Sun's convective envelope. Dimensional arguments indicate that the flows carry up heat at an average rate ∼ 10-3L⊙ which is not in conflict with observed changes in the irradiance. Five additional tests for g-modes and large-scale convection are given. An instability is described which undermines diffusion models of sunspot energy storage.

Electromagnetic damping of neutron star oscillations

view Abstract Citations (22) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electromagnetic damping of neutron star oscillations McDermott, P. N. ; Savedoff, M. P. ; van Horn, H. M. ; Zweibel, E. G. ; Hansen, C. J. Abstract A simple model of magnetic field perturbations driven by neutron star oscillations is used to estimate the electromagnetic power radiated by g-modes and torsional oscillations. The calculation assumes that the neutron star has a frozen-in magnetic field which is perturbed by the oscillatory motions of the surface. The disturbances propagate into the vacuum as outgoing electromagnetic waves. The relative effectiveness of Joule heating of the neutron star crust by pulsation-induced electric currents is estimated. It is concluded that electromagnetic damping is the dominant energy dissipation mechanism for quadrupole g-mode oscillations of neutron stars. For dipole spheroidal modes, both electromagnetic radiation and Joule heating are important, and there is no gravitational radiation emitted by these modes. Publication: The Astrophysical Journal Pub Date: June 1984 DOI: 10.1086/162152 Bibcode: 1984ApJ...281..746M Keywords: Magnetic Stars; Neutron Stars; Pulsars; Stellar Oscillations; Astrophysics; Electromagnetic Radiation; Magnetic Flux; Oscillation Dampers; Astrophysics full text sources ADS |

Nonradial g-mode oscillations of warm neutron stars

view Abstract Citations (110) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Nonradial g-mode oscillations of warm neutron stars McDermott, P. N. ; van Horn, H. M. ; Scholl, J. F. Abstract The adiabatic l =1 and 2 p, f, and g-mode oscillations of a numer of finite temperature neutron star models from the evolutionary sequences constructed by Richardson (1980) are compared. The general relativistic theory of spheroidal nonradial stellar oscillations is briefly reviewed, and an approximation termed the 'relativistic Cowling approximation' is introduced in analogy with its Newtonian counterpart. The resulting pulsation equations are solved in the limit of high spatial frequencies to obtain the local dispersion relation and to provide estimates of the periods of the p and g-mode oscillations. The numerical solutions of the full eigenvalue problem, still in Cowling approximation, are described. Calculations of p and f-mode periods, although the Cowling approximation is less satisfactory for these than for the g-modes, are included. The damping due to gravitational radiation for all modes, and due to neutrino emission for some of the modes, is estimated. Publication: The Astrophysical Journal Pub Date: May 1983 DOI: 10.1086/161006 Bibcode: 1983ApJ...268..837M Keywords: Hot Stars; Neutron Stars; Relativistic Theory; Stellar Models; Stellar Oscillations; Stellar Temperature; Adiabatic Equations; Approximation; Eigenvalues; Gravitational Waves; Neutrinos; Quadrupoles; Vibration Damping; Vibration Mode; Wave Dispersion; Astrophysics full text sources ADS |

Fast Oscillations in Variable X-Ray Sources and X-Ray Bursters

ABSTRACTThe properties of coherent oscillations with periods of tens of milliseconds that have been observed in seven variable X-ray sources and bursters are reviewed. Theoretical models for the generation of the oscillations are discussed. Difficulties with a magnetic rotator model are indicated. It is suggested that at least in several cases, g-mode oscillations of the neutron star provide a satisfactory explanation of the observations.

Fast variations in the period of a pulsar in the X-ray source 1H 1909+096

The known X-ray source 1H 1909+096 was found at a state of high intensity on 1977 October 15, when 5 ms accumulation data was transmitted by the HEAO A-1 experiment. During that time, this source was found to pulsate with a period of approx.15.25 ms. This period was found to change during the 10 s of scanning through the source. The expectation for this to occur at random is calculated and found to be very low (10/sup -11/). A theoreticaldiscussion and two models are presented: (1) nonradial g-mode oscillation, and (2) torsional oscillation in the solid crust of a neutron star.

Transient mass transfer caused by local surface heating in close binaries

The surge of mass from one component of a binary system resulting from local surface heating is analyzed. The impact of such surges on the companion can produce transient phenomena such as those seen in X-ray binaries, RS CVn objects, and cataclysmic variables. The heating may be caused by nonlinear g-mode oscillations or by X-ray heating by the companion in X-ray binaries, among other possible mechanisms. As an example, model calculations have been performed for a surge, triggered by a relatively moderate local heating, in a hypothetical X-ray binary; the results show that such a surge can account for X-ray turn-ons.