Tayler Instability of Magnetic Field as the Possible Reason for the Period Changes in Ap Star 56 Ari

The effect of the local time-varying magnetic field in our G measurement with the time-of-swing method is studied by magnifying the magnetic field to cause a perceptible change in the pendulum's period. The experimental result shows that the coefficients of the change in the period to the magnetic field are 37(1) and 12(1) ms/gauss in the two horizontal directions respectively, which means that the systematic uncertainty due to the local magnetic field is less than 0.4ppm in our G measurement.

Summary Six-component recording (three Earth current components and three magnetic field variation components) in the period range 10–200 s has been carried out for a continuous interval of 336 h at Mont St Hilaire, a mid-latitude station near Montreal, Canada. Power density spectra of all six components for consecutive hourly intervals for the total recording time have been computed on an analogue computer. Paper chart records were compared with the computer results. The analysis indicates that: (a) The distribution of power among the periods is not necessarily the same for all six components and there are often abrupt changes in the dominant periods of the six components from hour to hour. The changes from hour to hour in the power spectra indicate that there is no gradual change in the period, but instead changes in the relative power among the periods present. (b) There is a diurnal variation indicating a maximum near local noon in the continuity of events and in the relative power of the dominant periods, but there is no clear indication of a diurnal variation in the dominant periods of micropulsations. (c) The period of the dominant activity depends upon the three-hour range index Kp. The nature of the dependence varies among the six components. (d) The micropulsation activity in the period range 10–200 s at the mid-latitude station of Mont St Hilaire is divided into two main period bands: 20–80 s, and 120–200 s. The pulsations in the longer period band usually contain much more power than those of the shorter period group.

view Abstract Citations (129) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnetic Activity, Tides, and Orbital Period Changes in Close Binaries Applegate, James H. ; Patterson, Joseph Abstract It is proposed that a variable quadrupole moment produced by magnetic activity in the outer convection zone of one of the stars in a close binary is responsible for the orbital period changes which occur on a time scale of order 10 yr. In the model, magnetic activity modulates the orbital period, superposed on any long-term period changes that may occur. The model predicts that the orbital period changes should be periodic with the period of the activity cycle. If the period changes due to secular angular momentum loss and magnetic activity can be separated, a powerful new probe of stellar magnetic activity will emerge, tidal synchronization theory can be tested, and the progenitor population and evolution rate of cataclysmic variables can be determined. Publication: The Astrophysical Journal Pub Date: November 1987 DOI: 10.1086/185044 Bibcode: 1987ApJ...322L..99A Keywords: Eclipsing Binary Stars; Magnetoactivity; Periodic Variations; Stellar Magnetic Fields; Stellar Orbits; Angular Momentum; Quadrupoles; Secular Variations; Stellar Activity; Stellar Models; Astrophysics; STARS: ECLIPSING BINARIES; STARS: MAGNETIC full text sources ADS | data products SIMBAD (1)

We report the identification of cyclical changes in the orbital period of the eclipsing dwarf novae V2051 Ophiuchi and V4140 Sagitarii. We used sets of white dwarf mideclipse timings to construct observed-minus-calculated diagrams covering, respectively, 25 and 16 years of observations. The V2051 Oph data present cyclical variations that can be fitted by a linear plus sinusoidal function with period 22±2 yr and amplitude 17±3 s. The statistical significance of this period by an F-test is larger than 99.9 per cent. The V4140 Sgr data present cyclical variations of similar amplitude and period 6.9 ± 0.3 yr which are statistically significant at the 99.7 per cent level. We derive upper limits for secular period changes of | u P| < 3 × 10 12 and | u P| < 1.8 × 10 11 , respectively for V2051 Oph and V4140 Sgr. We combined our results with those in the literature to construct a diagram of the amplitude versus period of the modulation for a sample of 11 eclipsing cataclysmic variables (CVs). If the cyclical period changes are the consequence of a solar-type magnetic activity cycle in the secondary star, then magnetic activity is a widespread phenomenon in CVs, being equally common among long- and short-period systems. This gives independent evidence that the magnetic field (and activity) of the secondary stars of CVs do not disappear when they become fully convective. We also find that the fractional cycle period changes of the short-period CVs are systematically smaller than those of the long-period CVs.

We present the results of Doppler Imaging of the Ap star CU Vir in the silicon lines over the 1985–2011 time span, as well as multi-element imaging in the 2009/2011 epoch. The surface distribution of silicon in CU Vir exhibits stability over the approximately 26 years studied: the number, shape, and mutual distribution of the overabundance spots have remained unchanged. The modelling of the light curve based on the surface elemental distribution obtained with DI did not reveal any significant changes in the shape of the light curve that could explain the photometric phase shift observed in CU Vir. Consequently, the phase shifts and changes in the photometric period of CU Vir are caused by the rigid longitudinal drift of the surface-abundance structures. We performed simulations of the Tayler instability of the background magnetic field of CU Vir, and discuss the possibility of explaining the period variations by the drift of surface instability modes.

Magnetic activity cycles of a convective star in a close binary system may lead to orbit period changes through tidal spin-orbit coupling. An increase in the mean magnetic field throughout the convection zone provides an additional pressure support and increases the star's moment of inertia. In order for the system to conserve angular momentum instantaneously, the convective star must reduce its rotation rate. When this happens, the system is no longer synchronous; in particular, the equilibrium tide now suffers a phase lag. A tidal torque must act, transferring angular momentum from the star's rotation to the orbit, lengthening the period. When the field decays, these events reverse and the period shortens. Observed period changes in RS CVn-like systems are of the order delta p/p = 10 to the -6th, implying variations in the mean field strength at the base of the convection zone of approximately 1000 G over time scales of 10-30 yr. It is predicted that the strength of magnetic activity indicators will track orbital period changes in a given system: the period will be largest during the active portion of the magnetic cycle and smallest during the quiescent part.

Since the discovery of the first rapidly oscillating Ap (roAp) star in 1978 by Kurtz, this class of magnetic chemically-peculiar pulsators has grown to over two dozen. The eigenfrequency spectra of roAp stars (with periods of ∼ 6 – 15 min) are consistent with nonradialp-modes of low degreeand high overtonen, not unlike the Sun's five-minute oscillations seen in integrated light. However, unlike the Sun, the strong global dipole fields of roAp stars significantly affect the pulsations.Although much of the effort in the last decade has been towards detecting new roAp candidates and refining the frequencies of known variables, initial “seismic” analyses have already yielded important results. Measurements of fundamental frequency spacingsconstrain the luminosities and radii of some roAp stars. In addition, mode splitting provides: (1) an independent determination of rotation period, even in the absence of longer-term light variations; (2) limits on the rotational inclinationiand magnetic obliquityβ; and (3) an indication of the relativeinternalfield strengths of certain roAp stars. Very recently, the temperature - optical depth structure of the atmosphere of HR 3831 was inferred from optical and IR photometry of its oscillations.Judging from current developments, the next decade promises exciting results on both observational and theoretical fronts. Several roAp stars have now been monitored for over a decade, allowing us to investigate long-term period changes due to evolution, binarity, etc. Eigenfrequency models for stars in the mass and radius range appropriate for Ap stars are becoming available, as well as explicit treatments of the perturbations due to magnetic fields. Armed with these, we may be able to place some roAp stars on a theoretical(or “asteroseismological H-R“) diagram to derive independently their masses and main-sequence ages.

Variations in the magnetic pressure and flux blocking by starspots during the magnetic cycle of the cool semidetached component of an Algol binary may cause cyclic changes in the quadrupole moment and moment of inertia of the star which can cause alternate period changes. Since several different processes and timescales are involved, the orbital period changes may not correlate strongly with the indicators of magnetic activity. The structural changes in the semidetached component can also modulate the mass transfer rate. Sub-Keplerian velocities, supersonic turbulence, and high temperature regions in circumstellar material around the accreting star may all be a consequence of magnetic fields embedded in the flow. Models for the evolution of Algols which include the effects of angular momentum loss (AML) through a magnetized wind may have underestimated the AML rate by basing it on results from main sequence stars. Evolved stars appear to have higher AML rates, and there may be additional AML in a wind from the accretion disk.

Variations in the magnetic pressure and flux blocking by starspots during the magnetic cycle of the cool semidetached component of an Algol binary may cause cyclic changes in the quadrupole moment and moment of inertia of the star which can cause alternate period changes. Since several different processes and timescales are involved, the orbital period changes may not correlate strongly with the indicators of magnetic activity. The structural changes in the semidetached component can also modulate the mass transfer rate. Sub-Keplerian velocities, supersonic turbulence, and high temperature regions in circumstellar material around the accreting star may all be a consequence of magnetic fields embedded in the flow. Models for the evolution of Algols which include the effects of angular momentum loss (AML) through a magnetized wind may have underestimated the AML rate by basing it on results from main sequence stars. Evolved stars appear to have higher AML rates, and there may be additional AML in a wind from the accretion disk.

view Abstract Citations (55) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS What tides and flares do to RS Canum Venaticorum binaries. Decampli, W. M. ; Baliunas, S. L. Abstract The effects that anisotropic mass should have on the orbital and spin states of RS Canum Venaticorum binaries are discussed. In the absence of magnetic fields, orbital period changes reported for several RS Canum Venaticorum systems require dM/dt of about 0.000001 solar masses/yr. Magnetic braking can lower this required rate if the surface magnetic fields are not less than 1000 gauss. However, this requires a method much more powerful than tidal torques to convert spin angular momentum loss to orbital angular momentum loss. This possibility is important when interpreting the complicated light curves of these systems, and may contradict the Hall's 'drifting star spot' hypothesis. In addition, large mass-loss rates may result in significant self-absorption of quiescent soft X-rays observed from several of these binaries. Publication: The Astrophysical Journal Pub Date: June 1979 DOI: 10.1086/157140 Bibcode: 1979ApJ...230..815D Keywords: Eclipsing Binary Stars; Flare Stars; Stellar Mass Ejection; Tides; Angular Momentum; Light Curve; Stellar Magnetic Fields; X Ray Absorption; Astrophysics; Mass Loss:RS CVn Stars; Period Changes:RS CVn Stars; RS CVn Stars:Stellar Flares full text sources ADS | data products SIMBAD (7)

view Abstract Citations (14) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Interstellar grains and current flow in pulsar magnetospheres Cheng, A. F. Abstract Consequences of interstellar grain entry into radio pulsar magnetospheres are investigated. Ablation and ionization of grain matter within the light cylinder injects charges of both signs throughout the magnetosphere. Electron-positron pair creation cascades result for typical pulsar parameters. A quasi-steady current flow is maintained by these processes, which may yield a solution to the long-standing current closure problem in pulsar magnetospheres. The resulting pulsar model may provide an explanation for the striking observation that pulsar spindown rates are correlated with proper velocities. The observed correlation of the square root of the product of the period and the change in period with the pulsar proper motion is interpreted as a correlation between current flow and velocity arising from a dependence of current flow on the rate of interstellar grain destruction in the magnetosphere. In this model, grain destruction occurs within the light cylinder if the average neutron star temperature is near 10 to the 5th K. Furthermore, the true pulsar magnetic field exceeds the value conventionally inferred from the square root of the product of the period and the change in period if the current is less than the Goldreich-Julian (1969) value. In general, however, the model retains many of the phenomenological successes of earlier polar cap models. Publication: The Astrophysical Journal Pub Date: December 1985 DOI: 10.1086/163758 Bibcode: 1985ApJ...299..917C Keywords: Bow Waves; Interstellar Matter; Pulsar Magnetospheres; Pulsars; Radio Stars; Current Density; Electron-Positron Pairs; Neutron Stars; Photoelectric Emission; Shock Waves; Stellar Temperature; Thermal Radiation; Astrophysics full text sources ADS |

We present 132 h of new time-series photometric observations of the δ Scuti star CD−24 7599 acquired during 86 nights from 1993 to 1996 to study its frequency and amplitude variations. By using all published observations we demonstrate that the three dominating pulsation modes of the star can change their photometric amplitudes within one month at certain times, while the amplitudes can remain constant within the measurement errors at other times. CD−24 7599 also exhibits frequency variations, which do not show any correspondence between the different modes. The typical time-scale for the amplitude variations is found to be several hundred days, which is of the same order of magnitude as the inverse linear growth rates of a selected model. We find no evidence for periodic amplitude modulation of two of the investigated modes (f2 and f3), but f1 may exhibit periodic modulation. The latter result could be spurious and requires confirmation. The observed frequency variations may either be continuous or reflect sudden frequency jumps. No evidence for cyclical period changes is obtained. We exclude precession of the pulsation axis and oblique pulsation for the amplitude variations. Beating of closely spaced frequencies cannot explain the amplitude modulations of two of the modes, while it is possible for the third. Evolutionary effects, binarity, magnetic field changes or avoided crossings cannot be made responsible for the observed period changes. Only resonance between different modes may be able to explain the observations. However, at this stage a quantitative comparison is not possible. More observations, especially data leading to a definite mode identification and further measurements of the temporal behaviour of the amplitudes and frequencies of CD−24 7599, are required.

More than 1300 variables classified provisionally as first-overtone RR Lyrae pulsators in the MACHO variable-star database of the Large Magellanic Cloud (LMC) have been subjected to standard frequency analysis. Based on the remnant power in the prewhitened spectra, we found 70% of the total population to be monoperiodic. The remaining 30% (411 stars) are classified as one of nine types according to their frequency spectra. Several types of RR Lyrae pulsational behavior are clearly identified here for the first time. Together with the earlier discovered double-mode (fundamental and first-overtone) variables, this study increased the number of known double-mode stars in the LMC to 181. During the total 6.5 yr time span of the data, 10% of the stars showed strong period changes. The size, and in general also the patterns of the period changes, exclude a simple evolutionary explanation. We also discovered two additional types of multifrequency pulsators with low occurrence rates of 2% for each. In the first type, there remains one closely spaced component after prewhitening by the main pulsation frequency. In the second type, the number of remnant components is two; they are also closely spaced, and are symmetric in their frequency spacing relative to the central component. This latter type of variables are associated with their relatives among the fundamental pulsators, known as Blazhko variables. Their high frequency (≈20%) among the fundamental-mode variables versus the low occurrence rate of their first-overtone counterparts makes it more difficult to explain the Blazhko phenomenon by any theory depending mainly on the role of aspect angle or magnetic field. None of the current theoretical models are able to explain the observed close frequency components without invoking nonradial pulsation components in these stars.

We present 132 h of new time-series photometric observations of the δ Scuti star CD−24 7599 acquired during 86 nights from 1993 to 1996 to study its frequency and amplitude variations. By using all published observations we demonstrate that the three dominating pulsation modes of the star can change their photometric amplitudes within one month at certain times, while the amplitudes can remain constant within the measurement errors at other times. CD−24 7599 also exhibits frequency variations, which do not show any correspondence between the different modes. The typical time-scale for the amplitude variations is found to be several hundred days, which is of the same order of magnitude as the inverse linear growth rates of a selected model. We find no evidence for periodic amplitude modulation of two of the investigated modes (f2 and f3), but f1 may exhibit periodic modulation. The latter result could be spurious and requires confirmation. The observed frequency variations may either be continuous or reflect sudden frequency jumps. No evidence for cyclical period changes is obtained. We exclude precession of the pulsation axis and oblique pulsation for the amplitude variations. Beating of closely spaced frequencies cannot explain the amplitude modulations of two of the modes, while it is possible for the third. Evolutionary effects, binarity, magnetic field changes or avoided crossings cannot be made responsible for the observed period changes. Only resonance between different modes may be able to explain the observations. However, at this stage a quantitative comparison is not possible. More observations, especially data leading to a definite mode identification and further measurements of the temporal behaviour of the amplitudes and frequencies of CD−24 7599, are required.

view Abstract Citations (21) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Long-term changes in pulsar periods and the plasma in neutron star interiors. Easson, I. Abstract The paper examines the hypothesis that the plasma of charged particles in the fluid core corotates with the solid crust as a result of the large interior magnetic field over very long time scales comparable to the apparent age of pulsars. In order to enforce approximate corotation of the interior plasma in a neutron star with the crust, the magnetic field in the fluid interior must be stronger than a minimum value governed by the rotation period and its first and second derivatives. If the field is below the minimum required value, viscocity enforces approximate corotation through the formation of an Ekman layer and pumping into and out of it. The shear of the plasma velocity is sufficient to magnify any magnetic field in a small fraction of the apparent pulsar age, so all known pulsars should have interior magnetic fields at least as large as the minimum calculated here. Finally, the effective moment of inertia of the plasma in the fluid core is shown to depend on the structure of the field there, contrary to previous assumptions. Publication: The Astrophysical Journal Pub Date: October 1979 DOI: 10.1086/157432 Bibcode: 1979ApJ...233..711E Keywords: Astrophysics; Magnetohydrodynamics; Neutron Stars; Pulsars; Stellar Interiors; Boundary Layer Transition; Long Term Effects; Periodic Variations; Stellar Magnetic Fields; Stress Tensors; Astrophysics; Magnetic Fields:Stellar Interiors; Neutron Stars:Plasma; Pulsars:Pulse Structure full text sources ADS |