Global Pulsations Research Articles

Probing the core and envelope structure of DBV white dwarfs

We investigate the global pulsation properties of DBV white dwarf models that include both the double-layered envelope structure expected from time-dependent diffusion calculations, as well as a non-uniform C/O core expected from prior nuclear burning. We compare these models to otherwise identical models containing a pure C core to determine whether the addition of core structure leads to any significant improvement. Our double-layered envelope model fit to GD 358 that includes an adjustable C/O core is significantly better than our pure C core fit (7sigma improvement). We find a comparable improvement from fits to a second DBV star, CBS 114, though the values of the derived parameters may be more difficult to reconcile with stellar evolution theory. We find that our models are systematically cooler by 1,900 K relative to the similar models of Fontaine & Brassard (2002). Although a fit to their model reproduces the mass and envelope structure almost exactly, we are unable to reproduce the absolute quality of their fit to GD 358. Differences between the constitutive physics employed by the two models may account for both the temperature offset and the period residuals.

Geomagnetic pulsations caused by the magnetopause oscillations (Comparison of spacecraft and geomagnetic observations)

This paper is devoted to the study of long-period geomagnetic pulsations recorded after abrupt changes of the solar wind (SW) dynamic pressure. Data of simultaneous satellite observations within the “INTERBALL” program are used in the analysis of the magnetopause location and plasma and magnetic field parameters near the magnetopause. Geomagnetic pulsations are analyzed using data from several meridional chains of stations located in different longitudinal sectors. It is shown that experimental data confirm the Mishin's (1993) model of the excitation of damped radial oscillations of the magnetopause by SW pressure impulses that are recorded on the ground in the form of global geomagnetic pulsations. The period of these pulsations is proportional to the subsolar radius of the magnetosphere to the 4th power. It is shown that the pulsation period varies with time inversely as the sign of density variation. This is consistent with the Mishin's (1993) model.

Periodic variations of magnetosheath energetic electron flux associated with global Pc5 pulsations

Measurements from the Wind spacecraft in the solar wind, the Geotail spacecraft in the magnetosheath, and multiple magnetometers on the ground are combined to study a unique electron flux modulation event in the magnetosheath that occurred during an interval of global Pc5 waves. The electron event is characterized by a Pc5‐band oscillation occurring in the magnetic‐field‐aligned component of the electron flux (energy > 38 keV) while the local magnetic field and plasma density remained unperturbed. During this event the whole magnetosphere oscillated as inferred from magnetic field recorded at the CANOPUS, IMAGE, and 210° MM chains. At the ground stations that are mapped to the dayside and duskside magnetosphere, we find a one‐to‐one correspondence of the peaks in the horizontal component to those in the electron flux. At the stations mapped to the dawnside magnetosphere the observed frequency is twice that of the electron flux oscillation. The time‐lagged solar wind density and velocity at Wind were constant, which leads us to exclude the possibility that the waves were driven by ram pressure variations upstream of the bow shock. The Kelvin‐Helmholtz (K‐H) instability on the magnetopause is the more probable source of the Pc5 waves because (1) the solar wind velocity was ∼730 km s−1, well above the average value; (2) the ground‐based observations show that the Pc5 amplitude decreases with distance from the magnetopause; and (3) the dawnside and duskside magnetospheric flanks show oppositely polarized waves. The electron flux oscillation in the magnetosheath can be explained by intermittent magnetic connection between the spacecraft and the dawnside pulsating magnetopause. The ∼60 keV protons are continuously streaming throughout the whole interval, providing evidence for a continuously active source and permanent leakage through a tangential magnetopause discontinuity. It is worth noticing that at progressively higher energies the energetic particle angular distributions are gradually more isotropic because at higher energies the particles are strongly scattered in the magnetosheath.

On the stability of global non-radial pulsations of neutron stars

A neutron star is the cosmic nuclear object in which the energy of gravitational pull is brought to equilibrium by elastic energy stored in the neutron Fermi-continuum. Evidence for the viscoelastic behaviour of a stellar nuclear matter provides a seismological model of pulsar glitches interpreted as a sudden release of the elastic energy. In laboratory nuclear physics, the signatures of viscoelasticity of nuclear matter are found in the current investigations on the collective nuclear dynamics, in which a heavy nucleus is modelled by a spherical piece of viscoelastic Fermi-continuum compressed to the normal nuclear density. It is plausible to expect, therefore, that the motions of self-gravitating nuclear matter constituting the interior of neutron stars should be governed by the equations of an elastic solid, rather than by hydrodynamic equations describing the behaviour of gaseous plasma inside the main sequence stars. In this paper, we present arguments that elastodynamic equations, originally introduced in the context of nuclear collective dynamics, can provide a proper account of elasticity in the large scale motions of neutron matter under its own gravity. Emphasis is placed on mathematical physics underlying the constructive description of the continuum mechanics and the rheology of macroscopic nuclear matter. The capability of the elastodynamic approach is examined by analysis of oscillatory dynamics of a neutron star in the standard homogenous model, operating with a spherical mass of self-gravitating degenerate neutron matter whose viscoelastic behaviour is described in terms of the spheroidal and torsional gravitational-elastic eigenmodes, inherenly related to viscoelasticy. The energy variational principle is utilized to compute the frequencies of viscoelastic gravitational pulsations and their relaxation time. The method is demonstrated for both the idealized homogeneous model and the neutron star models constructed on realistic equations of state. Finally, we derive analytic conditions for the stability of a neutron star to linear elastic deformations accompanying the non-radial pulsations, and discuss the fingerprints of these pulsations in the electromagnetic activity of radiopulsars.

First Image of the Surface of a Star with the [ITAL]Hubble Space Telescope[/ITAL

A direct image of the surface of a star, Betelgeuse (=α Ori; M2 Iab), has been obtained with the Faint Object Camera on the Hubble Space Telescope. Images in two ~300 A-wide bands centered at 2550 and 2800 A cover ~10 resolution elements on the stellar disk. The ultraviolet diameter of Betelgeuse of 125 ± 5 mas at 2500 A (corresponding to 108 ± 4 mas, FWHM) is a factor of 2.2 larger than the optical diameter, thus indicating a substantially extended chromosphere in this supergiant. A single bright, unresolved area is present in the southwest quadrant of the disk (P.A. = 235°) in both images, with a peak amplitude of 1.3-1.8 as compared to the surrounding disk and a temperature differential in excess of 200 K. This feature may be the result of magnetic activity, atmospheric convection, or global pulsations and shock structures that heat the chromosphere. Spatially resolved spectroscopy of the Mg II λλ2795, 2802 doublet with the Goddard High Resolution Spectrograph suggests complicated dynamics, with outflowing material in the chromosphere indicated by the Mg II emission.

Line profile variations in M giants - Clues to mass-loss and chromospheric heating mechanisms

view Abstract Citations (8) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Line Profile Variations in M Giants: Clues to Mass-Loss and Chromospheric Heating Mechanisms Judge, P. G. ; Luttermoser, D. G. ; Neff, D. H. ; Cuntz, M. ; Stencel, R. E. Abstract Analysis is presented of time-series, high dispersion spectra of the Mg II, k, Ca II H, and K lines of the semiregular giants Rho Per (M4 II-III, periodicity of about 50 days), R Lyr (M5 III, period of about 46 days), and g Her (M6 III, period of about 90 days). The fine error sensor on the IUE satellite and ground based UBV photometry was used to relate line profile variations to photospheric variations. The above mentioned stars were selected to study the relative importance of convective motions and global stellar pulsations in determining the structure of the outer atmospheres. Small amplitude changes, but substantial changes in the profiles of Mg II and Ca II lines were detected. It is contended that the observed variability is due to changes in chromospheric conditions and not variations within the circumstellar shell. The picture of a steady state chromosphere, which is modulated on long time scales, is corroborated by these observations. Localized heating is found in g Her. Publication: The Astronomical Journal Pub Date: May 1993 DOI: 10.1086/116573 Bibcode: 1993AJ....105.1973J Keywords: Giant Stars; M Stars; Spectral Line Width; Stellar Mass Ejection; Stellar Motions; Stellar Oscillations; Astronomical Photometry; Atmospheric Heating; Chromosphere; Data Reduction; Iue; Stellar Envelopes; Ubv Spectra; Astrophysics; STARS: GIANTS; STARS: MASS-LOSS; STARS: CHROMOSPHERES full text sources ADS | data products SIMBAD (6) MAST (1) INES (1)

Chromospheric heating by acoustic shocks - A confrontation of GHRS observations of Alpha Tauri (K5 III) with AB initio calculations

We compare ab initio calculations of semiforbidden C II line profiles near 2325 A with recently published observations of the inactive red giant Alpha Tau (K5 III) obtained using the GHRS on board the Hubble Space Telescope. Our one-dimensional, time-dependent calculations assume that the chromosphere is heated by stochastic acoustic shocks generated by photospheric convection. We calculate various models using results from traditional (mixing length) convection zone calculations as input to hydrodynamical models. The semiforbidden C II line profiles and ratios provide sensitive diagnostics of chromospheric velocity fields, electron densities, and temperatures. We identify major differences between observed and computed line profiles which are related to basic gas dynamics and which are probably not due to technical modeling restrictions. If the GHRS observations are representative of chromospheric conditions at all epochs, then one (or more) of our model assumptions must be incorrect. Several possibilities are examined. We predict time variability of semiforbidden C II lines for comparison with observations. Based upon data from the IUE archives, we argue that photospheric motions associated with supergranulation or global pulsation modes are unimportant in heating the chromosphere of Alpha Tau.

160-min pulsation of the Sun: New observational results

The 1974–1988 Crimean measurements of the solar line-of-sight velocity continue to show the presence of a statistically significant periodicity P1 = 160.009 (±) min with an average harmonic amplitude of about 21 cm s−1. The period is supposed to be that of the global pulsation of the Sun but with a little-known physical mechanism of excitation.

Mean field escapers in non-equilibrium systems

We present some results of our study on non-equilibriumN-body systems that undergo initial collapse. This collapse is followed by global pulsations of the system. During these pulsations, the mean gravitational field fluctuates violently. Some particles pick up enough energy to be ejected from the system into the halo or even fly off to infinity. We discuss the region in phase space from which these mean field escapers originate and their energy frequency distribution for the illustrative case of a uniform spherical initial state. The pulsations lead to the production of shells in the halo.

A global pulsation event with conjugate study

On 17 July (day 198), 1982 a global long period pulsation was recorded over an extended array in the Northern Hemisphere and by magnetometers at the conjugate stations of Halley Bay and St Anthony, between 10:30 and 11:10 U.T. The pulsation appears to be simultaneously driven and highly damped, lasting for approximately five cycles with a period of about 450 s. Significant phase variations appear across the array and there are strong indications of an enhancement in amplitude of toroidal oscillations in the vicinity of the boundary of open/closed field lines. Analysis of the amplitude and phase variations between the stations suggests that the conjugate field line was oscillating in a fundamental quarter wave standing mode. Using whistler data, an estimate of the plasmapause position was obtained which indicates that the Halley/St Anthony field line was located in the innerplasmatrough. However, the anomalously long period and global behaviour suggests that the conjugate stations were situated some distance from the centre of the disturbance. Analysis of ISEE 3 data prior to the event indicated a doubling of solar wind pressure which may be correlated with the onset time of the event. In addition, a drop in solar wind velocity and switching of B z during the event indicate the possibility of a significant expansion of the magnetosphere with resultant rarefactional waves, the effects of which may be seen in the ground magnetograms towards the end of the event. It is suggested that this global disturbance was driven by large changes in solar wind conditions.

LOWER ATMOSPHERE CHANGES INFERRED FROM VARIATIONS IN BETELGEUSE'S LINEAR POLARIZATION

ABSTRACT The semiregular, late-type supergiant Betelgeuse has been intensively monitored over an eight-month interval from 1980-81. As was the case in the preceding observing season, significant ordered (as opposed to stochastic) polarization changes were detected. These changes are explained by the waxing and waning and motion of large-scale elements of the lower atmosphere. Ordered polarization position angle changes are attributed to changes in the orientation of the element. The observations are shown to be inconsistent with a single global nonradial pulsation mode, but are not inconsistent with local modes. These local nonradial pulsation modes are believed to be the cause of the observed polarization variations.