Solar internal rotation rate and the latitudinal variation of the tachocline     

H. M. Antia  Sarbani Basu  & S. M. Chitre 《Monthly notices of the Royal Astronomical Society》1998,298(2):543-556

A new set of accurately measured frequencies of solar oscillations is used to infer the rotation rate inside the Sun, as a function of radial distance as well as latitude. We have adopted a regularized least-squares technique with iterative refinement for both 1.5D inversion, using the splitting coefficients, and 2D inversion using individual m splittings. The inferred rotation rate agrees well with earlier estimates showing a shear layer just below the surface and another one around the base of the convection zone. The tachocline or the transition layer where the rotation rate changes from differential rotation in the convection zone to an almost latitudinally independent rotation rate in the radiative interior is studied in detail. No compelling evidence for any latitudinal variation in the position and width of the tachocline is found, although it appears that the tachocline probably shifts to a slightly larger radial distance at higher latitudes and possibly also becomes thicker. However, these variations are within the estimated errors and more accurate data would be needed to make a definitive statement about latitudinal variations.  相似文献   

Determining temperature amplitude as a function of depth in the atmospheres of rapidly oscillating Ap stars     

R. Medupe  & D. W. Kurtz 《Monthly notices of the Royal Astronomical Society》1998,299(2):371-378

We present numerical models based on realistic treatment of the intensity spectrum (from model atmospheres), and demonstrate that they are consistent with Kurtz and Medupe's recent formula in showing that limb darkening is too small an effect to explain the observed sharp decline of pulsation light amplitude with wavelength in rapidly oscillating Ap stars. Kurtz and Medupe's formula is shown to be a special form of Watson's earlier general formula for non-radial light variations of a star pulsating in any mode ( l m ). Using a technique suggested by Kurtz and Medupe we derive temperature semi-amplitude as a function of depth in the atmospheres of α Cir and HR 3831, assuming that we can neglect non-adiabatic effects.  相似文献   

Spectral line profile variations in the γ Doradus variable HD 164615: non-radial pulsations versus star-spots     

A. P. Hatzes 《Monthly notices of the Royal Astronomical Society》1998,299(2):403-409

High-resolution spectral data of the Fe  II 5318 Å line in the γ Doradus star HD 164615 are presented. These show systematic changes in the spectral lineshapes with the photometric period of 0.8133 d which are modelled using either non-radial pulsations or cool star-spots. The non-radial modes that can fit the lineshape changes have m degree of 2–4. However, only the m = 2 mode seems to be consistent with the amplitude of the radial velocity variations measured for this star. The star-spot model, although it can qualitatively fit the lineshape changes, is excluded as a possible hypothesis on the basis of (1) poorer fits to the observed spectral line profiles, (2) an inability to account for the large changes in the spectral linewidth as a function of phase, (3) a predicted radial velocity curve that looks qualitatively different from the observed one, and (4) a predicted photometric curve that is a factor of 5 larger than the observed light curve (and with the wrong qualitative shape). Finally, a 'Doppler image' (assuming cool spots) derived from a sequence of synthetic line profiles having non-radial pulsations results in an image that is almost identical to the Doppler image derived for HD 164615. These results provide strong evidence that non-radial pulsations are indeed the explanation for the variability of HD 164615 as well as the other γ Dor variables.  相似文献   

Oscillations in radiation-driven outflows     

Guy S. Miller  & Scott A. Grossman 《Monthly notices of the Royal Astronomical Society》1998,299(1):139-149

In this paper we investigate the dynamical behaviour of radiation-driven winds, specifically winds that arise when Compton scattering transfers momentum from the radiation field to the gas flow. Such winds occur during strong X-ray bursts from slowly accreting neutron stars, and also may be driven from the inner regions of a black hole or neutron star accretion disc when the mass transfer rate is very high. By linearizing the radiation hydrodynamic equations around steady spherical outflow, we evaluate the time-dependent response of these winds to perturbations introduced at their inner boundaries. We find that although radiation-driven winds are generally stable, they act as mechanical filters that should produce quasi-periodic oscillations or peaked noise in their radiation output when perturbations force them stochastically. This behaviour may underlie the photospheric oscillations observed in some strong Type I X-ray bursts.  相似文献   

Inversion of free oscillation data for an elliptical rotating earth in source moment tensor studies     

Jorge A. Mendiguren 《Pure and Applied Geophysics》1980,118(2):1192-1208

It is shown, through numerical experiments, that the effect of ignoring ellipticity and rotation of the earth when inverting free oscillation data in moment tensor studies, is to introduce spurious spectral components in the solution. For numerical simulation of the data conditions of the 1970 deep Colombian event the spurious part has isotropic and deviatoric components, besides a distortion in the phase spectra which may lead to artificial precursive source in the solution. The magnitude of the artificial isotropic component is of the order of 10% of the real source moment tensor, depending on the data set used in the inversion. Measuring the excitation of each mode at each station using the technique of integrating over the spectral peaks does not cancel completely the effect of multiplet splitting, in particular that splitting due to rotation of the earth. The effect of lateral heterogeneities was simulated by introducing arbitrary splitting parameters which produced scattering in the eigenperiods compatible with those observed. For that splitting the inversion rendered additional isotropic components.We conclude that the observed implosive precursor the the 1970 Colombian event could have been artificially introduced by ignoring rotation, ellipticity and lateral heterogeneities of the earth.A procedure to invert free oscillation data for an elliptical rotating earth is proposed. It could be applied to a laterally inhomogeneous earth if the excitation for that model could be computed. Its application to real data for the deep Colombian event indicates that we will need to improve our knowledge on the effect of rotation, ellipticity, lateral heterogeneities and anelasticity on free oscillation and their excitation in order to solve accurately for the six components of the moment tensor independently.  相似文献   

Models of α Centauri A and B with and without seismic constraints: time dependence of the mixing-length parameter     

M. Y&#;ld&#;z 《Monthly notices of the Royal Astronomical Society》2007,374(4):1264-1270

The α Centauri (α Cen) binary system is a well-known stellar system with very accurate observational constraints on the structure of its component stars. In addition to the classical non-seismic constraints, there are also seismic constraints for the interior models of α Cen A and B. These two types of constraint give very different values for the age of the system. While we obtain 8.9 Gyr for the age of the system from the non-seismic constraints, the seismic constraints imply that the age is about 5.6–5.9 Gyr. There may be observational or theoretical reasons for this discrepancy, which can be found by careful consideration of similar stars. The α Cen binary system, with its solar-type components, is also suitable for testing the stellar mass dependence of the mixing-length parameter for convection derived from the binaries of Hyades. The values of the mixing-length parameter for α Cen A and B are 2.10 and 1.90 for the non-seismic constraints. If we prioritize the seismic constraints, we obtain 1.64 and 1.91 for α Cen A and B, respectively. By taking into account these two contrasting cases for stellar mass dependence of the mixing-length parameter, we derive two expressions for its time dependence, which are also compatible with the mass dependence of the mixing-length parameter derived from the Hyades stars. For assessment, these expressions should be tested in other stellar systems and clusters.  相似文献   

On prospects for sounding activity cycles of Sun-like stars with acoustic modes     

W. J. Chaplin  Y. Elsworth  G. Houdek  R. New 《Monthly notices of the Royal Astronomical Society》2007,377(1):17-29

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Direct Propagation of Photospheric Acoustic <Emphasis Type="Italic">p</Emphasis> Modes into Nonmagnetic Solar Atmosphere     

C. Malins  R. Erdélyi 《Solar physics》2007,246(1):41-52

Solar p modes are one of the dominant types of coherent signals in Doppler velocity in the solar photosphere, with periods showing a power peak at five minutes. The propagation (or leakage) of these p-mode signals into the higher solar atmosphere is one of the key drivers of oscillatory motions in the higher solar chromosphere and corona. This paper examines numerically the direct propagation of acoustic waves driven harmonically at the photosphere, into the nonmagnetic solar atmosphere. Erdélyi et al. (Astron. Astrophys. 467, 1299, 2007) investigated the acoustic response to a single point-source driver. In the follow-up work here we generalise this previous study to more structured, coherent, photospheric drivers mimicking solar global oscillations. When our atmosphere is driven with a pair of point drivers separated in space, reflection at the transition region causes cavity oscillations in the lower chromosphere, and amplification and cavity resonance of waves at the transition region generate strong surface oscillations. When driven with a widely horizontally coherent velocity signal, cavity modes are caused in the chromosphere, surface waves occur at the transition region, and fine structures are generated extending from a dynamic transition region into the lower corona, even in the absence of a magnetic field.  相似文献   

Present and Future Observing Trends in Atmospheric Magnetoseismology     

D. Banerjee  R. Erdélyi  R. Oliver  E. O’Shea 《Solar physics》2007,246(1):3-29

With modern imaging and spectral instruments observing in the visible, EUV, X-ray, and radio wavelengths, the detection of oscillations in the solar outer atmosphere has become a routine event. These oscillations are considered to be the signatures of a wave phenomenon and are generally interpreted in terms of magnetohydrodynamic (MHD) waves. With multiwavelength observations from ground- and space-based instruments, it has been possible to detect waves in a number of different wavelengths simultaneously and, consequently, to study their propagation properties. Observed MHD waves propagating from the lower solar atmosphere into the higher regions of the magnetized corona have the potential to provide excellent insight into the physical processes at work at the coupling point between these different regions of the Sun. High-resolution wave observations combined with forward MHD modeling can give an unprecedented insight into the connectivity of the magnetized solar atmosphere, which further provides us with a realistic chance to reconstruct the structure of the magnetic field in the solar atmosphere. This type of solar exploration has been termed atmospheric magnetoseismology. In this review we will summarize some new trends in the observational study of waves and oscillations, discussing their origin and their propagation through the atmosphere. In particular, we will focus on waves and oscillations in open magnetic structures (e.g., solar plumes) and closed magnetic structures (e.g., loops and prominences), where there have been a number of observational highlights in the past few years. Furthermore, we will address observations of waves in filament fibrils allied with a better characterization of their propagating and damping properties, the detection of prominence oscillations in UV lines, and the renewed interest in large-amplitude, quickly attenuated, prominence oscillations, caused by flare or explosive phenomena.  相似文献   

A spectral atlas of post-main-sequence stars in ω Centauri: kinematics, evolution, enrichment and interstellar medium     

Jacco Th. van Loon  Floor van Leeuwen  Barry Smalley  rew W. Smith  Nicola A. Lyons  Iain McDonald  Martha L. Boyer 《Monthly notices of the Royal Astronomical Society》2007,382(3):1353-1374

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