scholarly journals Radial and nonradial oscillations of massive supergiants

2010 ◽  
Vol 6 (S272) ◽  
pp. 468-473
Author(s):  
Hideyuki Saio
Keyword(s):  
Convection Zone ◽  
Stellar Surface ◽  
Oscillatory Convection ◽  
Mode Instability ◽  
Low Degree ◽  
Theoretical Predictions ◽  
Considerable Range ◽  
Instability Boundary ◽  
Hr Diagram ◽  
Nonradial Oscillations

AbstractStability of radial and nonradial oscillations of massive supergiants is discussed. The kappa-mechanism and strange-mode instability excite oscillations having various periods in wide ranges of the upper part of the HR diagram. In addition, in very luminous (log L/L⊙ ≳ 5.9) models, monotonously unstable modes exist, which probably indicates the occurrence of optically thick winds. The instability boundary is not far from the Humphreys-Davidson limit. Furthermore, it is found that there exist low-degree (ℓ = 1, 2) oscillatory convection modes associated with the Fe-opacity peak convection zone, and they can emerge to the stellar surface so that they are very likely observable in a considerable range in the HR diagram. The convection modes have periods similar to g-modes, and their growth-times are comparable to the periods. Theoretical predictions are compared with some of the supergiant variables.

Meridional Circulation, Turbulence, and Diffusion Processes in Stars

1976 ◽  
Vol 32 ◽  
pp. 109-116 ◽  
Author(s):  
S. Vauclair
Keyword(s):  
Convection Zone ◽  
Diffusion Processes ◽  
Meridional Circulation ◽  
Diffusion Time ◽  
Work In Progress ◽  
First Results ◽  
Stellar Envelopes ◽  
And Diffusion ◽  
Turbulence And Diffusion ◽  
Hr Diagram

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.

Interactions between steady and oscillatory convection in mushy layers

2010 ◽  
Vol 645 ◽  
pp. 411-434 ◽  
Author(s):  
PETER GUBA ◽  
M. GRAE WORSTER
Keyword(s):  
Standing Waves ◽  
Mushy Layer ◽  
Two Dimensional ◽  
Oscillatory Convection ◽  
Mushy Layers ◽  
Theoretical Predictions ◽  
Convection Patterns ◽  
The Stability ◽  
Nonlinear Solutions ◽  
Steady Convection

We study nonlinear, two-dimensional convection in a mushy layer during solidification of a binary mixture. We consider a particular limit in which the onset of oscillatory convection just precedes the onset of steady overturning convection, at a prescribed aspect ratio of convection patterns. This asymptotic limit allows us to determine nonlinear solutions analytically. The results provide a complete description of the stability of and transitions between steady and oscillatory convection as functions of the Rayleigh number and the compositional ratio. Of particular focus are the effects of the basic-state asymmetries and non-uniformity in the permeability of the mushy layer, which give rise to abrupt (hysteretic) transitions in the system. We find that the transition between travelling and standing waves, as well as that between standing waves and steady convection, can be hysteretic. The relevance of our theoretical predictions to recent experiments on directionally solidifying mushy layers is also discussed.

scholarly journals PRISMA: A Mission to Study Interior and Surface of Stars

1994 ◽  
Vol 147 ◽  
pp. 540-544
Author(s):  
P. Lemaire
Keyword(s):  
Convection Zone ◽  
Stellar Surface ◽  
Direct Access ◽  
The Core ◽  
Active Structures ◽  
Deep Interior ◽  
Accuracy Of Measurements ◽  
The Waves ◽  
The Magnetic Field

AbstractNew technics such as asteroseismology are able to sound the deep interior of stars and to provide the data that will constrain the modelisation of the core. This information will be combined with data collected from the stellar surface which give direct access to measurements of the radiative losses, angular momentum losses and distribution of active structures. From the two sets of data, the key role of the convection zone will be clarified, as the convection zone excites the waves that propagate through the whole star and generates the magnetic field that structures the stellar surface. The PRISMA mission was developed to collect the data needed for detecting the oscillations by very accurate photometry (micromagnitude) and to derive the surface activity and rotation from accurate ultraviolet spectroscopy. A short description of the model payload is given with the observational constraints related to the needed accuracy of measurements. Following the non-selection by ESA in may 1993, some following perspectives are described.

scholarly journals Asymptotic properties of low degree gravity modes

1988 ◽  
Vol 123 ◽  
pp. 121-124
Author(s):  
Gabrielle Berthomieu ◽  
Janine Provost
Keyword(s):  
Convection Zone ◽  
Asymptotic Properties ◽  
Global Constraints ◽  
Type Model ◽  
Solar Model ◽  
Solar Convection Zone ◽  
Solar Convection ◽  
Deep Interior ◽  
Low Degree ◽  
Solar Type

Asymptotic properties of low degree gravity modes and their relation to the stratification of the model through the Brunt-Väissälä frequency are discussed for a solar type model and for a 10 M⊙ model. For the solar model, taking into account the quasiadiabaticity of the solar convection zone, it is shown that two global constraints on the deep interior of the solar model can be derived from a set of g-modes periods in the observed range. However modes with large periods, i.e. larger than those up to now observed, are required to obtain informations on the stratification just below the convection zone. For the 10 M⊙ model, the preliminary results show that the existence of a layer with a large mean molecular weight gradient destroys the equidistance of the periods of gravity modes of a given degree and that some informations on the properties of this layer can be obtained from the analysis of the g-modes periods.

scholarly journals IUE observations of stars in NGC 330 in the Small Magellanic Cloud

1991 ◽  
Vol 148 ◽  
pp. 339-340
Author(s):  
V. Caloi ◽  
A. Cassatella ◽  
V. Castellani ◽  
G. Klare
Keyword(s):  
Globular Cluster ◽  
Massive Star ◽  
Magellanic Cloud ◽  
Small Magellanic Cloud ◽  
Low Resolution ◽  
Star Evolution ◽  
Theoretical Predictions ◽  
Effective Temperatures ◽  
Massive Star Evolution ◽  
Hr Diagram

Low resolution spectra from 1200 to 3300Å have been obtained, with the IUE satellite, for seven blue giants and supergiants in the young, metal-poor globular cluster NGC 330 in the Small Magellanic Cloud (SMC). A further three spectra from the IUE archive have been added to the sample, covering a range of 2.5 mag. The effective temperatures and the local reddening in the SMC have been estimated, and the star positions in the theoretical HR diagram determined, and compared with theoretical predictions for massive star evolution. Substantial discrepancies have been found.

scholarly journals Nonlinear Inversion for the Hydrostatic Structure of the Solar Interior

1998 ◽  
Vol 185 ◽  
pp. 117-118
Author(s):  
K.I. Marchenkov ◽  
I.W. Roxburgh ◽  
S.V. Vorontsov
Keyword(s):  
Asymptotic Analysis ◽  
Born Approximation ◽  
Strong Influence ◽  
Convection Zone ◽  
Solar Interior ◽  
Nonlinear Inversion ◽  
Rapid Variation ◽  
Seismic Parameters ◽  
Asymptotic Description ◽  
Low Degree

The technique which we use is based on the “quasi-asymptotic” description of solar p-modes developed by Roxburgh & Vorontsov (1996). As a significant improvement of the standard asymptotic analysis, this description incorporates a Born approximation to allow for a detailed treatment of the regions of rapid variation of seismic parameters with depth (base of the convection zone) and strong influence of gravity perturbations (low-degree modes which penetrate deep into the solar core).

The Relationship of Oxygen Consumption to Age and Weight During the Post-Embryonic Growth of Locusta Migratoria L

1957 ◽  
Vol 34 (1) ◽  
pp. 29-41
Author(s):  
KENNETH U. CLARKE
Keyword(s):  
Oxygen Consumption ◽  
Locusta Migratoria ◽  
Live Weight ◽  
Normal Growth ◽  
Low Degree ◽  
Straight Lines ◽  
Relationship Of ◽  
High Degree ◽  
The Relationship ◽  
Considerable Range

The weight and oxygen consumption of individual locusts, six from each instar, have been measured at 12-hourly intervals throughout each stadium. An analysis of short duration changes shows that fluctuations in live weight are unavoidable in the growing animal, and that they are accompanied by fluctuations in oxygen consumption. In time the latter lag behind the former. As the fluctuations of these two variables are out of phase, there is a considerable range of oxygen consumption which must be regarded as normal for any given weight. Calculations are made to find the limits of these fluctuations compatible with normal growth. The changes that occur during a stadium can be represented by a straight trend line for both weight and oxygen consumption. Exceptions occur in the case of weight of the adult where two straight lines are necessary, one for the growing phase and one for the steady phase of this instar. In certain cases in the fifth instar two straight lines are necessary to express the trend in oxygen consumption which may show an abrupt change in the middle of the stadium. A high degree of correlation is found between the trend lines for weight and oxygen consumption in the early instars. A low degree is found in latter instars where the weight increases and the oxygen consumption remains nearly constant. A curve representing the changes of weight and oxygen consumption that occur during the growth of the locust has been constructed.

scholarly journals Distances of the Central Stars and their Position in the HR Diagram

1983 ◽  
Vol 103 ◽  
pp. 391-409 ◽  
Author(s):  
S.R. Pottasch
Keyword(s):  
Galactic Center ◽  
Planetary Nebulae ◽  
Central Star ◽  
Magellanic Clouds ◽  
Constant Mass ◽  
Theoretical Predictions ◽  
Statistical Distance ◽  
Central Stars ◽  
Hr Diagram

Determination of the distances to individual planetary nebulae are discussed. Especially those methods which are independent of assumed nebular properties (mass, absolute flux, etc.) are assembled and discussed. In this way, reasonable approximations to the distance can be obtained for about 50 planetary nebulae. The accuracy of the distances is tested by comparing nebular properties derived from these distances with the properties of nebulae at the galactic center or in the Magellanic clouds. A comparison is also made with the statistical distance determinations; the conclusion is that the assumption of constant mass often leads to an overestimate of the distance, while the assumption of constant Hβ flux leads to distances having individual uncertainties of up to a factor of 3.The central star temperature determination is summarized. Individual central stars are placed on the HR diagram and compared with theoretical predictions. Deductions concerning the evolution which can be made from the observations are discussed.

scholarly journals Magnetic effects on solar p-modes

1988 ◽  
Vol 123 ◽  
pp. 161-165
Author(s):  
W R Campbell ◽  
B Roberts
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Lower Limit ◽  
Convection Zone ◽  
Magnetic Effects ◽  
Numerical Studies ◽  
Low Degree ◽  
The Magnetic Field

Both analytical and numerical studies of the effect of a magnetic field at the base of the convection zone on p-modes are presented. It is argued that the recently reported changes in the low degree p-mode frequencies, from 1980 to 1984, may result from corresponding changes in the magnetic field strength. A lower limit of some 5 × 105–106 gauss is implied for the field strength at the base of the convection zone.

scholarly journals Excitation Mechanisms of Oscillations in Stars

1993 ◽  
Vol 137 ◽  
pp. 512-520 ◽  
Author(s):  
Yoji Osaki
Keyword(s):  
Heavy Element ◽  
Convection Zone ◽  
Acoustic Power ◽  
Turbulent Convection ◽  
Noise Generation ◽  
Excitation Mechanism ◽  
Stochastic Excitation ◽  
Solar Oscillations ◽  
Low Degree ◽  
Excitation Mechanisms

AbstractExcitation mechanisms of oscillations in stars are discussed, in particular on the problems of the Beta Cephei pulsations and of the solar oscillations. A long-standing mystery about the excitation mechanism of the Beta Cephei pulsations seems to have finally been solved, which is due to availability of the new “OPAL” opacities. The very mechanism is the classical к-mechanism, however, due to enhanced heavy element opacities at temperature around 2 × 105K. By using the new opacities, three independent groups have found pulsational instabilities for the fundamental radial mode and a few low-degree non-radial modes in Beta Cephei models. Stochastic excitation of solar oscillations by turbulent convection is then discussed. It is shown that the noise generation by quadrupole radiation concentrated at the top of the convection zone may well explain the frequency dependence of the observed solar acoustic power pumped into individual p-modes.

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