scholarly journals White Dwarf Seismology: Inverse Problem of g-Mode Oscillations

1988 ◽  
Vol 108 ◽  
pp. 86-87
Author(s):  
Hiromoto Shibahashi ◽  
Takashi Sekii ◽  
Steven Kawaler
Keyword(s):  
Inverse Problem ◽  
Internal Structure ◽  
White Dwarf ◽  
White Dwarfs ◽  
Research Field ◽  
The Sun ◽  
Pulsating Stars ◽  
Low Degree ◽  
Physical Quantities ◽  
Light Variability

Since light variability in white dwarfs was first discovered twenty years ago, eighteen DA white dwarfs, several pulsating DB white dwarfs, and hotter pre-white dwarfs have so far been found to be pulsating variables. The most conspicuous characteristics of pulsations in these stars are that they seem to consist of multiple g-modes of nonradial oscillations. Attention should be paid to multiplicity of modes. Stimulated by the success of helioseimology, a research field called ‘asteroseismology’, in which we may probe the internal structure of stars by means of observations of their oscillations, is going to develop. How well such a seismological approach succeeds is dependent on how many modes are observed in each of stars. Since the number of modes of an individual pulsating white dwarf is larger than those of other types of pulsating stars but for the Sun, the seismological study may be the most promising as to the white dwarfs. In fact, by applying the asymptotic relations among eigenfrequencies of high order g-modes with low degree, the degreel, and the radial ordern, Kawaler(1987a,b,c) succeeded to get some constraints on the physical quantities of some of pulsating white dwarfs.

scholarly journals White Dwarf Seismology at the Université de Montréal

1993 ◽  
Vol 139 ◽  
pp. 120-120
Author(s):  
G. Fontaine ◽  
P. Brassard ◽  
P. Bergeron ◽  
F. Wesemael
Keyword(s):  
Specific Heat ◽  
Cooling Rate ◽  
Internal Structure ◽  
White Dwarf ◽  
White Dwarfs ◽  
Period Change ◽  
Core Material ◽  
The Core ◽  
Core Composition ◽  
Chemical Stratification

Over the last several years, we have developed a comprehensive program aimed at better understanding the properties of pulsating DA white dwarfs (or ZZ Ceti stars). These stars are nonradial pulsators of the g-type, and their study can lead to inferences about their internal structure. For instance, the period spectrum of a white dwarf is most sensitive to its vertical chemical stratification, and one of the major goals of white dwarf seismology is to determine the thickness of the hydrogen layer that sits on top of a star. This can be done, in principle, by comparing in detail theoretical period spectra with the periods of the observed excited modes. Likewise, because the cooling rate of a white dwarf is very sensitive to the specific heat of its core material (and hence to its composition), it is possible to infer the core composition through measurements and interpretations of rates of period change in a pulsator.

scholarly journals The internal structure of ZZ Cet stars using quantitative asteroseismology: The case of R548

2013 ◽  
Vol 9 (S301) ◽  
pp. 285-288
Author(s):  
N. Giammichele ◽  
G. Fontaine ◽  
P. Brassard ◽  
S. Charpinet
Keyword(s):  
Internal Structure ◽  
White Dwarf ◽  
White Dwarfs ◽  
Optimization Technique ◽  
Seismic Model ◽  
The Core ◽  
Spectroscopic Analyses ◽  
Core Composition ◽  
Oscillation Modes ◽  
Chemical Stratification

AbstractWe explore quantitatively the low but sufficient sensitivity of oscillation modes to probe both the core composition and the details of the chemical stratification of pulsating white dwarfs. Until recently, applications of asteroseismic methods to pulsating white dwarfs have been far and few, and have generally suffered from an insufficient exploration of parameter space. To remedy this situation, we apply to white dwarfs the same double-optimization technique that has been used quite successfully in the context of pulsating hot B subdwarfs. Based on the frequency spectrum of the pulsating white dwarf R548, we are able to unravel in a robust way the unique onion-like stratification and the chemical composition of the star. Independent confirmations from both spectroscopic analyses and detailed evolutionary calculations including diffusion provide crucial consistency checks and add to the credibility of the inferred seismic model. More importantly, these results boost our confidence in the reliability of the forward method for sounding white dwarf internal structure with asteroseismology.

scholarly journals Nonradial and radial oscillations observed in non-emission line OB dwarfs and giants

1986 ◽  
Vol 7 ◽  
pp. 255-263
Author(s):  
Dietrich Baade
Keyword(s):  
Mass Loss ◽  
Emission Line ◽  
Internal Structure ◽  
Fundamental Problem ◽  
Spectral Lines ◽  
Driving Mechanism ◽  
The Sun ◽  
Ob Stars ◽  
Pulsating Stars ◽  
Reconnaissance Surveys

Only a decade ago, this talk could have concerned only the β Cephei stars which however populate a much more precisely defined strip in the Hertzsprung-Russel diagram (MED). But recent reconnaissance surveys (Smith 1977; Smith and Penrod 1984; Waelkens and Rufener 1985; Baade, in preparation) show that perhaps only one, if any, sizeable region of the upper HRD is devoid of nonradially pulsating stars. The identification of the driving mechanism is still pending (cf. the parallel talk by Osaki), and apparently our knowledge about the internal structure of OB stars is incomplete. But, turning that argument around, it also is indicative of how much may be learned about OB stars from and through the solution of that fundamental problem. This seismologial potential, the ubiquity of the phenomenon, and the effect, as suggested by recent observations of some stars, of the pulsations on the mass loss of OB stars make the oscillations of OB stars one of the most important problems of current astrophysics. On the observational side, rotationally broadened spectral lines, large amplitudes, comparatively long periods, and high luminosities permit information to be gathered which otherwise is accessible only for the sun.

scholarly journals Inverse problem: acoustic potential vs acoustic length

1988 ◽  
Vol 123 ◽  
pp. 133-136
Author(s):  
Hiromoto Shibahashi
Keyword(s):  
Inverse Problem ◽  
Integral Equation ◽  
Sound Velocity ◽  
Internal Structure ◽  
Asymptotic Method ◽  
The Sun ◽  
Quantization Rule ◽  
Rule Based ◽  
Deep Interior

By using the quantization rule based on the WKB asymptotic method, we present an integral equation to infer the form of the acoustic potential of a fixed ℓ as a function of the acoustic length. Since we analyze the acoustic potential itself by taking account of some factors other than the sound velocity and we can analyze the radial modes by this scheme as well as nonradial modes, this method improves the accuracy and effectiveness of the inverse problem to infer the internal structure of the Sun, in particular, the deep interior of the Sun.

scholarly journals An Example Demonstrating the Potential for Asteroseismology of DB White Dwarf Stars

1993 ◽  
Vol 139 ◽  
pp. 116-116
Author(s):  
P.A. Bradley ◽  
M.A. Wood
Keyword(s):  
Internal Structure ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Galactic Disk ◽  
Helium Surface ◽  
Dwarf Stars ◽  
University Of Texas ◽  
White Dwarf Stars ◽  
History Of

AbstractWe present the results of a parametric survey of evolutionary models of compositionally stratified white dwarfs with helium surface layers (DB white dwarfs). Because white dwarfs are the most common final end state of stellar evolution, determining their internal structure will offer us many clues about stellar evolution, the physics of matter under extreme conditions, plus the history of star formation and age of the local Galactic disk. As a first step towards determining the internal structure of DB white dwarf stars, we provide a comprehensive set of theoretical g-mode pulsation periods for comparison to observations.Because DB white dwarfs have a layered structure consisting of a helium layer overlying the carbon/oxygen core, some modes will have the same wavelength as the thickness of the helium layer, allowing a resonance to form. This resonance is called mode trapping (see Brassard et al. 1992 and references therein) and has directly observable consequences, because modes at or near the resonance have eigenfunctions and pulsation periods that are similar to each other. This results in much smaller period spacings between consecutive overtone modes of the same spherical harmonic index than the uniform period spacings seen between non-trapped modes. We demonstrate with an example how one can use the distribution of pulsation periods to determine the total stellar mass, the mass of the helium surface layer, and the extent of the helium/carbon and carbon/oxygen transition zones. With these tools, we have the prospect of being able to determine the structure of the observed DBV white dwarfs, once the requisite observations become available.We are grateful to C.J. Hansen, S.D. Kawaler, R.E. Nather, and D.E. Winget for their encouragement and many discussions. This research was supported by the National Science Foundation under grants 85-52457 and 90-14655 through the University of Texas and McDonald Observatory.

Diamonds in the Sky!

2019 ◽  
pp. 101-109
Author(s):  
Nicholas Mee
Keyword(s):  
Nuclear Fuel ◽  
White Dwarf ◽  
White Dwarfs ◽  
Star Trek ◽  
The Sun ◽  
New Class ◽  
The Earth ◽  
Night Sky

After consuming their nuclear fuel, most stars lose their outer envelopes and all that remains is the collapsed core of the star, an object known as a white dwarf. Ever since Galileo pointed a telescope at the night sky, each advance in telescope making has resulted in sensational discoveries. Alvan Clark & Sons ground some of the biggest telescope lenses ever made. Alvan Graham Clark discovered Sirius B while testing one of these lenses. Eddington deduced that Sirius B has a size similar to that of the Earth, but with the mass of the Sun, and was an example of a new class of stars—white dwarfs. The easiest white dwarf to see with a telescope orbits the star Keid. In Star Trek, the planet Vulcan orbits the star Keid A.

Seismic Inversions for White Dwarf Stars

2002 ◽  
Vol 185 ◽  
pp. 606-607
Author(s):  
M. Takata ◽  
M.H. Montgomery
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
The Sun ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
Inversion Methods ◽  
Space Observations

AbstractInversion methods have been used successfully for the Sun. The stars with the next richest set of observed frequencies are the white dwarfs. We consider here the viability of numerical inversions for these stars. We find that, while the number of presently observed modes in the white dwarf GD 358 is too small for structural inversions, such inversions would be possible if the frequencies of all modes with 1 ≤ l ≤ 3 were observed. This is possible for space observations by e.g. Eddington.

scholarly journals Pulsations in white dwarf stars

2013 ◽  
Vol 9 (S301) ◽  
pp. 273-280
Author(s):  
G. Fontaine ◽  
P. Bergeron ◽  
P. Brassard ◽  
S. Charpinet ◽  
P. Dufour ◽  
...  
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
Low Degree

AbstractWe first present a brief description of the six distinct families of pulsating white dwarfs that are now known. These are all opacity-driven pulsators showing low- to mid-order, low-degree gravity modes. We then discuss some recent highlights that have come up in the field of white dwarf asteroseismology.

scholarly journals On the Structure of Pre-White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 62-65
Author(s):  
D. Schönberner ◽  
R. Tylenda
Keyword(s):  
Internal Structure ◽  
White Dwarf ◽  
Planetary Nebula ◽  
White Dwarfs ◽  
Asymptotic Giant Branch ◽  
Horizontal Branch ◽  
The Creation ◽  
Pn Stage

White dwarfs (WD) are the final configurations of all stars up to initial masses between 5 and 9 M⊙. Two feeder channels for the creation of single WDs can be distinguished: Either evolution through the asymptotic giant branch (AGB) and the following planetary-nebula (PN) phase, or evolution from the horizontal branch through the hot subdwarf region. Prelimary estimates by Drilling and Schönberner (1985) and Heber (1986) indicate that the creation of WDs via the horizontal–branch channel is rather insignificant (few percent of the total WD birthrate) and can be neglected. Thus the evolution through the AGB determines the internal structure of single WDs, and the study of the PN stage serves to elucidate the inital conditions for the white-dwarf evolution.

scholarly journals Structure and Composition of White Dwarf Atmospheres and Convection Zones

1979 ◽  
Vol 53 ◽  
pp. 223-244
Author(s):  
K.H. Böhm
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Energy Transport ◽  
Practical Interest ◽  
Surface Flux ◽  
Surface Fluxes ◽  
The Sun ◽  
Surface Cooling ◽  
Additional Energy ◽  
Basic Properties

SummaryWe present a brief review of the basic properties of white dwarf atmospheres) convection zones and corona models emphasizing qualitative and intuitive aspects.1. Atmospheres: We restrict our discussion essentially to very hot and very cool atmospheres since these are especially interesting. With regard to the first type of objects we study the fundamental differences between DA and non-DA models and between their surface fluxes. We discuss the important role of electron scattering in determining the EUV spectra of these objects. The differences between DAs and non-DAs with regard to backwarming effect and surface cooling are summarized.In our discussion of very cool non-DA atmospheres we emphasize the importance of the additional energy transport mechanisms convection and conduction which should both be very effective for Teff < 4000K and which lead to a very flat temperature gradient. This small gradient must lead to a rather featureless surface flux.2. Convection Zones. After a survey of the basic numerical results in this field we investigate the question whether convection in white dwarfs has the same basic properties as convection in other stars. We find that contrary to intuitive expectations the Rayleigh number in very cool non-DAs is higher than in the sun (indicating very turbulent convection). The Prandtl number in these objects is 6 to 7 orders of magnitude higher than in the sun.3. Coronae. The basic methods of the calculation of coronae for white dwarfs are very briefly discussed. We present some results for DA and non-DA stars from unpublished work by D.O. Muchmore and the author. It uses revised values for the emissivities. Only non-DA coronae are of practical interest. DA coronae have much lower densities and temperatures. White dwarf coronae do not generate a stellar wind.

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