scholarly journals White Dwarf Evolution in Real Time: What Pulsating White Dwarfs Teach us About Stellar Evolution

1989 ◽  
Vol 114 ◽  
pp. 97-108 ◽  
Author(s):  
Steven D. Kawaler ◽  
Carl J. Hansen
Keyword(s):  
Real Time ◽  
Structural Properties ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Recent Progress ◽  
Short Review ◽  
Observational Constraints ◽  
Made In

The variable white dwarfs repeatedly force theory to conform to their observed properties so that further progress can be made in understanding the structure and evolution of all white dwarfs. We use the term “understanding” in a loose sense here because, as we will show, both observational constraints and interpretation of the observations vis-à-vis theory contribute to uncertainties in our understanding at this time. In any case, recent progress in this field (sometimes called white dwarf seismology) has provided some fascinating insights into the evolutionary and structural properties of white dwarfs and their progenitors. This short review is our attempt to describe recent progress made in the interaction of theory with observations.

scholarly journals Metal Abundances in Cool White Dwarfs and the Diffusion Theory

1979 ◽  
Vol 53 ◽  
pp. 165-178
Author(s):  
Gérard Vauclair
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Convection Zone ◽  
Diffusion Theory ◽  
Short Review ◽  
Convective Mixing ◽  
Substantial Progress ◽  
Metal Abundances ◽  
Cool White Dwarfs ◽  
Made In

In this theoretical review about cool white dwarfs, I will restrict myself to the problem of the metallic content in white dwarf outer layers. The first section will be a short review of what we know about the metal abundances. The hottest presently known white dwarf showing metal in its spectrum is the DB GD 40 (Te = 15 000 K). This temperature will be considered here as the hot boundary of the “cool” white dwarfs. Many efforts have been recently devoted to the understanding of these metal abundances. Section 2 will be a summary of recent calculations of diffusion time scales in both hydrogen and helium white dwarfs. It will be seen that diffusion is so efficient in white dwarf conditions that the convection zone which develops in the envelope as the effective temperature decreases along the cooling sequence is never deep enough to bring back to the surface the metals which had previously diffused downwards. A discussion of the carbon white dwarfs, also called λ 4670 stars, will be presented in section 3. Recent calculations show that the convective mixing between a helium envelope and a carbon core would produce λ 4670 composition for only very special conditions and for this reason we believe that this is an improbable explanation for this type of white dwarfs. We clearly need another physical mechanism to compete with diffusion and to maintain an observable amount of metals in some cool white dwarf atmospheres. We discuss in section 4 the competition between diffusion and accretion. This seems a very promising mechanism in spite of the fact that considerable improvements are still needed in the theory of accretion. Substantial progress has to be made in this direction. A few problems related to this model are invoked in the conclusion.

A catalog of 159,238 white dwarf ages

2019 ◽  
Vol 15 (S357) ◽  
pp. 188-191
Author(s):  
Ted von Hippel ◽  
Adam Moss ◽  
Isabelle Kloc ◽  
Natalie Moticska ◽  
Jimmy Sargent ◽  
...  
Keyword(s):  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Stellar Mass ◽  
Posterior Distributions ◽  
Trigonometric Parallaxes ◽  
On Line ◽  
Better Than

AbstractWe employ Pan-STARRS photometry, Gaia trigonometric parallaxes, modern stellar evolution and atmosphere models, and our Bayesian fitting approach to determine cooling and total ages for 159,238 white dwarfs. In many cases we are able to derive precise ages (better than 5%) for individual white dwarfs. These results are meant for broad use within the white dwarf and stellar astrophysics communities and we plan to make available on-line the posterior distributions for cooling age, total age, initial stellar mass, and other parameters.

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.

scholarly journals Recent progress on detecting understanding and controlling Pseudomonas syringae pv actinidiae a short review

2013 ◽  
Vol 66 ◽  
pp. 170-177 ◽  
Author(s):  
J.L. Vanneste
Keyword(s):  
Pseudomonas Syringae ◽  
Recent Progress ◽  
Short Review ◽  
Economic Importance ◽  
Whole Genome Sequence ◽  
Bacterial Canker ◽  
New Methods ◽  
The World ◽  
Epiphytic Survival ◽  
Made In

In the last few years the causal agent of bacterial canker of kiwifruit Pseudomonas syringae pv actinidiae (Psa) has become a global pathogen of economic importance Since the beginning of this global outbreak many laboratories in the world have been working on Psa Today it is known that Psa is not a homogeneous pathovar and tools that allow the distinction between biovars (subpathovar classification) have been developed The whole genome sequence of several strains of Psa has now been published Some of the assumptions on the life cycle (ports of entry epiphytic survival etc) made in the early days of the outbreak have now been confirmed Although few new methods have been found to control Psa there is now a better understanding of how to reduce the incidence of this disease This paper reviews the progress made in understanding the pathogen the disease and how to control it

scholarly journals A CHEOPS white dwarf transit search

2021 ◽  
Vol 651 ◽  
pp. L12
Author(s):  
Brett M. Morris ◽  
Kevin Heng ◽  
Alexis Brandeker ◽  
Andrew Swan ◽  
Monika Lendl
Keyword(s):  
Stellar Evolution ◽  
Point Spread Function ◽  
White Dwarf ◽  
Metal Pollution ◽  
White Dwarfs ◽  
Point Spread ◽  
Spread Function ◽  
Data Products ◽  
Red Giant

White dwarf spectroscopy shows that nearly half of white dwarf atmospheres contain metals that must have been accreted from planetary material that survived the red giant phases of stellar evolution. We can use metal pollution in white dwarf atmospheres as flags, signalling recent accretion, in order to prioritize an efficient sample of white dwarfs to search for transiting material. We present a search for planetesimals orbiting six nearby white dwarfs with the CHaracterising ExOPlanet Satellite (CHEOPS). The targets are relatively faint for CHEOPS, 11 mag < G < 12.8 mag. We used aperture photometry data products from the CHEOPS mission as well as custom point-spread function photometry to search for periodic variations in flux due to transiting planetesimals. We detect no significant variations in flux that cannot be attributed to spacecraft systematics, despite reaching a photometric precision of < 2 ppt in 60 s exposures on each target. We simulate observations to show that the small survey is sensitive primarily to Moon-sized transiting objects with periods between 3 h < P < 10 h, with radii of R ≳ 1000 km.

scholarly journals Carbon in the Envelopes of White Dwarfs

1979 ◽  
Vol 53 ◽  
pp. 188-191
Author(s):  
Francesca D’Antona
Keyword(s):  
Mass Loss ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Current Theory ◽  
Core Mass ◽  
Maximum Extension ◽  
Nuclear Burning ◽  
Remnant Mass

Current theory of stellar evolution predicts that stars of initial masses up to 4-6 M⊙ evolve into Carbon-Oxygen White Dwarfs surrounded by a Helium envelope and, possibly, by a Hydrogen envelope. It also predicts that the mass of the Helium envelope which remains on the star at the end of its double shell burning evolution is a function of the Carbon-Oxygen core mass (Paczynski 1975). It can be shown that this mass can be reduced – but only slightly – during the following evolution of the star towards the White Dwarf region, either by nuclear burning or by mass loss (D’Antona and Mazzitelli 1979). During the White Dwarf stage, Helium convection grows into White Dwarfs having Helium atmospheres. The maximum extension of Helium convective mass is a function of the mass of the star (Fontaine and Van Horn 1976; D’Antona and Mazzitelli 1975,1979). It turns out that the Helium envelope remnant mass is always at least three orders of magnitude larger than the maximum Helium convective mass, whatever the mass of the star may be. This statement is unlikely to be changed by refinements either in the theory of double shell burning or in the theory of White Dwarf envelope convection.

scholarly journals The effects of boundary conditions on stellar evolution

1987 ◽  
Vol 122 ◽  
pp. 463-464
Author(s):  
Amos Harpaz ◽  
Attay Kovetz ◽  
Giora Shaviv
Keyword(s):  
Boundary Conditions ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Surface Boundary ◽  
Surface Boundary Conditions

The effects of using different treatments of the surface boundary conditions are investigated in the context of the mass of He White Dwarfs. We find that since the White Dwarf progenitor is a star with a very extended atmosphere, the results are sensitive to the degree of accuracy implemented in the handling of the boundary conditions.

scholarly journals Mass Distribution and Luminosity Function of White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 1-14 ◽  
Author(s):  
Volker Weidemann ◽  
Jie W. Yuan
Keyword(s):  
Mass Loss ◽  
Mass Distribution ◽  
Stellar Evolution ◽  
White Dwarf ◽  
White Dwarfs ◽  
Luminosity Function ◽  
Present Situation ◽  
Single Well ◽  
Mass Dispersion

Ever since Graham’s Strömgren photometry (1972) demonstrated the existence of a single well defined cooling sequence of DA white dwarfs the question of the mass dispersion (or the width of the number-mass distribution) has been in the foreground of my studies (Weidemann, 1970, 1977).Indeed it turned out that the shape of the white dwarf mass distribution provides strong constraints on the theory of stellar evolution with mass loss, a fact which will be demonstrated again in the following lecture. It therefore seems worthwhile to dwell in some detail on the methods of its determination. For the benefit of the non-specialists I shall first present some of the historical results and then continue to discuss the present situation.

scholarly journals Column Accretion on to White Dwarfs

1983 ◽  
Vol 72 ◽  
pp. 181-197
Author(s):  
A. R. King
Keyword(s):  
Electron Energy ◽  
White Dwarf ◽  
White Dwarfs ◽  
Recent Progress ◽  
Energy Transport ◽  
X Ray

The problem of column accretion on to white dwarfs in the AM Her and DQ Her systems is reviewed. Particular attention is paid to recent progress in explaining the large soft X-ray - EUV fluxes observed in these systems in terms of nonlocal electron energy transport into the white dwarf photosphere.

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