scholarly journals White Dwarfs as Observed at High Signal to Noise

1988 ◽  
Vol 132 ◽  
pp. 175-183
Author(s):  
Jesse L. Greenstein
Keyword(s):  
Angular Momentum ◽  
White Dwarf ◽  
White Dwarfs ◽  
Temperature Scale ◽  
Surface Composition ◽  
High Signal ◽  
Brute Force ◽  
Signal To Noise ◽  
Magnetic Stars ◽  
Balmer Decrement

The goal is largely historical, 30 years of instrumental progress in a difficult new field, faint white dwarfs, and some results. High signal-to-noise spectrophotometry at 40–160 Å resolution revealed the separation between hydrogen- and helium-rich atmospheres, and provided a temperature scale from models. The white-dwarf color-luminosity relation proved narrow. Their simple spectra made brute-force averaging possible to 14th magnitude at good photographic resolution. Features as shallow as 5% and 200 Å wide included C2, but in magnetic stars some strong absorptions remain unidentified. Metals are deficient, gravitational diffusion setting the surface composition. The Palomar double CCD spectrograph can now give S/N ≈ 100 to 17m. Some polarized white dwarfs have Zeeman triplets in magnetic fields near 20 megagauss. In one, Zeeman components are shifted up to 2000 Å at 300 megagauss. Rotation is small in all white dwarfs, angular momentum mostly lost. Non-LTE cores of Hα, Hβ exist and permit improved gravitational redshifts. An evolutionary phenomenon is progressive steepening of the Balmer decrement below 7000 K, cool atmospheres being helium-dominated.

scholarly journals A magnetic white dwarf with five H α components

2019 ◽  
Vol 489 (3) ◽  
pp. 3648-3654 ◽  
Author(s):  
Mukremin Kilic ◽  
B Rolland ◽  
P Bergeron ◽  
Z Vanderbosch ◽  
P Benni ◽  
...  
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
White Dwarfs ◽  
Surface Composition ◽  
Degenerate System ◽  
Inhomogeneous Surface ◽  
Optical Photometry ◽  
Photometric Variability ◽  
Rotational Modulation ◽  
Complex Magnetic Field

ABSTRACT G183−35 is an unusual white dwarf that shows an H α line split into five components, instead of the usual three components seen in strongly magnetic white dwarfs. Potential explanations for the unusual set of lines include a double degenerate system containing two magnetic white dwarfs and/or rotational modulation of a complex magnetic field structure. Here, we present time-resolved spectroscopy of G183−35 obtained at the Gemini Observatory. These data reveal two sets of absorption lines that appear and disappear over a period of about 4 h. We also detect low-level (0.2 per cent) variability in optical photometry at the same period. We demonstrate that the spectroscopic and photometric variability can be explained by the presence of spots on the surface of the white dwarf and a change in the average field strength from about 4.6 to 6.2 MG. The observed variability is clearly due to G183−35’s relatively short spin period. However, rotational modulation of a complex magnetic field by itself cannot explain the changes seen in the central H α component. An additional source of variability in the line profiles, most likely due to a chemically inhomogeneous surface composition, is also needed. We propose further observations of similar objects to test this scenario.

scholarly journals Masses and Magnetic Fields of White Dwarfs in Cataclysmic Variables

1989 ◽  
Vol 114 ◽  
pp. 337-340
Author(s):  
J.P. Lasota ◽  
J.M. Hameury ◽  
A.R. King
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Magnetic Fields ◽  
White Dwarf ◽  
White Dwarfs ◽  
Cataclysmic Variables ◽  
Strong Support ◽  
Drastic Reduction ◽  
Secondary Star ◽  
Maximum Magnetic Field

We show that the existence of the AM Her period spike implies (i) a unique white dwarf mass ≃ 0.6 − 0.7M⊙ for most magnetic CV’s (ii) nova explosions remove exactly the accreted mass from magnetic white dwarfs, and (iii) the maximum magnetic field for most CV’s is ≤ 4 × 107 G. The existence of the spike is very strong support for the idea that the period gap results from a drastic reduction of angular momentum losses when the secondary star becomes fully convective.

scholarly journals The Coalescence of White Dwarfs And Type I Supernovae

1989 ◽  
Vol 114 ◽  
pp. 515-518
Author(s):  
Robert Mochkovitch ◽  
Mario Livio
Keyword(s):  
Angular Momentum ◽  
White Dwarf ◽  
Total Mass ◽  
White Dwarfs ◽  
Thick Disk ◽  
Initial System ◽  
Type I ◽  
Low Density ◽  
Coalescence Model ◽  
Merging Process

AbstractIn the context of the white dwarf coalescence model for type la supernovae, we compute post-coalescence configurations involving a thick disk, rotating around a central white dwarf (the original primary), having the same total mass, angular momentum and energy as the initial system. We show that carbon ignition in rather low density material (105 – 10° g.cm−3) can be triggered during the merging process itself or later, by dissipation due to turbulence in the disk. The evolution of the object following carbon ignition is very uncertain.

scholarly journals Rotation and Cataclysmic Variables

2004 ◽  
Vol 215 ◽  
pp. 551-560 ◽  
Author(s):  
Sumner Starrfield ◽  
Edward M. Sion ◽  
Paula Szkody
Keyword(s):  
Angular Momentum ◽  
Accretion Disk ◽  
White Dwarf ◽  
White Dwarfs ◽  
Binary Star ◽  
Center Of Mass ◽  
Cataclysmic Variables ◽  
Lagrangian Point ◽  
The Past ◽  
The Subject

Cataclysmic Variables are binary star systems and so are closely connected to the subject of this meeting. The stars revolve around the center of mass of the system. The gas lost by the secondary through the inner Lagrangian point enters the Roche lobe of the white dwarf with the angular momentum of the L1 point and, therefore, forms an accretion disk which rotates around the white dwarf. The gas must lose angular momentum to fall onto the white dwarf, and the white dwarf itself must rotate as it accretes infalling material and angular momentum and is gradually spun up. We will review what is known about these phenomena, and emphasize the new results about the white dwarfs that have been learned in the past few years.

scholarly journals Hervy Element Abundances Predicted by Radiative Support Theory in the Atmospheres of Hot White Dwarfs

1989 ◽  
Vol 114 ◽  
pp. 253-257
Author(s):  
P. Chayer ◽  
G. Fontaine ◽  
F. Wesemael
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Surface Composition ◽  
Pure Hydrogen ◽  
Evolution Theory ◽  
Gravitational Settling ◽  
Element Abundances ◽  
Support Theory ◽  
Theoretical Investigations ◽  
Early Phases

The surface composition of a white dwarf evolves as a result of the interaction of several mechanisms, the most important of which being gravitational settling. In the early phases of the evolution, theory shows that selective radiative levitation can occasionally defeat settling and, thus, prevent the formation of a pristine pure hydrogen (helium) atmospheric layer in a hot DA (non-DA) white dwarf (Fontaine and Michaud 1979; Vauclair, Vauclair, and Greenstein 1979). The exciting discovery of sharp metallic features in the ultraviolet spectra of several hot DA and non-DA stars alike resulting from the work of several investigators has provided the essential motivation for further theoretical investigations of radiative levitation in the atmospheres of white dwarfs. Additionnal impetus comes from the continuing investigations of hot DA white dwarfs carried out by Bruhweiler and Kondo which have already revealed a most interesting observational pattern of heavy elements in these stars (Bruhweiler 1985). Moreover the recent availability of theoretical equivalent widths of selected astrophysically important ultraviolet metal lines in hot DA white dwarfs (Henry, Shipman, and Wesemael 1985) makes a comparison between theory and observations -in at least this type of stars- a timely and useful exercise.

scholarly journals Gravitational waves from dynamical tides in white dwarf binaries

2019 ◽  
Author(s):  
L O McNeill ◽  
R A Mardling ◽  
B Müller
Keyword(s):  
Angular Momentum ◽  
Gravitational Wave ◽  
Gravitational Waves ◽  
Frequency Band ◽  
White Dwarf ◽  
White Dwarfs ◽  
Momentum Exchange ◽  
Tidal Dissipation ◽  
Tidal Forcing ◽  
Tidal Torques

Abstract We study the effect of tidal forcing on gravitational wave signals from tidally relaxed white dwarf pairs in the LISA, DECIGO and BBO frequency band (0.1 − 100 mHz). We show that for stars not in hydrostatic equilibrium (in their own rotating frames), tidal forcing will result in energy and angular momentum exchange between the orbit and the stars, thereby deforming the orbit and producing gravitational wave power in harmonics not excited in perfectly circular synchronous binaries. This effect is not present in the usual orbit-averaged treatment of the equilibrium tide, and is analogous to transit timing variations in multiplanet systems. It should be present for all LISA white dwarf pairs since gravitational waves carry away angular momentum faster than tidal torques can act to synchronize the spins, and when mass transfer occurs as it does for at least eight LISA verification binaries. With the strain amplitudes of the excited harmonics depending directly on the density profiles of the stars, gravitational wave astronomy offers the possibility of studying the internal structure of white dwarfs, complimenting information obtained from asteroseismology of pulsating white dwarfs. Since the vast majority of white-dwarf pairs in this frequency band are expected to be in the quasi-circular state, we focus here on these binaries, providing general analytic expressions for the dependence of the induced eccentricity and strain amplitudes on the stellar apsidal motion constants and their radius and mass ratios. Tidal dissipation and gravitation wave damping will affect the results presented here and will be considered elsewhere.

scholarly journals White Dwarf Rotation: Observations and Theory

2004 ◽  
Vol 215 ◽  
pp. 561-570 ◽  
Author(s):  
Steven D. Kawaler
Keyword(s):  
Angular Momentum ◽  
White Dwarf ◽  
White Dwarfs ◽  
Main Sequence ◽  
Observational Constraints ◽  
Agb Stars ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
The Many ◽  
Limiting Case

White dwarfs rotate. The angular momentum in single white dwarfs must originate early in the life of the star, but also must be modified (and perhaps severely modified) during the many stages of evolution between birth as a main–sequence star and final appearance as a white dwarf. Observational constraints on the rotation of single white dwarf stars come from traditional spectroscopy and from asteroseismology, with the latter providing hints of angular velocity with depth. Results of these observational determinations, that white dwarfs rotate with periods ranging from hours to days (or longer), tells us that the processes by which angular momentum is deposited and/or drained from the cores of AGB stars are complex. Still, one can place strong limits on these processes by considering relatively simple limiting cases for angular momentum evolution in prior stages, and on subsequent angular momentum evolution in the white dwarfs. These limiting-case constraints will be reviewed in the context of the available observations.

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