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 Hot DQ white dwarfs: a pulsational test of the mixing scenario for their formation

2009 ◽  
Vol 5 (H15) ◽  
pp. 370-370
Author(s):  
A. Romero ◽  
A. H. Córsico ◽  
L. G. Althaus ◽  
E. García-Berro
Keyword(s):  
Stability Analysis ◽  
White Dwarf ◽  
White Dwarfs ◽  
Evolutionary Models ◽  
Instability Strip ◽  
Dwarf Stars ◽  
Photometric Variability ◽  
New Class ◽  
White Dwarf Stars ◽  
Born Again

Hot DQ white dwarfs constitute a new class of white dwarf stars, uncovered recently within the framework of SDSS project. There exist nine of them, out of a total of several thousands white dwarfs spectroscopically identified. Recently, three hot DQ white dwarfs have been reported to exhibit photometric variability with periods compatible with pulsation g-modes. In this contribution, we presented the results of a non-adiabatic pulsation analysis of the recently discovered carbon-rich hot DQ white dwarf stars. Our study relies on the full evolutionary models of hot DQ white dwarfs recently developed by Althaus et al. (2009), that consistently cover the whole evolution from the born-again stage to the white dwarf cooling track. Specifically, we performed a stability analysis on white dwarf models from stages before the blue edge of the DBV instability strip (Teff ≈ 30000 K) until the domain of the hot DQ white dwarfs (18000-24000 K), including the transition DB→hot DQ white dwarf. We explore evolutionary models with M*= 0.585M⊙ and M* = 0.87M⊙, and two values of thickness of the He-rich envelope (MHe = 2 × 10−7M* and MHe = 10−8M*).

scholarly journals A spectroscopic test of the rotational modulation origin of periodic Kepler photometric variability of A-type stars

2020 ◽  
Vol 498 (2) ◽  
pp. 2456-2471
Author(s):  
J Sikora ◽  
G A Wade ◽  
J Rowe
Keyword(s):  
Light Curves ◽  
Surface Structures ◽  
Inhomogeneous Surface ◽  
Photometric Variability ◽  
Spectroscopic Monitoring ◽  
Rotational Modulation ◽  
Kepler Mission ◽  
Low Mass ◽  
Monitoring Survey ◽  
Modulated Light

ABSTRACT High-precision space-based photometry obtained by the Kepler and TESS missions has revealed evidence of rotational modulation associated with main-sequence (MS) A- and late B-type stars. Generally, such variability in these objects is attributed to inhomogeneous surface structures (e.g. chemical spots), which are typically linked to strong magnetic fields ($B\gtrsim 100\, {\rm G}$) visible at the surface. It has been reported that ≈44 per cent of all A-type stars observed during the Kepler mission exhibit rotationally modulated light curves. This is surprising considering that ≲10 per cent of all MS A-type stars are known to be strongly magnetic (i.e. they are Ap/Bp stars). We present a spectroscopic monitoring survey of 44 A- and late B-type stars reported to exhibit rotational modulation in their Kepler light curves. The primary goal of this survey is to test the hypothesis that the variability is rotational modulation by comparing each star’s rotational broadening (vsin i) with the equatorial velocities (veq) inferred from the photometric periods. We searched for chemical peculiarities and binary companions in order to provide insight into the origin of the apparent rotational modulation. We find that 14 stars in our sample have vsin i > veq and/or have low-mass companions that may contribute to or be responsible for the observed variability. Our results suggest that more than 10 per cent of all MS A- and late B-type stars may exhibit inhomogeneous surface structures; however, the incidence rate is likely ≲30 per cent.

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 Optical detection of the rapidly spinning white dwarf in V1460 Her

2021 ◽  
Author(s):  
Ingrid Pelisoli ◽  
R T Marsh ◽  
R P Ashley ◽  
Pasi Hakala ◽  
A Aungwerojwit ◽  
...  
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Binary Systems ◽  
Hubble Space Telescope ◽  
Cataclysmic Variable ◽  
Optical Photometry ◽  
Arrival Times ◽  
Precise Ephemeris ◽  
History Of

Abstract Accreting magnetic white dwarfs offer an opportunity to understand the interplay between spin-up and spin-down torques in binary systems. Monitoring of the white dwarf spin may reveal whether the white dwarf spin is currently in a state of near-equilibrium, or of uni-directional evolution towards longer or shorter periods, reflecting the recent history of the system and providing constraints for evolutionary models. This makes the monitoring of the spin history of magnetic white dwarfs of high interest. In this paper we report the results of a campaign of follow-up optical photometry to detect and track the 39 sec white dwarf spin pulses recently discovered in Hubble Space Telescope data of the cataclysmic variable V1460 Her. We find the spin pulsations to be present in g-band photometry at a typical amplitude of 0.4 per cent. Under favourable observing conditions, the spin signal is detectable using 2-meter class telescopes. We measured pulse-arrival times for all our observations, which allowed us to derive a precise ephemeris for the white dwarf spin. We have also derived an orbital modulation correction that can be applied to the measurements. With our limited baseline of just over four years, we detect no evidence yet for spin-up or spin-down of the white dwarf, obtaining a lower limit of $|P/\dot{P}| > 4\times 10^{7}$ years, which is already 4 to 8 times longer than the timescales measured in two other cataclysmic variable systems containing rapidly rotating white dwarfs, AE Aqr and AR Sco.

scholarly journals Discovery of a young pre-intermediate polar

2021 ◽  
Author(s):  
David J Wilson ◽  
Odette Toloza ◽  
John D Landstreet ◽  
Boris T Gänsicke ◽  
Jeremy J Drake ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
White Dwarf ◽  
White Dwarfs ◽  
Hubble Space Telescope ◽  
Cataclysmic Variables ◽  
Rotation Period ◽  
Field Measurements ◽  
The Magnetic Field

Abstract We present the discovery of a magnetic field on the white dwarf component in the detached post common envelope binary (PCEB) CC Cet. Magnetic white dwarfs in detached PCEBs are extremely rare, in contrast to the high incidence of magnetism in single white dwarfs and cataclysmic variables. We find Zeeman-split absorption lines in both ultraviolet Hubble Space Telescope (HST) spectra and archival optical spectra of CC Cet. Model fits to the lines return a mean magnetic field strength of 〈|B|〉 ≈ 600–700 kG. Differences in the best-fit magnetic field strength between two separate HST observations and the high v sin  i of the lines indicate that the white dwarf is rotating with a period ∼0.5 hours, and that the magnetic field is not axisymmetric about the spin axis. The magnetic field strength and rotation period are consistent with those observed among the intermediate polar class of cataclysmic variable, and we compute stellar evolution models that predict CC Cet will evolve into an intermediate polar in 7–17 Gyr. Among the small number of known PCEBs containing a confirmed magnetic white dwarf, CC Cet is the hottest (and thus youngest), with the weakest field strength, and cannot have formed via the recently proposed crystallisation/spin-up scenario. In addition to the magnetic field measurements, we update the atmospheric parameters of the CC Cet white dwarf via model spectra fits to the HST data and provide a refined orbital period and ephemeris from TESS photometry.

scholarly journals White Dwarf Pulsars and Very Massive Compact Ultra Magnetized White Dwarfs

2017 ◽  
Vol 45 ◽  
pp. 1760024 ◽  
Author(s):  
Edson Otoniel ◽  
R. V. Lobato ◽  
M. Malheiro ◽  
Bruno Franzon ◽  
Stefan Schramm ◽  
...  
Keyword(s):  
Magnetic Field ◽  
White Dwarf ◽  
White Dwarfs ◽  
Carbon Ions ◽  
Poloidal Magnetic Field ◽  
Relativistic Approach ◽  
Type Ia ◽  
General Relativistic ◽  
Ia Supernovae ◽  
The Magnetic Field

In this work, we discuss white dwarf pulsars found recently making also reference of the possibility of some SGRs/AXPs being part of this class of pulsars. We also study the properties of very massive compact ultra magnetized white dwarfs that could be the progenitors candidates of super luminous type Ia supernovae, and also a previous stage of these white dwarf pulsars before the magnetic field decay.The structure of this ultra magnetized white dwarfs is obtained by solving the Einstein-Maxwell equations with a poloidal magnetic field in a fully general relativistic approach. The stellar interior is composed of a regular crystal lattice made of carbon ions immersed in a degenerate relativistic electron gas. We find that magnetized white dwarfs violate the standard Chandrasekhar mass limit significantly. We obtain a maximum white dwarf mass of around [Formula: see text] with an equatorial radius [Formula: see text] Km, a central magnetic field of [Formula: see text] G and [Formula: see text] G at the stellar surface.

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 Breaking the 100 MG Barrier: The First High Field Magnetic CV

1997 ◽  
Vol 163 ◽  
pp. 409-412
Author(s):  
Paula Szkody ◽  
Gary D. Schmidt ◽  
Paul S. Smith ◽  
Andrew Silber ◽  
D.W. Hoard ◽  
...  
Keyword(s):  
Magnetic Field ◽  
High Magnetic Field ◽  
White Dwarf ◽  
High Field ◽  
Optical Spectra ◽  
Cataclysmic Variable ◽  
High State ◽  
Optical Photometry ◽  
X Ray ◽  
Cyclotron Emission

AbstractWe present results from IUE and optical spectra, optical photometry and circular polarimetry during high and low states of the highly luminous soft X-ray cataclysmic variable AR UMa that identifies the primary in this system as a white dwarf with a magnetic field of 230 MG. The high magnetic field likely threads accretion blobs all the way from the secondary to below the surface of the white dwarf, resulting in a lack of polarised cyclotron emission and an extreme soft-X-ray luminosity during the high state.

scholarly journals MASS OF HIGHLY MAGNETIZED WHITE DWARFS EXCEEDING THE CHANDRASEKHAR LIMIT: AN ANALYTICAL VIEW

2012 ◽  
Vol 27 (15) ◽  
pp. 1250084 ◽  
Author(s):  
ARITRA KUNDU ◽  
BANIBRATA MUKHOPADHYAY
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Landau Level ◽  
White Dwarf ◽  
White Dwarfs ◽  
High Surface ◽  
Average Energy ◽  
The Stability ◽  
Chandrasekhar Limit ◽  
Very High

In recent years a number of white dwarfs have been observed with very high surface magnetic fields. We can expect that the magnetic field in the core of these stars would be much higher (~1014 G ). In this paper, we analytically study the effect of high magnetic field on relativistic cold electron, and hence its effect on the stability and the mass–radius relation of a magnetic white dwarf. In strong magnetic fields, the equation of state of the Fermi gas is modified and Landau quantization comes into play. For relatively very high magnetic fields (with respect to the average energy density of matter) the number of Landau levels is restricted to one or two. We analyze the equation of states for magnetized electron degenerate gas analytically and attempt to understand the conditions in which transitions from the zeroth Landau level to first Landau level occurs. We also find the effect of the strong magnetic field on the star collapsing to a white dwarf, and the mass–radius relation of the resulting star. We obtain an interesting theoretical result that it is possible to have white dwarfs with mass more than the mass set by Chandrasekhar limit.

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.

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