An Isolated White Dwarf with a 70 s Spin Period

Abstract The photospheric metal pollution of white dwarfs is now well-established as the signature of the accretion of planetary debris. However, the origin of the trace hydrogen detected in many white dwarfs with helium atmospheres is still debated. Here, we report the analysis of GD 424: a metal-polluted, helium-atmosphere white dwarf with a large amount of trace hydrogen. We determined the atmospheric parameters using a hybrid analysis that combines the sensitivity of spectroscopy to the atmospheric composition, log (H/He), with that of photometry and astrometry to the effective temperature, Teff, and surface gravity, log g. The resulting white dwarf mass, radius, and cooling age are ${M_{\rm{WD}}}=0.77\pm 0.01\, {\rm{M}_{\odot}}$, ${R_{\rm{WD}}}=0.0109\pm 0.0001\, {\rm{R}_{\odot}}$, and τcool = 215 ± 10 Myr, respectively. We identified and measured the abundances of 11 photospheric metals and argue that the accretion event is most likely either in the increasing or steady state, and that the disrupted planetesimal resembles either CI chondrites or the bulk Earth in terms of its composition. We suggest that the observed 1.33 × 1022 g of trace hydrogen in GD 424 were at least partly acquired through accretion of water-rich planetary debris in an earlier accretion episode.

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TESS first look at evolved compact pulsators

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936340 ◽

2019 ◽

Vol 632 ◽

pp. A42 ◽

Cited By ~ 3

Author(s):

Keaton J. Bell ◽

Alejandro H. Córsico ◽

Agnès Bischoff-Kim ◽

Leandro G. Althaus ◽

Paul A. Bradley ◽

...

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

Compact Objects ◽

Stellar Structure ◽

Evolutionary Stage ◽

Pulsation Frequency ◽

Instability Strip ◽

Dwarf Stars ◽

Helium Atmosphere ◽

White Dwarf Stars

Context. Pulsation frequencies reveal the interior structures of white dwarf stars, shedding light on the properties of these compact objects that represent the final evolutionary stage of most stars. Two-minute cadence photometry from the Transiting Exoplanet Survey Satellite (TESS) records pulsation signatures from bright white dwarfs over the entire sky. Aims. As part of a series of first-light papers from TESS Asteroseismic Science Consortium Working Group 8, we aim to demonstrate the sensitivity of TESS data, by measuring pulsations of helium-atmosphere white dwarfs in the DBV instability strip, and what asteroseismic analysis of these measurements can reveal about their stellar structures. We present a case study of the pulsating DBV WD 0158−160 that was observed as TIC 257459955 with the two-minute cadence for 20.3 days in TESS Sector 3. Methods. We measured the frequencies of variability of TIC 257459955 with an iterative periodogram and prewhitening procedure. The measured frequencies were compared to calculations from two sets of white dwarf models to constrain the stellar parameters: the fully evolutionary models from LPCODE and the structural models from WDEC. Results. We detected and measured the frequencies of nine pulsation modes and eleven combination frequencies of WD 0158−160 to ∼0.01 μHz precision. Most, if not all, of the observed pulsations belong to an incomplete sequence of dipole (ℓ = 1) modes with a mean period spacing of 38.1 ± 1.0 s. The global best-fit seismic models from both LPCODE and WDEC have effective temperatures that are ≳3000 K hotter than archival spectroscopic values of 24 100–25 500 K; however, cooler secondary solutions are found that are consistent with both the spectroscopic effective temperature and distance constraints from Gaia astrometry. Conclusions. Our results demonstrate the value of the TESS data for DBV white dwarf asteroseismology. The extent of the short-cadence photometry enables reliably accurate and extremely precise pulsation frequency measurements. Similar subsets of both the LPCODE and WDEC models show good agreement with these measurements, supporting that the asteroseismic interpretation of DBV observations from TESS is not dominated by the set of models used. However, given the sensitivity of the observed set of pulsation modes to the stellar structure, external constraints from spectroscopy and/or astrometry are needed to identify the best seismic solutions.

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

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310009877 ◽

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*).

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Observation of a Variable, ZZ Ceti White Dwarf: GD154

International Astronomical Union Colloquium ◽

10.1017/s0252921100014196 ◽

1993 ◽

Vol 134 ◽

pp. 201-204

Author(s):

B. Pfeiffer ◽

G. Vauclair ◽

N. Dolez ◽

M. Chevreton ◽

J. R. Fremy ◽

...

Keyword(s):

Time Scales ◽

Effective Temperature ◽

White Dwarf ◽

White Dwarfs ◽

Nonlinear Phenomena ◽

Instability Strip ◽

Temperature Range

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

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Towards Ensemble Asteroseismology of the Pulsating DB White Dwarf Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100017231 ◽

2002 ◽

Vol 185 ◽

pp. 608-609

Author(s):

G. Handler ◽

M.A. Wood ◽

A. Nitta ◽

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

The Other ◽

Archival Data ◽

Dwarf Stars ◽

Helium Atmosphere ◽

Individual Stars ◽

White Dwarf Stars ◽

Binary Evolution ◽

Insight Into

The origin of the helium-atmosphere DB white dwarfs is still a matter of debate. In particular, the question is unresolved whether binary evolution produces a significant number of DBs. The pulsating DB white dwarfs (DBV stars) offer a complementary insight into this problem through asteroseismology; DBs descending from binaries will have different interior structures than DBs originating from single stars (Nitta & Winget, 1998).GD 358 is by far the best-observed pulsating DBV star, and the only one for which asteroseismology has been performed to date. This star’s structure has been shown to be inconsistent with an origin from binary evolution (Nitta & Winget, 1998), but most of the other DBVs are relatively poorly studied.We therefore analysed archival data on all DBVs and obtained new measurements of stars with very little data available (Table 1), firstly to identify suitable targets for asteroseismological investigations and secondly to examine the pulsation spectra of the DBVs as a group, following the works of Clemens (1994) and Kleinman (1995) on the pulsating DA white dwarfs. Our study also produced new seismological results on individual stars and promising targets for future Whole Earth Telescope (WET, Nather et al., 1990) runs.

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Asteroseismology of GD358 with complex core carbon and oxygen profiles

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316005950 ◽

2015 ◽

Vol 11 (A29B) ◽

pp. 489-492

Author(s):

Agnès Bischoff-Kim ◽

Judith L. Provencal

Keyword(s):

White Dwarf ◽

White Dwarfs ◽

Complete Data ◽

Time Dependent ◽

Archival Data ◽

Data Set ◽

Helium Atmosphere ◽

Complex Core ◽

Pure Helium ◽

Oxygen Profiles

AbstractGD 358 is the brightest (mv=13.7) and best studied helium atmosphere white dwarf pulsator. We present an analysis based on over 1000 hours of observations spanning 2007-2014 as well as archival data going back to 1982. From the complete data set, we identify a total of 27 independent frequencies and fit 14 of them as m=0 modes in our asteroseismic analysis. We add GD358 to a set of helium atmosphere white dwarfs fitted with similar models. With this consistent set, we can see a trend in the thickness of the pure helium layer that are quantitatively consistent with time-dependent diffusion calculations.

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CCD Time-Series Photometry of White Dwarf Stars

10.26686/wgtn.17007772.v1 ◽

2021 ◽

Author(s):

◽

Paul Robin Brian Chote

Keyword(s):

Data Reduction ◽

White Dwarf ◽

High Speed ◽

Energy Transport ◽

Hydrogen Atmosphere ◽

Rate Of Change ◽

Ccd Photometry ◽

Helium Atmosphere ◽

Convection Layer ◽

Radial Pulsations

<p>This thesis describes a practical programme that focused on CCD photometry of pulsating white dwarf (WD) stars. The first part of this thesis describes the development of two high-speed CCD photometer instruments and their data reduction pipeline, while the remainder describes the observation and analysis of several pulsating WDs and other targets. The two photometers (Puoko-nui North and South) share a common hardware design that is optimized for acquiring efficient photometry with integration periods of milliseconds through to minutes. The design integrates a commercial CCD (Charge-Coupled Device) camera and GPS (Global Positioning System) receiver with custom timing electronics and control software. The reduction and visualization software developed for these instruments provide detailed real-time information to the observer, and a streamlined data reduction pipeline. EC04207-4748 is a pulsating helium atmosphere WD that shows significant non-sinusoidal intensity variations. We show that the pulsation spectrum of this WD can be described by four independent pulsation eigenmodes plus linear combinations that arise from non-linear energy transport through a sub-surface convection layer. Our results are consistent with similar analyses that have been made for similar stars, and add an additional data point to the growing catalogue of these convection measurements. We argue that the convection layer depth may form a useful substitute for the effective temperatures of these WDs. GWLibrae is the class prototype of the accreting WD pulsators. These stars exist in cataclysmic variable (CV) systems, and show a mix of CV and pulsating WD-related phenomena. Our observations of GW Librae four - six years after its 2007 outburst show signs of quasi-stable intensity modulations that we believe may be caused by non-radial pulsations, but these are not convincingly explained by existing WD or CV models. L19-2 is a hydrogen atmosphere WD pulsator that shows extremely stable pulsation behaviour. We combine new observations with archival observations dating back to the mid 1970's, and derive a preliminary estimate of the period rate of change Ṗ for two of the pulsation modes in this target. We show a clear result for the main 192 s pulsation mode Ṗf₂ ≾ 10⁻¹⁴ s s-¹, and discuss the improvements that we plan to make in order to convincingly improve this constraint by an additional order of magnitude. Observations of other rapidly variable targets include two extremely low mass (ELM) WDs, which exhibit variability due to their orbital motion (J0751) as well as non-radial pulsations (J1518); the 33 ms optical period of the Crab Pulsar; the helium atmosphere WD pulsators EC05221-4725 and EC20058-5234; the stable hydrogen atmosphere pulsator G117–B15A; and the eclipsing sdB binary system PG1336-018.</p>

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An Explanation for the Euv Spectrum of Feige 24

International Astronomical Union Colloquium ◽

10.1017/s0252921100099905 ◽

1989 ◽

Vol 114 ◽

pp. 373-377 ◽

Cited By ~ 1

Author(s):

S. Vennes ◽

P. Chayer ◽

G. Fontaine ◽

F. Wesemael

Keyword(s):

White Dwarf ◽

Optical Spectrum ◽

Spectral Synthesis ◽

Model Atmosphere ◽

Atmospheric Composition ◽

Balmer Line ◽

Detailed Model ◽

Euv Spectrum ◽

M Dwarf

Feige 24 is a bright DA white dwarf which has been studied extensively both from ground-based and space-borne observatories. The best determination of its fundamental atmospheric parameters are that of Holberg, Wesemael, and Basile (1986) who have used detailed model atmosphere analyses in conjunction with optical, IUE, and Voyager data. They give log g = 7.23±0.35 and Te(103K) = 55±5. The question of the atmospheric composition is more involved as small traces of heavy elements would not be observable in the optical spectrum of such a hot, hydrogen-dominated atmosphere. If it were isolated, Feige 24 would presumably only show the usual bland optical spectrum of a typical DA white dwarf, i.e. the hydrogen Balmer line series, but the presence of a M dwarf companion complicates its spectrum. On the other hand, Feige 24 belongs to a handful of hot DA white dwarfs sufficiently bright that ultraviolet spectroscopy in the high resolution mode of the IUE has been possible. Following the theoretical expectation of Vauclair, Vauclair, and Greenstein (1979), it was discovered that the photosphere of Feige 24 contains small amounts of C, N, and Si (Dupree and Raymond 1982). Spectral synthesis techniques used by Wesemael, Henry, and Shipman (1984) indicate the following abundances: log(C/H)=−6.4±0.6, log(N/H)=−5.3±1.0, and log(Si/H)=−6.3±0.9. The most plausible explanation to account for these small abundances is the influence of selective radiative forces possibly coupled to a weak wind (Chayer et al. 1987).

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CCD Time-Series Photometry of White Dwarf Stars

10.26686/wgtn.17007772 ◽

2021 ◽

Author(s):

◽

Paul Robin Brian Chote

Keyword(s):

Data Reduction ◽

White Dwarf ◽

High Speed ◽

Energy Transport ◽

Hydrogen Atmosphere ◽

Rate Of Change ◽

Ccd Photometry ◽

Helium Atmosphere ◽

Convection Layer ◽

Radial Pulsations

<p>This thesis describes a practical programme that focused on CCD photometry of pulsating white dwarf (WD) stars. The first part of this thesis describes the development of two high-speed CCD photometer instruments and their data reduction pipeline, while the remainder describes the observation and analysis of several pulsating WDs and other targets. The two photometers (Puoko-nui North and South) share a common hardware design that is optimized for acquiring efficient photometry with integration periods of milliseconds through to minutes. The design integrates a commercial CCD (Charge-Coupled Device) camera and GPS (Global Positioning System) receiver with custom timing electronics and control software. The reduction and visualization software developed for these instruments provide detailed real-time information to the observer, and a streamlined data reduction pipeline. EC04207-4748 is a pulsating helium atmosphere WD that shows significant non-sinusoidal intensity variations. We show that the pulsation spectrum of this WD can be described by four independent pulsation eigenmodes plus linear combinations that arise from non-linear energy transport through a sub-surface convection layer. Our results are consistent with similar analyses that have been made for similar stars, and add an additional data point to the growing catalogue of these convection measurements. We argue that the convection layer depth may form a useful substitute for the effective temperatures of these WDs. GWLibrae is the class prototype of the accreting WD pulsators. These stars exist in cataclysmic variable (CV) systems, and show a mix of CV and pulsating WD-related phenomena. Our observations of GW Librae four - six years after its 2007 outburst show signs of quasi-stable intensity modulations that we believe may be caused by non-radial pulsations, but these are not convincingly explained by existing WD or CV models. L19-2 is a hydrogen atmosphere WD pulsator that shows extremely stable pulsation behaviour. We combine new observations with archival observations dating back to the mid 1970's, and derive a preliminary estimate of the period rate of change Ṗ for two of the pulsation modes in this target. We show a clear result for the main 192 s pulsation mode Ṗf₂ ≾ 10⁻¹⁴ s s-¹, and discuss the improvements that we plan to make in order to convincingly improve this constraint by an additional order of magnitude. Observations of other rapidly variable targets include two extremely low mass (ELM) WDs, which exhibit variability due to their orbital motion (J0751) as well as non-radial pulsations (J1518); the 33 ms optical period of the Crab Pulsar; the helium atmosphere WD pulsators EC05221-4725 and EC20058-5234; the stable hydrogen atmosphere pulsator G117–B15A; and the eclipsing sdB binary system PG1336-018.</p>

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