scholarly journals Temperatures for Hot and Pulsating Helium-Rich (DB) White Dwarfs Obtained with the IUE Observatory

1985 ◽  
Vol 87 ◽  
pp. 387-390
Author(s):  
J. Liebert ◽  
F. Wesemael ◽  
C.J. Hansen ◽  
G. Fontaine ◽  
H.L. Shipman ◽  
...  
Keyword(s):  
White Dwarfs ◽  
Instability Strip ◽  
Energy Distributions ◽  
Helium Atmosphere ◽  
Pulsating Stars ◽  
Blue Edge ◽  
Red Edge ◽  
Ultraviolet Energy

AbstractUltraviolet energy distributions are analyzed for several hot, helium atmosphere DB white dwarfs, including the four known pulsating stars which define an empirical DB instability strip. Temperatures are derived exclusively from fits to the ultraviolet energy distributions. The blue edge of the empirical DB instability strip lies at 30,000 ± 4,000 K, and the red edge lies near 24,000 ± 2,000 K. The hottest DB star — and the only known one hotter than the instability strip — is PG0112+104 at or above 30,000 K. This leaves no known helium-atmosphere degenerate stars in the interval 30,000 ≤ Te ≤ 45.000K.

Spectroscopic Studies and Atmospheric Parameters of ZZ Ceti Stars

1989 ◽  
Vol 114 ◽  
pp. 244-248
Author(s):  
D. Daou ◽  
F. Wesemael ◽  
P. Bergeron ◽  
G. Fontaine ◽  
J. B. Holberg
Keyword(s):  
Strong Evidence ◽  
White Dwarfs ◽  
Narrow Range ◽  
Spectroscopic Studies ◽  
Atmospheric Parameters ◽  
Instability Strip ◽  
Pulsating Stars ◽  
Evolutionary Phase ◽  
Effective Temperatures

The pulsating ZZ Ceti stars cover a narrow range of effective temperatures along the cooling sequence of DA white dwarfs (see, eg., Winget and Fontaine 1982). Fast-photometric searches for pulsating stars in that class have provided strong evidence that the ZZ Ceti phase is an evolutionary phase through which all cooling DA stars will eventually go through (Fontaine et al. 1982). Recent investigations, based on optical or ultraviolet photometry and spectrophotometry, have set the boundaries of the instability strip at temperatures near 10,000-11,000 K and 12,000-13,000 K, respectively (McGraw 1979; Greenstein 1982; Weidemann and Koester 1984; Fontaine et al. 1985; Wesemael, Lamontagne, and Fontaine 1986; Lamontagne, Wesemael, and Fontaine 1987, 1988).

scholarly journals An Isolated White Dwarf with a 70 s Spin Period

2021 ◽  
Vol 923 (1) ◽  
pp. L6
Author(s):  
Mukremin Kilic ◽  
Alekzander Kosakowski ◽  
Adam G. Moss ◽  
P. Bergeron ◽  
Annamarie A. Conly
Keyword(s):  
White Dwarf ◽  
High Speed ◽  
White Dwarfs ◽  
Tangential Velocity ◽  
Large Mass ◽  
Atmospheric Composition ◽  
Instability Strip ◽  
Detailed Model ◽  
Helium Atmosphere ◽  
Spin Period

Abstract We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high-speed photometry of three ultramassive white dwarfs within 100 pc and discovered significant variability in one. SDSS J221141.80+113604.4 is a 1.27 M ⊙ (assuming a CO core) magnetic white dwarf that shows 2.9% brightness variations in the BG40 filter with a 70.32 ± 0.04 s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength of B d = 15 MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long-term monitoring of the spin evolution of J2211+1136 and other fast-spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the monoperiodic variables found within the instability strip may be fast-spinning white dwarfs impersonating ZZ Ceti pulsators.

scholarly journals An Ultraviolet Look at the Blue Edge of the ZZ Ceti Instability Strip

1989 ◽  
Vol 114 ◽  
pp. 240-243
Author(s):  
R. Lamontagne ◽  
F. Vesemael ◽  
G. Fontaine ◽  
G. Vegner ◽  
E. P. Nelan
Keyword(s):  
White Dwarfs ◽  
Theoretical Calculations ◽  
Stellar Mass ◽  
Instability Strip ◽  
Optical Photometry ◽  
Model Calculations ◽  
Narrow Temperature Range ◽  
Blue Edge ◽  
Ultraviolet Observations ◽  
Variability Region

It has already been shown that most, and probably all, of the DA white dwarfs become variable in a narrow temperature range as they cool down (Fontaine et al. 1982). Optical photometry and spectrophotometry has led to several determinations of the boundaries of this instability strip. The strip has been found to cover the range 10300 - 13600 K (McGraw 1979), 10400 - 12100 K (Greenstein 1982), 10000 - 13000 K (Weidemann and Koester 1984) and 11000 - 13000 K (Fontaine et al. 1985). Theoretical calculations show that the location of the blue edge is very sensitive to the efficiency of convection used in the unpertubed models (Winget et al. 1982; Winget and Fontaine 1982; Fontaine, Tassoul, and Wesemael 1984). Also, the sharpness of this boundary depends on the range of stellar mass and thickness of the hydrogen envelope found in ZZ Ceti stars. Recently, Wesemael, Lamontagne, and Fontaine (1986) and Lamontagne, Wesemael, and Fontaine (1987) have obtained and compared ultraviolet observations of several DA white dwarfs, in or near the instability strip, with published model calculations from Nelan and Wegner (1985), hereafter NW, and Koester et al. (1985), hereafter KWZV. They determined the boundaries of the variability region at 11400 - 12500 K or 11700 - 13000 K depending on which grid was used. We present here a reanalysis of these IUE observations with an improved grid of model atmospheres in order to define more precisely the location of the blue edge.

scholarly journals Non-Pulsating Stars and the Population I and II Instability Strips

1974 ◽  
Vol 59 ◽  
pp. 59-60
Author(s):  
Arthur N. Cox ◽  
James E. Tabor ◽  
David S. King
Keyword(s):  
Evolutionary Theory ◽  
Equations Of State ◽  
High Accuracy ◽  
Instability Strip ◽  
Classical Cepheid ◽  
Helium Content ◽  
Pulsating Stars ◽  
Rr Lyrae ◽  
Red Edge ◽  
Scuti Stars

With specially computed detailed tables of equations of state and opacities, the instability strips for δ Scuti stars and Cepheids of population I and RR Lyrae and W Virginis stars of population II have been compared using the linear pulsation theory. Uncertainties in the observed strip locations and sometimes the mode of the observed pulsations do not allow high accuracy in fixing helium contents or the variables masses. Nevertheless, if masses close to those given by evolutionary theory are used, the helium content in population II objects is likely less than Y = 0.25. A helium content of close to zero would put the theoretical blue edge of the instability strip to the red of the observed red edge, and have all the hotter stars which are in the strip as non-pulsating. For population I, Y can be more than 0.3, (more than 0.4 if half evolutionary masses are used), but if a given star has Y less than about 0.2 (full mass) or 0.25 (half mass), it can appear in the dwarf and classical Cepheid strips as non-pulsating.

scholarly journals TESS first look at evolved compact pulsators

2019 ◽  
Vol 632 ◽  
pp. A42 ◽  
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.

scholarly journals The Effects of Time Dependant Convection on White Dwarf Radial Pulsations

1989 ◽  
Vol 114 ◽  
pp. 115-118
Author(s):  
S. Starrfield ◽  
A. N. Cox
Keyword(s):  
White Dwarf ◽  
Time Scale ◽  
White Dwarfs ◽  
Pure Hydrogen ◽  
Helium Surface ◽  
Surface Layers ◽  
Instability Strip ◽  
Blue Edge ◽  
Pure Helium ◽  
Radial Pulsations

AbstractWe have investigated the effects of relaxing the normal assumption of frozen in convection on studies of radial instabilities in 0.6M⊙ carbon-oxygen white dwarfs with either pure hydrogen layers overlying pure helium layers or 0.6M⊙ carbon-oxygen white dwarfs with pure helium surface layers. In this paper we assume that convection can adjust to the pulsation at a rate determined by the time scale of a convective eddy. Using this assumption in our analysis stabilizes most of the modes in both the DA and DB radial instability strips. We also find that the blue edge of the DA radial instability strip, assuming frozen in convection, is between 12,0O0K and 13,000K. The blue edge for the DB radial instability strip (frozen in convection) is between 32,000K and 33,000K.

On the Red Edge of the δ Scuti Instability Strip

2002 ◽  
Vol 2 (5) ◽  
pp. 441-448 ◽  
Author(s):  
Yan Xu ◽  
Zhi-Ping Li ◽  
Li-Cai Deng ◽  
Da-Run Xiong
Keyword(s):  
Instability Strip ◽  
Red Edge

scholarly journals The McDonald Observatory search for pulsating sdA stars

2018 ◽  
Vol 617 ◽  
pp. A6 ◽  
Author(s):  
K. J. Bell ◽  
I. Pelisoli ◽  
S. O. Kepler ◽  
W. R. Brown ◽  
D. E. Winget ◽  
...  
Keyword(s):  
Time Series ◽  
White Dwarfs ◽  
Main Sequence ◽  
Pulsating Stars ◽  
Blue Stragglers ◽  
Rr Lyrae ◽  
Stellar Pulsations ◽  
Low Mass ◽  
New Variables

Context. The nature of the recently identified “sdA” spectroscopic class of stars is not well understood. The thousands of known sdAs have H-dominated spectra, spectroscopic surface gravity values between main sequence stars and isolated white dwarfs, and effective temperatures below the lower limit for He-burning subdwarfs. Most are likely products of binary stellar evolution, whether extremely low-mass white dwarfs and their precursors or blue stragglers in the halo. Aims. Stellar eigenfrequencies revealed through time series photometry of pulsating stars sensitively probe stellar structural properties. The properties of pulsations exhibited by sdA stars would contribute substantially to our developing understanding of this class. Methods. We extend our photometric campaign to discover pulsating extremely low-mass white dwarfs from the McDonald Observatory to target sdA stars classified from SDSS spectra. We also obtain follow-up time series spectroscopy to search for binary signatures from four new pulsators. Results. Out of 23 sdA stars observed, we clearly detect stellar pulsations in 7. Dominant pulsation periods range from 4.6 min to 12.3 h, with most on timescales of approximately one hour. We argue specific classifications for some of the new variables, identifying both compact and likely main sequence dwarf pulsators, along with a candidate low-mass RR Lyrae star. Conclusions. With dominant pulsation periods spanning orders of magnitude, the pulsational evidence supports the emerging narrative that the sdA class consists of multiple stellar populations. Since multiple types of sdA exhibit stellar pulsations, follow-up asteroseismic analysis can be used to probe the precise evolutionary natures and stellar structures of these individual subpopulations.

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 The Status of the Absolute Calibration of Stellar Fluxes Between 912 and 1200 Å

1985 ◽  
Vol 111 ◽  
pp. 479-483
Author(s):  
R. S. Polidan ◽  
J. B. Holberg
Keyword(s):  
Planetary Nebula ◽  
White Dwarfs ◽  
Main Sequence ◽  
Absolute Calibration ◽  
Energy Distributions ◽  
B Stars ◽  
The Status ◽  
The Absolute ◽  
Flux Calibration

Recent results have shed new light on the status of the calibration of absolute stellar fluxes between 912 and 1200 Å. Observations of hot white dwarfs, subdwarfs and planetary nebula nuclei with the Voyager ultraviolet spectrometers provide evidence that the current calibration agrees very well with extrapolations of IUE energy distributions shortwards of 1200 Å. Voyager observations of main sequence B-stars used as flux calibration sources have revealed that many are variable in brightness in the 912–1200 Å region. We conclude there is no current observational motivation for any revision of the 912 to 1200 Å calibration described by Holberg et al. (1982).

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