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).

Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars

1999 ◽  
Vol 516 (2) ◽  
pp. 887-891 ◽  
Author(s):  
A. Beauchamp ◽  
F. Wesemael ◽  
P. Bergeron ◽  
G. Fontaine ◽  
R. A. Saffer ◽  
...  
Keyword(s):  
White Dwarfs ◽  
Spectroscopic Studies ◽  
Instability Strip

scholarly journals On the formation of hydrogen-deficient low-mass white dwarfs

2020 ◽  
Vol 638 ◽  
pp. A30
Author(s):  
Tiara Battich ◽  
Leandro G. Althaus ◽  
Alejandro H. Córsico
Keyword(s):  
White Dwarfs ◽  
Evolutionary History ◽  
Main Sequence ◽  
Asymptotic Giant Branch ◽  
Instability Strip ◽  
Giant Star ◽  
Thermal Pulse ◽  
Low Mass ◽  
Effective Temperatures ◽  
Red Giant

Context. Two of the possible channels for the formation of low-mass (M⋆ ≲ 0.5 M⊙) hydrogen-deficient white dwarfs are the occurrence of a very-late thermal pulse after the asymptotic giant-branch phase or a late helium-flash onset in an almost stripped core of a red giant star. Aims. We aim to asses the potential of asteroseismology to distinguish between the hot flasher and the very-late thermal pulse scenarios for the formation of low-mass hydrogen-deficient white dwarfs. Methods. We computed the evolution of low-mass hydrogen-deficient white dwarfs from the zero-age main sequence in the context of the two evolutionary scenarios. We explore the pulsation properties of the resulting models for effective temperatures characterizing the instability strip of pulsating helium-rich white dwarfs. Results. We find that there are significant differences in the periods and in the period spacings associated with low radial-order (k ≲ 10) gravity modes for white-dwarf models evolving within the instability strip of the hydrogen-deficient white dwarfs. Conclusions. The measurement of the period spacings for pulsation modes with periods shorter than ∼500 s may be used to distinguish between the two scenarios. Moreover, period-to-period asteroseismic fits of low-mass pulsating hydrogen-deficient white dwarfs can help to determine their evolutionary history.

scholarly journals Searching for ZZ Ceti white dwarfs in the Gaia survey

2019 ◽  
Vol 15 (S357) ◽  
pp. 123-126
Author(s):  
Olivier Vincent ◽  
Pierre Bergeron ◽  
David Lafrenière
Keyword(s):  
White Dwarf ◽  
White Dwarfs ◽  
Physical Parameters ◽  
Hydrogen Line ◽  
Instability Strip ◽  
Dwarf Stars ◽  
White Dwarf Stars ◽  
Effective Temperatures

AbstractThe Gaia satellite recently released parallax measurements for nearly 400,000 white dwarf stars, allowing for precise measurements of their physical parameters. By combining these parallaxes with Pan-STARRS and CFIS-u photometry, we measured the effective temperatures and surface gravities for all white dwarfs within 100 pc and identified a sample of ZZ Ceti white dwarf candidates within the instability strip. We report the results of a photometric follow-up, currently under way, aimed at identifying new ZZ Ceti stars among this sample using the PESTO camera attached to the 1.6-m telescope at the Mont Mégantic Observatory. Our goal is to verify that ZZ Ceti stars occupy a region in the logg-Teff plane where no nonvariable stars are found, supporting the idea that ZZ Ceti pulsators represent a phase through which all hydrogen-line (DA) white dwarfs must evolve.

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.

scholarly journals Asteroseismological analysis of the ultra-massive ZZ Ceti stars BPM 37093, GD 518, and SDSS J0840+5222

2019 ◽  
Vol 632 ◽  
pp. A119
Author(s):  
Alejandro H. Córsico ◽  
Francisco C. De Gerónimo ◽  
María E. Camisassa ◽  
Leandro G. Althaus
Keyword(s):  
Chemical Composition ◽  
White Dwarf ◽  
White Dwarfs ◽  
Intrinsic Property ◽  
Stellar Mass ◽  
Evolutionary Origin ◽  
Instability Strip ◽  
Single Star ◽  
The Core ◽  
Effective Temperatures

Context. Ultra-massive (≳1 M⊙) hydrogen-rich (DA) white dwarfs are expected to have a substantial portion of their cores in a crystalline state at the effective temperatures characterising the ZZ Ceti instability strip (Teff ∼ 12 500 K) as a result of Coulomb interactions in very dense plasmas. Asteroseismological analyses of these white dwarfs can provide valuable information related to the crystallisation process, the core chemical composition, and the evolutionary origin of these stars. Aims. We present a thorough asteroseismological analysis of the ultra-massive ZZ Ceti star BPM 37093, which exhibits a rich period spectrum, on the basis of a complete set of fully evolutionary models that represent ultra-massive oxygen/neon (ONe) core DA white dwarf stars harbouring a range of hydrogen (H) envelope thicknesses. We also carry out preliminary asteroseismological inferences on two other ultra-massive ZZ Ceti stars that exhibit fewer periods, GD 518, and SDSS J0840+5222. Methods. We considered g-mode adiabatic pulsation periods for ultra-massive ONe-core DA white dwarf models with stellar masses in the range 1.10 ≲ M⋆/M⊙ ≲ 1.29, effective temperatures in the range 10 000 ≲ Teff ≲ 15 000 K, and H-envelope thicknesses in the interval −10 ≲ log(MH/M⋆)≲ − 6. We explored the effects of employing different H-envelope thicknesses on the mode-trapping properties of our ultra-massive ONe-core DA white dwarf models and performed period-to-period fits to ultra-massive ZZ Ceti stars with the aim of finding an asteroseismological model for each target star. Results. We find that the trapping cycle and trapping amplitude are larger for thinner H envelopes, and that the asymptotic period spacing is longer for thinner H envelopes. We find a mean period spacing of ΔΠ ∼ 17 s in the data of BPM 37093, which is likely to be associated with ℓ = 2 modes. However, we are not able to put constraints on the stellar mass of BPM 37093 using this mean period spacing due to the simultaneous sensitivity of ΔΠ with M⋆, Teff, and MH, which is an intrinsic property of DAV stars. We find asteroseismological models for the three objects under analysis, two of them (BPM 37093 and GD 518) characterised by canonical (thick) H envelopes, and the third one (SDSS J0840+5222) with a thinner H envelope. The effective temperature and stellar mass of these models are in agreement with the spectroscopic determinations. The percentage of crystallised mass for these asteroseismological models is 92%, 97%, and 81% for BPM 37093, GD 518, and SDSS J0840+5222, respectively. We also derive asteroseismological distances which differ somewhat from the astrometric measurements of Gaia for these stars. Conclusions. Asteroseismological analyses like the one presented in this paper could lead to a more complete understanding of the processes occurring during crystallisation inside white dwarfs. Also, such analyses could make it possible to deduce the core chemical composition of ultra-massive white dwarfs and, in this way, to infer their evolutionary origin, such as the correlation between a star’s ONe core and its having originated through single-star evolution or a carbon/oxygen (CO) core indicating the star is the product of a merger of the two components of a binary system. However, in order to achieve these objectives, it is necessary to find a greater number of pulsating ultra-massive WDs and to carry out additional observations of known pulsating stars to detect more pulsation periods. Space missions such as TESS can provide a great boost towards achieving these aims.

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*).

Effective Temperatures of White Dwarfs

White Dwarfs ◽  
1971 ◽  
pp. 67-76 ◽  
Author(s):  
J. B. Oke ◽  
H. L. Shipman
Keyword(s):  
White Dwarfs ◽  
Effective Temperatures

scholarly journals Discovery of a second pulsating intermediate helium-enriched sdOB star

2019 ◽  
Vol 623 ◽  
pp. L12 ◽  
Author(s):  
M. Latour ◽  
E. M. Green ◽  
G. Fontaine
Keyword(s):  
Galactic Halo ◽  
Local Thermodynamic Equilibrium ◽  
Variable Stars ◽  
Light Curves ◽  
Atmospheric Parameters ◽  
Instability Strip ◽  
Long Period ◽  
Non Local ◽  
Halo Population ◽  
Low Amplitude

We present the discovery of long-period, low-amplitude, g-mode pulsations in the intermediate He-rich hot subdwarf (sdOB) star Feige 46. So far, only one other He-enriched sdOB star (LS IV−14 ° 116) was known to exhibit such pulsations. From our ground-based light curves of Feige 46, we extracted five independent periodicities ranging from 2294 s to 3400 s. We fit our optical spectrum of the star with our grid of non-local thermodynamic equilibrium (NLTE) model atmospheres and derived the following atmospheric parameters: Teff = 36120 ± 230 K, log g = 5.93 ± 0.04, and log N(He)/N(H) = −0.32 ± 0.03 (formal fitting errors only). These parameters are very similar to those of LS IV−14 ° 116 and place Feige 46 well outside of the instability strip where the hydrogen-rich g-mode sdB pulsators are found. We used the Gaia parallax and proper motion of Feige 46 to perform a kinematic analysis of this star and found that it likely belongs to the Galactic halo population. This is most certainly an intriguing and interesting result given that LS IV−14 ° 116 is also a halo object. The mechanism responsible for the pulsations in these two peculiar objects remains unclear, but a possible scenario involves the ϵ-mechanism. Although they are the only two members in their class of variable stars, these pulsators appear to have more in common than just their pulsation properties.

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