Periodic Variations in Theo–Cdiagrams of Five Pulsation Frequencies of the DB White Dwarf Ec 20058–5234 (original) (raw)
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Highlights of Astronomy, 1980
It has been known for a long time that white dwarfs are pulsationally unstable if nuclear burning takes place in their envelopes. Perturbation of energy generation rate promotes pulsational instability and this effect is frequently referred to as e-mechanism. In recent years, with the advent of high-speed photometry, many rapidly varying white dwarfs have been discovered. However, periods of variability were found to be significantly longer than the periods of radial pulsations which were the only type of oscillations considered before the discovery. Furthermore, the case of e-mechanism as being responsible for the observed variability has never been made strong for any of the observed objects. Variable white dwarfs are found among: 1° single DA-type objects in the effective temperature range 10000-15000K; 2° members of close, usually but not always, cataclysmic binary systems. Although, following an early suggestion by Warner and Robinson (1972), the excitation of nonradial oscillation is postulated in both cases, the two types represent very different physical situations and they will be discussed here separately. There are 12 single variable white dwarfs known to date, and they form a well defined type of variable stars called ZZ Ceti. Comprehensive surveys of the properties of these stars are available in review articles by Nather (1979) and Robinson (1979). Incidence of ZZ Ceti among white dwarfs is similar to incidence of pulsating variables among early post-main sequence or horizontal branch stars. Namely, it is confined to a narrow range of effective temperatures and it is not related to any peculiarity of the objects. ZZ Ceti stars with their low amplitudes and multiperiodicity bear most resemblance to 6 Scuti-type variables. Amplitude changes occurring on the time scales of hours or days are frequently observed among ZZ Ceti stars. Also in the case of 6 Scuti stars relatively rapid amplitude variations have been reported, but their reality was questioned by Fitch (1976). The variable white dwarfs, however, differ sharply from other pulsating stars in the modes they choose to pulsate. Periods of variability *0n leave from the Copernicus Astronomical Center,
The Search for Planets Around Pulsating White Dwarf Stars
White Dwarfs, 2003
We are implementing a proven technique to search for extrasolar planets with a different sensitivity from other methods, using the pulsations of white dwarf stars as intrinsic clocks. We will search more than one hundred stars over the next decade for the effects of reflex orbital motion in changing light travel times and accelerations caused by any planets orbiting these stars.
2008
The pulsating DA white dwarfs are the coolest degenerate stars that undergo self-driven oscillations. Understanding their interior structure will help us to understand the previous evolution of the star. To this end, we report the analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA white dwarf star EC 14012−1446 acquired during four observing epochs in three different years, including a coordinated three-site campaign. A total of 19 independent frequencies in the star's light variations together with 148 combination signals up to fifth order could be detected. We are unable to obtain the period spacing of the normal modes and therefore a mass estimate of the star, but we infer a fairly short rotation period of 0.61 ±0.03 d, assuming the rotationally split modes are = 1. The pulsation modes of the star undergo amplitude and frequency variations, in the sense that modes with higher radial overtone show more pronounced variability and that amplitude changes are always accompanied by frequency variations. Most of the second-order combination frequencies detected have amplitudes that are a function of their parent mode amplitudes, but we found a few cases of possible resonantly excited modes. We point out the complications in the analysis and interpretation of data sets of pulsating white dwarfs that are affected by combination frequencies of the form f A + f B − f C intruding into the frequency range of the independent modes.
The temporal changes of the pulsational periods of the pre-white dwarf PG 1159-035
Astronomy and Astrophysics, 2008
Context. PG 1159-035, a pre-white dwarf with T eff ≃ 140 000 K, is the prototype of the PG 1159 spectroscopic class and the DOV pulsating class. Pulsating pre-white dwarf stars evolve rapidly: the effective surface temperature decreases rapidly, the envelope contracts and the inner structure experiences stratification due to gravitational settling. These changes in the star generate variations in its oscillation periods. The measurement of temporal change in the oscillation periods,Ṗ, allows us to estimate directly rates of stellar evolutionary changes, such as the cooling rate and the envelope contraction rate, providing a way to test and refine evolutionary models for pre-white dwarf pulsating stars. Aims. Previously, only two pulsation modes of the highest amplitudes for PG 1159-035 have had theirṖ measured: the 516.0 s and the 539.3 s modes. We measured theṖ of a larger number of pulsation modes, increasing the number of constraints for evolutionary studies of PG 1159-035. We attempted to use the secular variations in the periods of multiplets to calculate the variation in the rotational period, the envelope contraction rate, and the cooling rate of the star. Methods. The period variations were measured directly from the PG 1159-035 observational data and refined by the (O-C) method. Results. We measured 27 pulsation mode period changes. The periods varied at rates of between 1 and 100 ms/yr, and several can be directly measured with a relative standard uncertainty below 10%. For the 516.0 s mode (the highest in amplitude) in particular, not only the value ofṖ can be measured directly with a relative standard uncertainty of 2%, but the second order period change,P, can also be calculated reliably. By using the (O-C) method, we refined theṖs and estimated thePs for six other pulsation periods. As a first application, we calculated the change in the PG 1559-035 rotation period,Ṗ rot = (−2.13 ± 0.05) × 10 −6 ss −1 , the envelope contraction rateṘ = (−2.2 ± 0.5) × 10 −13 R ⊙ s −1 , and the cooling rateṪ = −1.42 × 10 −3 Ks −1 .
The Astrophysical Journal, 2010
Nonradial pulsations in the primary white dwarfs of cataclysmic variables can now potentially allow us to explore the stellar interior of these accretors using stellar seismology. In this context, we conducted a multi-site campaign on the accreting pulsator SDSS J161033.64-010223.3 (V386 Ser) using seven observatories located around the world in May 2007 over a duration of 11 days. We report the best fit periodicities here, which were also previously observed in 2004, suggesting their underlying stability. Although we did not uncover a sufficient number of independent pulsation modes for a unique seismological fit, our campaign revealed that the dominant pulsation mode at 609 s is an evenly spaced triplet. The even nature of the triplet is suggestive of rotational splitting, implying an enigmatic rotation period of about 4.8 days. There are two viable alternatives assuming the triplet is real: either the period of 4.8 days is representative of the rotation period of the entire star with implications for the angular momentum evolution of these systems, or it is perhaps an indication of differential rotation with a fast rotating exterior and slow rotation deeper in the star. Investigating the possibility that a changing period could mimic a triplet suggests that this scenario is improbable, but not impossible.
A 5.3-minute-period pulsing white dwarf in a binary detected from radio to X-rays
arXiv (Cornell University), 2023
White dwarf stars are the most common stellar fossils. When in binaries, they make up the dominant form of compact object binary within the Galaxy and can offer insight into different aspects of binary formation and evolution. One of the most remarkable white dwarf binary systems identified to date is AR Scorpii (henceforth AR Sco). AR Sco is composed of an M-dwarf star and a rapidly-spinning white dwarf in a 3.56-hour orbit. It shows pulsed emission with a period of 1.97 minutes over a broad range of wavelengths, which led to it being known as a white dwarf pulsar. Both the pulse mechanism and the evolutionary origin of AR Sco provide challenges to theoretical models. Here we report the discovery of the first sibling of AR Sco, J191213.72−441045.1 (henceforth J1912−4410), which harbours a white dwarf in a 4.03-hour orbit with an M-dwarf and exhibits pulsed emission with a period of 5.30 minutes. This discovery establishes binary white dwarf pulsars as a class and provides support for proposed formation models for white dwarf pulsars. The white dwarf pulsar AR Sco is detected over a broad range of wavelengths, from radio 1 to X-rays 2. The spin-down of its rapidly-rotating white dwarf provides enough energy to power the pulses 3 , but the exact driving mechanism is not fully understood. Unlike in neutron star pulsars, where no companion is required, binarity seems to play an important role in AR Sco's pulses. The observed periodicity of 1.97 min is consistent with a reprocessed frequency, the beat frequency between the 1.95 min spin period of the white dwarf and the 3.56-hour orbital period. This suggests that interaction between the white dwarf and the M-dwarf is behind the pulse mechanism. Proposed models for the origin of emission include the surface or
Pulsating white dwarfs: new insights
The Astronomy and Astrophysics Review, 2019
Stars are extremely important astronomical objects that constitute the pillars on which the Universe is built, and as such, their study has gained increasing interest over the years. White dwarf stars are not the exception. Indeed, these stars constitute the final evolutionary stage for more than 95 per cent of all stars. The Galactic population of white dwarfs conveys a wealth of information about several fundamental issues and are of vital importance to study the structure, evolution and chemical enrichment of our Galaxy and its components-including the star formation history of the Milky Way. Several important studies have emphasized the advantage of using white dwarfs as reliable clocks to date a variety of stellar populations in the solar neighborhood and in the nearest stellar clusters, including the thin and thick disks, the Galactic spheroid and the system of globular and open clusters. In addition, white dwarfs are tracers of the evolution of planetary systems along several phases of stellar evolution. Not less relevant than these applications, the study of matter at high densities has benefited from our detailed knowledge about evolutionary and observational properties of white dwarfs. In this sense, white dwarfs are used as laboratories for astro-particle physics, being their interest focused on physics beyond the standard model, that is, neutrino physics, axion physics and also radiation from "extra dimensions", and even crystallization. The last decade has witnessed a great progress in the study of white dwarfs. In particular, a wealth of information of these stars from different surveys has
GW Librae: a unique laboratory for pulsations in an accreting white dwarf
Monthly Notices of the Royal Astronomical Society, 2016
Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at 275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits highamplitude variability on an 4.4 h timescale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to 30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.