The 1051 s period of the interacting binary white dwarf amcvn

The close binary system AM CVn consists of two white dwarfs; the lower mass white dwarf, a helium white dwarf, is transferring mass to its higher mass companion. The light curve of AM CVn has a double humped variation with a period of 1051 s that previously was identified with the orbital period of the system. An earlier measurement of the 1051 s period seemed to show that it was increasing rapidly, too rapidly to be easily understood. We have, therefore, remeasured the rate of change of the 1051 s period. We find that the period is changing two orders of magnitude more slowly than previously thought: dP/dt= (–3.2±0.6) 10ˉ¹² s sˉ¹• Since the mass losing star in AM CVn is a white dwarf, the orbital period of the system must be increasing. We have found that the 1051 s period is decreasing, and, therefore, it cannot be caused by orbital motion. We reinterpret the 1051 s period as the rotation period of the accreting white dwarf, which must then be magnetized. We have also found a 1011.4 s period in the light curve that may be related to the orbital period, but cannot itself be the orbital period. Finally, we show that the double humped light curve may really be a single humped light curve with a period of 525.5s

The 1051 s period of the interacting binary white dwarf amcvn

The close binary system AM CVn consists of two white dwarfs; the lower mass white dwarf, a helium white dwarf, is transferring mass to its higher mass companion. The light curve of AM CVn has a double humped variation with a period of 1051 s that previously was identified with the orbital period of the system. An earlier measurement of the 1051 s period seemed to show that it was increasing rapidly, too rapidly to be easily understood. We have, therefore, remeasured the rate of change of the 1051 s period. We find that the period is changing two orders of magnitude more slowly than previously thought: dP/dt= (–3.2±0.6) 10ˉ¹² s sˉ¹• Since the mass losing star in AM CVn is a white dwarf, the orbital period of the system must be increasing. We have found that the 1051 s period is decreasing, and, therefore, it cannot be caused by orbital motion. We reinterpret the 1051 s period as the rotation period of the accreting white dwarf, which must then be magnetized. We have also found a 1011.4 s period in the light curve that may be related to the orbital period, but cannot itself be the orbital period. Finally, we show that the double humped light curve may really be a single humped light curve with a period of 525.5s

Asteroseismology of a star cooled by neutrino emission : the pulsating pre-white dwarf PG 0122+200

Observation of g-mode pulsations in the variable pre-white dwarf (GW Virginis) stars provides a unique means to probe their interiors and to study the late stages of stellar evolution. Multisite campaigns have in several cases proved highly successful in decoding preÈwhite dwarf light curves. Three previous attempts to untangle the pulsation spectrum of the coolest GW Virginis star, PG 0122+200, confirmed the existence of multiple g-modes but left the fundamental period spacing and therefore the star's mass and luminosity in doubt. We present an analysis based on new observations of PG 0122+200 obtained during a Whole Earth Telescope (WET) campaign conducted in the fall of 1996. Although our coverage was, because of bad weather, far poorer than in previous WET campaigns, we confirm the previous result that PG 0122+200 rotates once in 1.6±0.1 days. The most likely period spacing supported by the data implies a mass of 0.69±0.03 Mʘ. Based on the best seismology we can currently do, the cooling of PG 0122+200 is dominated by neutrino losses. This is not true for all pre-white dwarf stars and makes PG 0122+200 the prime candidate for learning useful physics. Constraints placed on the cooling rate of PG 0122+200 by future measurement of dII/dt could provide a unique test of the standard theory of lepton interactions in the (experimentally unexplored) region of phase-space appropriate to pre-white dwarf interiors

The everchanging pulsating white dwarf GD358

We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree l = 1 and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude l = 2 mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point during the 1996 observations, most of the pulsation energy was transferred into the radial order k = 8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k = 9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale

Understanding the cool DA white dwarf pulsator G29-38

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post-main-sequence evolution, along with their cooling rates, which will allow us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DA variables (DAVs), which have not been explored previously through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning 10 yr, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modeling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown that its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modeling, thereby joining the rest of the known white dwarf pulsators

2006 Whole Earth Telescope observations of GD358 : a new look at the prototype DBV

We report on the analysis of 436.1 hr of nearly continuous high-speed photometry on the pulsating DB white dwarf GD358 acquired with the Whole Earth Telescope (WET) during the 2006 international observing run, designated XCOV25. The Fourier transform (FT) of the light curve contains power between 1000 and 4000 μHz, with the dominant peak at 1234 μHz. We find 27 independent frequencies distributed in 10 modes, as well as numerous combination frequencies. Our discussion focuses on a new asteroseismological analysis of GD358, incorporating the 2006 data set and drawing on 24 years of archival observations. Our results reveal that, while the general frequency locations of the identified modes are consistent throughout the years, the multiplet structure is complex and cannot be interpreted simply as l =l modes in the limit of slow rotation. The high-k multiplets exhibit significant variability in structure, amplitude and frequency. Any identification of the m components for the high-k multiplets is highly suspect. The k = 9 and 8 modes typically do show triplet structure more consistent with theoretical expectations. The frequencies and amplitudes exhibit some variability, but much less than the high-k modes. Analysis of the k = 9 and 8 multiplet splittings from 1990 to 2008 reveal a long-term change in multiplet splittings coinciding with the 1996 sforzando event, where GD358 dramatically altered its pulsation characteristics on a timescale of hours. We explore potential implications, including the possible connections between convection and/or magnetic fields and pulsations.We suggest future investigations, including theoretical investigations of the relationship between magnetic fields, pulsation, growth rates, and convection

Constraining the evolution of ZZ Ceti

We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf ) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt ≤ (5:5 ± 1:9) x 10 15 s s-ˡ, after correcting for proper motion. Our results are consistent with previous _PP values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is |P/PP| ≥ 1.2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions

Whole earth telescope observations of am canum venaticorum-discoseismology at last

We report the results of 143.2 hours of time-series photometry over a 12 day period for AM CVn (= HZ 29) as part of the Whole Earth Telescope (WET) project.ˡ This star is believed to be an ultra-short period cataclysmic binary. In the temporal spectrum of the light curve we find a series of 5 harmonically related frequency mo dulations, some with sidebands with a constant frequency spacing of 20.8 μHz always on the high-frequency side. The set of harmonics has a fundamental frequency of 951 μHz. No modulation is detected at this frequency in the light curve. In addition, modulations with frequencies 972.5 and 988.9 μHz are detected with low amplitudes. The structure of the dominant 1903 Hz modulation explains part of the “phase jitter” observed earlier. The amplitude of this peak is modulated with a period of 13.32 ± 0.05 hrs

Whole earth telescope observations of am canum venaticorum-discoseismology at last

We report the results of 143.2 hours of time-series photometry over a 12 day period for AM CVn (= HZ 29) as part of the Whole Earth Telescope (WET) project.ˡ This star is believed to be an ultra-short period cataclysmic binary. In the temporal spectrum of the light curve we find a series of 5 harmonically related frequency mo dulations, some with sidebands with a constant frequency spacing of 20.8 μHz always on the high-frequency side. The set of harmonics has a fundamental frequency of 951 μHz. No modulation is detected at this frequency in the light curve. In addition, modulations with frequencies 972.5 and 988.9 μHz are detected with low amplitudes. The structure of the dominant 1903 Hz modulation explains part of the “phase jitter” observed earlier. The amplitude of this peak is modulated with a period of 13.32 ± 0.05 hrs

The unusual helium variable AM Canum Venaticorum

The unusual variable star AM CVn has puzzled astronomers for over 40 years. This object, both a photometric and spectroscopic variable, is believed to contain a pair of hydrogen-deficient white dwarfs of extreme mass ratio, transferring material via an accretion disk. We examine the photometric properties of AM CVn, analyzing 289 hours of high-speed photometric data spanning 1976 to 1992. The power spectrum displays significant peaks at 988.7, 1248.8, 1902.5, 2853.8, 3805.2, 4756.5, and 5707.8 μHz (1011.4, 800.8, 525.6, 350.4, 262.8, 210.2, and 175.2 s). We find no detectable power at 951.3 μHz (1051 s), the previously reported main frequency. The 1902.5, 2853.9, and 3805.2 μHz peaks are multiplets, with frequency splitting in each case of 20.77 ± 0.05 μHz. The 1902.5 μHz seasonal pulse shapes are identical, within measurement noise, and maintain the same amplitude and phase as a function of color. We have determined the dominant frequency to be 1902.509802 ± 0.00001 μHz, with p = + 1.71 (±0.04) X 10-11 s s-ˡ. We discuss the implications of these findings on a model forAM CVn