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Search Results: 1 - 10 of 297321 matches for " J. Provencal "
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Searching for Planets with White Dwarf Pulsations: Spurious Detections
J. Dalessio,J. L. Provencal,H. S. Shipman
Physics , 2011, DOI: 10.1063/1.3570985
Abstract: We present 13 years of pulsation timing measurements of the DBV white dwarf EC 2005-5234. Each of the four O-C diagrams mimic the sinusoidal behavior typically attributed to a planet + WD system. However, the amplitude and phase of the O-C variations are inconsistent with each other. We discuss the impact of this result on timing based WD planet searches.
Seven-Period Asteroseismic Fit of the Kepler DBV
Agnes Kim,Roy Ostensen,J. J. Hermes,Judith Provencal
Physics , 2014,
Abstract: We present a new, better-constrained asteroseismic analysis of the helium-atmosphere (DB) white dwarf discovered in the field of view of the original Kepler mission. Observations obtained over the course of two years yield at least seven independent modes, two more than were found in the discovery paper for the object. With several triplets and doublets, we are able to fix the $\ell$ and $\rm{m}$ identification of several modes before performing the fitting, greatly reducing the number of assumptions we must make about mode identification. We find a very thin helium layer for this relatively hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling. At least a few of the modes appear to be stable on evolutionary timescales and could allow us to obtain a measurement of the rate of cooling with monitoring of the star over the course of the next few years with ground-based follow-up.
Time series photometry of the helium atmosphere pulsating white dwarf EC 04207-474
P. Chote,D. J. Sullivan,M. H. Montgomery,J. L. Provencal
Physics , 2014, DOI: 10.1093/mnras/stt180
Abstract: We present the analysis of 71 hours of high quality time-series CCD photometry of the helium atmosphere pulsating white dwarf (DBV) EC 04207-4748 obtained using the facilities at Mt John University Observatory in New Zealand. The photometric data set consists of four week-long observing sessions covering the period March to November 2011. A Fourier analysis of the lightcurves yielded clear evidence of four independent eigenmodes in the star with the dominant mode having a period of 447 s. The lightcurve variations exhibit distinct nonsinusoidal shapes, which results in significant harmonics of the dominant frequency appearing in the Fourier transforms. These observed variations are interpreted in terms of nonlinear contributions from the energy flux transmission through the subsurface convection zone in the star. Our modelling of this mechanism, using the methods first introduced by Montgomery (2005), yields a time-averaged convective response time of tau_0 ~ 150 s for the star, and this is shown to be broadly consistent with a MLT/alpha parameter value between 0.8 and 1.2. It is argued that for the DBV pulsators the measured value of tau_0 is a better estimate of the relative stellar surface temperatures than those obtained via spectroscopic techniques.
Evidence for Temperature Change and Oblique Pulsation from Light Curve Fits of the Pulsating White Dwarf GD 358
M. H. Montgomery,J. L. Provencal,A. Kanaan,Anjum S. Mukadam,S. E. Thompson,J. Dalessio,H. L. Shipman,D. E. Winget,S. O. Kepler,D. Koester
Physics , 2010, DOI: 10.1088/0004-637X/716/1/84
Abstract: Convective driving, the mechanism originally proposed by Brickhill (1991, 1983) for pulsating white dwarf stars, has gained general acceptance as the generic linear instability mechanism in DAV and DBV white dwarfs. This physical mechanism naturally leads to a nonlinear formulation, reproducing the observed light curves of many pulsating white dwarfs. This numerical model can also provide information on the average depth of a star's convection zone and the inclination angle of its pulsation axis. In this paper, we give two sets of results of nonlinear light curve fits to data on the DBV GD 358. Our first fit is based on data gathered in 2006 by the Whole Earth Telescope (WET); this data set was multiperiodic, containing at least 12 individual modes. Our second fit utilizes data obtained in 1996, when GD 358 underwent a dramatic change in excited frequencies accompanied by a rapid increase in fractional amplitude; during this event it was essentially monoperiodic. We argue that GD 358's convection zone was much thinner in 1996 than in 2006, and we interpret this as a result of a short-lived increase in its surface temperature. In addition, we find strong evidence of oblique pulsation using two sets of evenly split triplets in the 2006 data. This marks the first time that oblique pulsation has been identified in a variable white dwarf star.
Atmospheric parameters and carbon abundance for hot DB white dwarfs
Detlev Koester,Judi Provencal,Boris T. G?nsicke
Physics , 2014, DOI: 10.1051/0004-6361/201424231
Abstract: Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. This is particularly unfortunate, because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - detected by Dufour et al. (2007), with spectra dominated by carbon lines. The analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution white dwarfs of spectral classes DB and hot DQ. High resolution UV spectra of five DBs are studied with model atmospheres. We determine stellar parameters and abundances or upper limits of C and Si. These objects are compared with cooler DBs below 20000K. We find photospheric C and no other heavy elements - with extremely high limits on the C/Si ratio - in two of the five hot DBs. We compare various explanations for this unusual composition, which have been proposed in the literature: accretion of interstellar or circumstellar matter, radiative levitation, carbon dredge-up from deeper interior below the helium layer, and a residual stellar wind. None of these explanations is completely satisfactory, and the problem of the origin of the hot DQ remains an open question.
First Kepler results on compact pulsators VIII: Mode identifications via period spacings in $g-$mode pulsating Subdwarf B stars
M. D. Reed,A. Baran,A. C. Quint,S. D. Kawaler,S. J. O'Toole,J. Telting,S. Charpinet,C. Rodriguez-Lopez,R. H. Ostensen,J. L. Provencal,E. S. Johnson,S. E. Thompson,C. Allen,C. K. Middour,H. Kjeldsen,J. Christensen-Dalsgaard
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.18532.x
Abstract: We investigate the possibility of nearly-equally spaced periods in 13 hot subdwarf B (sdB) stars observed with the Kepler spacecraft and one observed with CoRoT. Asymptotic limits for gravity (g-)mode pulsations provide relationships between equal period spacings of modes with differing degrees and relationships between periods of the same radial order but differing degrees. Period transforms, Kolmogorov-Smirnov tests, and linear least-squares fits have been used to detect and determine the significance of equal period spacings. We have also used Monte Carlo simulations to estimate the likelihood that the detected spacings could be produced randomly. Period transforms for nine of the Kepler stars indicate ell=1 period spacings, with five also showing peaks for ell=2 modes. 12 stars indicate ell=1 modes using the Kolmogorov-Smirnov test while another shows solely ell=2 modes. Monte Carlo results indicate that equal period spacings are significant in 10 stars above 99% confidence and 13 of the 14 are above 94% confidence. For 12 stars, the various methods find consistent regular period spacing values to within the errors, two others show some inconsistencies, likely caused by binarity, and the last has significant detections but the mode assignment disagrees between methods. We find a common ell=1 period spacing spanning a range from 231 to 272 s allowing us to correlate pulsation modes with 222 periodicities and that the ell=2 period spacings are related to the ell=1 spacings by the asymptotic relationship $1/\sqrt{3}$. We briefly discuss the impact of equal period spacings which indicate low-degree modes with a lack of significant mode trappings.
Precursor flares in OJ 287
P. Pihajoki,M. Valtonen,S. Zola,A. Liakos,M. Drozdz,M. Winiarski,W. Ogloza,D. Koziel-Wierzbowska,J. Provencal,K. Nilsson,A. Berdyugin,E. Lindfors,R. Reinthal,A. Sillanp??,L. Takalo,M. M. M. Santangelo,H. Salo,S. Chandra,S. Ganesh,K. S. Baliyan,S. A. Coggins-Hill,A. Gopakumar
Physics , 2012, DOI: 10.1088/0004-637X/764/1/5
Abstract: We have studied three most recent precursor flares in the light curve of the blazar OJ 287 while invoking the presence of a precessing binary black hole in the system to explain the nature of these flares. Precursor flare timings from the historical light curves are compared with theoretical predictions from our model that incorporate effects of an accretion disk and post-Newtonian description for the binary black hole orbit. We find that the precursor flares coincide with the secondary black hole descending towards the accretion disk of the primary black hole from the observed side, with a mean z-component of approximately z_c = 4000 AU. We use this model of precursor flares to predict that precursor flare of similar nature should happen around 2020.96 before the next major outburst in 2022.
Pulsational Mapping of Calcium Across the Surface of a White Dwarf
Susan E. Thompson,M. H. Montgomery,T. von Hippel,A. Nitta,J. Dalessio,J. Provencal,W. Strickland,J. A. Holtzman,A. Mukadam,D. Sullivan,T. Nagel,D. Koziel-Wierzbowska,S. Zola,T. Kundera,M. Winiarski,M. Drozdz,E. Kuligowska,W. Ogloza,Zs. Bognar,G. Handler,A. Kanaan,T. Ribeira,R. Rosen,D. Reichart,J. Haislip,B. N. Barlow,B. H. Dunlap,K. Ivarsen,A. LaCluyze,F. Mullally
Physics , 2010, DOI: 10.1088/0004-637X/714/1/296
Abstract: We constrain the distribution of calcium across the surface of the white dwarf star G29-38 by combining time series spectroscopy from Gemini-North with global time series photometry from the Whole Earth Telescope. G29-38 is actively accreting metals from a known debris disk. Since the metals sink significantly faster than they mix across the surface, any inhomogeneity in the accretion process will appear as an inhomogeneity of the metals on the surface of the star. We measure the flux amplitudes and the calcium equivalent width amplitudes for two large pulsations excited on G29-38 in 2008. The ratio of these amplitudes best fits a model for polar accretion of calcium and rules out equatorial accretion.
Periodic Variations in the O-C Diagrams of Five Pulsation Frequencies of the DB White Dwarf EC 20058-5234
James Dalessio,Denis Sullivan,Judi Provencal,Harry Shipman,Tiri Sullivan,Dave Kilkenny,Luciano Fraga,Ramotholo Sefako
Physics , 2013, DOI: 10.1088/0004-637X/765/1/5
Abstract: Variations in the pulsation arrival time of five independent pulsation frequencies of the DB white dwarf EC 20058-5234 individually imitate the effects of reflex motion induced by a planet or companion but are inconsistent when considered in unison. The pulsation frequencies vary periodically in a 12.9 year cycle and undergo secular changes that are inconsistent with simple neutrino plus photon-cooling models. The magnitude of the periodic and secular variations increases with the period of the pulsations, possibly hinting that the corresponding physical mechanism is located near the surface of the star. The phase of the periodic variations appears coupled to the sign of the secular variations. The standards for pulsation-timing-based detection of planetary companions around pulsating white dwarfs, and possibly other variables such as subdwarf B stars, should be re-evaluated. The physical mechanism responsible for this surprising result may involve a redistribution of angular momentum or a magnetic cycle. Additionally, variations in a supposed combination frequency are shown to match the sum of the variations of the parent frequencies to remarkable precision, an expected but unprecedented confirmation of theoretical predictions.
Understanding the Cool DA White Dwarf, G29-38
S. J. Kleinman,R. E. Nather,D. E. Winget,J. C. Clemens,P. A. Bradley,A. Kanaan,J. L. Provencal,C. F. Claver,T. K. Watson,K. Yanagida,A. Nitta,J. S. Dixson,M. A. Wood,A. D. Grauer,B. P. Hine,G. Fontaine,James Liebert,D. J. Sullivan,D. T. Wickramasinghe,N. Achilleos. T. M. K. Marar,S. Seetha,B. N. Ashoka,E. Meistas,E. M. Leibowitz,P. Moskalik,J. Krzesinski,J. -E. Solheim,A. Bruvold,D. W. Kurtz,B. Warner,Peter Martinez,G. Vauclair,N. Dolez,M. Chevreton,M. A. Barstow,S. O. Kepler,O. Giovannini,T. Augusteijn,C. J. Hansen,S. D. Kawaler
Physics , 1997, DOI: 10.1086/305259
Abstract: 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, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, 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. Modelling 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 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 modelling, thereby joining the rest of the known white dwarf pulsators.
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