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Convection is a highly turbulent, three dimensional process that is traditionally treated using a simple, local, time independent description. Convection is one of the largest sources of theoretical uncertainty in stellar modeling. We outline recent progress in studies using pulsating white dwarfs to constrain convection and calibrate mixing length theory.

We aim to investigate variations in the arrival time of coherent stellar pulsations due to the light-travel time effect to test for the presence of sub-stellar companions. Those companions are the key to one possible formation scenario of apparently single sub-dwarf B stars. We made use of an extensive set of ground-based observations of the four large amplitude p-mode pulsators DW Lyn, V1636 Ori, QQ Vir, and V541 Hya. Observations of the TESS space telescope are available on two of the targets. The timing method compares the phase of sinusoidal fits to the full multi-epoch light curves with phases from the fit of a number of subsets of the original time series. Observations of the TESS mission do not sample the pulsations well enough to be useful due to the (currently) fixed two-minute cadence. From the ground-based observations, we infer evolutionary parameters from the arrival times. The residual signals show many statistically significant periodic signals, but no clear evidence for changes in arrival time induced by sub-stellar companions. The signals can be explained partly by mode beating effects. We derive upper limits on companion masses set by the observational campaign., 22 pages, 23 figures, accepted for publication in A&A

GD 358 is the brightest (mv=13.7) and best studied helium atmosphere white dwarf pulsator. We present an analysis based on over 1000 hours of observations spanning 2007-2014 as well as archival data going back to 1982. From the complete data set, we identify a total of 27 independent frequencies and fit 14 of them as m=0 modes in our asteroseismic analysis. We add GD358 to a set of helium atmosphere white dwarfs fitted with similar models. With this consistent set, we can see a trend in the thickness of the pure helium layer that are quantitatively consistent with time-dependent diffusion calculations.

We present a new analysis of the rapidly oscillating Ap (roAp) star, 2MASS J$19400781-4420093$ (J1940; $V=13.1$). The star was discovered using SuperWASP broadband photometry to have a frequency of 176.39 d$^{-1}$ (2041.55 $\mu$Hz; $P = 8.2$ min; Holdsworth et al. 2014a) and is shown here to have a peak-to-peak amplitude of 34 mmag. J1940 has been observed during three seasons at the South African Astronomical Observatory, and has been the target of a Whole Earth Telescope campaign. The observations reveal that J1940 pulsates in a distorted quadrupole mode with unusual pulsational phase variations. A higher signal-to-noise ratio spectrum has been obtained since J1940's first announcement, which allows us to classify the star as A7 Vp Eu(Cr). The observing campaigns presented here reveal no pulsations other than the initially detected frequency. We model the pulsation in J1940 and conclude that the pulsation is distorted by a magnetic field of strength $1.5$ kG. A difference in the times of rotational maximum light and pulsation maximum suggests a significant offset between the spots and pulsation axis, as can be seen in roAp stars., Comment: 14 pages, 16 figures, 12 tables, accepted for publication in MNRAS

White dwarfs represent the end point of stellar evolution for the majority of stars. As such, they are excellent astrophysical laboratories. They are structurally simple, with electron degenerate cores surrounded by thin surface layers of helium and/or hydrogen. The g-mode pulsations provide a window into their internal structure. The Whole Earth Telescope has been conducting a long-term survey of pulsating white dwarfs with the goal of providing an empirical map of convection parameters across the DA and DB instability strips. We present an overview of white dwarf asteroseismology, and discuss the current status of our survey.

We report on the analysis of 34 years of photometric observations of the pulsating helium atmosphere white dwarf GD358. The complete data set includes archival data from 1982-2006, and 1195.2 hours of new observations from 2007- 2016. From this data set, we extract 15 frequencies representing g-mode pulsation modes, adding 4 modes to the 11 modes known previously. We present evidence that these 15 modes are ell = 1 modes, 13 of which belong to a consecutive sequence in radial overtone k. We perform a detailed asteroseismic analysis using models that include parameterized, complex carbon and oxygen core composition profiles to fit the periods. Recent spectroscopic analyses place GD358 near the red edge of the DBV instability strip, at 24,000 plus or minus 500 K and a log g of 7.8 plus or minus 0.08 dex. The surface gravity translates to a mass range of 0.455 to 0.540 solar masses. Our best fit model has a temperature of 23,650 K and a mass of 0.5706 solar masses. That is slightly more massive than suggested by most the recent spectroscopy. We find a pure helium layer mass of 10^-5.50, consistent with the result of previous studies and the outward diffusion of helium over time., Comment: 94 pages, 14 figures, 7 tables, Accepted for publication in the ApJ

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.

We analyse the light curve in the R band of the blazar OJ287, gathered during the 2015/2016 observing season. We did a search for quasi-periodic oscillations (QPOs) using several methods over a wide range of timescales. No statistically significant periods were found in the high-frequency domain both in the ground-based data and in Kepler observations. In the longer-period domain, the Lomb–Scargle periodogram revealed several peaks above the 99% significance level. The longest one—about 95 days—corresponds to the innermost stable circular orbit (ISCO) period of the more massive black hole. The 43-day period could be an alias, or it can be attributed to accretion in the form of a two-armed spiral wave.

We analyse the light curve in the R-band gathered during the 2015/16 observing season of OJ287. We did a search for QPOs using several methods in wide time domain. No statistically significant periods were found in the high frequency domain both in the ground data and also in K2 observations. In the longer periods domain the Lomb-Scargle periodogram revealed several peaks above the 99\% significance level. The longest one, about 95-day, corresponds to the ISCO period of the more massive SMBH. The 43-day period could be an alias or can be attributed to accretion in the form of two armed spiral wave.

We present the detection of non-radial oscillations in a hot, helium-atmosphere white dwarf using 78.7 d of nearly uninterrupted photometry from the Kepler space telescope. With an effective temperature >30,000 K, PG 0112+104 becomes the hottest helium-atmosphere white dwarf known to pulsate. The rich oscillation spectrum of low-order g-modes includes clear patterns of rotational splittings from consecutive sequences of dipole and quadrupole modes, which can be used to probe the rotation rate with depth in this highly evolved stellar remnant. We also measure a surface rotation rate of 10.17404 hr from an apparent spot modulation in the K2 data. With two independent measures of rotation, PG 0112+104 provides a remarkable test of asteroseismic inference., 8 pages, 7 figures, accepted for publication in the Astrophysical Journal

PG 1351+489 is one of the 20 DBVs ― pulsating helium-atmosphere white dwarf stars ― known and has the simplest power spectrum for this class of star, making it a good candidate to study cooling rates. We report accurate period determinations for the main peak at 489.334 48 s and two other normal modes using data from the Whole Earth Telescope (WET) observations of 1995 and 2009. In 2009, we detected a new pulsation mode and the main pulsation mode exhibited substantial change in its amplitude compared to all previous observations. We were able to estimate the star's rotation period, of 8.9 h, and discuss a possible determination of the rate of period change of (2.0 ± 0.9) x 10 ―13 s s ―1 , the first such estimate for a DBV.

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.

Advanced ovarian carcinoma in early progression (6 months) (AOCEP) is considered resistant to most cytotoxic drugs. Gemcitabine (GE) and oxaliplatin (OXA) have shown single-agent activity in relapsed ovarian cancer. Their combination was tested in patients with AOCEP in phase II study. Fifty patients pre-treated with platinum-taxane received q3w administration of OXA (100 mg m(-2), d1) and GE (1000 mg m(-2), d1, d8, 100-min infusion). Patient characteristics were a : median age 64 years (range 46-79),and 1 (84%) or 2 (16%) earlier lines of treatment. Haematological toxicity included grade 3-4 neutropaenia (33%), anaemia (8%), and thrombocytopaenia (19%). Febrile neutropaenia occurred in 3%. Non-haematological toxicity included grade 2-3 nausea or vomiting (34%), grade 3 fatigue (25%),and grade 2 alopecia (24%). Eighteen (37%) patients experienced response. Median progression-free (PF) and overall survivals (OS) were 4.6 and 11.4 months, respectively. The OXA-GE combination has high activity and acceptable toxicity in AOCEP patients. A comparison of the doublet OXA-GE with single-agent treatment is warranted.

We obtained 74 hours of time-resolved CCD photometry of the pulsating DA white dwarf star WD 1524-0030 from three different sites well separated in longitude. We found evidence for amplitude variability with relative changes of ±10% and detected a total of 15 independent and 10 combination frequencies in our light curves. The large number of excited modes, the high amplitudes and nonsinusoidal light curves, the apparent brightness and the equatorial location on the sky make WD 1524-0030 an attractive target for future campaigns with the goal of asteroseismology and nonlinear light curve fitting.

The Stellar Photometry Assistant, SPA, is a stand alone software package for time-series photometry reduction and analysis slated for an initial test release in spring 2008. The goal of SPA is to be simple, powerful, and intuitive. SPA was born out of complications in studying the pulsating DB white dwarf EC20058-5234 (QU Tel) due to the proximity of its nearby companions. SPA also addresses the Whole Earth Telescope’s (WET) demand for large scale rapid data reduction from multiple sites. SPA is being developed in Matlab by the Delaware Asteroseismologic Research Center (DARC) in collaboration with the University of Delaware and the Mount Cuba Astronomical Observatory. The need for SPA is addressed, and key features of the program are listed and discussed.

To determine the feasibility of two chemotherapy regimens in elderly patients with advanced ovarian carcinoma (AOC).Eighty-three patientsor=70 years were previously enrolled in a trial evaluating carboplatin and cyclophosphamide (CC). On the basis of identical eligibility criteria, 75 further patients were enrolled in a trial evaluating carboplatin and paclitaxel (Taxol) (CP). The primary end point of these studies was the feasibility of six courses of chemotherapy. Comprehensive geriatric assessment (CGA) parameters were assessed in terms of prognostic factors.More patients in the CC group presented with performance status of two or more, depression symptoms, use of co-medications, hypoalbuminemia, abnormal Mini-Mental Status score, or sub-optimal surgery. Both regimens appeared feasible: 75.6% in the CC group and 68.1% in the CP group completed six courses. CC and CP groups had similar overall survival (OS). Independent prognostic factors of poorer OS were the following: increasing age (P = 0.013), depression symptoms at baseline (P0.001), International Federation of Gynecology and Obstetrics stage IV (P = 0.001), and use of paclitaxel (P = 0.025).As this is a non-randomised retrospective review of two consecutive studies, no firm conclusion can be drawn. It seems, however, that in elderly patients with AOC the use of paclitaxel results in more toxicity. CGA parameters and particularly emotional disorders might help to determine a priori the risk/benefit ratio of chemotherapy in this patient population.

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 l and 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 relative hot DB, which adds evidence to the hypothesis that helium diffuses outward during DB cooling.

We are trying to reduce the largest uncertainties in using white dwarf stars as Galactic chronometers by understanding the details of carbon crystalliazation that currently result in a 1-2 Gyr uncertainty in the ages of the oldest white dwarf stars. We expect the coolest white dwarf stars to have crystallized interiors, but theory also predicts hotter white dwarf stars, if they are massive enough, will also have some core crystallization. BPM 37093 is the first discovered of only a handful of known massive white dwarf stars that are also pulsating DAV, or ZZ Ceti, variables. Our approach is to use the pulsations to constrain the core composition and amount of crystallization. Here we report our analysis of 4 hours of continuous time series spectroscopy of BPM 37093 with Gemini South combined with simultaneous time-series photometry from Mt. John (New Zealand), SAAO, PROMPT, and Complejo Astronomico El Leoncito (CASLEO, Argentina)., La lista completa de autores que integran el documento puede consultarse en el archivo, Facultad de Ciencias Astronómicas y Geofísicas

Background: Data from prospective clinical trials are needed to better define standards of care in elderly patients with advanced ovarian carcinoma and to demonstrate the interest of Comprehensive Geriatric Assessment (CGA) in this fragile and heterogeneous population. Patients and methods: From July 1998 to October 2000, 83 advanced ovarian carcinoma patients >70 years old received carboplatin AUC 5 and cyclophosphamide 600 mg/m2, on day 1 of six 28-day cycles. The clinical and biological geriatric covariates prospectively studied were: comorbidities, comedications, cognitive functions (Mini-Mental test), nutritional status and autonomy. Results: Patient characteristics were: median age 76 years, serous histology (73%), FIGO stage III (75%), optimal initial surgery (21%) and performance status (PS) ≥2 (44%). Sixty patients (72%) received six chemotherapy cycles without severe toxicity (STox) or tumor progression. Multivariate analysis retained three factors as independent predictors of STox: symptoms of depression at baseline (P = 0.006), dependence (P = 0.048) and PS ≥2 (P = 0.026). Independent prognostic factors identified for overall survival (Cox model) were depression (P = 0.003), FIGO stage IV (P = 0.007) and more than six different comedications per day (P = 0.043). Conclusion: CGA could predict STox and overall survival of elderly advanced ovarian carcinoma patients.

Procyon B is one of a select group of white dwarf stars in visual binaries that are used to put stringent constraints on the fundamental physics of stellar degeneracy. We present Space Telescope Imaging Spectrograph observations of this elusive white dwarf. Our spectra cover 1800-10000 A and contain carbon Swan bands, Mg II, and numerous iron features. Procyon B is therefore a rare DQZ white dwarf. Our analysis of the spectrum yields an effective temperature of 7740 ± 50 K and a radius of 0.01234 ± 0.00032 R☉. We discuss the implications of these results on observational support for the white dwarf mass-radius relation.

The roles played by carbon and hydrogen in helium atmosphere (DB) white dwarf evolution is not well understood. We present GHRS observations (centered at 1215 and 1335 A) of the hot helium white dwarfs PG 0112+104 (≈30,000 K) and GD 190 (≈23,000 K) and EUVE observations of the well-studied DB GD 358 (≈27,000 K) and revisit pre-COSTAR observations of GD 358. We detect Lyα and carbon absorption features in all three of our hot DB white dwarfs. Using the non-LTE model atmosphere code TLUSTY, we calculate carbon and hydrogen surface abundances and mass fractions. We discuss the implications of our analysis for two aspects of DB evolution. First, we find hydrogen surface abundances corresponding to mass fractions over an order of magnitude too low to explain the observed lack of helium atmosphere white dwarfs between 45,000 and 30,000 K unless we allow extensive mixing in DB atmospheres. Second, the photospheric carbon in all three objects cannot be explained by interstellar accretion or conventional convective dredge-up. We present evidence for substantial horizontal motions in two of our objects that may explain both the presence of carbon and the lower than predicted hydrogen surface abundance.

We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k=14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for l=2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set.

The parallax and astrometric orbit of Procyon have been redetermined from PDS measurements of over 250 photographic plates spanning 83 years, with roughly 600 exposures used in the solution. These data are combined with two modern measurements of the primary–white dwarf separation, one utilizing a ground-based coronagraph, the other, the Planetary Camera (PC) of the Hubble Space Telescope. Together with the redetermined astrometric orbit and parallax, these yield new estimates of the component masses. The derived masses are 1.497 ± 0.037 M⊙ for the primary and 0.602 ± 0.015M⊙ for the white dwarf secondary. These mass values are heavily weighted by the PC separation measurement, which, while being somewhat discordant with the ground-based measures, we argue is more precise and more accurate and thus deserving of its greater weight. This stated, the long-standing discrepancy between previous determinations of the observed mass of Procyon A (1.75 M⊙) and the value supported by stellar evolution models (1.50 M⊙) appears to be reconciled.

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 pre-white 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 M☉. 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 dΠ/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 astrometric satellite HIPPARCOS has determined the distance to the important white dwarf 40 Eridani B with an accuracy surpassing 1%. We use this datum to redetermine the mass and radius of this star and find that M = 0.501 ± 0.011 M☉ and R = 0.0136 ± 0.00024 R☉. These values, the most accurately determined masses and radii for any white dwarf star, place 40 Eri B securely on the zero-temperature carbon core mass-radius relation, considerably improving observational confirmation of the theory of stellar degeneracy. The mass is now sufficiently high that binary star evolutionary channels are no longer required to account for it. The data exclude the possibility that the surface hydrogen layer is as thick as 10-4 M☉.

Procyon B is one of a select group of white dwarf stars which are members of visual binary systems and represents a critically important object for testing the fundamental physics of stellar degeneracy. We have accurate measurements of the binary period and the white dwarf mass. However, the extreme brightness ratio and the small separation between primary and secondary makes accurate ground-based determinations of basic characteristics, such as effective temperature and chemical composition, which we require to derive stellar radius, impossible to obtain. Furthermore, since the age of this system is known, a temperature determination for Procyon B constrains its cooling age, providing an estimate of the progenitor mass, a stringent test of theories of stellar mass loss. We present the results of photometry from a series of WFPC2 images, from which we determine an apparent magnitude of mV = 10.92 ± 0.05, and an effective temperature of 8688 ± 200 K, based on Hubble Space Telescope equivalent UBVRI filter measurements. Our more uncertain, problematic, narrow filter results argue a helium composition is more plausible for Procyon B. The stellar radius we derive from these parameters, assuming a mass of 0.622 ± 0.023 M☉, is 0.0096 ± 0.0005 R☉, inconsistent with the carbon-core, zero-temperature mass-radius relation of Hamada & Salpeter. This implies that Procyon B has a heavier core than carbon, calling into question the assumption of carbon-core composition commonly used for white dwarf stars.

We present our analysis of 240 hr of white-light, high-speed photometry of the dwarf nova-like helium variable PG 1346+082 (CR Boo). We identify two frequencies in the low-state power spectrum, at 679.670 ± 0.004 μHz and 669.887 ± 0.008 μHz. The 679.670 μHz variation is coherent over at least a 2 week time span, the first demonstration of a phase-coherent photometric variation in any dwarf nova-like interacting binary white dwarf system. The high-state power spectrum contains a complex fundamental with a frequency similar, but not identical, to the low-state spectrum, and a series of harmonics not detected in low state. We also uncover an unexpected dependence of the high-frequency power's amplitude and frequency structure on overall system magnitude. We discuss these findings in light of the general AM CVn system model, particularly the implications of the high-order harmonics on future studies of disk structure, mass transfer, and disk viscosity.

This paper presents the results of high speed photo­ metric observations of the cool variable DA white dwarf (DAV) GD 154 obtained with the Whole Earth Telescope. GD 154 is one of the c;oolest pulsating DA white dwarfs and its study is important for understanding the red edge of the ZZ Ceti insta­ bility strip. Its power spectrum is dominated by three indepen­ dent modes (P 1 = 1186.5s, Pz = 1088.6s and P3 = 402.6s), and their harmonics and linear combinations. None of the half-integer harmonics reported in previous observations were present during the WET campaign. We propose that the ob­ served modes are trapped in the thin outer hydrogen layers. From the resulting identification of the pulsation modes, one derives an estimate of the rotation period (2.3 days) and of the mass ofthe outer hydrogen layer (2 X w-IO M *).

We report the discovery of a new AM CVn system on the basis of broadband photometry obtained with the Kepler satellite supplemented by ground-based optical spectroscopy. Initially retained on Kepler target lists as a potential compact pulsator, the blue object SDSS J190817.07+394036.4 (KIC 004547333) has turned out to be a high-state AM CVn star showing the He-dominated spectrum of its accretion disk significantly reddened by interstellar absorption. We constructed new grids of NLTE synthetic spectra for accretion disks in order to analyze our spectroscopic observations. From this analysis, we infer preliminary estimates of the rate of mass transfer, the inclination angle of the disk, and the distance to the system. The AM CVn nature of the system is also evident in the Kepler light curve, from which we extracted 11 secure periodicities. The luminosity variations are dominated by a basic periodicity of 938.507 s, likely to correspond to a superhump modulation. The light curve folded on the period of 938.507 s exhibits a pulse shape that is very similar to the superhump wavefront seen in AM CVn itself, which is a high-state system and the prototype of the class. Our Fourier analysis also suggests the likely presence of a quasi-periodic oscillation similar to those already observed in some high-state AM CVn systems. Furthermore, some very low-frequency, low-amplitude aperiodic photometric activity is likely present, which is in line with what is expected in accreting binary systems. Inspired by previous work, we further looked for and found some intriguing numerical relationships between the 11 secure detected frequencies, in the sense that we can account for all of them in terms of only three basic clocks. This is further evidence in favor of the AM CVn nature of the system.

We present 13 years of pulsation timing measurements of the DBV white dwarf EC 20058−5234. 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.

The ZZ Ceti stars form a class of variable white dwarfs: the hydrogen dominated atmosphere ones, which do pulsate in an instability strip in the effective temperature range 13000K-11500K. We know 22 such ZZ Ceti white dwarfs. Their variations are caused by nonradial g-mode pulsations with periods are in the range 100-1000 seconds.A subsample of the ZZ Ceti stars shows amplitude variations on time scales of the order of one month. These variations could be driven by nonlinear phenomena.

We present 13 years of pulsation timing measurements of the DBV white dwarf EC20058‐5234. These measurements are part of a campaign to measure the plasmon neutrino emission rate for this object. Our data is inconsistent with a simple photon plus neutrino WD cooling model. Under a certain set of assumptions, we constrain the neutrino emission rate. We describe our methodology and address the impact of our data on pulsation timing based planetary searches.

EC14012-1446 is a hydrogen atmosphere (DA) white dwarf pulsator. Its rich pulsation spectrum displays a range of excited modes with complex multiplet structure, in addition to numerous combination frequencies. In April 2008, EC14012-1446 was the primary target of XCOV26. We obtained over 300 hrs of nearly continuous high speed photometry with the goal of using the nonlinear pulse shapes to empirically determine the parameters of the convection zone. The Fourier transform (FT) of the light curve contains power between 1000 to 4000 μHz, with the dominant peak at 1234 μHz. We find 13 independent frequencies distributed in 8 modes, as well as a myriad of combination frequencies. In the following, we present preliminary results and lay the groundwork for future investigation leading to light curve fitting of EC14012-1446.

On the basis of our preliminary analysis of the harmonic structure, we suggest that the 1493 s oscillation is either the orbital period of the system or the rotation period of a magnetized, accreting white dwarf. The 1471 s is most probably a nonradial oscillation of the primary white dwarf since its character mimics that of oscillations seen in single DB variables.

Context. PG 1159-035, a pre-white dwarf with Teff ≃ 140000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to measure the stellar mass, the rotational period and the inclination of the rotation axis, to estimate an upper limit for the magnetic field, and even to obtain information about the inner stratification of the star. Aims. We have three principal aims: to increase the number of detected and identified pulsation modes in PG 1159-035, study trapping of the star's pulsation modes, and to improve or constrain the determination of stellar parameters. Methods. We used all available WET photometric data from 1983, 1985, 1989. 1993 and 2002 to identify the pulsation periods. Results. We identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M/M⊙= 0.59 ±0.02 for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at rc/R* = 0.83 ± 0.05. From the multiplet splitting, we calculated the rotational period Prot = 1.3920 ± 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for ℓ = 1 modes and less than 50% for ℓ = 2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars., La lista completa de autores que integran el documento puede consultarse en el archivo., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

The pulsating DAV white dwarf G185-32 was observed with the Whole Earth Telescope in 1992, during XCov8. We briefly report on the weighted Fourier transform of the data obtained.

We present results from a multisite (‘Whole Earth Telescope’) photometric campaign on PG 1336−018, the close eclipsing binary system containing a pulsating subdwarf B (sdB) star. The main part of the campaign (1999 April) resulted in ~172 h of observations, representing a coverage of about 47 per cent, and additional data were obtained outside the core campaign. Periodogram analysis shows that the light variations are dominated by three frequencies near 5757, 5585 and 5369 μHz (~174, 179 and 186s, respectively), although many frequencies are present, particularly in the range 5000–6000 μHz (~200–170 s). We identify, with some confidence, 28 frequencies down to a semi-amplitude of 0.0005 in fractional intensity (equivalent to about 0.5 mmag). It is clear that the pulsation frequencies of PG 1336−018 have changed substantially since the 1996 discovery observations were made, and that amplitude changes occur, at least in the dominant three frequencies, on relatively short time-scales (of the order of a day). On the assumption that the pulsating star is phase-locked in the binary system, we have searched for rotational splitting of frequencies near the orbital and half of the orbital period, but the results are confused by aliasing at those frequencies (due to the data gaps caused by the eclipses). A preliminary model qualitatively matches the distribution of frequencies in PG 1336−018, with some good individual correspondences, but cannot be considered adequate because geometric cancellation should hide some of the modes which are apparently detected. Analysis of the pulsations during eclipse recovers three of the strongest modes, but the limited eclipse data – which can, at best, be only about 9 per cent of the total – do not allow mode identification at this stage. Simulations indicate that an overall coverage of about 80 per cent would be required for this to be viable. An attempt was made to determine phase shifts in the pulsation frequencies as a way of directly measuring the size of the binary orbit, but the uncertainties in the method are comparable to the light travel time across the orbit (probably less than a second).

The pulsating PG 1159 planetary nebula central star RXJ 2117+3412 has been observed over three successive seasons of a multisite photometric campaign. The asteroseismological analysis of the data, based on the 37 identified l = 1 modes among the 48 independent pulsation frequencies detected in the power spectrum, leads to the derivation of the rotational splitting, the period spacing and the mode trapping cycle and amplitude, from which a number of fundamental parameters can be deduced. The average rotation period is 1.16 ± 0.05 days. The trend for the rotational splitting to decrease with increasing periods is incompatible with a solid body rotation. The total mass is 0.56⁺⁰˙⁰²₋₀.₀₄ Mѳ and the He-rich envelope mass fraction is in the range 0.013–0.078 M*. The luminosity derived from asteroseismology is log(L/Lѳ) = 4.05⁺⁰˙²³₋₀.₃₂ and the distance 760⁺²³⁰₋₂₃₅ pc. At such a distance, the linear size of the planetary nebulae is 2.9 ± 0.9 pc. The role of mass loss on the excitation mechanism and its consequence on the amplitude variations is discussed.

We report the analysis of 154 hours of nearly continuous high-speed photometric data on the pulsating DB white dwarf (DBV) GD 358 obtained during the Whole Earth Telescope (WET) run of May 1990. The Fourier transform (FT) of the light curve is dominated by power in the range from 1200 – 1700μHz with more than 180 significant peaks in the total transform. We also see significant power at the sums and differences of the dominant frequencies, indicating the importance of nonlinear behavior. We can use this data to obtain an accurate total stellar mass, and surface He layer mass. The implied surface He layer mass, if correct, provides a significant and surprising challenge to stellar evolution theory, as well as the theory of chemical mixing.

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 2 yr 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 l and 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.

We carried out an extensive CCD photometry campaign of the open cluster NGC 3293 that contains eleven known beta Cephei stars. Preliminary results indicate that none of these is singly periodic. Several objects are among the most multiperiodic of these massive pulsators, giving us strong hope to perform precision asteroseismology in an open cluster for the first time. We also report a peculiar group of variables in NGC 3293 that is located near the low-luminosity end of the beta Cephei instability strip. The variability periods of these stars are too long for classical beta Cephei pulsation, but too short for binarity or rotational effects, or for SPB-type pulsation. In addition, we discovered about a dozen delta Scuti stars in the field.

For the ultra-short period binary system AM CVn we obtained time-resolved UV spectroscopy, spanning λ 1260 A to λ 155 A, with the HST Faint Object Spectrograph. Strong, asymetric features of NV and CIV gave direct evidence of mass loss from the system via a stellar wind. AM CVn’s UV spectrum is dominated by features of silicon and maybe iron. The spectra have been binned giving 125 flux points spanning approximately 7 hours. The strongest UV variation corresponds to a modulation period of 1014 + 2 s, and has an amplitude much larger than the 1011.5 s modulation observed in the visible part of the spectrum. This strong UV modulation indicates a different origin for this modulation than for the harmonics of the 1051 s period observed in white light photometry, and maybe we for the first time observe pulsations on the accreting primary star.

Procyon B represents a critical object for testing the fundamental physics of stellar degeneracy. Although this important star is the third brightest white dwarf known, we know very little about it, as Nature has conspired to place it a maximum of 5 arcsec from one of the brightest stars in the sky, rendering ground-based observations of this object nearly impossible. We have a rough visual magnitude of mv≈10.7 (Eggen & Greenstein 1965). The system has a period of 40.82±0.06 yrs, and the white dwarf mass is 0.622±0.023 M⊙ (Girard et al. 1996). Walker et al. (1994) utilize speckle interferometry, and their High Resolution Camera to obtain mv = 11.3, B — V = +0.26, and B — I = +0.62. Unfortunately, their technique is not designed with photometry in mind, and the authors do not quote error estimates. This brief introduction outlines the extent of our knowledge of Procyon B. We do not know its temperature, chemical composition, or accurate visual magnitude.

G29-38 (WD2326+049) is one of the brighter members of the class of variable DA or ZZ Ceti stars. It has one of the largest amplitudes in its variations and shows the typical complex variable lightcurves of other largeamplitude DAVs. Zuckerman and Becklin (1987) found an infrared excess in the stars spectrum above 2, µm. Two different models have been proposed to explain this excess: a brown dwarf companion or a near-by dust cloud. The Whole Earth Telescope observed this star in 1988 and Winget et al. (1990) reported a large phase change in the largest amplitude oscillation, indicating the presence of a massive companion. On the other hand, radial velocity measurements showed that there is no massive companion in the system, although a low-mass orbiting object is still possible (Barnbaum and Zuckerman 1992). However, recent photometric observations over a five year time span (Kleinman et al. 1994) did not show any evidence of orbital motion in the system.

We present preliminary results from Whole Earth Telescope (WET) data on the DBV star PG1115+158. In the Fourier Transform of this data, we have tentatively identified peaks arranged in multiplets with common frequency splitting. The average period spacing between these multiplets is ~ 40s, very similar to the multiplet spacing found in the DBV GD358. Theoretical models show l = 1 modes with ~ 40s period spacing between modes of consecutive k for white dwarfs with niasses near 0.6M ⊙. If we assume the identified multiplets are l = 1 modes, then the rotation rate of the star is 0.45 days. Furthermore, the deviations of the period spacing between modes from the mean period spacing indicates that, like GD358, the surface Helium layer is thin, less than 10-4 M *.