A Survey for Photometric Variability in Isolated Magnetic White Dwarfs—Measuring their Spin Periods

We present the initial findings of a photometric survey of isolated magnetic white dwarfs (MWDs) carried out with the 1.0m Jacobus Kapteyn Telescope. Of our sample of 30 MWDs, we have observed variability in 17 (57%) over our observed timescales (minutes to years), with a further 11 requiring more data, and two that are non-variable at the 1% level. In total we have discovered possible variability in 15 targets that has not been reported before in the literature, and we have measured the spin period of five objects in our sample to within a few percent. We find no correlation between spin period, mass or temperature, but there may be a weak negative correlation between period and field strength for the short-period targets. We have identified 14 MWDs with low field strengths and low temperatures, which are candidates for having star spots on their surfaces and should be followed up with polarimetry. We have also found that three low-field, high temperature MWDs are unexpectedly variable, with no obvious mechanism to cause this

ULTRACAM photometry of the eclipsing cataclysmic variable OU Vir

We present high-speed, three-colour photometry of the faint eclipsing cataclysmic variable OU Vir. For the first time in OU Vir, separate eclipses of the white dwarf and bright spot have been observed. We use timings of these eclipses to derive a purely photometric model of the system, obtaining a mass ratio of q = 0.175 +/- 0.025, an inclination of i = 79.2 +/- 0.7 degrees and a disc radius of Rd/a = 0.2315 +/- 0.0150. We separate the white dwarf eclipse from the lightcurve and, by fitting a blackbody spectrum to its flux in each passband, obtain a white dwarf temperature of T = 21700 +/- 1200 K and a distance of D = 650 +/- 210 pc. Assuming that the primary obeys the Nauenberg (1972) mass-radius relation for white dwarfs and allowing for temperature effects, we also find a primary mass Mw/Msun = 0.90 +/- 0.19, primary radius Rw/Rsun = 0.0097 +/- 0.0031 and orbital separation a/Rsun = 0.75 +/- 0.05.Comment: 8 pages LaTeX, 6 figures. Accepted by MNRAS; erratum added at end. Mon.Not.Roy.Astron.Soc. 347 (2004) 1173, erratum in pres

Constraints on the Lifetimes of Disks Resulting from Tidally Destroyed Rocky Planetary Bodies

Spitzer IRAC observations of 15 metal-polluted white dwarfs reveal infrared excesses in the spectral energy distributions of HE 0110-5630, GD 61, and HE 1349-2305. All three of these stars have helium-dominated atmospheres, and their infrared emissions are consistent with warm dust produced by the tidal destruction of (minor) planetary bodies. This study brings the number of metal-polluted, helium and hydrogen atmosphere white dwarfs surveyed with IRAC to 53 and 38 respectively. It also nearly doubles the number of metal-polluted helium-rich white dwarfs found to have closely orbiting dust by Spitzer. From the increased statistics for both atmospheric types with circumstellar dust, we derive a typical disk lifetime of log[t_{disk} (yr)] = 5.6+-1.1 (ranging from 3*10^4 - 5*10^6 yr). This assumes a relatively constant rate of accretion over the timescale where dust persists, which is uncertain. We find that the fraction of highly metal-polluted helium-rich white dwarfs that have an infrared excess detected by Spitzer is only 23 per cent, compared to 48 per cent for metal-polluted hydrogen-rich white dwarfs, and we conclude from this difference that the typical lifetime of dusty disks is somewhat shorter than the diffusion time scales of helium-rich white dwarf. We also find evidence for higher time-averaged accretion rates onto helium-rich stars compared to the instantaneous accretion rates onto hydrogen-rich stars; this is an indication that our picture of evolved star-planetary system interactions is incomplete. We discuss some speculative scenarios that can explain the observations.Comment: 20 pages, 9 figures, accepted to be published in Ap

Photometric Variability and Rotation in Magnetic White Dwarfs

We present a search for long term (months—years) photometric variability in a sample of ten isolated magnetic white dwarfs using observations taken with the Liverpool Robotic Telescope between March 2005 and January 2007. These stars had previously been found to be photometrically stable on short (hours—one week) timescales [1]. We construct differential light curves for each target and then use CLEAN and Lomb‐Scargle periodograms to determine any periodicity that may be present. Photometric variability is detected in two of the targets during the observed timescale—G 240–72 and G 227–28. We find no variability in the remaining eight targets above the 1% level. Finally, we search for any correlations between the spin periods and intrinsic physical properties of magnetic white dwarfs, such as the magnetic field strength, temperature, mass and age

The Mid-Infrared Spectrum of the Short Orbital Period Polar EF Eridani from the Spitzer Space Telescope

We present the first mid-infrared (5.5-14.5 micron) spectrum of a highly magnetic cataclysmic variable, EF Eridani, obtained with the Infrared Spectrograph on the Spitzer Space Telescope. The spectrum displays a relatively flat, featureless continuum. A spectral energy distribution model consisting of a 9500 K white dwarf, L5 secondary star, cyclotron emission corresponding to a B~13 MG white dwarf magnetic field, and an optically thin circumbinary dust disk is in reasonable agreement with the extant 2MASS, IRAC, and IRS observations of EF Eri. Cyclotron emission is ruled out as a dominant contributor to the infrared flux density at wavelengths >3 microns. The spectral energy distribution longward of ~5 microns is dominated by dust emission. Even longer wavelength observations would test the model's prediction of a continuing gradual decline in the circumbinary disk-dominated region of the spectral energy distribution.Comment: To be published in The Astrophysical Journa

Spitzer Space Telescope Observations of Circumbinary Dust Disks around Polars

We present Spitzer Space Telescope IRAC photometry of the magnetic cataclysmic variables EF Eri, MR Ser, VV Pup, V834 Cen, GG Leo and V347 Pav. When we combine our results with the 2MASS data, we find that at least five of the polars have flux densities in the mid-IR in excess of the emission expected from the stellar components alone. We are unable to model this mid-IR excess with cyclotron emission, but we can recreate the observed spectral energy distributions with the inclusion of a simple circumbinary dust disk model. Importantly, we find that the masses of our modelled disks are approximately 12 orders of magnitude lower than required to significantly affect CV evolution. The accretion disk-less polars are ideal places to search for these disks, since the luminous accretion disk in most CVs would drown out the faint IR signature of the cooler, dimmer circumbinary disks

Perceived exercise barriers are reduced and benefits are improved with lifestyle modification in overweight and obese women with polycystic ovary syndrome: a randomised controlled trial

Background: This study assessed the perceived benefits and barriers to exercise participation in overweight and obese women with polycystic ovary syndrome (PCOS) and monitored changes in response to a lifestyle intervention. Methods: Forty-three overweight/obese PCOS women (Age, 30.3(6.2) yrs; BMI, 36.4(5.6) kg/m2) were randomised to one of three 20-week lifestyle programs: diet only (DO, n = 13), diet and aerobic exercise (DA, n = 11) and diet and combined aerobic-resistance exercise (DC, n = 19). Exercise Benefits/Barriers Scale (EBBS), weight, aerobic fitness, depression and PCOS specific health-related quality of life were measured. Results: Barriers score was related to depression (r = 0.45, P = 0.002) and aerobic fitness (r = −0.32, P = 0.04), while benefits score was related to aerobic fitness (r = 0.41, P = 0.007). EBBS, benefits and barriers scores improved overtime (P ≤ 0.001). Benefits subscales psychological outlook and social interaction increased (P ≤ 0.001) and life enhancement and preventative health did not change (P ≥ 0.3). Physical performance increased only in DA (P = 0.009). There were no differences between treatments for any of the other subscales (P ≥ 0.2). Barriers subscales exercise milieu, time expenditure and physical exertion reduced (P ≤ 0.003) and family discouragement did not change (P = 0.6). Conclusions: This study demonstrated that lifestyle modification consisting of an energy-restricted diet with or without exercise training improved the perceived benefits from and barriers to exercise.Rebecca L. Thomson, Jonathan D. Buckley and Grant D. Brinkwort

Analytical Method of Approximating the Motion of a Spinning Vehicle with Variable Mass and Inertia Properties Acted Upon by Several Disturbing Parameters

An analytical method has been developed which approximates the dispersion of a spinning symmetrical body in a vacuum, with time-varying mass and inertia characteristics, under the action of several external disturbances-initial pitching rate, thrust misalignment, and dynamic unbalance. The ratio of the roll inertia to the pitch or yaw inertia is assumed constant. Spin was found to be very effective in reducing the dispersion due to an initial pitch rate or thrust misalignment, but was completely Ineffective in reducing the dispersion of a dynamically unbalanced body

Rotational period of WD1953-011 - a magnetic white dwarf with a star spot

WD1953-011 is an isolated, cool (7920 +/- 200K, Bergeron, Legget & Ruiz, 2001) magnetic white dwarf (MWD) with a low average field strength (~70kG, Maxted et al. 2000) and a higher than average mass (~0.74 M_sun, Bergeron et al. 2001). Spectroscopic observations taken by Maxted et al. (2000) showed variations of equivalent width in the Balmer lines, unusual in a low field white dwarf. Here we present V band photometry of WD1953-011 taken at 7 epochs over a total of 22 months. All of the datasets show a sinusoidal variation of approximately 2% peak-to-peak amplitude. We propose that these variations are due to a star spot on the MWD, analogous to a sunspot, which is affecting the temperature at the surface, and therefore its photometric magnitude. The variations have a best-fit period over the entire 22 months of 1.4418 days, which we interpret as the rotational period of the WD.Comment: (1) University of Southampton, (2) University of Warwick, (3) University of Nijmegen, (4) Keele University, (5) University of Leicester. 6 pages, 5 figs, accepted MNRA

A Spitzer Space Telescope Study of the Debris Disks around four SDSS White Dwarfs

We present Spitzer Space Telescope data of four isolated white dwarfs that were previously known to harbor circumstellar gaseous disks. IRAC photometry shows a significant infrared excess in all of the systems, SDSS0738+1835, SDSS0845+2257, SDSS1043+0855 and SDSS1617+1620, indicative of a dusty extension to those disks. The 4.5-micron excesses seen in SDSS0738, SDSS0845, and SDSS1617 are 7.5, 5.7 and 4.5 times the white dwarf contribution, respectively. In contrast, in SDSS1043, the measured flux density at 4.5 microns is only 1.7 times the white dwarf contribution. We compare the measured IR excesses in the systems to models of geometrically thin, optically thick disks, and find that we are able to match the measured SEDs to within 3 sigma of the uncertainties, although disks with unfeasibly hot inner dust temperatures generally provide a better fit than those below the dust sublimation temperature. Possible explanations for the dearth of dust around SDSS1043+0855 are briefly discussed. Including our previous study of SDSS1228+1040, all five white dwarfs with gaseous debris disks have significant amounts of dust around them. It is evident that gas and dust can coexist around these relatively warm, relatively young white dwarfs.Comment: 20 pages, including 4 figures. Accepted to Ap