Analysis of IUE spectra of helium-rich white dwarf stars

We studied the class of DB white dwarf stars, using re-calibrated UV spectra for thirty four DBs obtained with the IUE satellite. By comparing the observed energy distributions with model atmospheres, we simultaneously determine spectroscopic distances (d), effective temperature (Teff), and surface gravities (log g). Using parallax measurements and previous determinations of Teff and log g from optical spectra, we can study whether the atmospheres of eleven DB stars are consistent with pure He or have a small amount of H contamination. We also report on our observations of seventeen stars with Teff close to the DB instability strip through time series photometry and found them to be non variable within our detection limits.Comment: 8 pages, 5 figure

Precise mass and radius values for the white dwarf and low mass M dwarf in the pre-cataclysmic binary NN Serpentis

We derive precise system parameters for the pre-cataclysmic binary, NN Ser. From light curve fitting we find an orbital inclination of i = 89.6 +/- 0.2 deg. From the HeII absorption line we find K_{WD}= 62.3 +/- 1.9 km/s. The irradiation-induced emission lines from the surface of the secondary star give a range of observed radial velocities. The corrected values give a radial velocity of K_{sec}= 301 +/- 3 km/s, with an error dominated by the systematic effects of the model. This leads to a binary separation of a = 0.934 +/- 0.009 R_{sun}, radii of R_{WD} = 0.0211 +/- 0.0002 R_{sun} and R_{sec} = 0.149 +/- 0.002 R_{sun} and masses of M_{WD} = 0.535 +/- 0.012 M_{sun} and M_{sec} = 0.111 +/- 0.004 M_{sun}. The masses and radii of both components of NN Ser were measured independently of any mass-radius relation. For the white dwarf, the measured mass, radius and temperature show excellent agreement with a `thick' hydrogen layer of fractional mass M_{H}/{M}_{WD} = 10^{-4}. The measured radius of the secondary star is 10% larger than predicted by models, however, correcting for irradiation accounts for most of this inconsistency, hence the secondary star in NN Ser is one of the first precisely measured very low mass objects to show good agreement with models. ULTRACAM r', i' and z' photometry taken during the primary eclipse determines the colours of the secondary star as (r'-i')_{sec}= 1.4 +/- 0.1 and (i'-z')_{sec} = 0.8 +/- 0.1 which corresponds to a spectral type of M4 +/- 0.5. This is consistent with the derived mass, demonstrating that there is no detectable heating of the unirradiated face, despite intercepting radiative energy from the white dwarf which exceeds its own luminosity by over a factor of 20.Comment: 20 pages, 17 figures, 8 tables, minor changes, accepted for publication in MNRA

Whole Earth Telescope discovery of a strongly distorted quadrupole pulsation in the largest amplitude rapidly oscillating Ap star

We present a new analysis of the rapidly oscillating Ap (roAp) star, 2MASS J19400781 − 4420093 (J1940; V = 13.1). The star was discovered using SuperWASP broadband photometry to have a frequency of 176.39 d−1 (2041.55 µHz; 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 Ob- servatory, 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

Pulsational Mapping of Calcium Across the Surface of a White Dwarf

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.Comment: Accepted to the Astrophysical Journal. 16 pages, 10 figures

Characterizing the pulsations of the ZZ Ceti star KUV 02464+3239

We present the results on period search and modeling of the cool DAV star KUV 02464+3239. Our observations resolved the multiperiodic pulsational behaviour of the star. In agreement with its position near the red edge of the DAV instability strip, it shows large amplitude, long period pulsation modes, and has a strongly non-sinusoidal light curve. We determined 6 frequencies as normal modes and revealed remarkable short-term amplitude variations. A rigorous test was performed for the possible source of amplitude variation: beating of modes, effect of noise, unresolved frequencies or rotational triplets. Among the best-fit models resulting from a grid search, we selected 3 that gave l=1 solutions for the largest amplitude modes. These models had masses of 0.645, 0.650 and 0.680 M_Sun. The 3 `favoured' models have M_H between 2.5x10^-5 - 6.3x10^-6 M_* and give 14.2 - 14.8 mas seismological parallax. The 0.645 M_Sun (11400 K) model also matches the spectroscopic log g and T_eff within 1 sigma. We investigated the possibility of mode trapping and concluded that while it can explain high amplitude modes, it is not required.Comment: 11 pages, 8 figures, accepted for publication in MNRA

Vertically resolved aerosol optical properties over the ARM SGP site

We will present an overview of early airborne results obtained aboard the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRP AS) Twin Otter aircraft during the Atmospheric Radiation Measurement (ARM) program aerosol intensive observation period in May 2003

Searches for Gravitational Waves from Binary Neutron Stars: A Review

A new generation of observatories is looking for gravitational waves. These waves, emitted by highly relativistic systems, will open a new window for ob- servation of the cosmos when they are detected. Among the most promising sources of gravitational waves for these observatories are compact binaries in the final min- utes before coalescence. In this article, we review in brief interferometric searches for gravitational waves emitted by neutron star binaries, including the theory, instru- mentation and methods. No detections have been made to date. However, the best direct observational limits on coalescence rates have been set, and instrumentation and analysis methods continue to be refined toward the ultimate goal of defining the new field of gravitational wave astronomy.Comment: 30 pages, 5 Figures, to appear in "Short-Period Binary Stars: Observations, Analyses, and Results", Ed.s Eugene F. Milone, Denis A. Leahy, David W. Hobil

An asteroseismic test of diffusion theory in white dwarfs

The helium-atmosphere (DB) white dwarfs are commonly thought to be the descendants of the hotter PG1159 stars, which initially have uniform He/C/O atmospheres. In this evolutionary scenario, diffusion builds a pure He surface layer which gradually thickens as the star cools. In the temperature range of the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still taking place, allowing asteroseismic tests of the theory. We have obtained dual-site observations of the pulsating DB star CBS114, to complement existing observations of the slightly cooler star GD358. We recover the 7 independent pulsation modes that were previously known, and we discover 4 new ones to provide additional constraints on the models. We perform objective global fitting of our updated double-layered envelope models to both sets of observations, leading to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory. These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory, linking the DB white dwarfs to PG1159 stars.Comment: 7 pages, 3 figures, 3 tables, accepted for publication in A&

Hot DAVs : a probable new class of pulsating white dwarf stars

We have discovered a pulsating DA white dwarf at the lower end of the temperature range 45 000–30 000 K where a few helium atmosphere white dwarfs are known. There are now three such pulsators known, suggesting that a new class of theoretically predicted pulsating white dwarf stars exists. We name them the hot DAV stars. From high-speed photometric observations with the ULTRACAM photometer on the 4.2-m William Herschel Telescope, we show that the hydrogen atmosphere white dwarf star WD1017−138 pulsates in at least one mode with a frequency of 1.62 mHz (a period of 624 s). The amplitude of that mode was near 1 mmag at a 10σ confidence level on one night of observation and an 8.4σ confidence level on a second night. The combined data have a confidence level of 11.8σ. This supports the two other detections of hot DAV stars previously reported. From three Very Large Telescope Ultraviolet and Visual Echelle Spectrograph spectra we confirm also that WD1017−138 is a hydrogen atmosphere white dwarf with no trace of helium or metals with Teff = 32 600 K, log g = 7.8 (cgs) and M = 0.55 M⊙. The existence of pulsations in these DA white dwarfs at the cool edge of the 45 000–30 000 K temperature range supports the thin hydrogen layer model for the deficit of helium atmosphere white dwarfs in this range. DA white dwarfs with thick hydrogen layers do not have the superadiabatic, chemically inhomogeneous (μ-gradient) zone that drives pulsation in this temperature range. The potential for higher amplitude hot DAV stars exists; their discovery would open the possibility of a direct test of the explanation for the deficit of helium atmosphere white dwarfs at these temperatures by asteroseismic probing of the atmospheric layers of the hot DAV stars. A search for pulsation in a further 22 candidates with ULTRACAM on the European Southern Observatory New Technology Telescope gave null results for pulsation at precisions in the range 0.5–3 mmag, suggesting that the pulsation amplitudes in such stars are relatively low, hence near the detection limit with the ground-based telescopes used in the surve

Resolving Sirius-like binaries with the Hubble Space Telescope

We have imaged seventeen recently discovered Sirius-like binary systems with HST/WFPC2 and resolved the white dwarf secondary in eight cases. Most of the implied orbital periods are of order several hundred years, but in three cases (56 Per, Zeta Cygni and REJ1925-566) the periods are short enough that it may be possible to detect orbital motion within a few years. It will then be possible to derive dynamically determined masses for the white dwarfs, and potentially these stars could be used as stringent tests of the mass-radius relation and initial-final mass relation.Comment: To appear in Proceedings of the 12th European Workshop on White Dwarfs, eds. H. Shipman and J. Provenca