J. C. Clemens

Mode identification from time-resolved spectroscopy of the pulsating white dwarf G29-38

We have used time-resolved spectroscopy to measure the colour dependence of pulsation amplitudes in the DAV white dwarf G29-38. Model atmospheres predict that mode amplitudes should change with wavelength in a manner that depends on the spherical harmonic degree l of the mode. This dependence arises from the convolution of mode geometry with wavelength-dependent limb darkening. Our analysis of the six largest normal modes detected in Keck observations of G29-38 reveals one mode with a colour dependence different from the other five, permitting us to identify the l-value of all six modes and to test the model predictions. The Keck observations also show pulsation amplitudes that are unexpectedly asymmetric within absorption lines. We show that these asymmetries arise from surface motions associated with the non-radial pulsations (which are discussed in detail in a companion paper). By incorporating surface velocity fields into line profile calculations, we are able to produce models that more closely resemble the observations.

Mode Identification of Pulsating White Dwarfs Using the Hubble Space Telescope

We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV stars G226-29 and G185-32 and for the pulsating DBV star PG 1351+489 with the Hubble Space Telescope Faint Object Spectrograph to compare the ultraviolet to the optical pulsation amplitude and determine the pulsation indices. We find that for essentially all observed pulsation modes, the amplitude rises to the ultraviolet as the theoretical models predict for l = 1 nonradial g-modes. We do not find any pulsation mode visible only in the ultraviolet, nor any modes whose phase flips by 180° in the ultraviolet, as would be expected if high l pulsations were excited. We find one periodicity in the light curve of G185-32, at 141 s, which does not fit theoretical models for the change of amplitude with wavelength of g-mode pulsations.

EMPIRICAL DETERMINATION OF CONVECTION PARAMETERS IN WHITE DWARFS. I. WHOLE EARTH TELESCOPE OBSERVATIONS OF EC14012-1446 ∗

We report on an analysis of 308.3?hr of high-speed photometry targeting the pulsating DA white dwarf EC14012-1446. The data were acquired with the Whole Earth Telescope during the 2008 international observing run XCOV26. The Fourier transform of the light curve contains 19 independent frequencies and numerous combination frequencies. The dominant peaks are 1633.907, 1887.404, and 2504.897 ?Hz. Our analysis of the combination amplitudes reveals that the parent frequencies are consistent with modes of spherical degree l = 1. The combination amplitudes also provide m identifications for the largest amplitude parent frequencies. Our seismology analysis, which includes 2004-2007 archival data, confirms these identifications, provides constraints on additional frequencies, and finds an average period spacing of 41?s. Building on this foundation, we present nonlinear fits to high signal-to-noise light curves from the SOAR 4.1?m, McDonald 2.1?m, and KPNO 2?m telescopes. The fits indicate a time-averaged convective response timescale of ?0 = 99.4 ? 17?s, a temperature exponent N = 85 ? 6.2, and an inclination angle of ? i = 329 ? 32. We present our current empirical map of the convective response timescale across the DA instability strip.

EC 10246−2707: an eclipsing subdwarf B + M dwarf binary

We announce the discovery of a new eclipsing hot subdwarf B + M dwarf binary, EC 10246-2707, and present multi-colour photometric and spectroscopic observations of this system. Similar to other HW Vir-type binaries, the light curve shows both primary and secondary eclipses, along with a strong reflection effect from the M dwarf; no intrinsic light contribution is detected from the cool companion. The orbital period is 0.1185079936 +/- 0.0000000009 days, or about three hours. Analysis of our time-series spectroscopy reveals a velocity semi-amplitude of K_1 = 71.6 +/- 1.7 km/s for the sdB and best-fitting atmospheric parameters of Teff = 28900 +/- 500 K, log g = 5.64 +/- 0.06, and log[N(He)/N(H)] = -2.5 +/- 0.2. Although we cannot claim a unique solution from modeling the light curve, the best-fitting model has an sdB mass of 0.45 Msun and a cool companion mass of 0.12 Msun. These results are roughly consistent with a canonical-mass sdB and M dwarf separated by a ~ 0.84 Rsun. We find no evidence of pulsations in the light curve and limit the amplitude of rapid photometric oscillations to 7.2 x 10^(-12). If EC 10246-2707 evolves into a cataclysmic variable, its period should fall below the famous CV period gap.

The peculiar pulsations of PY vulpeculae

The pulsating white dwarf star PY Vul (G185-32) exhibits pulsation modes with peculiar properties that set it apart from other variable stars in the ZZ Ceti (variable DA white dwarf [DAV]) class. These peculiarities include a low total pulsation amplitude, a mode with bizarre amplitudes in the ultraviolet, and a mode harmonic that exceeds the amplitude of its fundamental. Here we present optical time-series spectroscopy of PY Vul acquired with the Keck II Low Resolution Imaging Spectrograph. Our analysis has revealed that the mode with unusual UV amplitudes also has distinguishing characteristics in the optical. Comparison of its line profile variations to models suggests that this mode has a spherical degree of 4. We show that all the other peculiarities in this star are accounted for by a dominant pulsation mode of l = 4 and propose this hypothesis as a solution to the mysteries of PY Vul.

High-Resolution Spectroscopy of the Pulsating White Dwarf G29-38

We present the analysis of time-resolved, high-resolution spectra of the cool white dwarf pulsator, G29-38. From measuring the Doppler shifts of the Hα core, we detect velocity changes as large as 16.5 km s-1 and conclude that they are due to the horizontal motions associated with the g-mode pulsations on the star. We detect seven pulsation modes from the velocity time series and identify the same modes in the flux variations. We discuss the properties of these modes and use the advantage of having both velocity and flux measurements of the pulsations to test the convective driving theory proposed for DAV stars. Our data show limited agreement with the expected relationships between the amplitudes and phases of the velocity and flux modes. Unexpectedly, the velocity curve shows evidence for harmonic distortion, in the form of a peak in the Fourier transform whose frequency is the exact sum of the two largest frequencies. Combination frequencies are a characteristic feature of the Fourier transforms of light curves of G29-38, but before now they have not been detected in the velocities, nor does published theory predict that they should exist. We compare our velocity combination frequency to combination frequencies found in the analysis of light curves of G29-38 and discuss what might account for the existence of velocity combinations with the properties we observe. We also use our high-resolution spectra to determine whether either rotation or pulsation can explain the truncated shape observed for the DAV stars line core. We are able to eliminate both mechanisms: the average spectrum does not fit the rotationally broadened model, and the time series of spectra provides proof that the pulsations do not significantly truncate the line.

A SECOND CASE OF OUTBURSTS IN A PULSATING WHITE DWARF OBSERVED BY KEPLER

We present observations of a new phenomenon in pulsating white dwarf stars: large-amplitude outbursts at timescales much longer than the pulsation periods. The cool (

A new look at the pulsating DB white dwarf GD 358: Line-of-sight velocity measurements and constraints on model atmospheres

{T}_{\mathrm{eff}}

A puzzling periodicity in the pulsating DA white dwarf G 117-B15A ⋆

= 11,060 K), hydrogen-atmosphere pulsating white dwarf PG 1149+057 was observed nearly continuously for more than 78.8 day by the extended Kepler mission in K2 Campaign 1. The target showed 10 outburst events, recurring roughly every 8 day and lasting roughly 15 hr, with maximum flux excursions up to 45% in the Kepler bandpass. We demonstrate that the outbursts affect the pulsations and therefore must come from the white dwarf. Additionally, we argue that these events are not magnetic reconnection flares, and are most likely connected to the stellar pulsations and the relatively deep surface convection zone. PG 1149+057 is now the second cool pulsating white dwarf to show this outburst phenomenon, after the first variable white dwarf observed in the Kepler mission, KIC 4552982. Both stars have the same effective temperature, within the uncertainties, and are among the coolest known pulsating white dwarfs of typical mass. These outbursts provide fresh observational insight into the red edge of the DAV instability strip and the eventual cessation of pulsations in cool white dwarfs.

A New Small-amplitude Variable Hot DQ White Dwarf

We report on our findings of the bright, pulsating, helium atmospherewhite dwarf GD 358, based on time-resolved optical spectrophotometry. Weidentify 5 real pulsation modes and at least 6 combination modes atfrequencies consistent with those found in previous observations. Themeasured Doppler shifts from our spectra show variations with amplitudesof up to 5.5 km s-1 at the frequencies inferred from the fluxvariations. We conclude that these are variations in the line-of-sightvelocities associated with the pulsational motion. We use the observedflux and velocity amplitudes and phases to test theoretical predictionswithin the convective driving framework, and compare these with similarobservations of the hydrogen atmosphere white dwarf pulsators (DAVs).The wavelength dependence of the fractional pulsation amplitudes(chromatic amplitudes) allows us to conclude that all five real modesshare the same spherical degree, most likely, l=1. This is consistentwith previous identifications based solely on photometry. We find that ahigh signal-to-noise mean spectrum on its own is not enough to determinethe atmospheric parameters and that there are small but significantdiscrepancies between the observations and model atmospheres. The sourceof these remains to be identified. While we infer Teff =24 kKand log g ~ 8.0 from the mean spectrum, the chromatic amplitudes, whichare a measure of the derivative of the flux with respect to thetemperature, unambiguously favour a higher effective temperature, 27 kK,which is more in line with independent determinations from ultra-violetspectra.The data presented herein were obtained at the W.M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W.M. Keck Foundation. (Less)