84 research outputs found
Mode Identification from Combination Frequency Amplitudes in ZZ Ceti Stars
The lightcurves of variable DA stars are usually multi-periodic and
non-sinusoidal, so that their Fourier transforms show peaks at eigenfrequencies
of the pulsation modes and at sums and differences of these frequencies. These
combination frequencies provide extra information about the pulsations, both
physical and geometrical, that is lost unless they are analyzed. Several
theories provide a context for this analysis by predicting combination
frequency amplitudes. In these theories, the combination frequencies arise from
nonlinear mixing of oscillation modes in the outer layers of the white dwarf,
so their analysis cannot yield direct information on the global structure of
the star as eigenmodes provide. However, their sensitivity to mode geometry
does make them a useful tool for identifying the spherical degree of the modes
that mix to produce them. In this paper, we analyze data from eight hot,
low-amplitude DAV white dwarfs and measure the amplitudes of combination
frequencies present. By comparing these amplitudes to the predictions of the
theory of Goldreich & Wu, we have verified that the theory is crudely
consistent with the measurements. We have also investigated to what extent the
combination frequencies can be used to measure the spherical degree (ell) of
the modes that produce them. We find that modes with ell > 2 are easily
identifiable as high ell based on their combination frequencies alone.
Distinguishing between ell=1 and 2 is also possible using harmonics. These
results will be useful for conducting seismological analysis of large ensembles
of ZZ Ceti stars, such as those being discovered using the Sloan Digital Sky
Survey. Because this method relies only on photometry at optical wavelengths,
it can be applied to faint stars using 4 m class telescopes.Comment: 73 pages, 22 figures, accepted in the Ap
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 H-alpha
core, we detect velocity changes as large as 16.5 km/s 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
amplitude 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 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 if either rotation or
pulsation can explain the truncated shape observed for the DAV star's 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.Comment: 24 pages, 9 figures, Accepted for publication in ApJ (June
Gravity-Modes in ZZ Ceti Stars: I.Quasiadiabatic Analysis of Overstability
We analyze the stability of g-modes in variable white dwarfs with hydrogen
envelopes. In these stars, the radiative layer contributes to mode damping
because its opacity decreases upon compression and the amplitude of the
Lagrangian pressure perturbation increases outward. The overlying convective
envelope is the seat of mode excitation because it acts as an insulating
blanket with respect to the perturbed flux that enters it from below. A crucial
point is that the convective motions respond to the instantaneous pulsational
state. Driving exceeds damping by as much as a factor of two provided
, where is the radian frequency of the mode and
with being the thermal time constant
evaluated at the base of the convective envelope. As a white dwarf cools, its
convection zone deepens, and modes of lower frequency become overstable.
However, the deeper convection zone impedes the passage of flux perturbations
from the base of the convection zone to the photosphere. Thus the photometric
variation of a mode with constant velocity amplitude decreases. These factors
account for the observed trend that longer period modes are found in cooler
DAVs. The linear growth time, ranging from hours for the longest period
observed modes ( minutes) to thousands of years for those of
shortest period ( minutes), probably sets the time-scale for
variations of mode amplitude and phase. This is consistent with observations
showing that longer period modes are more variable than shorter period ones.
Our investigation confirms many results obtained by Brickhill in his pioneering
studies of ZZ Cetis.Comment: 26 pages, including 5 figures, uses aaspp4.sty, submitted to Ap
Nonlinear effects in time-resolved spectra of DAVs
Numerical simulations of light curves of variable DA white dwarfs (ZZ Ceti
stars) predict flux amplitudes with surface distributions different from the
spherical harmonics of the pulsation mode in deeper layers. In contrast to the
results of the perturbation analysis by Goldreich and Wu this is also true for
the fundamental period of the flux variation. As a consequence normalized
amplitude spectra depend not only on the mode number l but also on pulsation
amplitude and inclination. Another new result is that with increasing amplitude
of the pressure variation below the convection zone the flux variation at the
surface goes through a maximum and then decreases again
Time-resolved optical spectroscopy of the pulsating DA white dwarf HS 0507+0434B: New constraints on mode identification and pulsation properties
We present a detailed analysis of time-resolved optical spectra of the ZZ
Ceti white dwarf, HS 0507+0434B. Using the wavelength dependence of observed
mode amplitudes, we deduce the spherical degree, l, of the modes, most of which
have l=1. The presence of a large number of combination frequencies (linear
sums or differences of the real modes) enabled us not only to test theoretical
predictions but also to indirectly infer spherical and azimuthal degrees of
real modes that had no observed splittings. In addition to the above, we
measure line-of-sight velocities from our spectra. We find only marginal
evidence for periodic modulation associated with the pulsation modes: at the
frequency of the strongest mode in the lightcurve, we measure an amplitude of
2.6+/-1.0 km/s, which has a probability of 2% of being due to chance; for the
other modes, we find lower values. Our velocity amplitudes and upper limits are
smaller by a factor of two compared to the amplitudes found in ZZ Psc. We find
that this is consistent with expectations based on the position of HS
0507+0434B in the instability strip. Combining all the available information
from data such as ours is a first step towards constraining atmospheric
properties in a convectionally unstable environment from an observational
perspective.Comment: 16 pages, 12 figs.; accepted for publication in A&
A new look at the pulsating DB white dwarf GD 358:Line-of-sight velocity measurements and constraints on model atmospheres
We report on our findings of the bright, pulsating, helium atmosphere white
dwarf GD 358, based on time-resolved optical spectrophotometry. We identify 5
real pulsation modes and at least 6 combination modes at frequencies consistent
with those found in previous observations. The measured Doppler shifts from our
spectra show variations with amplitudes of up to 5.5 km/s at the frequencies
inferred from the flux variations. We conclude that these are variations in the
line-of-sight velocities associated with the pulsational motion. We use the
observed flux and velocity amplitudes and phases to test theoretical
predictions within the convective driving framework, and compare these with
similar observations 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 modes share the same
spherical degree, most likely, l=1. This is consistent with previous
identifications based solely on photometry. We find that a high signal-to-noise
mean spectrum on its own is not enough to determine the atmospheric parameters
and that there are small but significant discrepancies between the observations
and model atmospheres. The source of these remains to be identified. While we
infer T_eff=24kK and log g~8.0 from the mean spectrum, the chromatic
amplitudes, which are a measure of the derivative of the flux with respect to
the temperature, unambiguously favour a higher effective temperature, 27kK,
which is more in line with independent determinations from ultra-violet
spectra.Comment: 14 pages, 11 figures; accepted for publication in A&
Prospects for Measuring Differential Rotation in White Dwarfs Through Asteroseismology
We examine the potential of asteroseismology for exploring the internal
rotation of white dwarf stars. Data from global observing campaigns have
revealed a wealth of frequencies, some of which show the signature of
rotational splitting. Tools developed for helioseismology to use many solar
p-mode frequencies for inversion of the rotation rate with depth are adapted to
the case of more limited numbers of modes of low degree. We find that the small
number of available modes in white dwarfs, coupled with the similarity between
the rotational-splitting kernels of the modes, renders direct inversion
unstable. Accordingly, we adopt what we consider to be plausible functional
forms for the differential rotation profile; this is sufficiently restrictive
to enable us to carry out a useful calibration. We show examples of this
technique for PG 1159 stars and pulsating DB white dwarfs. Published frequency
splittings for white dwarfs are currently not accurate enough for meaningful
inversions; reanalysis of existing data can provide splittings of sufficient
accuracy when the frequencies of individual peaks are extracted via
least-squares fitting or multipeak decompositions. We find that when mode
trapping is evident in the period spacing of g modes, the measured splittings
can constrain dOmega/dr.Comment: 26 pages, 20 postscript figures. Accepted for publication in The
Astrophysical Journa
The Peculiar Pulsations of PY Vul
The pulsating white dwarf star PY Vul (G~185-32) exhibits pulsation modes
with peculiar properties that set it apart from other variable stars in the ZZ
Ceti (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 LRIS 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 four. 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.Comment: 30 pages, 14 figures, Accepted for publication in Ap
Re-defining the Empirical ZZ Ceti Instability Strip
We use the new ZZ Ceti stars (hydrogen atmosphere white dwarf variables;
DAVs) discovered within the Sloan Digital Sky Survey (Mukadam et al. 2004) to
re-define the empirical ZZ Ceti instability strip. This is the first time since
the discovery of white dwarf variables in 1968 that we have a homogeneous set
of spectra acquired using the same instrument on the same telescope, and with
consistent data reductions, for a statistically significant sample of ZZ Ceti
stars. The homogeneity of the spectra reduces the scatter in the spectroscopic
temperatures and we find a narrow instability strip of width ~950K, from
10850--11800K. We question the purity of the DAV instability strip as we find
several non-variables within. We present our best fit for the red edge and our
constraint for the blue edge of the instability strip, determined using a
statistical approach.Comment: 14 pages, 5 pages, ApJ paper, accepte
The pulsating DA white dwarf star EC 14012-1446: results from four epochs of time-resolved photometry
The pulsating DA white dwarfs are the coolest degenerate stars that undergo
self-driven oscillations. Understanding their interior structure will help to
understand the previous evolution of the star. To this end, we report the
analysis of more than 200 h of time-resolved CCD photometry of the pulsating DA
white dwarf star EC 14012-1446 acquired during four observing epochs in three
different years, including a coordinated three-site campaign. A total of 19
independent frequencies in the star's light variations together with 148
combination signals up to fifth order could be detected. We are unable to
obtain the period spacing of the normal modes and therefore a mass estimate of
the star, but we infer a fairly short rotation period of 0.61 +/- 0.03 d,
assuming the rotationally split modes are l=1. The pulsation modes of the star
undergo amplitude and frequency variations, in the sense that modes with higher
radial overtone show more pronounced variability and that amplitude changes are
always accompanied by frequency variations. Most of the second-order
combination frequencies detected have amplitudes that are a function of their
parent mode amplitudes, but we found a few cases of possible resonantly excited
modes. We point out the complications in the analysis and interpretation of
data sets of pulsating white dwarfs that are affected by combination
frequencies of the form f_A+f_B-f_C intruding into the frequency range of the
independent modes.Comment: 14 pages, 6 figures, 6 tables. MNRAS, in pres
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