1,048 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
Persistence in Cluster--Cluster Aggregation
Persistence is considered in diffusion--limited cluster--cluster aggregation,
in one dimension and when the diffusion coefficient of a cluster depends on its
size as . The empty and filled site persistences are
defined as the probabilities, that a site has been either empty or covered by a
cluster all the time whereas the cluster persistence gives the probability of a
cluster to remain intact. The filled site one is nonuniversal. The empty site
and cluster persistences are found to be universal, as supported by analytical
arguments and simulations. The empty site case decays algebraically with the
exponent . The cluster persistence is related to the
small behavior of the cluster size distribution and behaves also
algebraically for while for the behavior is
stretched exponential. In the scaling limit and with fixed the distribution of intervals of size between
persistent regions scales as , where is the average interval size and . For finite the
scaling is poor for , due to the insufficient separation of the two
length scales: the distances between clusters, , and that between
persistent regions, . For the size distribution of persistent regions
the time and size dependences separate, the latter being independent of the
diffusion exponent but depending on the initial cluster size
distribution.Comment: 14 pages, 12 figures, RevTeX, submitted to Phys. Rev.
Coaxial Jets and Sheaths in Wide-Angle-Tail Radio Galaxies
We add 20, 6 and 3.6 cm wavelength VLA observations of two WATs, 1231+674 and
1433+553, to existing VLA data at 6 and 20 cm, in order to study the variations
of spectral index as a function of position. We apply the spectral tomography
process that we introduced in our analysis of 3C67, 3C190 and 3C449. Both
spectral tomography and polarization maps indicate that there are two distinct
extended components in each source. As in the case of 3C449, we find that each
source has a flat spectrum jet surrounded by a steeper spectrum sheath. The
steep components tend to be more highly polarized than the flat components. We
discuss a number of possibilities for the dynamics of the jet/sheath systems,
and the evolution of their relativistic electron populations. While the exact
nature of these two coaxial components is still uncertain, their existence
requires new models of jets in FR I sources and may also have implications for
the dichotomy between FR Is and FR IIs.Comment: 29 text pages plus 13 figures. Scheduled for publication in May 10,
1999 Ap
First Attempt at Spectroscopic Detection of Gravity Modes in a Long-Period Pulsating Subdwarf B Star -- PG 1627+017
In the first spectroscopic campaign for a PG 1716 variable (or long-period
pulsating subdwarf B star), we succeeded in detecting velocity variations due
to g-mode pulsations at a level of 1.0-1.5 km/s.The observations were obtained
during 40 nights on 2-m class telescopes in Arizona, South Africa,and
Australia. The target,PG1627+017, is one of the brightest and largest amplitude
stars in its class.It is also the visible component of a post-common envelope
binary.Our final radial velocity data set includes 84 hours of time-series
spectroscopy over a time baseline of 53 days. Our derived radial velocity
amplitude spectrum, after subtracting the orbital motion, shows three potential
pulsational modes 3-4 sigma above the mean noise level, at 7201.0s,7014.6s and
7037.3s.Only one of the features is statistically likely to be real,but all
three are tantalizingly close to, or a one day alias of, the three strongest
periodicities found in the concurrent photometric campaign. We further
attempted to detect pulsational variations in the Balmer line amplitudes. The
single detected periodicity of 7209 s, although weak, is consistent with
theoretical expectations as a function of wavelength.Furthermore, it allows us
to rule out a degree index of l= 3 or l= 5 for that mode. Given the extreme
weakness of g-mode pulsations in these stars,we conclude that anything beyond
simply detecting their presence will require larger telescopes,higher
efficiency spectral monitoring over longer time baselines,improved longitude
coverage, and increased radial velocity precision.Comment: 39 pages, 9 figures, 4 tables, ApJ accepted. See postscript for full
abtrac
Unusual Dynamical Scaling in the Spatial Distribution of Persistent Sites in 1D Potts Models
The distribution, n(k,t), of the interval sizes, k, between clusters of
persistent sites in the dynamical evolution of the one-dimensional q-state
Potts model is studied using a combination of numerical simulations, scaling
arguments, and exact analysis. It is shown to have the scaling form n(k,t) =
t^{-2z} f(k/t^z), with z= max(1/2,theta), where theta(q) is the persistence
exponent which characterizes the fraction of sites which have not changed their
state up to time t. When theta > 1/2, the scaling length, t^theta, for the
interval-size distribution is larger than the coarsening length scale, t^{1/2},
that characterizes spatial correlations of the Potts variables.Comment: RevTex, 11 page
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
MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence
We report on the analysis of new and previously published MMT optical spectra
of the black hole binary XTE J1118+480 during the decline from the 2000
outburst to true quiescence. From cross-correlation with template stars, we
measure the radial velocity of the secondary to derive a new spectroscopic
ephemeris. The observations acquired during approach to quiescence confirm the
earlier reported modulation in the centroid of the double-peaked Halpha
emission line. Additionally, our data combined with the results presented by
Zurita et al. (2002) provide support for a modulation with a periodicity in
agreement with the expected precession period of the accretion disk of ~52 day.
Doppler images during the decline phase of the Halpha emission line show
evidence for a hotspot and emission from the gas stream: the hotspot is
observed to vary its position, which may be due to the precession of the disk.
The data available during quiescence show that the centroid of the Halpha
emission line is offset by about -100 km/s from the systemic velocity which
suggests that the disk continues to precess. A Halpha tomogram reveals emission
from near the donor star after subtraction of the ring-like contribution from
the accretion disk which we attribute to chromospheric emission. No hotspot is
present suggesting that accretion from the secondary has stopped (or decreased
significantly) during quiescence. Finally, a comparison is made with the black
hole XRN GRO J0422+32: we show that the Halpha profile of this system also
exhibits a behaviour consistent with a precessing disk.Comment: 11 pages, 5 figures, accepted by Ap
Cluster persistence in one-dimensional diffusion--limited cluster--cluster aggregation
The persistence probability, , of a cluster to remain unaggregated is
studied in cluster-cluster aggregation, when the diffusion coefficient of a
cluster depends on its size as . In the mean-field the
problem maps to the survival of three annihilating random walkers with
time-dependent noise correlations. For the motion of persistent
clusters becomes asymptotically irrelevant and the mean-field theory provides a
correct description. For the spatial fluctuations remain relevant
and the persistence probability is overestimated by the random walk theory. The
decay of persistence determines the small size tail of the cluster size
distribution. For the distribution is flat and, surprisingly,
independent of .Comment: 11 pages, 6 figures, RevTeX4, submitted to Phys. Rev.
Persistence in q-state Potts model: A Mean-Field approach
We study the Persistence properties of the T=0 coarsening dynamics of one
dimensional -state Potts model using a modified mean-field approximation
(MMFA). In this approximation, the spatial correlations between the interfaces
separating spins with different Potts states is ignored, but the correct time
dependence of the mean density of persistent spins is imposed. For this
model, it is known that follows a power-law decay with time, where is the -dependent persistence exponent. We
study the spatial structure of the persistent region within the MMFA. We show
that the persistent site pair correlation function has the scaling
form for all values of the persistence
exponent . The scaling function has the limiting behaviour () and (). We then show within the
Independent Interval Approximation (IIA) that the distribution of
separation between two consecutive persistent spins at time has the
asymptotic scaling form where the
dynamical exponent has the form =max(). The behaviour of
the scaling function for large and small values of the arguments is found
analytically. We find that for small separations where =max(), while for large
separations , decays exponentially with . The
unusual dynamical scaling form and the behaviour of the scaling function is
supported by numerical simulations.Comment: 11 pages in RevTeX, 10 figures, submitted to Phys. Rev.
Ensemble Characteristics of the ZZ Ceti stars
We present the observed pulsation spectra of all known non-interacting ZZ
Ceti stars (hydrogen atmosphere white dwarf variables; DAVs) and examine
changes in their pulsation properties across the instability strip. We confirm
the well established trend of increasing pulsation period with decreasing
effective temperature across the ZZ Ceti instability strip. We do not find a
dramatic order of magnitude increase in the number of observed independent
modes in ZZ Ceti stars, traversing from the hot to the cool edge of the
instability strip; we find that the cool DAVs have one more mode on average
compared to the hot DAVs. We confirm the initial increase in pulsation
amplitude at the blue edge, and find strong evidence of a decline in amplitude
prior to the red edge. We present the first observational evidence that ZZ Ceti
stars lose pulsation energy just before pulsations shut down at the empirical
red edge of the instability strip.Comment: ApJ, in press. (26 pages, 3 figures
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