24 research outputs found
Alfven node-free vibrations of white dwarf in the model of solid star with toroidal magnetic field
In the context of the white dwarf asteroseismology, we investigate
vibrational properties of a non-convective solid star with an axisymmetric
purely toroidal intrinsic magnetic field of two different shapes. Focus is laid
on regime of node-free global Lorentz-force-driven vibrations about symmetry
axis at which material displacements have one and the same form as those for
nodeless spheroidal and torsional vibrations restored by Hooke's force of
elastic shear stresses. Particular attention is given to the even-parity
poloidal Alfven modes whose frequency spectra are computed in analytic form
showing how the purely toroidal magnetic fields completely buried beneath the
star surface can manifest itself in seismic vibrations of non-magnetic white
dwarfs. The obtained spectral formulae are discussed in juxtaposition with
those for Alfven modes in the solid star model with the poloidal, homogeneous
internal and dipolar external, magnetic field whose inferences are relevant to
Alfven vibrations in magnetic white dwarfs.Comment: Accepted for publication in Astrophysics & Space Scienc
Limits on mode coherence in pulsating DA white dwarfs due to a nonstatic convection zone
The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes that are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finite-amplitude pulsations. Models predict that the mass of convection zones in pulsating hydrogen-atmosphere (DAV) white dwarfs is very sensitive to temperature (i.e., M_CZâT_eff^-90), leading to the possibility that even low-amplitude pulsators may experience significant nonlinear effects. In particular, the outer turning point of finite-amplitude g-mode pulsations can vary with the local surface temperature, producing a reflected wave that is out of phase with what is required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. In this paper we show that (1) whether a mode is calculated to propagate to the base of the convection zone is an accurate predictor of its width in the Fourier spectrum, (2) the phase shifts produced by reflection from the outer turning point are large enough to produce significant damping, and (3) amplitudes and periods are predicted to increase from the blue edge to the middle of the instability strip, and subsequently decrease as the red edge is approached. This amplitude decrease is in agreement with the observational data while the period decrease has not yet been systematically studied.Published versio
WD1032+011, an inflated brown dwarf in an old eclipsing binary with a white dwarf
We present the discovery of only the third brown dwarf known to eclipse a non-accreting white dwarf. Gaia parallax information and multicolour photometry confirm that the white dwarf is cool (9950 ± 150 K) and has a low mass (0.45 ± 0.05 Mâ), and spectra and light curves suggest the brown dwarf has a mass of 0.067 ± 0.006 Mâ (70MJup) and a spectral type of L5 ± 1. The kinematics of the system show that the binary is likely to be a member of the thick disc and therefore at least 5-Gyr old. The high-cadence light curves show that the brown dwarf is inflated, making it the first brown dwarf in an eclipsing white dwarf-brown dwarf binary to be so
Fourier Analysis of Gapped Time Series: Improved Estimates of Solar and Stellar Oscillation Parameters
Quantitative helio- and asteroseismology require very precise measurements of
the frequencies, amplitudes, and lifetimes of the global modes of stellar
oscillation. It is common knowledge that the precision of these measurements
depends on the total length (T), quality, and completeness of the observations.
Except in a few simple cases, the effect of gaps in the data on measurement
precision is poorly understood, in particular in Fourier space where the
convolution of the observable with the observation window introduces
correlations between different frequencies. Here we describe and implement a
rather general method to retrieve maximum likelihood estimates of the
oscillation parameters, taking into account the proper statistics of the
observations. Our fitting method applies in complex Fourier space and exploits
the phase information. We consider both solar-like stochastic oscillations and
long-lived harmonic oscillations, plus random noise. Using numerical
simulations, we demonstrate the existence of cases for which our improved
fitting method is less biased and has a greater precision than when the
frequency correlations are ignored. This is especially true of low
signal-to-noise solar-like oscillations. For example, we discuss a case where
the precision on the mode frequency estimate is increased by a factor of five,
for a duty cycle of 15%. In the case of long-lived sinusoidal oscillations, a
proper treatment of the frequency correlations does not provide any significant
improvement; nevertheless we confirm that the mode frequency can be measured
from gapped data at a much better precision than the 1/T Rayleigh resolution.Comment: Accepted for publication in Solar Physics Topical Issue
"Helioseismology, Asteroseismology, and MHD Connections
A white dwarf with transiting circumstellar material far outside the Roche limit
We report the discovery of a white dwarf exhibiting deep, irregularly shaped transits, indicative of circumstellar planetary debris. Using Zwicky Transient Facility DR2 photometry of ZTF J013906.17+524536.89 and follow-up observations from the Las Cumbres Observatory, we identify multiple transit events that recur every â107.2 days, much longer than the 4.5â4.9 hr orbital periods observed in WD 1145+017, the only other white dwarf known with transiting planetary debris. The transits vary in both depth and duration, lasting 15â25 days and reaching 20%â45% dips in flux. Optical spectra reveal strong Balmer lines, identifying the white dwarf as a DA with T_eff=10,530 ± 140K and log(g) =7.86 ± 0.06. A Ca ii K absorption feature is present in all spectra both in and out of transit. Spectra obtained during one night at roughly 15% transit depth show increased Ca ii K absorption with a model atmospheric fit suggesting [Ca/H] = â4.6 ± 0.3, whereas spectra taken on three nights out of transit have [Ca/H] of â5.5, â5.3, and â4.9 with similar uncertainties. While the Ca ii K line strength varies by only 2Ï, we consider a predominantly interstellar origin for Ca absorption unlikely. We suggest a larger column density of circumstellar metallic gas along the line of site or increased accretion of material onto the white dwarf's surface are responsible for the Ca absorption, but further spectroscopic studies are required. In addition, high-speed time series photometry out of transit reveals variability with periods of 900 and 1030 s, consistent with ZZ Ceti pulsations.Published versio
Multi-Site Observations of the DAV White Dwarf R 548
The pulsating DA white dwarf R 548 was observed for 46 h in October 1993 in an eight-site campaign. New peaks near the known doublets in the Fourier transform are found
Asteroseismology of Eclipsing Binary Stars in the Kepler Era
Eclipsing binary stars have long served as benchmark systems to measure
fundamental stellar properties. In the past few decades, asteroseismology - the
study of stellar pulsations - has emerged as a new powerful tool to study the
structure and evolution of stars across the HR diagram. Pulsating stars in
eclipsing binary systems are particularly valuable since fundamental properties
(such as radii and masses) can determined using two independent techniques.
Furthermore, independently measured properties from binary orbits can be used
to improve asteroseismic modeling for pulsating stars in which mode
identifications are not straightforward. This contribution provides a review of
asteroseismic detections in eclipsing binary stars, with a focus on space-based
missions such as CoRoT and Kepler, and empirical tests of asteroseismic scaling
relations for stochastic ("solar-like") oscillations.Comment: 28 pages, 12 figures, 2 tables; Proceedings of the AAS topical
conference "Giants of Eclipse" (AASTCS-3), July 28 - August 2 2013, Monterey,
C
Pulsating White Dwarf Stars and Precision Asteroseismology
Galactic history is written in the white dwarf stars. Their surface
properties hint at interiors composed of matter under extreme conditions. In
the forty years since their discovery, pulsating white dwarf stars have moved
from side-show curiosities to center stage as important tools for unraveling
the deep mysteries of the Universe. Innovative observational techniques and
theoretical modeling tools have breathed life into precision asteroseismology.
We are just learning to use this powerful tool, confronting theoretical models
with observed frequencies and their time rate-of-change. With this tool, we
calibrate white dwarf cosmochronology; we explore equations of state; we
measure stellar masses, rotation rates, and nuclear reaction rates; we explore
the physics of interior crystallization; we study the structure of the
progenitors of Type Ia supernovae, and we test models of dark matter. The white
dwarf pulsations are at once the heartbeat of galactic history and a window
into unexplored and exotic physics.Comment: 70 pages, 11 figures, to be published in Annual Review of Astronomy
and Astrophysics 200
Limits on Mode Coherence Due to a Non-static Convection Zone
The 21st European Workshop on White Dwarfs was held in Austin, TX from July 23rd to 27th of 2018The standard theory of pulsations deals with the
frequencies and growth rates of infinitesimal perturbations
in a stellar model. Modes which are calculated
to be linearly driven should increase their
amplitudes exponentially with time; the fact that
nearly constant amplitudes are usually observed
is evidence that nonlinear mechanisms inhibit the
growth of finite amplitude pulsations. Models predict
that the mass of DAV convection zones is very
sensitive to temperature (i.e., MCZ is proportional to T -90
eff ), leading
to the possibility that even âsmall amplitudeâ
pulsators may experience significant nonlinear effects.
In particular, the outer turning point of finite amplitude
g-mode pulsations can vary with the
local surface temperature, producing a reflected
wave that is slightly out of phase with that required
for a standing wave. This can lead to a lack of coherence
of the mode and a reduction in its global
amplitude. We compute the size of this effect for
specific examples and discuss the results in the context
of Kepler and K2 observations.Astronom