14,694 research outputs found
Mixing and Overshooting in Surface Convection Zones of DA White Dwarfs: First Results from ANTARES
We present results of a large, high resolution 3D hydrodynamical simulation
of the surface layers of a DA white dwarf (WD) with K and
using the ANTARES code, the widest and deepest such simulation to
date. Our simulations are in good agreement with previous calculations in the
Schwarzschild-unstable region and in the overshooting region immediately
beneath it. Farther below, in the wave-dominated region, we find that the rms
horizontal velocities decay with depth more rapidly than the vertical ones.
Since mixing requires both vertical and horizontal displacements, this could
have consequences for the size of the region that is well mixed by convection,
if this trend is found to hold for deeper layers. We discuss how the size of
the mixed region affects the calculated settling times and inferred
steady-state accretion rates for WDs with metals observed in their atmospheres.Comment: This is a pre-copyedited, author-produced PDF of an article accepted
for publication in Monthly Notices of the Royal Astronomical Society
following peer review (author accepted manuscript). 13 pages, 14 figure
Sensor performance analysis
The theory is described and the equations required to design are developed and the performance of electro-optical sensor systems that operate from the visible through the thermal infrared spectral regions are analyzed. Methods to compute essential optical and detector parameters, signal-to-noise ratio, MTF, and figures of merit such as NE delta rho and NE delta T are developed. A set of atmospheric tables are provided to determine scene radiance in the visible spectral region. The Planck function is used to determine radiance in the infrared. The equations developed were incorporated in a spreadsheet so that a wide variety of sensor studies can be rapidly and efficiently conducted
Constraining the Surface Inhomogeneity and Settling Times of Metals on Accreting White Dwarfs
Due to the short settling times of metals in DA white dwarf atmospheres, any
white dwarfs with photospheric metals must be actively accreting. It is
therefore natural to expect that the metals may not be deposited uniformly on
the surface of the star. We present calculations showing how the temperature
variations associated with white dwarf pulsations lead to an observable
diagnostic of the surface metal distribution, and we show what constraints
current data sets are able to provide. We also investigate the effect that
time-variable accretion has on the metal abundances of different species, and
we show how this can lead to constraints on the gravitational settling times.Comment: 4 pages, 5 figures, accepted for publication in the Astrophysical
Journal Letters, updated reference
A New Technique for Probing Convection in Pulsating White Dwarf Stars
In this paper we demonstrate how pulsating white dwarfs can be used as an
astrophysical laboratory for empirically constraining convection in these
stars. We do this using a technique for fitting observed non-sinusoidal light
curves, which allows us to recover the thermal response timescale of the
convection zone (its "depth") as well as how this timescale changes as a
function of effective temperature. We also obtain constraints on mode
identifications for the pulsation modes, allowing us to use asteroseismology to
study the interior structure of these stars. Aspects of this approach may have
relevance for other classes of pulsators, including the Cepheids and RR Lyrae
stars.Comment: 8 pages, 7 figures, accepted for publication in the Ap
Driving in ZZ Ceti stars - Problem solved?
There is a fairly tight correlation between the pulsation periods and
effective temperatures of ZZ Ceti stars (cooler stars have longer periods).
This seems to fit the theoretical picture, where driving occurs in the partial
ionization zone, which lies deeper and deeper within the star as it cools. It
is reasonable to assume that the pulsation periods should be related to the
thermal timescale in the region where driving occurs. As that region sinks
further down below the surface, that thermal timescale increases. Assuming this
connection, the pulsation periods could provide an additional way to determine
effective temperatures, independent of spectroscopy. We explore this idea and
find that in practice, things are not so simple.Comment: 4 pages, 3 figure
The turbulent generation of outward traveling Alfvenic fluctuations in the solar wind
From an analysis of the incompressible MHD equations, it is concluded that the frequent observation of outward propagating Alfvenic fluctuations in the solar wind can arise from early stages of in situ turbulent evolution, and need not reflect coronal processes
The experience of drowning
Internationally, drowning is a leading cause of accidental death that features in many legal cases. In these cases, possible mitigations and the 'pain and suffering' in terms of the duration and subjective experience of drowning are often pivotal in determining levels of compensation and outcome. As a result, there is a requirement to understand the stages of the drowning process, and the duration and physiological and subjective responses associated with each stage. In this short review we focus on these issues
- …