1,715 research outputs found
Analytical and experimental studies of thermionically emitting electrodes in contact with dense, seeded plasmas
Interactions are considered between a moving,
alkali-metal seeded, dense plasma and a
metallic electrode whose surface properties are
influenced by the absorption of seed particles. The
plasma behavior is governed by a set of differential
equations, which are coupled to the surface
through the boundary conditions. These conditions
are obtained by utilizing the particle desorption
rate expressions of Levine and Gyftopoulos. The
solution of the problem yields the state of the surface
as well as the spatial distribution of plasma
properties. In particular, electrode voltage drops
are predicted, which indicate whether the electrode
operates in a thermionic or arc mode. The
method has been applied to a potassiwn-seeded
argon plasma in contact with a tungsten electrode
in a stagnation flow geometry. The results show
that the plasma - surface interaction may lead to
large electrode currents at moderate voltage drops.
These currents can be up to an order of magnitude
greater than what the random electron current
would be at the surface under conditions of
perfect thermodynamic equilibrium at the surface
temperature. R.esults of a comparable experiment
show reasonably good agreement with the theory
Keck Speckle Imaging of the White Dwarf G29-38: No Brown Dwarf Companion Detected
The white dwarf Giclas 29-38 has attracted much attention due to its large
infrared excess and the suggestion that excess might be due to a companion
brown dwarf. We observed this object using speckle interferometry at the Keck
telescope, obtaining diffraction-limited resolution (55 milliarcseconds) at K
band, and found it unresolved. Assuming the entire K band excess is due to a
single point-like companion, we place an upper limit on the binary separation
of 30 milliarcseconds, or 0.42 AU at the star's distance of 14.1 pc. This
result, combined with astroseismological data and other images of G29-38,
supports the hypothesis that the source of the near-infrared excess is not a
cool companion but a dust cloud.Comment: 7 pages, 2 figure
The pre-cataclysmic variable, LTT 560
Aims. System parameters of the object LTT560 are determined in order to clarify its nature and evolutionary status.
Methods. We apply time-series photometry to reveal orbital modulations of the light curve, time-series spectroscopy to measure radial velocities of features from both the primary and the secondary star, and flux-calibrated spectroscopy to derive temperatures of both components.
Results. We find that LTT 560 is composed of a low temperature (T ⌠7500 K) DA white dwarf as the primary and an M5.5±1 mainsequence star as the secondary component. The current orbital period is Porb = 3.54(07) h.We derive a mass ratio Msec/Mwd = 0.36(03) and estimate the distance to d = 25â40 pc. Long-term variation of the orbital light curve and an additional Hα emission component
on the white dwarf indicate activity in the system, probably in the form of flaring and/or accretion events
Contribution of White Dwarfs to Cluster Masses
I present a literature search through 31 July 1997 of white dwarfs (WDs) in
open and globular clusters. There are 36 single WDs and 5 WDs in binaries known
among 13 open clusters, and 340 single WDs and 11 WDs in binaries known among
11 globular clusters. From these data I have calculated WD mass fractions for
four open clusters (the Pleiades, NGC 2168, NGC 3532, and the Hyades) and one
globular cluster (NGC 6121). I develop a simple model of cluster evolution that
incorporates stellar evolution but not dynamical evolution to interpret the WD
mass fractions. I augment the results of my simple model with N-body
simulations incorporating stellar evolution (Terlevich 1987; de la Feunte
Marcos 1996; Vesperini & Heggie 1997). I find that even though these clusters
undergo moderate to strong kinematical evolution the WD mass fraction is
relatively insensitive to kinematical evolution. By comparing the cluster mass
functions to that of the Galactic disk, and incorporating plausibility
arguments for the mass function of the Galactic halo, I estimate the WD mass
fraction in these two populations. I assume the Galactic disk is ~10 Gyrs old
(Winget et al. 1987; Liebert, Dahn, & Monet 1988; Oswalt et al. 1996) and that
the Galactic halo is ~12 Gyrs old (Reid 1997b; Gratton et al. 1997; Chaboyer et
al. 1998), although the WD mass fraction is insensitive to age in this range. I
find that the Galactic halo should contain 8 to 9% (alpha = -2.35) or perhaps
as much as 15 to 17% (alpha = -2.0) of its stellar mass in the form of WDs. The
Galactic disk WD mass fraction should be 6 to 7% (alpha = -2.35), consistent
with the empirical estimates of 3 to 7% (Liebert, Dahn, & Monet 1988; Oswalt et
al. 1996). (abridged)Comment: 20 pages, uuencoded gunzip'ed latex + 3 postscrip figures, to be
published in AJ, April, 199
Mode identification of Pulsating White Dwarfs using the HST
We have obtained time-resolved ultraviolet spectroscopy for the pulsating DAV
stars G226-29 and G185-32, and for the pulsating DBV star PG1351+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 non-radial g-modes. We do
not find any pulsation mode visible only in the ultraviolet, nor any modes
whose phase flips by 180 degrees; 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.Comment: Accepted for publication in the Astrophysical Journal, Aug 200
In-situ corrosion studies on selected high-level waste packaging materials under simulated disposal conditions in rock salt formations
Mouse maps of gene expression in the brain
Comprehensive atlases of gene expression in the mouse brain will provide a great resource for neurobiology
The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing
We determine the ratio of helium- to hydrogen-atmosphere white dwarf stars as
a function of effective temperature from a model atmosphere analysis of the
infrared photometric data from the Two Micron All Sky Survey combined with
available visual magnitudes. Our study surpasses any previous analysis of this
kind both in terms of the accuracy of the Teff determinations as well as the
size of the sample. We observe that the ratio of helium- to hydrogen-atmosphere
white dwarfs increases gradually from a constant value of ~0.25 between Teff =
15,000 K and 10,000 K to a value twice as large in the range 10,000 > Teff >
8000 K, suggesting that convective mixing, which occurs when the bottom of the
hydrogen convection zone reaches the underlying convective helium envelope, is
responsible for this gradual transition. The comparison of our results with an
approximate model used to describe the outcome of this convective mixing
process implies hydrogen mass layers in the range log M_H/M_tot = -10 to -8 for
about 15% of the DA stars that survived the DA to DB transition near Teff ~
30,000 K, the remainder having presumably more massive layers above log
M_H/M_tot ~ -6.Comment: 30 pages, 11 figures, accepted for publication in the Astrophysical
Journa
White Dwarfs in Globular Clusters: HST Observations of M4
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258
white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial
distances from the cluster center were observed and sizeable WD populations
were found in all three. The location of these WDs in the color-magnitude
diagram, their mean mass of 0.51()M, and their luminosity
function confirm basic tenets of stellar evolution theory and support the
results from current WD cooling theory. The WDs are used to extend the cluster
main-sequence mass function upward to stars that have already completed their
nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and
found to be at most a few percent of all the main-sequence stars. The most
ancient WDs found are about 9 Gyr old, a level which is set solely by the
photometric limits of our data. Even though this is less than the age of M4, we
discuss how these cooling WDs can eventually be used to check the turnoff ages
of globular clusters and hence constrain the age of the Universe.Comment: 46 pages, latex, no figures included, figures available at
ftp://ftp.astro.ubc.ca/pub/richer/wdfig.uu size 2.7Mb. To be published in the
Astrophysical Journa
New Insights into the Problem of the Surface Gravity Distribution of Cool DA White Dwarfs
We review at length the longstanding problem in the spectroscopic analysis of
cool hydrogen-line (DA) white dwarfs (Teff < 13,000 K) where gravities are
significantly higher than those found in hotter DA stars. The first solution
that has been proposed for this problem is a mild and systematic helium
contamination from convective mixing that would mimic the high gravities. We
constrain this scenario by determining the helium abundances in six cool DA
white dwarfs using high-resolution spectra from the Keck I 10-m telescope. We
obtain no detections, with upper limits as low as He/H = 0.04 in some cases.
This allows us to put this scenario to rest for good. We also extend our model
grid to lower temperatures using improved Stark profiles with non-ideal gas
effects from Tremblay & Bergeron and find that the gravity distribution of cool
objects remains suspiciously high. Finally, we find that photometric masses
are, on average, in agreement with expected values, and that the high-log g
problem is so far unique to the spectroscopic approach.Comment: 44 pages, 14 figures, accepted for publication in the Astrophysical
Journa
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