2,543 research outputs found
Towards an Empirical Determination of the ZZ Ceti Instability Strip
We present atmospheric parameters for a large sample of DA white dwarfs that
are known to be photometrically constant. For each star, we determine the
effective temperature and surface gravity by comparing high signal-to-noise
ratio optical spectra to the predictions of detailed model atmosphere
calculations. We also report the successful prediction and detection of
photometric variability in G232-38 based on similar Teff and log g
determinations. The atmospheric parameters derived for this sample of constant
stars as well as those for the known sample of bright ZZ Ceti stars (now
boosted to a total of 39) have been obtained in a highly homogeneous way. We
combine them to study the empirical red and blue edges as well as the purity of
the ZZ Ceti instability strip. We find that the red edge is rather well
constrained whereas there exists a rather large range of possibilities for the
slope of the blue edge. Furthermore, the ZZ Ceti instability strip that results
from our analysis contains no nonvariable white dwarfs. Our sample of constant
stars is part of a much broader spectroscopic survey of bright (V < 17) DA
white dwarfs, which we have recently undertaken. We also present here some
preliminary results of this survey. Finally, we revisit the analysis by Mukadam
et al. of the variable and nonvariable DA stars uncovered as part of the Sloan
Digital Sky Survey. Their erroneous conclusion of an instability strip
containing several nonvariable stars is traced back to the low signal-to-noise
ratio spectroscopic observations used in that survey.Comment: 43 pages, 2 tables, 14 figures, accepted for publication in the
Astrophysical Journa
Search For Oxygen in Cool DQ White Dwarf Atmospheres
We report new infrared spectroscopic observations of cool DQ white dwarfs by
using Coolspec on the 2.7m Harlan-Smith Telescope. DQs have helium-rich
atmospheres with traces of molecular carbon thought to be the result of
convective dredge-up from their C/O interiors. Recent model calculations
predict that oxygen should also be present in DQ atmospheres in detectable
amounts. Our synthetic spectra calculations for He-rich white dwarfs with
traces of C and O indicate that CO should be easily detected in the cool DQ
atmospheres if present in the expected amounts. Determination of the oxygen
abundance in the atmosphere will reveal the C/O ratio at the core/envelope
boundary, constraining the important and uncertain ^{12}C(alpha,gamma)^{16}O
reaction rate.Comment: 2 pages, 2 figures, to appear in proceedings of the 13th European
Workshop on White Dwarf
Evidence for multiple processes contributing to the Perruchet effect: Response priming and associative learning.
The Perruchet effect constitutes a robust demonstration that it is possible to dissociate
conditioned responding and expectancy in a random partial reinforcement design across a
variety of human associative learning paradigms. This dissociation has been interpreted as
providing evidence for multiple processes supporting learning, with expectancy driven by
cognitive processes that lead to a Gambler's fallacy, and the pattern of conditioned
responding (CRs) the result of an associative learning process. An alternative explanation is
that the pattern of CRs is the result of exposure to the unconditioned stimulus (US). In three
human eyeblink conditioning experiments we examined these competing explanations of the
Perruchet effect by employing a differential conditioning design and varying the degree to
which the two conditioned stimuli (CS) were discriminable. Across all of these experiments
there was evidence for a component of the CRs being strongly influenced by recent
reinforcement, in a way that was not demonstrably influenced by manipulations of CS
discriminability, which suggests a response priming mechanism contributes to the Perruchet
effect. However, the complete pattern of results and an analysis of the results from previously
published studies are also consistent with there being an associative contribution to the effect.This research was supported by grant DP1096437 from the Australian Research
Council to G. Weidemann, an ESRC Doctoral Training Grant to A. McAndrew and I. P.L.
McLaren, and an EPS Study visit grant awarded to A. McAndrew
Neuronal Activity in the Human Subthalamic Nucleus Encodes Decision Conflict during Action Selection
The subthalamic nucleus (STN), which receives excitatory inputs from the cortex and has direct connections with the inhibitory pathways\ud
of the basal ganglia, is well positioned to efficiently mediate action selection. Here, we use microelectrode recordings captured during\ud
deep brain stimulation surgery as participants engage in a decision task to examine the role of the human STN in action selection. We\ud
demonstrate that spiking activity in the STN increases when participants engage in a decision and that the level of spiking activity\ud
increases with the degree of decision conflict. These data implicate the STN as an important mediator of action selection during decision\ud
processes.\u
The Formation Rate, Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio were obtained
of a complete sample of 347 DA white dwarfs from the Palomar Green (PG) Survey.
Fits of observed Balmer lines to synthetic spectra calculated from
pure-hydrogen model atmospheres were used to obtain robust values of Teff, log
g, masses, radii, and cooling ages. The luminosity function of the sample,
weighted by 1/Vmax, was obtained and compared with other determinations. The
mass distribution of the white dwarfs is derived, after important corrections
for the radii of the white dwarfs in this magnitude-limited survey and for the
cooling time scales. The formation rate of DA white dwarfs from the PG is
estimated to be 0.6x10^(-12) pc^(-3) yr^(-1). Comparison with predictions from
a theoretical study of the white dwarf formation rate for single stars
indicates that >80% of the high mass component requires a different origin,
presumably mergers of lower mass double degenerate stars. In order to estimate
the recent formation rate of all white dwarfs in the local Galactic disk,
corrections for incompleteness of the PG, addition of the DB-DO white dwarfs,
and allowance for stars hidden by luminous binary companions had to be applied
to enhance the rate. An overall formation rate of white dwarfs recently in the
local Galactic disk of 1.15+/-0.25x10^(-12) pc^(-3) yr^(-1) is obtained. Two
recent studies of samples of nearby Galactic planetary nebulae lead to
estimates around twice as high. Difficulties in reconciling these
determinations are discussed.Comment: 73 pages, 18 figures, accepted for publication in the ApJ Supplemen
CHURCH AND STATE COLLABORATION TO POLITICIZE HOMOPHOBIA A CASE OF POLAND AND RUSSIA
This thesis explores how homosexuality has been reframed as a political and moral “other” by far-right movements and religious institutions in Poland and Russia. I leverage theories about democratic backsliding, otherization, and religious nationalism to argue that the purpose of this alliance is to gain or keep political power. I untangle how nationalist groups, political parties, and religious institutions in Poland and Russia became aligned in this campaign of politicized homophobia. In Poland, the Catholic hierarchy’s colluded with newly formed nationalist parties to demonize the growing LGBTQ community and gain political influence through appealing to nationalist and religious feelings. In Russia, the Putin regime feared losing popularity due to economic setbacks and thus co-opted the Russian Orthodox Church language of religious nationalism. The Putin regime thus made LGBTQ rights a “wedge issue” to shore up approval ratings among the religious.Master of Art
From Young and Hot to Old and Cold: Comparing White Dwarf Cooling Theory to Main Sequence Stellar Evolution in Open Clusters
I explore the current ability of both white dwarf cooling theory and main
sequence stellar evolution theory to accurately determine stellar population
ages by comparing ages derived using both techniques for open clusters ranging
from 0.1 to 4 Gyr. I find good agreement between white dwarf and main sequence
evolutionary ages over the entire age range currently available for study. I
also find that directly comparing main sequence turn-off ages to white dwarf
ages is only weakly sensitive to realistic levels of errors in cluster
distance, metallicity, and reddening. Additional detailed comparisons between
white dwarf and main sequence ages have tremendous potential to refine and
calibrate both of these important clocks, and I present new simulations of
promising open cluster targets. The most demanding requirement for these white
dwarf studies are very deep (V > 25-28) cluster observations made necessary by
the faintness of the oldest white dwarfs.Comment: 25 pages, incl. 10 figures, ApJ accepted for April, 200
Life Products of Stars
We attempt to document complete energetic transactions of stars in their
life. We calculate photon and neutrino energies that are produced from stars in
their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art
stellar evolution code, tracing the evolution continuously from pre-main
sequence gravitational contraction to white dwarfs. We also catalogue
gravitational and thermal energies and helium, and heavier elements that are
stored in stars and those ejected into interstellar space in each evolutionary
phase.Comment: 26 pages, including 8 figures and 3 tables. Submitted to ApJ
Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs
We present an analysis of spectroscopic and photometric data for cool DQ
white dwarfs based on improved model atmosphere calculations. In particular, we
revise the atmospheric parameters of the trigonometric parallax sample of
Bergeron et al.(2001), and discuss the astrophysical implications on the
temperature scale and mean mass, as well as the chemical evolution of these
stars. We also analyze 40 new DQ stars discovered in the first data release of
the Sloan Digital Sky Survey.Comment: 6 pages,3 figures, 14th European Workshop on White Dwarfs, ASP
Conference Series, in pres
C/O white dwarfs of very low mass: 0.33-0.5 Mo
The standard lower limit for the mass of white dwarfs (WDs) with a C/O core
is roughly 0.5 Mo. In the present work we investigated the possibility to form
C/O WDs with mass as low as 0.33 Mo. Both the pre-WD and the cooling evolution
of such nonstandard models will be described.Comment: Submitted to the "Proceedings of the 16th European White Dwarf
Workshop" (to be published JPCS). 7 pages including 13 figure
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