368 research outputs found
An asteroseismic test of diffusion theory in white dwarfs
The helium-atmosphere (DB) white dwarfs are commonly thought to be the
descendants of the hotter PG1159 stars, which initially have uniform He/C/O
atmospheres. In this evolutionary scenario, diffusion builds a pure He surface
layer which gradually thickens as the star cools. In the temperature range of
the pulsating DB white dwarfs (T_eff ~ 25,000 K) this transformation is still
taking place, allowing asteroseismic tests of the theory. We have obtained
dual-site observations of the pulsating DB star CBS114, to complement existing
observations of the slightly cooler star GD358. We recover the 7 independent
pulsation modes that were previously known, and we discover 4 new ones to
provide additional constraints on the models. We perform objective global
fitting of our updated double-layered envelope models to both sets of
observations, leading to determinations of the envelope masses and pure He
surface layers that qualitatively agree with the expectations of diffusion
theory. These results provide new asteroseismic evidence supporting one of the
central assumptions of spectral evolution theory, linking the DB white dwarfs
to PG1159 stars.Comment: 7 pages, 3 figures, 3 tables, accepted for publication in A&
Evolutionary Timescale of the DAV G117-B15A: The Most Stable Optical Clock Known
We observe G117-B15A, the most precise optical clock known, to measure the
rate of change of the main pulsation period of this blue-edge DAV white dwarf.
Even though the obtained value is only within 1 sigma, Pdot = (2.3 +/- 1.4) x
10^{-15} s/s, it is already constraining the evolutionary timescale of this
cooling white dwarf star.Comment: Accepted for publication in ApJ
Genetic-Algorithm-based Asteroseismological Analysis of the DBV White Dwarf GD 358
White dwarf asteroseismology offers the opportunity to probe the structure
and composition of stellar objects governed by relatively simple physical
principles. The observational requirements of asteroseismology have been
addressed by the development of the Whole Earth Telescope (WET), but the
analytical procedures still need to be refined before this technique can yield
the complete physical insight that the data can provide. Toward this end, we
have utilized a genetic-algorithm-based optimization method to fit our models
to the observed pulsation frequencies of the DBV white dwarf GD 358 obtained
with the WET in 1990. This new approach has finally exploited the sensitivity
of our models to the core composition, and will soon yield some interesting
constraints on nuclear reaction cross-sections.Comment: 8 pages using emulateapj.sty, 4 figures (1 color), 2 tables. Accepted
for publication in Ap
Optimal placement of a limited number of observations for period searches
Robotic telescopes present the opportunity for the sparse temporal placement
of observations when period searching. We address the best way to place a
limited number of observations to cover the dynamic range of frequencies
required by an observer. We show that an observation distribution geometrically
spaced in time can minimise aliasing effects arising from sparse sampling,
substantially improving signal detection quality. The base of the geometric
series is however a critical factor in the overall success of this strategy.
Further, we show that for such an optimal distribution observations may be
reordered, as long as the distribution of spacings is preserved, with almost no
loss of quality. This implies that optimal observing strategies can retain
significant flexibility in the face of scheduling constraints, by providing
scope for on-the-fly adaptation. Finally, we present optimal geometric
samplings for a wide range of common observing scenarios, with an emphasis on
practical application by the observer at the telescope. Such a sampling
represents the best practical empirical solution to the undersampling problem
that we are aware of. The technique has applications to robotic telescope and
satellite observing strategies, where target acquisition overheads mean that a
greater total target exposure time (and hence signal-to-noise) can often in
practice be achieved by limiting the number of observations.Comment: 8 pages with 16 figure
New Pulsating DB White Dwarf Stars from the Sloan Digital Sky Survey
We are searching for new He atmosphere white dwarf pulsators (DBVs) based on
the newly found white dwarf stars from the spectra obtained by the Sloan
Digital Sky Survey. DBVs pulsate at hotter temperature ranges than their better
known cousins, the H atmosphere white dwarf pulsators (DAVs or ZZ Ceti stars).
Since the evolution of white dwarf stars is characterized by cooling,
asteroseismological studies of DBVs give us opportunities to study white dwarf
structure at a different evolutionary stage than the DAVs. The hottest DBVs are
thought to have neutrino luminosities exceeding their photon luminosities
(Winget et al. 2004), a quantity measurable through asteroseismology.
Therefore, they can also be used to study neutrino physics in the stellar
interior. So far we have discovered nine new DBVs, doubling the number of
previously known DBVs. Here we report the new pulsators' lightcurves and power
spectra.Comment: 15 pages, 2 figures, 3 tables, ApJ accepte
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
Gene therapy with Angiotensin-(1-9) preserves left ventricular systolic function after myocardial infarction
BACKGROUND: Angiotensin-(1-9) [Ang-(1-9)] is a novel peptide of the counter-regulatory axis of the renin angiotensin system previously demonstrated to have therapeutic potential in hypertensive cardiomyopathy when administered via osmotic minipump in mice. Here, we investigate whether gene transfer of Ang-(1-9) is cardioprotective in a murine model of myocardial infarction (MI).
OBJECTIVES: To evaluate effects of Ang-(1-9) gene therapy on myocardial structural and functional remodeling post infarction.
METHODS: C57BL/6 mice underwent permanent left anterior descending coronary artery ligation and cardiac function was assessed using echocardiography for 8 weeks followed by a terminal measurement of left ventricular (LV) pressure-volume loops. Ang-(1-9) was delivered by adeno-associated viral vector via single tail vein injection immediately following induction of MI. Direct effects of Ang-(1-9) on cardiomyocyte excitation–contraction coupling and cardiac contraction were evaluated in isolated mouse and human cardiomyocytes and in an ex vivo Langendorff perfused whole heart model.
RESULTS: Gene delivery of Ang-(1-9) significantly reduced sudden cardiac death post-MI. Pressure–volume measurements revealed complete restoration of end systolic pressure, ejection fraction, end systolic volume and the end diastolic pressure–volume relationship by Ang-(1-9) treatment. Stroke volume and cardiac output were significantly increased versus sham. Histological analysis revealed only mild effects on cardiac hypertrophy and fibrosis, but a significant increase in scar thickness. Direct assessment of Ang-(1-9) on isolated cardiomyocytes demonstrated a positive inotropic effect via increasing calcium transient amplitude and increasing contractility. Ang-(1-9) increased contraction in the Langendorff model through a protein kinase A-dependent mechanism.
CONCLUSIONS: Our novel findings show that Ang-(1-9) gene therapy preserves LV systolic function post-MI, restoring cardiac function. Furthermore, Ang-(1-9) has a direct effect on cardiomyocyte
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calcium handling through a protein kinase A-dependent mechanism. These data highlight Ang-(1-9) gene therapy as a potential new strategy in the context of MI
Discovery of a Nova-Like Cataclysmic Variable in the Kepler Mission Field
We announce the identification of a new cataclysmic variable star in the
field of the Kepler Mission, KIC J192410.81+445934.9. This system was
identified during a search for compact pulsators in the Kepler field.
High-speed photometry reveals coherent large-amplitude variability with a
period of 2.94 h. Rapid, large-amplitude quasi-periodic variations are also
detected on time scales of ~1200 s and ~650 s. Time-resolved spectroscopy
covering one half photometric period shows shallow, broad Balmer and He I
absorption lines with bright emission cores as well as strong He II and Bowen
blend emission. Radial velocity variations are also observed in the Balmer and
He I emission lines that are consistent with the photometric period. We
therefore conclude that KIC J192410.81+445934.9 is a nova-like variable of the
UX UMa class in or near the period gap, and it may belong to the rapidly
growing subclass of SW Sex systems. Based on 2MASS photometry and companion
star models, we place a lower limit on the distance to the system of ~500 pc.
Due to limitations of our discovery data, additional observations including
spectroscopy and polarimetry are needed to confirm the nature of this object.
Such data will help to further understanding of the behavior of nova-like
variables in the critical period range of 3-4 h, where standard cataclysmic
variable evolutionary theory finds major problems. The presence of this system
in the Kepler mission field-of-view also presents a unique opportunity to
obtain a continuous photometric data stream of unparalleled length and
precision on a cataclysmic variable system.Comment: Accepted for publication in the Astronomical Journal. 8 pages, 7
figures, uses emulateapj
Probing the internal rotation of pre-white dwarf stars with asteroseismology: the case of PG 122+200
We put asteroseismological constraints on the internal rotation profile of
the GW Vir (PG1159-type) star PG 0122+200. To this end we employ a
state-of-the-art asteroseismological model for this star and we assess the
expected frequency splittings induced by rotation adopting a forward approach
in which we compare the theoretical frequency separations with the observed
ones assuming different types of plausible internal rotation profiles. We also
employ two asteroseismological inversion methods for the inversion of the
rotation profile of PG 0122+200. We find evidence for differential rotation in
this star. We demonstrate that the frequency splittings of the rotational
multiplets exhibited by PG 0122+200 are compatible with a rotation profile in
which the central regions are spinning about 2.4 times faster than the stellar
surface.Comment: 8 pages, 6 figures, 2 tables. To be published in MNRA
Time Changes with the Embodiment of Another’s Body Posture
The aim of the present study was to investigate whether the perception of presentation durations of pictures of different body postures was distorted as function of the embodied movement that originally produced these postures. Participants were presented with two pictures, one with a low-arousal body posture judged to require no movement and the other with a high-arousal body posture judged to require considerable movement. In a temporal bisection task with two ranges of standard durations (0.4/1.6 s and 2/8 s), the participants had to judge whether the presentation duration of each of the pictures was more similar to the short or to the long standard duration. The results showed that the duration was judged longer for the posture requiring more movement than for the posture requiring less movement. However the magnitude of this overestimation was relatively greater for the range of short durations than for that of longer durations. Further analyses suggest that this lengthening effect was mediated by an arousal effect of limited duration on the speed of the internal clock system
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