2,442 research outputs found
Biases in Expansion Distances of Novae Arising from the Prolate Geometry of Nova Shells
(abridged) Expansion distances (or expansion parallaxes) for classical novae
are based on comparing a measurement of the shell expansion velocity,
multiplied by the time since outburst, with some measure of the angular size of
the shell. We review and formalize this method in the case of prolate
spheroidal shells. We present expressions for the maximum line-of-sight
velocity from a complete, expanding shell and for its projected major and minor
axes, in terms of the intrinsic axis ratio and the inclination of the polar
axis to the line of sight. For six distinct definitions of ``angular size'', we
tabulate the error in distance that is introduced under the assumption of
spherical symmetry (i.e., without correcting for inclination and axis ratio).
The errors can be significant and systematic, affecting studies of novae
whether considered individually or statistically. Each of the six estimators
overpredicts the distance when the polar axis is close to the line of sight,
and most underpredict the distance when the polar axis is close to the plane of
the sky. The straight mean of the projected semimajor and semiminor axes gives
the least distance bias for an ensemble of randomly oriented prolate shells.
The best individual expansion distances, however, result from a full
spatio-kinematic modeling of the nova shell. We discuss several practical
complications that affect expansion distance measurements of real nova shells.
Nova shell expansion distances be based on velocity and angular size
measurements made contemporaneously if possible, and the same ions and
transitions should be used for the imaging and velocity measurements. We
emphasize the need for complete and explicit reporting of measurement
procedures and results, regardless of the specific method used.Comment: 21 pages, LaTeX, uses aasms4.sty, to be published in Publ. Astron.
Soc. of the Pacific, May 200
Bounds for the time to failure of hierarchical systems of fracture
For years limited Monte Carlo simulations have led to the suspicion that the
time to failure of hierarchically organized load-transfer models of fracture is
non-zero for sets of infinite size. This fact could have a profound
significance in engineering practice and also in geophysics. Here, we develop
an exact algebraic iterative method to compute the successive time intervals
for individual breaking in systems of height in terms of the information
calculated in the previous height . As a byproduct of this method,
rigorous lower and higher bounds for the time to failure of very large systems
are easily obtained. The asymptotic behavior of the resulting lower bound leads
to the evidence that the above mentioned suspicion is actually true.Comment: Final version. To appear in Phys. Rev. E, Feb 199
Scaling in the time-dependent failure of a fiber bundle with local load sharing
We study the scaling behaviors of a time-dependent fiber-bundle model with
local load sharing. Upon approaching the complete failure of the bundle, the
breaking rate of fibers diverges according to ,
where is the lifetime of the bundle, and is a quite
universal scaling exponent. The average lifetime of the bundle scales
with the system size as , where depends on the
distribution of individual fiber as well as the breakdown rule.Comment: 5 pages, 4 eps figures; to appear in Phys. Rev.
Failure Probabilities and Tough-Brittle Crossover of Heterogeneous Materials with Continuous Disorder
The failure probabilities or the strength distributions of heterogeneous 1D
systems with continuous local strength distribution and local load sharing have
been studied using a simple, exact, recursive method. The fracture behavior
depends on the local bond-strength distribution, the system size, and the
applied stress, and crossovers occur as system size or stress changes. In the
brittle region, systems with continuous disorders have a failure probability of
the modified-Gumbel form, similar to that for systems with percolation
disorder. The modified-Gumbel form is of special significance in weak-stress
situations. This new recursive method has also been generalized to calculate
exactly the failure probabilities under various boundary conditions, thereby
illustrating the important effect of surfaces in the fracture process.Comment: 9 pages, revtex, 7 figure
The Post-Common Envelope and Pre-Cataclysmic Binary PG 1224+309
We have made extensive spectroscopic and photometric observations of PG
1224+309, a close binary containing a DA white dwarf primary and an M4+
secondary. The H alpha line is in emission due to irradiation of the M-star by
the hot white dwarf and is seen to vary around the orbit. From the radial
velocities of the H alpha line we derive a period of P = 0.258689 +/- 0.000004
days and a semi-amplitude of K_Halpha = 160 +/- 8 km/s. We estimate a
correction Delta_K = 21 +/- 2 km/s, where K_M = K_Halpha + Delta_K. Radial
velocity variations of the white dwarf reveal a semi-amplitude of K_WD = 112
+/- 14 km/s. The blue spectrum of the white dwarf is well fit by a synthetic
spectrum having T_eff = 29,300 K and log(g) = 7.38. The white dwarf contributes
97% of the light at 4500 Angstroms and virtually all of the light blueward of
3800 Angstroms. No eclipses are observed. The mass inferred for the white dwarf
depends on the assumed mass of the thin residual hydrogen envelope: 0.40 < M_WD
< 0.45 solar masses for hydrogen envelope masses of 0 < M_H < 4.0E-4 solar
masses. We argue that the mass of the white dwarf is closer to 0.45 solar
masses, hence it appears that the white dwarf has a relatively large residual
hydrogen envelope. The mass of the M-star is then M_M = 0.28 +/- 0.05 solar
masses, and the inclination is i = 77 +/- 7 degrees. We discuss briefly how PG
1224+309 may be used to constrain theories of close binary star evolution, and
the past and future histories of PG 1224+309 itself. The star is both a
``post-common envelope'' star and a ``pre-cataclysmic binary'' star. Mass
transfer by Roche-lobe overflow should commence in about 10 Gyr.Comment: 17 pages, 8 figures, AAS LaTeX, to appear in AJ, March 199
Bubble collisions and measures of the multiverse
To compute the spectrum of bubble collisions seen by an observer in an
eternally-inflating multiverse, one must choose a measure over the diverging
spacetime volume, including choosing an "initial" hypersurface below which
there are no bubble nucleations. Previous calculations focused on the case
where the initial hypersurface is pushed arbitrarily deep into the past.
Interestingly, the observed spectrum depends on the orientation of the initial
hypersurface, however one's ability observe the effect rapidly decreases with
the ratio of inflationary Hubble rates inside and outside one's bubble. We
investigate whether this conclusion might be avoided under more general
circumstances, in particular placing the observer's bubble near the initial
hypersurface. We find that it is not. As a point of reference, a substantial
appendix reviews relevant aspects of the measure problem of eternal inflation.Comment: 24 pages, two figures, plus 16-page appendix with one figure; v2:
minor improvements and clarifications, conclusions unchanged (version to
appear in JCAP
Clinically Actionable Hypercholesterolemia and Hypertriglyceridemia in Children with Nonalcoholic Fatty Liver Disease
OBJECTIVE:
To determine the percentage of children with nonalcoholic fatty liver disease (NAFLD) in whom intervention for low-density lipoprotein cholesterol or triglycerides was indicated based on National Heart, Lung, and Blood Institute guidelines.
STUDY DESIGN:
This multicenter, longitudinal cohort study included children with NAFLD enrolled in the National Institute of Diabetes and Digestive and Kidney Diseases Nonalcoholic Steatohepatitis Clinical Research Network. Fasting lipid profiles were obtained at diagnosis. Standardized dietary recommendations were provided. After 1 year, lipid profiles were repeated and interpreted according to National Heart, Lung, and Blood Institute Expert Panel on Integrated Guidelines for Cardiovascular Health and Risk Reduction. Main outcomes were meeting criteria for clinically actionable dyslipidemia at baseline, and either achieving lipid goal at follow-up or meeting criteria for ongoing intervention.
RESULTS:
There were 585 participants, with a mean age of 12.8 years. The prevalence of children warranting intervention for low-density lipoprotein cholesterol at baseline was 14%. After 1 year of recommended dietary changes, 51% achieved goal low-density lipoprotein cholesterol, 27% qualified for enhanced dietary and lifestyle modifications, and 22% met criteria for pharmacologic intervention. Elevated triglycerides were more prevalent, with 51% meeting criteria for intervention. At 1 year, 25% achieved goal triglycerides with diet and lifestyle changes, 38% met criteria for advanced dietary modifications, and 37% qualified for antihyperlipidemic medications.
CONCLUSIONS:
More than one-half of children with NAFLD met intervention thresholds for dyslipidemia. Based on the burden of clinically relevant dyslipidemia, lipid screening in children with NAFLD is warranted. Clinicians caring for children with NAFLD should be familiar with lipid management
Morphological characterization of shocked porous material
Morphological measures are introduced to probe the complex procedure of shock
wave reaction on porous material. They characterize the geometry and topology
of the pixelized map of a state variable like the temperature. Relevance of
them to thermodynamical properties of material is revealed and various
experimental conditions are simulated. Numerical results indicate that, the
shock wave reaction results in a complicated sequence of compressions and
rarefactions in porous material. The increasing rate of the total fractional
white area roughly gives the velocity of a compressive-wave-series.
When a velocity is mentioned, the corresponding threshold contour-level of
the state variable, like the temperature, should also be stated. When the
threshold contour-level increases, becomes smaller. The area increases
parabolically with time during the initial period. The curve goes
back to be linear in the following three cases: (i) when the porosity
approaches 1, (ii) when the initial shock becomes stronger, (iii) when the
contour-level approaches the minimum value of the state variable. The area with
high-temperature may continue to increase even after the early
compressive-waves have arrived at the downstream free surface and some
rarefactive-waves have come back into the target body. In the case of energetic
material ... (see the full text)Comment: 3 figures in JPG forma
Dynamical Backaction of Microwave Fields on a Nanomechanical Oscillator
We measure the response and thermal motion of a high-Q nanomechanical
oscillator coupled to a superconducting microwave cavity in the
resolved-sideband regime where the oscillator's resonance frequency exceeds the
cavity's linewidth. The coupling between the microwave field and mechanical
motion is strong enough for radiation pressure to overwhelm the intrinsic
mechanical damping. This radiation-pressure damping cools the fundamental
mechanical mode by a factor of 5 below the thermal equilibrium temperature in a
dilution refrigerator to a phonon occupancy of 140 quanta.Comment: 4 pages, 4 figure
A search for cool subdwarfs: Stellar parameters for 134 candidates
The results of a search for cool subdwarfs are presented. Kinematic (U, V,
and W) and stellar parameters (Teff, log g, [Fe/H], and V_t) are derived for
134 candidate subdwarfs based on high resolution spectra. The observed stars
span 4200K < Teff < 6400K and -2.70 < [Fe/H] < 0.25 including only 8 giants
(log g < 4.0). Of the sample, 100 stars have MgH bands present in their
spectra. The targets were selected by their large reduced proper-motion, the
offset from the solar metallicity main sequence, or culled from the literature.
We confirm the claims made by Ryan (1989) regarding the NLTT catalog being a
rich source of subdwarfs and verify the success of the reduced proper-motion
constraint in identifying metal-poor stars.Comment: 31 pages, 10 figures, Accepted for publication in PAS
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