6,355 research outputs found
The roles of kinetic theory and gas-surface interactions in measurements of upper-atmospheric density
Kinetic theory and gas-surface interactions in measurements of upper atmospheric densit
Dynamical Formation of Close Binaries During the Pre-main-sequence Phase
Solar-type binaries with short orbital periods ( 1 -
10 days; 0.1 AU) cannot form directly via fragmentation of
molecular clouds or protostellar disks, yet their component masses are highly
correlated, suggesting interaction during the pre-main-sequence (pre-MS) phase.
Moreover, the close binary fraction of pre-MS stars is consistent with that of
their MS counterparts in the field ( = 2.1%). Thus we can infer
that some migration mechanism operates during the early pre-MS phase (
5 Myr) that reshapes the primordial separation distribution. We test
the feasibility of this hypothesis by carrying out a population synthesis
calculation which accounts for two formation channels: Kozai-Lidov (KL)
oscillations and dynamical instability in triple systems. Our models
incorporate (1) more realistic initial conditions compared to previous studies,
(2) octupole-level effects in the secular evolution, (3) tidal energy
dissipation via weak-friction equilibrium tides at small eccentricities and via
non-radial dynamical oscillations at large eccentricities, and (4) the larger
tidal radius of a pre-MS primary. Given a 15% triple star fraction, we simulate
a close binary fraction from KL oscillations alone of
0.4% after = 5 Myr, which increases to 0.8% by
= 5 Gyr. Dynamical ejections and disruptions of unstable coplanar
triples in the disk produce solitary binaries with slightly longer periods
10 - 100 days. The remaining 60% of close binaries with
outer tertiaries, particularly those in compact coplanar configurations with
log (days) 2 - 5 ( 50 AU), can be
explained only with substantial extra energy dissipation due to interactions
with primordial gas.Comment: Accepted by ApJ; 23 pages; 8 figures; this version incorporates
changes made to address comments by refere
Extension of modern control system theory Final report
Analysis and synthesis techniques for linear systems and methods of minimizing sensitivity of optimal control systems to large parameter variation
Human Factors Considerations in System Design
Human factors considerations in systems design was examined. Human factors in automated command and control, in the efficiency of the human computer interface and system effectiveness are outlined. The following topics are discussed: human factors aspects of control room design; design of interactive systems; human computer dialogue, interaction tasks and techniques; guidelines on ergonomic aspects of control rooms and highly automated environments; system engineering for control by humans; conceptual models of information processing; information display and interaction in real time environments
Impact of inter-correlated initial binary parameters on double black hole and neutron star mergers
The distributions of the initial main-sequence binary parameters are one of
the key ingredients in obtaining evolutionary predictions for compact binary
(BH-BH / BH-NS / NS-NS) merger rates. Until now, such calculations were done
under the assumption that initial binary parameter distributions were
independent. Here, we implement empirically derived inter-correlated
distributions of initial binary parameters primary mass (M1), mass ratio (q),
orbital period (P), and eccentricity (e). Unexpectedly, the introduction of
inter-correlated initial binary parameters leads to only a small decrease in
the predicted merger rates by a factor of 2 3 relative to the previously
used non-correlated initial distributions. The formation of compact object
mergers in the isolated classical binary evolution favors initial binaries with
stars of comparable masses (q = 0.5 1) at intermediate orbital periods (log
P (days) = 2 4). New distributions slightly shift the mass ratios towards
smaller values with respect to the previously used flat q distribution, which
is the dominant effect decreasing the rates. New orbital periods only
negligibly increase the number of progenitors. Additionally, we discuss the
uncertainty of merger rate predictions associated with possible variations of
the massive-star initial mass function (IMF). We argue that evolutionary
calculations should be normalized to a star formation rate (SFR) that is
obtained from the observed amount of UV light at wavelength 1500{\AA} (SFR
indicator). In this case, contrary to recent reports, the uncertainty of the
IMF does not affect the rates by more than a factor of 2. Any change to the IMF
slope for massive stars requires a change of SFR in a way that counteracts the
impact of IMF variations on the merger rates. In contrast, we suggest that the
uncertainty in cosmic SFR at low metallicity can be a significant factor at
play.Comment: accepted for publication in A&
Renal Osteodystrophy or Kidney-Induced Osteoporosis?
PURPOSE OF REVIEW:
Chronic kidney disease (CKD) affects nearly 10% of the population. The incidence of fractures in population studies demonstrate an increase with worsening stages of kidney disease suggesting specific CKD related causes of fracture.
RECENT FINDINGS:
The increase in fractures with CKD most likely represents disordered bone quality due to the abnormal bone remodeling from renal osteodystrophy. There is also an increase in fractures with age in patients with CKD, suggesting that patients with CKD also have many fracture risk factors common to patients without known CKD. Osteoporosis is defined by the National Institutes of Health as "A skeletal disorder characterized by compromised bone strength predisposing to an increased risk of fracture. Bone strength reflects the integration of two main features: bone quantity and bone quality." Thus, CKD-related fractures can be considered a type of osteoporosis-where the bone quality is additionally impaired above that of age/hormonal-related osteoporosis. Perhaps using the term CKD-induced osteoporosis, similar to steroid-induced osteoporosis, will allow patients with CKD to be studied in trials investigating therapeutic agents. In this series, we will examine how CKD-induced osteoporosis may be diagnosed and treated
Calcium as a cardiovascular toxin in CKD-MBD
Disordered calcium balance and homeostasis are common in patients with chronic kidney disease. Such alterations are commonly associated with abnormal bone remodeling, directly and indirectly. Similarly, positive calcium balance may also be a factor in the pathogenesis of extra skeletal soft tissue and arterial calcification. Calcium may directly affect cardiac structure and function through direct effects to alter cell signaling due to abnormal intracellular calcium homeostasis 2) extra-skeletal deposition of calcium and phosphate in the myocardium and small cardiac arterioles, 3) inducing cardiomyocyte hypertrophy through calcium and hormone activation of NFAT signaling mechanisms, and 4) increased aorta calcification resulting in chronic increased afterload leading to hypertrophy. Similarly, calcium may alter vascular smooth muscle cell function and affect cell signaling which may predispose to a proliferative phenotype important in arteriosclerosis and arterial calcification. Thus, disorders of calcium balance and homeostasis due to CKD-MBD may play a role in the high cardiovascular burden observed in patients with CKD
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