6,145 research outputs found
Formation of Stellar Clusters and the Importance of Thermodynamics for Fragmentation
We discuss results from numerical simulations of star cluster formation in
the turbulent interstellar medium (ISM). The thermodynamic behavior of the
star-forming gas plays a crucial role in fragmentation and determines the
stellar mass function as well as the dynamic properties of the nascent stellar
cluster. This holds for star formation in molecular clouds in the solar
neighborhood as well as for the formation of the very first stars in the early
universe. The thermodynamic state of the ISM is a result of the balance between
heating and cooling processes, which in turn are determined by atomic and
molecular physics and by chemical abundances. Features in the effective
equation of state of the gas, such as a transition from a cooling to a heating
regime, define a characteristic mass scale for fragmentation and so set the
peak of the initial mass function of stars (IMF). As it is based on fundamental
physical quantities and constants, this is an attractive approach to explain
the apparent universality of the IMF in the solar neighborhood as well as the
transition from purely primordial high-mass star formation to the more normal
low-mass mode observed today.Comment: 10 pages, invited review, to appear in Dynamical Evolution of Dense
Stellar Systems, Proceed. of the IAU Symp. 246 (Capri, Sept. 2007), eds.
E.Vesperini, M. Giersz, and A. Sill
Shear Thickening and Scaling of the Elastic Modulus in a Fractal Colloidal System with Attractive Interactions
Dilute oil dispersions of fractal carbon black particles with attractive Van
der Waals interactions display continuous shear thickening followed by shear
thinning at high shear rates. The shear thickening transition occurs at
and is driven by hydrodynamic
breakup of clusters. Pre-shearing dispersions at shear rates
produces enhanced-modulus gels where and is directly proportional to the residual stress
in the gel measured at a fixed sample age. The observed data can be accounted
for using a simple scaling model for the breakup of fractal clusters under
shear stress.Comment: 5 pages, 5 figures; v2: treating low shear rate date separately;
edited title; reworked figure
On the rotating wave approximation in the adiabatic limit
I revisit a longstanding question in quantum optics; When is the rotating
wave approximation justified? In terms of the Jaynes-Cummings and Rabi models I
demonstrate that the approximation in general breaks down in the adiabatic
limit regardless of system parameters. This is explicitly shown by comparing
Berry phases of the two models, where it is found that this geometrical phase
is strictly zero in the Rabi model contrary to the non-trivial Berry phase of
the Jaynes-Cummings model. The source of this surprising result is traced back
to different topologies in the two models.Comment: 8 pages, 3 figure
Community Health: A Patient Satisfaction Inquiry at Puentes de Salud, a Health and Wellness Clinic Centering the Latinx Community in Philadelphia, PA
Free healthcare clinic models contribute the health and wellness of a community providing essential preventative and acute healthcare screening and diagnosis, mitigating negative health outcomes and higher healthcare costs for the individual. Current research supports optimizing community health through knowledge gained from frequent assessment of patient satisfaction and perceived barriers to healthcare. This study examined the level of patient satisfaction at Puentes de Salud (Puentes), a nonprofit healthcare clinic serving Philadelphia’s Latinx migrant community. Utilizing a validated and reliable survey instrument that captures patient satisfaction with healthcare, Spanish or English- speaking patients (n= 79) 18-89 years of age completed a self-administered survey. In addition, a demographic questionnaire with open ended clinic inquiry questions was developed to assesses healthcare access barriers. Participants reported a higher level of satisfaction with higher number of clinic visits but did not report higher scores with more years associated with the clinic. Appointment reminders, improving communication/ answering phone calls, and access to specialty services were suggested to optimize participant health. Improving patient satisfaction and identifying perceived barriers to healthcare presents a unique opportunity to incorporate community needs at Puentes to ensure future growth and utilization of the clinic
Mixed Plastics Waste to Ethylene and Propylene Feedstocks
Circular recycle of waste plastic holds significant environmental benefit in reducing the need for crude oil feed to produce plastic monomers and in addressing massive global accumulation of plastic waste. A two-stage cracking process is here explored for the reduction of long-chain polyethylene (PE), polypropylene (PP), and polystyrene (PS) to ethylene and propylene. The reaction yields useful byproducts, such as liquid fuel used to sustain the high energy demands of the process, and pressurized steam. A feed of 70 MT/day of PE, PP, and PS is assumed to be treated first in a rotary kiln pyrolysis reactor and secondly in a steam-cracking unit for the formation of short-chain unsaturated hydrocarbons. 41% of the feedstock by weight is converted to either ethylene or propylene. Due to the random nature of cracking, a pilot plant is deemed necessary to better understand this conversion. Heat integration is explored extensively throughout the cracking to employ other process products as fuel sources. A novel separation train and refrigeration cycle are then designed to isolate the two products of interest. The process is found not to be profitable, with an Internal Rate of Return of -4.74%, Net Present Value of - $18.8MM, and Return on Investment (ROI) in the third year of operation of -2.12%. However, a circular monomers facility holds significant value environmentally, and options are thus explored to potentially reduce the cost or make the process profitable
Protostellar Collapse with Various Metallicities
The thermal and chemical evolution of gravitationally collapsing protostellar
clouds is investigated, focusing attention on their dependence on metallicity.
Calculations are carried out for a range of metallicities spanning the local
interstellar value to zero. During the time when clouds are transparent to
continuous radiation, the temperatures are higher for those with lower
metallicity, reflecting lower radiative ability. However, once the clouds
become opaque, in the course of the adiabatic contraction of the transient
cores, their evolutionary trajectories in the density-temperature plane
converge to a unique curve that is determined by only physical constants. The
trajectories coincide with each other thereafter. Consequently, the size of the
stellar core at the formation is the same regardless of the gas composition of
the parent cloud.Comment: 30 pages. The Astrophysical Journal, 533, in pres
Effects of No-Tillage Production Practices on Crop Yields as Influenced by Crop and Growing Environment Factors
This paper evaluated differences between yields of no-tillage compared to conventional or reduced tillage and their associated downside risk. Six crops were evaluated along with how those yields and risks differed by various environmental factors such geographic location, precipitation, soil type and how long the practice had been used.no-tillage, conservation, conventional tillage, downside-risk, yield, Agribusiness, Environmental Economics and Policy, Farm Management, Land Economics/Use, Production Economics, Risk and Uncertainty,
Resonant ion-pair formation in electron recombination with HF^+
The cross section for resonant ion-pair formation in the collision of
low-energy electrons with HF^+ is calculated by the solution of the
time-dependent Schrodinger equation with multiple coupled states using a wave
packet method. A diabatization procedure is proposed to obtain the electronic
couplings between quasidiabatic potentials of ^1Sigma^+ symmetry for HF. By
including these couplings between the neutral states, the cross section for
ion-pair formation increases with about two orders of magnitude compared with
the cross section for direct dissociation. Qualitative agreement with the
measured cross section is obtained. The oscillations in the calculated cross
section are analyzed. The cross section for ion-pair formation in electron
recombination with DF^+ is calculated to determine the effect of isotopic
substitution.Comment: 12 pages, 12 figure
Three-dimensional imaging and detection efficiency performance of orthogonal coplanar CZT strip detectors
We report on recent three-dimensional imaging performance and detection efficiency measurements obtained with 5 mm thick prototype CdZnTe detectors fabricated with orthogonal coplanar anode strips. In previous work, we have shown that detectors fabricated using this design achieve both very good energy resolution and sub-millimeter spatial resolution with fewer electronic channels than are required for pixel detectors. As electron-only devices, like pixel detectors, coplanar anode strip detectors can be fabricated in the thickness required to be effective imagers for photons with energies in excess of 500 keV. Unlike conventional double-sided strip detectors, the coplanar anode strip detectors require segmented contacts and signal processing electronics on only one surface. The signals can be processed to measure the total energy deposit and the photon interaction location in three dimensions. The measurements reported here provide a quantitative assessment of the detection capabilities of orthogonal coplanar anode strip detectors
Fcc-bcc transition for Yukawa interactions determined by applied strain deformation
Calculations of the work required to transform between bcc and fcc phases
yield a high-precision bcc-fcc transition line for monodisperse point Yukawa
(screened-Couloumb) systems. Our results agree qualitatively but not
quantitatively with previously published simulations and phenomenological
criteria for the bcc-fcc transition. In particular, the bcc-fcc-fluid triple
point lies at a higher inverse screening length than previously reported.Comment: RevTex4, 9 pages, 6 figures. Discussion of phase coexistence
extended, a few other minor clarifications added, referencing improved.
Accepted for publication by Physical Review
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