380 research outputs found
Microcanonical solution of lattice models with long range interactions
We present a general method to obtain the microcanonical solution of lattice
models with long range interactions. As an example, we apply it to the long
range Ising chain, focusing on the role of boundary conditions.Comment: 6 pages, proceedings of the NEXT 2001 conferenc
Viscous diffusion and photoevaporation of stellar disks
The evolution of a stellar disk under the influence of viscous evolution,
photoevaporation from the central source, and photoevaporation by external
stars is studied. We take the typical parameters of TTSs and the Trapezium
Cluster conditions. The photoionizing flux from the central source is assumed
to arise both from the quiescent star and accretion shocks at the base of
stellar magnetospheric columns, along which material from the disk accretes.
The accretion flux is calculated self-consistently from the accretion mass loss
rate. We find that the disk cannot be entirely removed using only viscous
evolution and photoionization from the disk-star accretion shock. However, when
FUV photoevaporation by external massive stars is included the disk is removed
in 10^6 -10^7yr; and when EUV photoevaporation by external massive stars is
included the disk is removed in 10^5 - 10^6yr.
An intriguing feature of photoevaporation by the central star is the
formation of a gap in the disk at late stages of the disk evolution. As the gap
starts forming, viscous spreading and photoevaporation work in resonance.
There is no gap formation for disks nearby external massive stars because the
outer annuli are quickly removed by the dominant EUV flux. On the other hand,
at larger, more typical distances (d>>0.03pc) from the external stars the flux
is FUV dominated. As a consequence, the disk is efficiently evaporated at two
different locations; forming a gap during the last stages of the disk
evolution.Comment: 27 pages, 11 figures, accepted for publication in Ap
Chemical Abundance Study of One Red Giant Star in NGC 5694 : A Globular Cluster with Dwarf Spheroidals' Chemical Signature?
We report the abundance analysis of one red giant branch star in the
metal-poor outer halo globular cluster NGC 5694. We obtain [Fe/H] = -1.93,
based on the ionized lines, and our metallicity measurement is in good
agreement with previous estimates. We find that [Ca+Ti/2Fe] and [Cu/Fe] of NGC
5694 are about 0.3 -- 0.4 dex lower than other globular clusters with similar
metallicities, but similar to some LMC clusters and stars in some dwarf
spheroidal galaxies. Differences persist, however, in the abundances of neutron
capture elements. The unique chemical abundance pattern and the large
Galactocentric distance (30 kpc) and radial velocity (-138.6 +/- 1.0 km/sec)
indicate that NGC 5694 had an extragalactic origin.Comment: ApJL accepte
Inequivalence of ensembles in a system with long range interactions
We study the global phase diagram of the infinite range Blume-Emery-Griffiths
model both in the canonical and in the microcanonical ensembles. The canonical
phase diagram is known to exhibit first order and continuous transition lines
separated by a tricritical point. We find that below the tricritical point,
when the canonical transition is first order, the phase diagrams of the two
ensembles disagree. In this region the microcanonical ensemble exhibits energy
ranges with negative specific heat and temperature jumps at transition
energies. These results can be extended to weakly decaying nonintegrable
interactions.Comment: Revtex, 4 pages with 3 figures, submitted to Phys. Rev. Lett., e-mail
[email protected]
Review of genicular artery embolization, radiofrequency ablation, and cryoneurolysis in the management of osteoarthritis-related knee pain
Osteoarthritis (OA) of the knee represents one of the most common diseases in the world, affecting an estimated 14 million people in the United States alone. Exercise therapy and oral pain medication are first-line treatments but have limited efficacy. Next-line treatments such as intra-articular injections are limited in durability. Moreover, total knee replacements, although effective, require surgical intervention, which has considerable variability in patient satisfaction. Novel minimally invasive image-guided interventions are becoming more widespread for treating OA-related knee pain. Recent studies of these interventions have revealed promising results, minor complications, and reasonable patient satisfaction. In this study, published manuscripts were reviewed in the field of minimally invasive, image-guided interventions for OA-related knee pain, with a focus on genicular artery embolization, radiofrequency ablation, and cryoneurolysis. Recent studies have demonstrated a significant decrease in pain-related symptoms following these interventions. Reported complications were mild in the reviewed studies. Image-guided interventions for OA-related knee pain exist as valuable options for patients who fail other therapies, may not be good surgical candidates, or wish to avoid surgical intervention. Further studies with randomization and an increased length of follow-up are needed to better characterize outcomes following these minimally invasive therapies
Equilibrium and Dynamical Evolution of Self-Gravitating System Embedded in a Potential Well
Isothermal and self-gravitating systems bound by non-conducting and
conducting walls are known to be unstable if the density contrast between the
center and the boundary exceeds critical values. We investigate the equilibrium
and dynamical evolution of isothermal and self-gravitating system embedded in
potential well, which can be the situation of many astrophysical objects such
as the central parts of the galaxies, or clusters of galaxies with potential
dominated by dark matter, but is still limited to the case where the potential
well is fixed during the evolution. As the ratio between the depth of
surrounding potential well and potential of embedded system becomes large, the
potential well becomes effectively the same boundary condition as conducting
wall, which behaves like a thermal heat bath. We also use the direct N-body
simulation code, NBODY6 to simulate the dynamical evolution of stellar system
embedded in potential wells and propose the equilibrium models for this system.
In deep potential well, which is analogous to the heat bath with high
temperature, the embedded self-gravitating system is dynamically hot, and
loosely bound or can be unbound since the kinetic energy increases due to the
heating by the potential well. On the other hand, the system undergoes core
collapse by self-gravity when potential well is shallow. Binary heating can
stop the collapse and leads to the expansion, but the evolution is very slow
because the potential as a heat bath can absorb the energy generated by the
binaries. The system can be regarded as quasi-static. Density and velocity
dispersion profiles from the N-body simulations in the final quasi-equilibrium
state are similar to our equilibrium models assumed to be in thermal
equilibrium with the potential well.Comment: 12 pages, 12 figures, Submitted to MNRA
High-Resolution Spectroscopy in Tr37: Gas Accretion Evolution in Evolved Dusty Disks
Using the Hectochelle multifiber spectrograph, we have obtained
high-resolution (R~34,000) spectra in the Halpha region for a large number of
stars in the 4 Myr-old cluster Tr 37, containing 146 previously known members
and 26 newly identified ones. We present the Halpha line profiles of all
members, compare them to our IR observations of dusty disks (2MASS/JHK + IRAC +
MIPS 24 micron), use the radial velocities as a membership criterion, and
calculate the rotational velocities. We find a good correlation between the
accretion-broadened profiles and the presence of protoplanetary disks, noting
that a small fraction of the accreting stars presents broad profiles with
Halpha equivalent widths smaller than the canonical limit separating CTTS and
WTTS. The number of strong accretors appears to be lower than in younger
regions, and a large number of CTTS have very small accretion rates
(dM/dt<10^{-9} Msun/yr). Taking into account that the spectral energy
distributions are consistent with dust evolution (grain growth/settling) in the
innermost disk, this suggests a parallel evolution of the dusty and gaseous
components. We also observe that about half of the "transition objects" (stars
with no IR excesses at wavelengths shorter than ~6 micron) do not show any
signs of active accretion, whereas the other half is accreting with accretion
rates <10^{-9} Msun/yr. These zero or very low accretion rates reveal important
gas evolution and/or gas depletion in the innermost disk, which could be
related to grain growth up to planetesimal or even planet sizes. Finally, we
examine the rotational velocities of accreting and non accreting stars, finding
no significant differences that could indicate disk locking at these ages.Comment: 51 pages, 13 (reduced resolution) figures, 2 tables. AJ in pres
Two-Component Fokker-Planck Models for the Evolution of Isolated Globular Clusters
Two-component (normal and degenerate stars) models are the simplest
realization of clusters with a mass spectrum because high mass stars evolve
quickly into degenerates, while low mass stars remain on the main-sequence for
the age of the universe. Here we examine the evolution of isolated globular
clusters using two-component Fokker-Planck (FP) models that include heating by
binaries formed in tidal capture and in three-body encounters. Three-body
binary heating dominates and the postcollapse expansion is self-similar, at
least in models with total mass M <= 3 x 10^5 M_\odot, initial half-mass radius
r_{h,i} >= 5 pc, component mass ratio m_2/m_1 <= 2, and number ratio N_1/N_2 <=
300 when m_2=1.4 M_\odot. We derive scaling laws for \rho_c, v_c, r_c, and r_h
as functions of m_1/m_2, N, M, and time t from simple energy-balance arguments,
and these agree well with the FP simulations. We have studied the conditions
under which gravothermal oscillations (GTOs) occur. If E_{tot} and E_c are the
energies of the cluster and of the core, respectively, and t_{rh} and t_c are
their relaxation times, then \epsilon \equiv (E_{tot}/t_{rh})/(E_c/t_{rc}) is a
good predictor of GTOs: all models with \epsilon>0.01 are stable, and all but
one with \epsilon < 0.01 oscillate. We derive a scaling law for \epsilon
against N and m_1/m_2 and compared with our numerical results. Clusters with
larger m_2/m_1 or smaller N are stabler.Comment: 15 pages (LaTeX) with 8 figures. To appear in ApJ March 10, 1998
issu
IRAC Observations of Taurus Pre-Main Sequence Stars
We present infrared photometry obtained with the IRAC camera on the Spitzer
Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in
the Taurus star-forming region. We find a clear separation in some IRAC
color-color diagrams between objects with and without disks. A few
``transition'' objects are noted, which correspond to systems in which the
inner disk has been evacuated of small dust. Separating pure disk systems from
objects with remnant protostellar envelopes is more difficult at IRAC
wavelengths, especially for objects with infall at low rates and large angular
momenta. Our results generally confirm the IRAC color classification scheme
used in previous papers by Allen et al. and Megeath et al. to distinguish
between protostars, T Tauri stars with disks, and young stars without (inner)
disks. The observed IRAC colors are in good agreement with recent improved disk
models, and in general accord with models for protostellar envelopes derived
from analyzing a larger wavelength region. We also comment on a few Taurus
objects of special interest. Our results should be useful for interpreting IRAC
results in other, less well-studied star-forming regions.Comment: 29 pages 10 figures, to appear in Ap
Accretion Disks Around Young Objects. II. Tests of Well-Mixed Models with Ism Dust
We construct detailed vertical structure models of irradiated accretion disks
around T Tauri stars with interstellar medium dust uniformly mixed with gas.
The dependence of the structure and emission properties on mass accretion rate,
viscosity parameter, and disk radius is explored using these models. The
theoretical spectral energy distributions (SEDs) and images for all
inclinations are compared with observations of the entire population of
Classical T Tauri stars (CTTS) and Class I objects in Taurus. In particular, we
find that the median near-infrared fluxes can be explained within the errors
with the most recent values for the median accretion rates for CTTS. We further
show that the majority of the Class I sources in Taurus cannot be Class II
sources viewed edge-on because they are too luminous and their colors would be
consistent with disks seen only in a narrow range of inclinations. Our models
appear to be too geometrically thick at large radii, as suggested by: (a)
larger far-infrared disk emission than in the typical SEDs of T Tauri stars;
(b) wider dark dust lanes in the model images than in the images of HH30 and HK
Tau/c; and (c) larger predicted number of stars extincted by edge-on disks than
consistent with current surveys. The large thickness of the model is a
consequence of the assumption that dust and gas are well-mixed, suggesting that
some degree of dust settling may be required to explain the observations.Comment: 41 pages, 13 figures, accepted in Ap
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