13,198 research outputs found
Non-equilibrium temperatures in steady-state systems with conserved energy
We study a class of non-equilibrium lattice models describing local
redistributions of a globally conserved quantity, which is interpreted as an
energy. A particular subclass can be solved exactly, allowing to define a
statistical temperature T_{th} along the same lines as in the equilibrium
microcanonical ensemble. We compute the response function and find that when
the fluctuation-dissipation relation is linear, the slope T_{FD}^{-1} of this
relation differs from the inverse temperature T_{th}^{-1}. We argue that T_{th}
is physically more relevant than T_{FD}, since in the steady-state regime, it
takes equal values in two subsystems of a large isolated system. Finally, a
numerical renormalization group procedure suggests that all models within the
class behave similarly at a coarse-grained level, leading to a new parameter
which describes the deviation from equilibrium. Quantitative predictions
concerning this parameter are obtained within a mean-field framework.Comment: 16 pages, 2 figures, submitted to Phys. Rev.
Tracing Slow Winds from T Tauri Stars via Low Velocity Forbidden Line Emission
Using Keck/HIRES spectra {\Delta}v ~ 7 km/s, we analyze forbidden lines of [O
I] 6300 {\AA}, [O I] 5577 {\AA} and [S II] 6731 {\AA} from 33 T Tauri stars
covering a range of disk evolutionary stages. After removing a high velocity
component (HVC) associated with microjets, we study the properties of the low
velocity component (LVC). The LVC can be attributed to slow disk winds that
could be magnetically (MHD) or thermally (photoevaporative) driven. Both of
these winds play an important role in the evolution and dispersal of
protoplanetary material.
LVC emission is seen in all 30 stars with detected [O I] but only in 2 out of
eight with detected [S II] , so our analysis is largely based on the properties
of the [O I] LVC. The LVC itself is resolved into broad (BC) and narrow (NC)
kinematic components. Both components are found over a wide range of accretion
rates and their luminosity is correlated with the accretion luminosity, but the
NC is proportionately stronger than the BC in transition disks.
The FWHM of both the BC and NC correlates with disk inclination, consistent
with Keplerian broadening from radii of 0.05 to 0.5 AU and 0.5 to 5 AU,
respectively. The velocity centroids of the BC suggest formation in an MHD disk
wind, with the largest blueshifts found in sources with closer to face-on
orientations. The velocity centroids of the NC however, show no dependence on
disk inclination. The origin of this component is less clear and the evidence
for photoevaporation is not conclusive
Microscopic theory for the glass transition in a system without static correlations
We study the orientational dynamics of infinitely thin hard rods of length L,
with the centers-of-mass fixed on a simple cubic lattice with lattice constant
a.We approximate the influence of the surrounding rods onto dynamics of a pair
of rods by introducing an effective rotational diffusion constant D(l),l=L/a.
We get D(l) ~ [1-v(l)], where v(l) is given through an integral of a
time-dependent torque-torque correlator of an isolated pair of rods. A glass
transition occurs at l_c, if v(l_c)=1. We present a variational and a
numerically exact evaluation of v(l).Close to l_c the diffusion constant
decreases as D(l) ~ (l_c-l)^\gamma, with \gamma=1. Our approach predicts a
glass transition in the absence of any static correlations, in contrast to
present form of mode coupling theory.Comment: 6 pages, 3 figure
Recommended from our members
The Overlap-Dirac Operator: Topology and Chiral Symmetry Breaking
We review the spectral flow techniques for computing the index of the overlap Dirac operator including results relevant for SUSY Yang-Mills theories. We describe properties of the overlap Dirac operator, and methods to implement it numerically. We use the results from the spectral flow to illuminate the difficulties in numerical calculations involving domain wall and overlap fermions
Cluster algorithms
Cluster algorithms for classical and quantum spin systems are discussed. In
particular, the cluster algorithm is applied to classical O(N) lattice actions
containing interactions of more than two spins. The performance of the
multi-cluster and single--cluster methods, and of the standard and improved
estimators are compared. (Lecture given at the summer school on `Advances in
Computer Simulations', Budapest, July 1996.)Comment: 17 pages, Late
Kinematics of Multigrid Monte Carlo
We study the kinematics of multigrid Monte Carlo algorithms by means of
acceptance rates for nonlocal Metropolis update proposals. An approximation
formula for acceptance rates is derived. We present a comparison of different
coarse-to-fine interpolation schemes in free field theory, where the formula is
exact. The predictions of the approximation formula for several interacting
models are well confirmed by Monte Carlo simulations. The following rule is
found: For a critical model with fundamental Hamiltonian H(phi), absence of
critical slowing down can only be expected if the expansion of
in terms of the shift psi contains no relevant (mass) term. We also introduce a
multigrid update procedure for nonabelian lattice gauge theory and study the
acceptance rates for gauge group SU(2) in four dimensions.Comment: 28 pages, 8 ps-figures, DESY 92-09
- …