549 research outputs found
Computers and Liquid State Statistical Mechanics
The advent of electronic computers has revolutionised the application of
statistical mechanics to the liquid state. Computers have permitted, for
example, the calculation of the phase diagram of water and ice and the folding
of proteins. The behaviour of alkanes adsorbed in zeolites, the formation of
liquid crystal phases and the process of nucleation. Computer simulations
provide, on one hand, new insights into the physical processes in action, and
on the other, quantitative results of greater and greater precision. Insights
into physical processes facilitate the reductionist agenda of physics, whilst
large scale simulations bring out emergent features that are inherent (although
far from obvious) in complex systems consisting of many bodies. It is safe to
say that computer simulations are now an indispensable tool for both the
theorist and the experimentalist, and in the future their usefulness will only
increase.
This chapter presents a selective review of some of the incredible advances
in condensed matter physics that could only have been achieved with the use of
computers.Comment: 22 pages, 2 figures. Chapter for a boo
Constraints on OH Megamaser Excitation from a Survey of OH Satellite Lines
We report the results of a full-Stokes survey of all four 18 cm OH lines in
77 OH megamasers (OHMs) using the Arecibo Observatory. This is the first survey
of OHMs that included observations of the OH satellite lines; only 4 of the 77
OHMs have existing satellite line observations in the literature. In 5 sources,
satellite line emission is detected, with 3 of the 5 sources re-detections of
previously published sources. The 2 sources with new detections of satellite
line emission are IRAS F10173+0829, which was detected at 1720 MHz, and IRAS
F15107+0724, for which both the 1612 MHz and 1720 MHz lines were detected. In
IRAS F15107+0724, the satellite lines are partially conjugate, as 1720 MHz
absorption and 1612 MHz emission have the same structure at some velocities
within the source, along with additional broader 1612 MHz emission. This is the
first observed example of conjugate satellite lines in an OHM. In the remaining
sources, no satellite line emission is observed. The detections and upper
limits are generally consistent with models of OHM emission in which all of the
18 cm OH lines have the same excitation temperature. There is no evidence for a
significant population of strong satellite line emitters among OHMs.Comment: 9 pages, accepted to Ap
Parsec-scale magnetic fields in Arp 220
We present the first very-long-baseline interferometry (VLBI) detections of
Zeeman splitting in another galaxy. We used Arecibo Observatory, the Green Bank
Telescope, and the Very Long Baseline Array to perform dual-polarization
observations of OH maser lines in the merging galaxy Arp 220. We measured
magnetic fields of 1-5 mG associated with three roughly parsec-sized
clouds in the nuclear regions of Arp 220. Our measured magnetic fields have
comparable strengths and the same direction as features at the same velocity
identified in previous Zeeman observations with Arecibo alone. The agreement
between single dish and VLBI results provides critical validation of previous
Zeeman splitting observations of OH megamasers that used a single large dish.
The measured magnetic field strengths indicate that magnetic energy densities
are comparable to gravitational energy in OH maser clouds. We also compare our
total intensity results to previously published VLBI observations of OH
megamasers in Arp 220. We find evidence for changes in both structure and
amplitude of the OH maser lines that are most easily explained by variability
intrinsic to the masing region, rather than variability produced by
interstellar scintillation. Our results demonstrate the potential for using
high-sensitivity VLBI to study magnetic fields on small spatial scales in
extragalactic systems.Comment: 9 pages, accepted to MNRA
Path integral Monte Carlo simulations for rigid rotors and their application to water
In this work the path integral formulation for rigid rotors, proposed by
M\"user and Berne [Phys. Rev. Lett. {\bf 77}, 2638 (1996)], is described in
detail. It is shown how this formulation can be used to perform Monte Carlo
simulations of water. Our numerical results show that whereas some properties
of water can be accurately reproduced using classical simulations with an
empirical potential which, implicitly, includes quantum effects, other
properties can only be described quantitatively when quantum effects are
explicitly incorporated. In particular, quantum effects are extremely relevant
when it comes to describing the equation of state of the ice phases at low
temperatures, the structure of the ices at low temperatures, and the heat
capacity of both liquid water and the ice phases. They also play a minor role
in the relative stability of the ice phases.Comment: to appear in Molecular Physics (2011
- …