2,771 research outputs found
Equilibration in long-range quantum spin systems from a BBGKY perspective
The time evolution of -spin reduced density operators is studied for a
class of Heisenberg-type quantum spin models with long-range interactions. In
the framework of the quantum Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY)
hierarchy, we introduce an unconventional representation, different from the
usual cluster expansion, which casts the hierarchy into the form of a
second-order recursion. This structure suggests a scaling of the expansion
coefficients and the corresponding time scales in powers of with the
system size , implying a separation of time scales in the large system
limit. For special parameter values and initial conditions, we can show
analytically that closing the BBGKY hierarchy by neglecting -spin
correlations does never lead to equilibration, but gives rise to quasi-periodic
time evolution with at most independent frequencies. Moreover, for the
same special parameter values and in the large- limit, we solve the complete
recursion relation (the full BBGKY hierarchy), observing a superexponential
decay to equilibrium in rescaled time .Comment: 3 figure
Molecules in the Circumstellar Disk Orbiting BP Piscium
BP Psc is a puzzling late-type, emission-line field star with large infrared
excess. The star is encircled and enshrouded by a nearly edge-on, dust
circumstellar disk, and displays an extensive jet system similar to those
associated with pre-main sequence (pre-MS) stars. We conducted a mm-wave
molecular line survey of BP Psc with the 30 m telescope of the Institut de
Radio Astronomie Millimetrique (IRAM). We detected lines of 12CO and 13CO and,
possibly, very weak emission from HCO+ and CN; HCN, H2CO, and SiO are not
detected. The CO line profiles of BP Psc are well fit by a model invoking a
disk in Keplerian rotation. The mimumum disk gas mass, inferred from the 12CO
line intensity and 13CO/12CO line ratio, is ~0.1 Jupiter masses. The weakness
of HCO+ and CN (relative to 13CO) stands in sharp contrast to the strong HCO+
and CN emission that characterizes most low-mass, pre-main sequence stars that
have been the subjects of molecular emission-line surveys, and is suggestive of
a very low level of X-ray-induced molecular ionization within the BP Psc disk.
These results lend some support to the notion that BP Psc is an evolved star
whose circumstellar disk has its origins in a catastrophic interaction with a
close companion.Comment: 6 pages, 4 figures; to appear in Astronomy & Astrophysic
AND Protocols Using Only Uniform Shuffles
Secure multi-party computation using a deck of playing cards has been a
subject of research since the "five-card trick" introduced by den Boer in 1989.
One of the main problems in card-based cryptography is to design
committed-format protocols to compute a Boolean AND operation subject to
different runtime and shuffle restrictions by using as few cards as possible.
In this paper, we introduce two AND protocols that use only uniform shuffles.
The first one requires four cards and is a restart-free Las Vegas protocol with
finite expected runtime. The second one requires five cards and always
terminates in finite time.Comment: This paper has appeared at CSR 201
VLA Imaging of the Disk Surrounding the Nearby Young Star TW Hya
The TW Hya system is perhaps the closest analog to the early solar nebula. We
have used the Very Large Array to image TW Hya at wavelengths of 7mm and 3.6 cm
with resolutions 0.1 arcseconds (about 5 AU) and 1.0 arcseconds (about 50 AU),
respectively. The 7mm emission is extended and appears dominated by a dusty
disk of radius larger than 50 AU surrounding the star. The 3.6 cm emission is
unresolved and likely arises from an ionized wind or gyrosynchrotron activity.
The dust spectrum and spatially resolved 7mm images of the TW Hya disk are
fitted by a simple model with temperature and surface density described by
radial power laws, and . These
properties are consistent with an irradiated gaseous accretion disk of mass
with an accretion rate and viscosity parameter . The estimates of
mass and mass accretion rates are uncertain as the gas-to-dust ratio in the TW
Hya disk may have evolved from the standard interstellar value.Comment: 13 pages, 3 figures, accepted by ApJ Letter
Hellmut Fritzsche
In a letter he wrote in June 2018, the month of his passing, Hellmut Fritzsche, professor emeritus at the University of Chicago, reflected on his life and marriage: “[My wife] Sybille and I were so very fortunate to have been selected in 1950 to the group of European students who were invited by the US government to come to the United States for one year. This is how we met. How immensely admirable was the generosity of the United States! We both had a rich and rewarding life. We are grateful for the opportunities given to us and for the wonderful life and inspiring circle of friends who questioned and supported us intellectually.” Those of us in that circle were richly rewarded for having experienced Hellmut’s inquisitive mind and vibrant nature
Hellmut Fritzsche
In a letter he wrote in June 2018, the month of his passing, Hellmut Fritzsche, professor emeritus at the University of Chicago, reflected on his life and marriage: “[My wife] Sybille and I were so very fortunate to have been selected in 1950 to the group of European students who were invited by the US government to come to the United States for one year. This is how we met. How immensely admirable was the generosity of the United States! We both had a rich and rewarding life. We are grateful for the opportunities given to us and for the wonderful life and inspiring circle of friends who questioned and supported us intellectually.” Those of us in that circle were richly rewarded for having experienced Hellmut’s inquisitive mind and vibrant nature
- …