1,193 research outputs found
Phase transitions in a gas of anyons
We continue our numerical Monte Carlo simulation of a gas of closed loops on
a 3 dimensional lattice, however now in the presence of a topological term
added to the action corresponding to the total linking number between the
loops. We compute the linking number using certain notions from knot theory.
Adding the topological term converts the particles into anyons. Using the
correspondence that the model is an effective theory that describes the
2+1-dimensional Abelian Higgs model in the asymptotic strong coupling regime,
the topological linking number simply corresponds to the addition to the action
of the Chern-Simons term. We find the following new results. The system
continues to exhibit a phase transition as a function of the anyon mass as it
becomes small \cite{mnp}, although the phases do not change the manifestation
of the symmetry. The Chern-Simons term has no effect on the Wilson loop, but it
does affect the {\rm '}t Hooft loop. For a given configuration it adds the
linking number of the 't Hooft loop with all of the dynamical vortex loops to
the action. We find that both the Wilson loop and the 't Hooft loop exhibit a
perimeter law even though there are no massless particles in the theory, which
is unexpected.Comment: 6 pages, 5 figure
Models of Metal Poor Stars with Gravitational Settling and Radiative Accelerations: I. Evolution and Abundance Anomalies
Evolutionary models have been calculated for Pop II stars of 0.5 to
1.0 from the pre-main-sequence to the lower part of the giant branch.
Rosseland opacities and radiative accelerations were calculated taking into
account the concentration variations of 28 chemical species, including all
species contributing to Rosseland opacities in the OPAL tables. The effects of
radiative accelerations, thermal diffusion and gravitational settling are
included. While models were calculated both for Z=0.00017 and 0.0017, we
concentrate on models with Z=0.00017 in this paper. These are the first Pop II
models calculated taking radiative acceleration into account. It is shown that,
at least in a 0.8 star, it is a better approximation not to let Fe
diffuse than to calculate its gravitational settling without including the
effects of . In the absence of any turbulence outside of
convection zones, the effects of atomic diffusion are large mainly for stars
more massive than 0.7. Overabundances are expected in some stars with
\teff \ge 6000K. Most chemical species heavier than CNO are affected. At 12
Gyr, overabundance factors may reach 10 in some cases (e.g. for Al or Ni) while
others are limited to 3 (e.g. for Fe). The calculated surface abundances are
compared to recent observations of abundances in globular clusters as well as
to observations of Li in halo stars. It is shown that, as in the case of Pop I
stars, additional turbulence appears to be present.Comment: 40 pages, 17 color figures, to appear in The Astrophysical Journal,
April 2002 (paper with original high resolution figures can be found at
http://www.cerca.umontreal.ca/~richer/Fichiersps/popII_1.ps
Star formation in Perseus
We present a complete survey of current star formation in the Perseus
molecular cloud, made at 850 and 450 micron with SCUBA at the JCMT. Covering 3
deg^2, this submillimetre continuum survey for protostellar activity is second
in size only to that of rho Ophiuchus (Johnstone et al. 2004). Complete above
0.4 msun (5 sigma detection in a 14'' beam), we detect a total of 91 protostars
and prestellar cores. Of these, 80% lie in clusters, representative of star
formation across the Galaxy. Two of the groups of cores are associated with the
young stellar clusters IC348 and NGC1333, and are consistent with a steady or
reduced star formation rate in the last 0.5 Myr, but not an increasing one. In
Perseus, 40--60% of cores are in small clusters (< 50 msun) and isolated
objects, much more than the 10% suggested from infrared studies. Complementing
the dust continuum, we present a C^18O map of the whole cloud at 1' resolution.
The gas and dust show filamentary structure of the dense gas on large and small
scales, with the high column density filaments breaking up into clusters of
cores. The filament mass per unit length is 5--11 msun per 0.1 pc. Given these
filament masses, there is no requirement for substantial large scale flows
along or onto the filaments in order to gather sufficient material for star
formation. We find that the probability of finding a submillimetre core is a
strongly increasing function of column density, as measured by C^18O integrated
intensity, prob(core) proportional to I^3.0. This power law relation holds down
to low column density, suggesting that there is no A_v threshold for star
formation in Perseus, unless all the low-A_v submm cores can be demonstrated to
be older protostars which have begun to lose their natal molecular cloud.Comment: 15 pages, 5 figures, bibtex file scubasf_astroph.bbl, included tex
files SSA-clusters-sorted-tidy.te
ALMA Capabilities for Observations of Spectral Line Emission
The Atacama Large Millimeter/submillimeter Array (ALMA) combines large
collecting area and location on a high dry site to provide it with unparalleled
potential for sensitive millimeter/submillimeter spectral line observations.
Its wide frequency coverage, superb receivers and flexible spectrometer will
ensure that its potential is met. Since the 1999 meeting on ALMA
Science\cite{RefA}, the ALMA team has substantially enhanced its capability for
line observations. ALMA's sensitivity increased when Japan joined the project,
bringing the 16 antennas of the Atacama Compcat Array (ACA), equivalent to
eight additional 12m telescopes. The first four receiver cartridges for the
baseline ALMA (Japan's entry has brought two additional bands to ALMA's
receiver retinue) have been accepted, with performance above the
already-challenging specifications. ALMA's flexibility has increased with the
enhancement of the baseline correlator with additional channels and
flexibility, and with the addition of a separate correlator for the ACA. As an
example of the increased flexibility, ALMA is now capable of
multi-spectral-region and multi-resolution modes. With the former, one might
observe e.g. four separate transitions anywhere within a 2 GHz band with a high
resolution bandwidth. With the latter, one might simultaneously observe with
low spectral resolution over a wide bandwidth and with high spectral resolution
over a narrow bandwidth; this mode could be useful for observations of
pressure-broadened lines with narrow cores, for example. Several science
examples illustrate ALMA's potential for transforming millimeter and
submillimeter astronomy.Comment: 6 pages, for the Proceedings of Science with ALMA: a new era for
Astrophysics: International Conference, 2006 held 13 - 17 November 2006 in
Madrid, Spai
Data acquisition electronics and reconstruction software for real time 3D track reconstruction within the MIMAC project
Directional detection of non-baryonic Dark Matter requires 3D reconstruction
of low energy nuclear recoils tracks. A gaseous micro-TPC matrix, filled with
either 3He, CF4 or C4H10 has been developed within the MIMAC project. A
dedicated acquisition electronics and a real time track reconstruction software
have been developed to monitor a 512 channel prototype. This autotriggered
electronic uses embedded processing to reduce the data transfer to its useful
part only, i.e. decoded coordinates of hit tracks and corresponding energy
measurements. An acquisition software with on-line monitoring and 3D track
reconstruction is also presented.Comment: Proceedings of TWEPP-11, Vienna, Austria, 26-30 September 201
A {\mu}-TPC detector for the characterization of low energy neutron fields
The AMANDE facility produces monoenergetic neutron fields from 2 keV to 20
MeV for metrological purposes. To be considered as a reference facility,
fluence and energy distributions of neutron fields have to be determined by
primary measurement standards. For this purpose, a micro Time Projection
Chamber is being developed to be dedicated to measure neutron fields with
energy ranging from 8 keV up to 1 MeV. In this work we present simulations
showing that such a detector, which allows the measurement of the ionization
energy and the 3D reconstruction of the recoil nucleus, provides the
determination of neutron energy and fluence of these neutron fields
The Dwarf Irregular Galaxy UGC 7636 Exposed: Stripping At Work In The Virgo Cluster
We present the results of optical spectroscopy of a newly discovered H II
region residing in the H I gas cloud located between the dwarf irregular galaxy
UGC 7636 and the giant elliptical galaxy NGC 4472 in the Virgo Cluster. By
comparing UGC 7636 with dwarf irregular galaxies in the field, we show that the
H I cloud must have originated from UGC 7636 because (1) the oxygen abundance
of the cloud agrees with that expected for a galaxy with the blue luminosity of
UGC 7636, and (2) M_{H I}/L_B for UGC 7636 becomes consistent with the measured
oxygen abundance of the cloud if the H I mass of the cloud is added back into
UGC 7636. It is likely that tides from NGC 4472 first loosened the H I gas,
after which ram-pressure stripping removed the gas from UGC 7636.Comment: 12 pages, 2 eps figures (AASTeX 5.0); accepted for publication in ApJ
Letter
A search for solar-like oscillations in the Am star HD 209625
The goal is to test the structure of hot metallic stars, and in particular
the structure of a near-surface convection zone using asteroseismic
measurements. Indeed, stellar models including a detailed treatement of the
radiative diffusion predict the existence of a near-surface convection zone in
order to correctly reproduce the anomalies in surface abundances that are
observed in Am stars. The Am star HD 209625 was observed with the Harps
spectrograph mounted on the 3.6-m telescope at the ESO La Silla Observatory
(Chile) during 9 nights in August 2005. This observing run allowed us to
collect 1243 radial velocity (RV) measurements, with a standard deviation of
1.35 m/s. The power spectrum associated with these RV measurements does not
present any excess. Therefore, either the structure of the external layers of
this star does not allow excitation of solar-like oscillations, or the
amplitudes of the oscillations remain below 20-30 cm/s (depending on their
frequency range).Comment: 5 pages, 4 figures, A&A accepte
- …