14 research outputs found
Quantum Clock Synchronization: a Multi-Party Protocol
We present a multi-party quantum clock synchronization protocol that utilizes
shared prior entanglement and broadcast of classical information to synchronize
spatially separated clocks. Notably, it is necessary only for any one party to
publish classical information. Consequently, the efficacy of the method is
independent of the relative location of the parties. The suggested protocol is
robust and does not require precise sequencing of procedural steps.Comment: 3 page
The Magnetic Field in Taurus Probed by Infrared Polarization
We present maps of the plane-of-sky magnetic field within two regions of the
Taurus molecular cloud: one in the dense core L1495/B213 filament, the other in
a diffuse region to the west. The field is measured from the polarization of
background starlight seen through the cloud. In total, we measured 287
high-quality near-infrared polarization vectors in these regions. In
L1495/B213, the percent polarization increases with column density up to Av ~ 9
mag, the limits of our data. The Radiative Torques model for grain alignment
can explain this behavior, but models that invoke turbulence are inconsistent
with the data. We also combine our data with published optical and
near-infrared polarization measurements in Taurus. Using this large sample, we
estimate the strength of the plane-of-sky component of the magnetic field in
nine subregions. This estimation is done with two different techniques that use
the observed dispersion in polarization angles. Our values range from 5-82
microgauss and tend to be higher in denser regions. In all subregions, the
critical index of the mass-to-magnetic flux ratio is sub-unity, implying that
Taurus is magnetically supported on large scales (~2 pc). Within the region
observed, the B213 filament makes a sharp turn to the north and the direction
of the magnetic field also takes a sharp turn, switching from being
perpendicular to the filament to becoming parallel. This behavior can be
understood if we are observing the rim of a bubble. We argue that it has
resulted from a supernova remnant associated with a recently discovered nearby
gamma-ray pulsar.Comment: Accepted into the Astrophysical Journal. 20 pages in emulateapj
format including 10 figures and 4 table
The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds
We have investigated the time scale for formation of molecular clouds by
examining the conversion of HI to H2 using a time-dependent model. H2 formation
on dust grains and cosmic ray and photo destruction are included in
one-dimensional model slab clouds which incorporate time-independent density
and temperature distributions. We calculate 21cm spectral line profiles seen in
absorption against a background provided by general Galactic HI emission, and
compare the model spectra with HI Narrow Self-Absorption, or HINSA, profiles
absorbed in a number of nearby molecular clouds. The time evolution of the HI
and H2 densities is dramatic, with the atomic hydrogen disappearing in a wave
propagating from the central, denser regions which have a shorter H2 formation
time scale, to the edges, where the density is lower and the time scale for H2
formation longer. The model 21cm spectra are characterized by very strong
absorption at early times, when the HI column density through the model clouds
is extremely large. The minimum time required for a cloud to have evolved to
its observed configuration, based on the model spectra, is set by the
requirement that most of the HI in the outer portions of the cloud, which
otherwise overwhelms the narrow absorption, be removed. The characteristic time
that has elapsed since cloud compression and initiation of the HI to H2
conversion is a few x 10^{14} s or ~ 10^7 yr. This sets a minimum time for the
age of these molecular clouds and thus for the star formation that may take
place within them
An Improved Technique for Measurement of Cold HI in Molecular Cloud Cores
The presence of atomic gas mixed with molecular species in a "molecular"
cloud may significantly affect its chemistry, the excitation of some species,
and can serve as probe of the cloud's evolution. Cold neutral atomic hydrogen
(HI) in molecular clouds is revealed by its self absorption of background
galactic HI 21-cm emission. The properties of this gas can be investigated
quantitatively through observation of HI Narrow Self-Absorption (HINSA). In
this paper, we present a new technique for measuring atomic gas physical
parameters from HINSA observations that utilizes molecular tracers to guide the
HINSA extraction. This technique offers a significant improvement in the
precision with which HI column densities can be determined over previous
methods, and it opens several new avenues of study of relevance to the field of
star formation.Comment: Accepted for Astrophycal Journal 8/200
The GALFA-HI Survey: Data Release 1
We present the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) survey, and
its first full data release (DR1). GALFA-HI is a high resolution (~ 4'), large
area (13000 deg^2), high spectral resolution (0.18 km/s), wide band (-700 <
v_LSR < +700 km/s) survey of the Galactic interstellar medium in the 21-cm line
hyperfine transition of neutral hydrogen conducted at Arecibo Observatory.
Typical noise levels are 80 mK RMS in an integrated 1 km/s channel. GALFA-HI is
a dramatic step forward in high-resolution, large-area Galactic HI surveys, and
we compare GALFA-HI to past, present, and future Galactic HI surveys. We
describe in detail new techniques we have developed to reduce these data in the
presence of fixed pattern noise, gain variation, and inconsistent beam shapes,
and we show how we have largely mitigated these effects. We present our first
full data release, covering 7520 square degrees of sky and representing 3046
hours of integration time, and discuss the details of these data.Comment: Accepted to the ApJ