1,956 research outputs found
High Energy Colliding Beams; What Is Their Future?
The success of the first few years of LHC operations at CERN, and the
expectation of more to come as the LHC performance improves, are already
leading to discussions of what should be next for both proton-proton and
electron-positron colliders. In this discussion I see too much theoretical
desperation caused by the so far unsuccessful hunt for what is beyond the
Standard Model, and too little of the necessary interaction of the accelerator,
experimenter, and theory communities necessary for a scientific and engineering
success. Here, I give my impressions of the problem, its possible solution, and
what is needed to have both a scientifically productive and financially viable
future.Comment: 12 pages, 0 figure
Charting the Course for Elementary Particle Physics
It was the best of times; it was the worst of times is the way Dickens begins
the Tale of Two Cities. The line is appropriate to our time in particle
physics. It is the best of times because we are in the midst of a revolution in
understanding, the third to occur during my career. It is the worst of times
because accelerator facilities are shutting down before new ones are opening,
restricting the opportunity for experiments, and because of great uncertainty
about future funding. My task today is to give you a view of the most important
opportunities for our field under a scenario that is constrained by a tight
budget. It is a time when we cannot afford the merely good, but must give first
priority to the really important.Comment: AAAS National Meeting, San Francisco, 2007, Symposium, A New Frontier
in Particle Physics, 15 pages, 8 figure
Infrared Studies of Molecular Shocks in the Supernova Remnant HB21: I. Thermal Admixture of Shocked H_2 Gas in the North
We present near- and mid-infrared observations on the shock-cloud interaction
region in the northern part of the supernova remnant HB21, performed with the
InfraRed Camera (IRC) aboard AKARI satellite and the Wide InfraRed Camera
(WIRC) at the Palomar 5 m telescope. The IRC 7 um (S7), 11 um (S11), and 15 um
(L15) band images and the WIRC H2 v = 1 -> 0 S(1) 2.12 um image show similar
shock-cloud interaction features. We chose three representative regions, and
analyzed their IRC emissions through comparison with H2 line emissions of
several shock models. The IRC colors are well explained by the thermal
admixture model of H2 gas--whose infinitesimal H2 column density has a
power-law relation with the temperature T, dN ~ T^-b dT--with n(H2) ~ 10^3
cm^-3, b ~ 3, and N(H2 ;T > 100K) ~ 3x10^20 cm^-2. The derived b value may be
understood by a bow shock picture, whose shape is cycloidal (cuspy) rather than
paraboloidal. However, this picture raises another issue that the bow shocks
must reside within ~0.01 pc size-scale, smaller than the theoretically
expected. Instead, we conjectured a shocked clumpy interstellar medium picture,
which may avoid the sizescale issue while explaining the similar model
parameters. The observed H2 v = 1 -> 0 S(1) intensities are a factor of ~17 -
33 greater than the prediction from the power-law admixture model. This excess
may be attributed to either an extra component of hot H2 gas or to the effects
of collisions with hydrogen atoms, omitted in our power-law admixture model,
both of which would increase the population in the v = 1 level of H2.Comment: 14 pages, 9 figures, ApJ accepted, higher resolution @
http://astro.snu.ac.kr/~jhshinn/ms.pd
The abundances of polyacetylenes towards CRL618
We present a mid-infrared high spectral resolution spectrum of CRL618 in the
frequency ranges 778-784 and 1227-1249 cm^-1 (8.01-8.15 and 12.75-12.85 um)
taken with the Texas Echelon-cross-Echelle Spectrograph (TEXES) and the
Infrared Telescope Facility (IRTF). We have identified more than 170
ro-vibrational lines arising from C2H2, HCN, C4H2, and C6H2. We have found no
unmistakable trace of C8H2. The line profiles display a complex structure
suggesting the presence of polyacetylenes in several components of the
circumstellar envelope (CSE). We derive total column densities of 2.5 10^17,
3.1 10^17, 2.1 10^17, 9.3 10^16 cm^-2, and < 5 10^16 cm^-2 for HCN, C2H2, C4H2,
C6H2, and C8H2, respectively. The observations indicate that both the
rotational and vibrational temperatures in the innermost CSE depend on the
molecule, varying from 100 to 350 K for the rotational temperatures and 100 to
500 K for the vibrational temperatures. Our results support a chemistry in the
innermost CSE based on radical-neutral reactions triggered by the intense UV
radiation field.Comment: 9 pages, 4 figures, 1 table; accepted for publication in The
Astrophysical Journa
Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1
The high- and intermediate-velocity interstellar clouds (HVCs/IVCs) are
tracers of energetic processes in and around the Milky Way. Clouds with
near-solar metallicity about one kpc above the disk trace the circulation of
material between disk and halo (the Galactic Fountain). The Magellanic Stream
consists of gas tidally extracted from the SMC, tracing the dark matter
potential of the Milky Way. Several other HVCs have low-metallicity and appear
to trace the continuing accretion of infalling intergalactic gas. These
assertions are supported by the metallicities (0.1 to 1 solar) measured for
about ten clouds in the past decade. Direct measurements of distances to HVCs
have remained elusive, however. In this paper we present four new distance
brackets, using VLT observations of interstellar \CaII H and K absorption
toward distant Galactic halo stars. We derive distance brackets of 5.0 to 11.7
kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8
to 15.1 kpc for complex GCP (also known as the Smith Cloud or HVC40-15+100 and
with still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associated
with the return flow of the Fountain in the Perseus Arm, and 1.8 to 3.8 kpc for
cloud g1, which appears to be in the outflow phase of the Fountain. Our
measurements further demonstrate that the Milky Way is accreting substantial
amounts of gaseous material, which influences the Galaxy's current and future
dynamical and chemical evolution.Comment: Accepted by Ap
The 21cm "Outer Arm" and the Outer-Galaxy High-Velocity Clouds: Connected by Kinematics, Metallicity, and Distance
Using high-resolution ultraviolet spectra obtained with the HST/Space
Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic
Explorer, we study the metallicity, kinematics, and distance of the gaseous
"Outer Arm" (OA) and the high-velocity clouds (HVCs) in the outer Galaxy. We
detect the OA in a variety of absorption lines toward two QSOs, H1821+643 and
HS0624+6907. We search for OA absorption toward eight Galactic stars and detect
it in one case, which constrains the OA Galactocentric radius to 9<R_{G}<18
kpc. We also detect HVC Complex G, which is projected near the OA at a similar
velocity, in absorption toward two stars; Complex G is therefore in the same
region at R_{G} = 8 - 10 kpc. HVC Complex C is known to be at a similar
Galactocentric radius. Toward H1821+643, the low-ionization absorption lines
are composed of multiple narrow components, indicating the presence of several
cold clouds and rapid cooling and fragmentation. Some of the highly ionized gas
is also surprisingly cool. Accounting for ionization corrections, we find that
the OA metallicity is Z=0.2-0.5 Z_{solar}, but nitrogen is underabundant and
some species are possibly mildly depleted by dust. The similarity of the OA
metallicity, Galactocentric location, and kinematics to those of the adjacent
outer-Galaxy HVCs, including high velocities that are not consistent with
Galactic rotation, suggests that the OA and outer-Galaxy HVCs could have a
common origin.Comment: Accepted for publication in the Astrophysical Journa
Rotation Curves of Spiral Galaxies
Rotation curves of spiral galaxies are the major tool for determining the
distribution of mass in spiral galaxies. They provide fundamental information
for understanding the dynamics, evolution and formation of spiral galaxies. We
describe various methods to derive rotation curves, and review the results
obtained. We discuss the basic characteristics of observed rotation curves in
relation to various galaxy properties, such as Hubble type, structure,
activity, and environment.Comment: 40 pages, 6 gif figures; Ann. Rev. Astron. Astrophys. Vol. 39, p.137,
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