9,292 research outputs found
Meson and Quark Degrees of Freedom and the Radius of the Deuteron
The existing experimental data for the deuteron charge radius are discussed.
The data of elastic electron scattering are inconsistent with the value
obtained in a recent atomic physics experiment. Theoretical predictions based
on a nonrelativistic description of the deuteron with realistic nucleon-nucleon
potentials and with a rather complete set of meson-exchange contributions to
the charge operator are presented. Corrections arising from the quark-gluon
substructure of the nucleon are explored in a nonrelativistic quark model; the
quark-gluon corrections, not accounted for by meson exchange, are small. Our
prediction for the deuteron charge radius favors the value of a recent atomic
physics experiment.Comment: 20 pages, LaTeX, 4 Postscript figures, to appear in Few-Body-System
Relationship between the column density distribution and evolutionary class of molecular clouds as viewed by ATLASGAL
We present the first study of the relationship between the column density
distribution of molecular clouds within nearby Galactic spiral arms and their
evolutionary status as measured from their stellar content. We analyze a sample
of 195 molecular clouds located at distances below 5.5 kpc, identified from the
ATLASGAL 870 micron data. We define three evolutionary classes within this
sample: starless clumps, star-forming clouds with associated young stellar
objects, and clouds associated with HII regions. We find that the N(H2)
probability density functions (N-PDFs) of these three classes of objects are
clearly different: the N-PDFs of starless clumps are narrowest and close to
log-normal in shape, while star-forming clouds and HII regions exhibit a
power-law shape over a wide range of column densities and log-normal-like
components only at low column densities. We use the N-PDFs to estimate the
evolutionary time-scales of the three classes of objects based on a simple
analytic model from literature. Finally, we show that the integral of the
N-PDFs, the dense gas mass fraction, depends on the total mass of the regions
as measured by ATLASGAL: more massive clouds contain greater relative amounts
of dense gas across all evolutionary classes.Comment: Accepted for publication in A&A (25th June 15) 23 pages, 12 figures.
Additional appendix figures will appear in the journal version of this pape
The Herschel/PACS view of disks around low-mass stars in Chamaleon-I
Circumstellar disks are expected to be the birthplaces of planets. The
potential for forming one or more planets of various masses is essentially
driven by the initial mass of the disks. We present and analyze Herschel/PACS
observations of disk-bearing M-type stars that belong to the young ~2 Myr old
Chamaleon-I star forming region. We used the radiative transfer code RADMC to
successfully model the SED of 17 M-type stars detected at PACS wavelengths. We
first discuss the relatively low detection rates of M5 and later spectral type
stars with respect to the PACS sensitivity, and argue their disks masses, or
flaring indices, are likely to be low. For M0 to M3 stars, we find a relatively
broad range of disk masses, scale heights, and flaring indices. Via a
parametrization of dust stratification, we can reproduce the peak fluxes of the
10 m emission feature observed with Spitzer/IRS, and find that disks
around M-type stars may display signs of dust sedimentation. The Herschel/PACS
observations of low-mass stars in Cha-I provide new constraints on their disk
properties, overall suggesting that disk parameters for early M-type stars are
comparable to those for more massive stars (e.g., comparable scale height and
flaring angles). However, regions of the disks emitting at about 100 m may
still be in the optically thick regime, preventing direct determination of disk
masses. Thus the modeled disk masses should be considered as lower limits.
Still, we are able to extend the wavelength coverage of SED models and start
characterizing effects such as dust sedimentation, an effort leading the way
towards ALMA observations of these low-mass stars
Gaps, Rings, and Non-Axisymmetric Structures in Protoplanetary Disks - From Simulations to ALMA Observations
Recent observations by the Atacama Large Millimeter/submillimeter Array
(ALMA) of disks around young stars revealed distinct asymmetries in the dust
continuum emission. In this work we want to study axisymmetric and
non-axisymmetric structures, evocated by the magneto-rotational instability in
the outer regions of protoplanetary disks. We combine the results of
state-of-the-art numerical simulations with post-processing radiative transfer
(RT) to generate synthetic maps and predictions for ALMA. We performed
non-ideal global 3D MHD stratified simulations of the dead-zone outer edge
using the FARGO MHD code PLUTO. The stellar and disk parameters are taken from
a parameterized disk model applied for fitting high-angular resolution
multi-wavelength observations of circumstellar disks. The 2D temperature and
density profiles are calculated consistently from a given surface density
profile and Monte-Carlo radiative transfer. The 2D Ohmic resistivity profile is
calculated using a dust chemistry model. The magnetic field is a vertical net
flux field. The resulting dust reemission provides the basis for the simulation
of observations with ALMA. The fiducial model develops a large gap followed by
a jump in surface density located at the dead-zone outer edge. The jump in
density and pressure is strong enough to stop the radial drift of particles. In
addition, we observe the generation of vortices by the Rossby wave instability
(RWI) at the jumps location close to 60 AU. The vortices are steadily generated
and destroyed at a cycle of 40 local orbits. The RT results and simulated ALMA
observations predict the feasibility to observe such large scale structures
appearing in magnetized disks without having a planet.Comment: Language update, added comments, added citations, in press. (A&A
Kinematic and Thermal Structure at the onset of high-mass star formation
We want to understand the kinematic and thermal properties of young massive
gas clumps prior to and at the earliest evolutionary stages of high-mass star
formation. Do we find signatures of gravitational collapse? Do we find
temperature gradients in the vicinity or absence of infrared emission sources?
Do we find coherent velocity structures toward the center of the dense and cold
gas clumps? To determine kinematics and gas temperatures, we used ammonia,
because it is known to be a good tracer and thermometer of dense gas. We
observed the NH(1,1) and (2,2) lines within seven very young high-mass
star-forming regions with the VLA and the Effelsberg 100m telescope. This
allows us to study velocity structures, linewidths, and gas temperatures at
high spatial resolution of 3-5, corresponding to 0.05 pc. We find on
average cold gas clumps with temperatures in the range between 10 K and 30 K.
The observations do not reveal a clear correlation between infrared emission
peaks and ammonia temperature peaks. We report an upper limit for the linewidth
of 1.3 km s, at the spectral resolution limit of our VLA
observation. This indicates a relatively low level of turbulence on the scale
of the observations. Velocity gradients are present in almost all regions with
typical velocity differences of 1 to 2 km s and gradients of 5 to 10 km
s pc. These velocity gradients are smooth in most cases, but
there is one exceptional source (ISOSS23053), for which we find several
velocity components with a steep velocity gradient toward the clump centers
that is larger than 30 km s pc. This steep velocity gradient is
consistent with recent models of cloud collapse. Furthermore, we report a
spatial correlation of ammonia and cold dust, but we also find decreasing
ammonia emission close to infrared emission sources.Comment: 20 pages, 10 figure
Triggered/sequential star formation? A multi-phase ISM study around the prominent IRDC G18.93-0.03
G18.93-0.03 is a prominent dust complex within an 0.8deg long filament, with
the molecular clump G18.93/m being IR dark from near IR wavelength up to 160mu.
Spitzer composite images show an IR bubble spatially associated with G18.93. We
use GRS 13CO and IRAM 30m H13CO+ data to disentangle the spatial structure of
the region. From ATLASGAL submm data we calculate the gas mass, while we use
the H13CO+ line width to estimate its virial mass. Using HERSCHEL data we
produce temperature maps from fitting the SED. With the MAGPIS 20cm and
SuperCOSMOS Halpha data we trace the ionized gas, and the VGPS HI survey
provides information on the atomic hydrogen gas. We show that the bubble is
spatially associated with G18.93, located at a kinematic near distance of
3.6kpc. With 280Msun, the most massive clump within G18.93 is G18.93/m. The
virial analysis shows that it may be gravitationally bound and has neither
Spitzer young stellar objects nor mid-IR point sources within. Fitting the SED
reveals a temperature distribution that decreases towards its center, but
heating from the ionizing source puts it above the general ISM temperature. We
find that the bubble is filled by HII gas, ionized by an O8.5 star. Between the
ionizing source and the IR dark clump G18.93/m we find a layered structure,
from ionized to atomic to molecular hydrogen, revealing a PDR. Furthermore, we
identify an additional velocity component within the bubble's 8mu emission rim
at the edge of the infrared dark cloud and speculate that it might be shock
induced by the expanding HII region. While the elevated temperature allows for
the build-up of larger fragments, and the shock induced velocity component may
lead to additional turbulent support, we do not find conclusive evidence that
the massive clump G18.93/m is prone to collapse because of the expanding HII
region.Comment: Accepted for publication in A&
Long, Bellows-Free Vertical Helium Transfer Lines for the LHC Cryogenic System
The cryogenic system for the Large Hadron Collider (LHC) under construction at CERN will include four new vertical helium transfer lines connecting the new helium refrigerators to the underground areas. These four transfer lines will be installed between a refrigerator on the surface and an interconnection box located 80 m to 145 m underground. They consist of a vacuum jacket, a thermal screen and four internal helium pipes. Due to space and accessibility limitations, the lines have been specified without bellows or bends of any kind in the long vertical part; the thermal contractions must be compensated at the surface only. The displacement due to these contractions amounts to more than 35 cm in one case, and all four internal pipes, as well as the thermal screen, must be able to contract and expand independently. The lines will be built and installed by a consortium of Linde AG and Babcock Noell Nuclear GmbH. Their technical design choices are presented together with expected performance
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