346 research outputs found
Local heat flux and energy loss in a 2D vibrated granular gas
We performed event-driven simulations of a two-dimensional granular gas
between two vibrating walls and directly measured the local heat flux and
energy dissipation rate in the stationary state. Describing the local heat flux
as a function of the coordinate x in the direction perpendicular to the driving
walls, we use a generalization of Fourier's law, q_x(x) = kappa d_x T(x) + mu
d_x rho(x), to relate the local heat flux to the local gradients of the
temperature and density. This ansatz accounts for the fact that density
gradients also generate heat flux, not only temperature gradients. The
transport coefficients kappa and mu are assumed to be independent of x, and we
check the validity of this assumption in the simulations. Both kappa and mu are
determined for different system parameters, in particular, for a wide range of
coefficients of restitution. We also compare our numerical results to existing
hydrodynamic theories. Agreement is found for kappa for very small
inelasticities only. Beyond this region, kappa and mu exhibit a striking
non-monotonic behavior.Comment: 8 pages, 5 figure
Local Equation of State and Velocity Distributions of a Driven Granular Gas
We present event-driven simulations of a granular gas of inelastic hard disks
with incomplete normal restitution in two dimensions between vibrating walls
(without gravity). We measure hydrodynamic quantities such as the stress
tensor, density and temperature profiles, as well as velocity distributions.
Relating the local pressure to the local temperature and local density, we
construct a local constitutive equation. For strong inelasticities the local
constitutive relation depends on global system parameters, like the volume
fraction and the aspect ratio. For moderate inelasticities the constitutive
relation is approximately independent of the system parameters and can hence be
regarded as a local equation of state, even though the system is highly
inhomogeneous with heterogeneous temperature and density profiles arising as a
consequence of the energy injection. Concerning the local velocity
distributions we find that they do not scale with the square root of the local
granular temperature. Moreover the high-velocity tails are different for the
distribution of the x- and the y-component of the velocity, and even depend on
the position in the sample, the global volume fraction, and the coefficient of
restitution.Comment: 14 pages, 14 figures of which Figs. 13a-f and Fig. 14 are archived as
separate .gif files due to upload-size limitations. A version of the paper
including all figures in better quality can be downloaded at
http://www.theorie.physik.uni-goettingen.de/~herbst/download/LocEqSt.ps.gz
(3.8 MB, ps.gz) or at
http://www.theorie.physik.uni-goettingen.de/~herbst/download/LocEqSt.pdf (4.9
MB, pdf
Rotational spectroscopy of the thioformaldehyde isotopologues HCS and HCS in four interacting excited vibrational states and an account on the rotational spectrum of thioketene, HCCS
An investigation of the rotational spectrum of the interstellar molecule
thioformaldehyde between 110 and 377 GHz through a pyrolysis reaction revealed
a multitude of absorption lines assignable to HCS and HCS in
their lowest four excited vibrational states besides lines of numerous
thioformaldehyde isotopologues in their ground vibrational states reported
earlier as well as lines pertaining to several by-products. Additional
transitions of HCS in its lowest four excited vibrational states were
recorded in selected regions between 571 and 1386 GHz. Slight to strong
Coriolis interactions occur between all four vibrational states with the
exception of the two highest lying states because both are totally symmetric
vibrations. We present combined analyses of the ground and the four interacting
states for our rotational data of HCS and HCS. The HCS data
were supplemented with two sets of high-resultion IR data in two separate
analyses. The state has been included in analyses of Coriolis
interactions of low-lying fundamental states of HCS for the first time and
this improved the quality of the fits substantially. We extended furthermore
assignments in of transition frequencies of thioketene in its ground
vibrational state.Comment: 23 pages including figures, tables, and references; Mol. Phys.,
accepted (for the Tim Lee memorial issue
Mopra line survey mapping of NGC6334I and I(N) at 3mm
A 5'x5' region encompassing NGC6334I and I(N) has been mapped at a wavelength
of 3mm (from 83.5 to 115.5GHz) with the Mopra telescope at an angular
resolution between 33 arcsec and 36 arcsec. This investigation has made use of
the recently installed 3mm MMIC receiver and the Mopra Spectrometer (MOPS) with
broadband capabilities permitting total coverage of the entire frequency range
with just five different observations. In total, the spatial distribution of
nineteen different molecules, ions and radicals, along with additional selected
isotopologues have been studied. Whilst most species trace the sites of star
formation, CH_3CN appears to be most closely associated with NGC6334I and I(N).
Both CN and C_2H appear to be widespread, tracing gas that is not associated
with active star formation. Both N_2H^+ and HC_3N closely resemble dust
continuum emission, showing they are reliable tracers of dense material, as
well as the youngest stages of high mass star formation. Hot (E_u/k>100K)
thermal CH_3OH emission is preferentially found towards NGC6334I, contrasting
with I(N), where only cold (E_u/k<22K) thermal CH_3OH emission is found.Comment: Accepted by MNRA
First astronomical detection of the CF+ ion
We report the first astronomical detection of the CF+ (fluoromethylidynium)
ion obtained by recent observations of its J = 1 - 0 (102.6 GHz), J = 2 - 1
(205.2 GHz), and J = 3 - 2 (307.7 GHz) pure rotational emissions toward the
Orion Bar. Our search for CF+, carried out using the IRAM 30m and APEX 12m
telescopes, was motivated by recent theoretical models that predict CF+
abundances of a few x E-10 in UV-irradiated molecular regions where C+ is
present. The measurements confirm the predictions. They provide support for our
current theories of interstellar fluorine chemistry, which suggest that
hydrogen fluoride should be ubiquitous in interstellar gas clouds.Comment: 2 pages, 1 figure (uses iaus.sty), to appear in IAU Symposium No.
231, Astrochemistry - Recent Successes and Current Challenges, eds. D. C.
Lis, G. A. Blake & E. Herbst (Cambridge Univ. Press
Arcsecond resolution images of the chemical structure of the low-mass protostar IRAS 16293-2422
It remains a key challenge to establish the molecular content of different
components of low-mass protostars, like their envelopes and disks, and how this
depends on the evolutionary stage and/or environment of the young stars.
Observations at submillimeter wavelengths provide a direct possibility to study
the chemical composition of low-mass protostars through transitions probing
temperatures up to a few hundred K in the gas surrounding these sources. This
paper presents a large molecular line survey of the deeply embedded
protostellar binary IRAS 16293-2422 from the Submillimeter Array (SMA) -
including images of individual lines down to approximately 1.5-3" (190-380 AU)
resolution. More than 500 individual transitions are identified related to 54
molecular species (including isotopologues) probing temperatures up to about
550 K. Strong chemical differences are found between the two components in the
protostellar system with a separation between, in particular, the sulfur- and
nitrogen-bearing species and oxygen-bearing complex organics. The action of
protostellar outflow on the ambient envelope material is seen in images of CO
and SiO and appear to influence a number of other species, including
(deuterated) water, HDO. The effects of cold gas-phase chemistry is directly
imaged through maps of CO, N2D+ and DCO+, showing enhancements of first DCO+
and subsequently N2D+ in the outer envelope where CO freezes-out on dust
grains.Comment: Accepted for publication in A&A, 30 pages, 22 figure
TIMASSS: The IRAS16293-2422 Millimeter And Submillimeter Spectral Survey. I. Observations, calibration and analysis of the line kinematics
While unbiased surveys observable from ground-based telescopes have
previously been obtained towards several high mass protostars, very little
exists on low mass protostars. To fill up this gap, we carried out a complete
spectral survey of the bands at 3, 2, 1 and 0.8 mm towards the solar type
protostar IRAS16293-2422. The observations covered about 200\,GHz and were
obtained with the IRAM-30m and JCMT-15m telescopes. Particular attention was
devoted to the inter-calibration of the obtained spectra with previous
observations. All the lines detected with more than 3 sigma and free from
obvious blending effects were fitted with Gaussians to estimate their basic
kinematic properties. More than 4000 lines were detected (with sigma \geq 3)
and identified, yielding a line density of approximatively 20 lines per GHz,
comparable to previous surveys in massive hot cores. The vast majority (~2/3)
of the lines are weak and due to complex organic molecules. The analysis of the
profiles of more than 1000 lines belonging 70 species firmly establishes the
presence of two distinct velocity components, associated with the two objects,
A and B, forming the IRAS16293-2422 binary system. In the source A, the line
widths of several species increase with the upper level energy of the
transition, a behavior compatible with gas infalling towards a ~1 Mo object.
The source B, which does not show this effect, might have a much lower central
mass of ~0.1 Mo. The difference in the rest velocities of both objects is
consistent with the hypothesis that the source B rotates around the source A.
This spectral survey, although obtained with single-dish telescope with a low
spatial resolution, allows to separate the emission from 2 different
components, thanks to the large number of lines detected. The data of the
survey are public and can be retrieved on the web site
http://www-laog.obs.ujf-grenoble.fr/heberges/timasss.Comment: 41 pages (26 pages of online Tables), 7 Tables and 6 Figure
Charged Boson Stars and Vacuum Instabilities
We consider charged boson stars and study their effect on the structure of
the vacuum. For very compact particle like ``stars", with constituent mass
close to the Planck mass , i.e. , we argue that there is a limiting total electric charge ,
which, primarily, is due to the formation of a pion condensate (, where is the fine structure constant and is the
electric charge of the positron). If the charge of the ``star" is larger than
we find numerical evidence for a complete screening indicating a limiting
charge for a very compact object.
There is also a less efficient competing charge screening mechanism due to
spontaneous electron-positron pair creation in which case . Astrophysical and cosmological abundances of charged compact
boson stars are briefly discussed in terms of dark matter.Comment: latex,29p,9 figs not included can be sent by fax on request,ITP
92-24,ZU-TH-38/9
Herschel observations of extra-ordinary sources: Detection of Hydrogen Fluoride in absorption towards Orion~KL
We report a detection of the fundamental rotational transition of hydrogen
fluoride in absorption towards Orion KL using Herschel/HIFI. After the removal
of contaminating features associated with common molecules ("weeds"), the HF
spectrum shows a P-Cygni profile, with weak redshifted emission and strong
blue-shifted absorption, associated with the low-velocity molecular outflow. We
derive an estimate of 2.9 x 10^13 cm^-2 for the HF column density responsible
for the broad absorption component. Using our best estimate of the H2 column
density within the low-velocity molecular outflow, we obtain a lower limit of
~1.6 x 10^-10 for the HF abundance relative to hydrogen nuclei, corresponding
to 0.6% of the solar abundance of fluorine. This value is close to that
inferred from previous ISO observations of HF J=2--1 absorption towards Sgr B2,
but is in sharp contrast to the lower limit of 6 x 10^-9 derived by Neufeld et
al. (2010) for cold, foreground clouds on the line of sight towards G10.6-0.4.Comment: 5 pages, 3 figures, paper to be published in the Herschel special
issue of A&A letter
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