1,179 research outputs found
The Spectral Energy Distribution of Self-gravitating Interstellar Clouds I. Spheres
We derive the spectral energy distribution (SED) of dusty, isothermal, self
gravitating, stable and spherical clouds externally heated by the ambient
interstellar radiation field. For a given radiation field and dust properties,
the radiative transfer problem is determined by the pressure of the surrounding
medium and the cloud mass expressed as a fraction of the maximum stable cloud
mass above which the clouds become gravitational unstable.
To solve the radiative transfer problem a ray-tracing code is used to
accurately derive the light distribution inside the cloud. This code considers
both non isotropic scattering on dust grains and multiple scattering events.
The dust properties inside the clouds are assumed to be the same as in the
diffuse interstellar medium in our galaxy. We analyse the effect of the
pressure, the critical mass fraction, and the ISRF on the SED and present
brightness profiles in the visible, the IR/FIR and the submm/mm regime with the
focus on the scattered emission and the thermal emission from PAH-molecules and
dust grains.Comment: accepted for publication in ApJS, May 2008, v176n1 issu
A Corona Australis cloud filament seen in NIR scattered light II: Comparison with sub-millimeter data
We study a northern part of the Corona Australis molecular cloud that
consists of a filament and a dense sub-millimetre core inside the filament. Our
aim is to measure dust temperature and sub-mm emissivity within the region. We
also look for confirmation that near-infrared (NIR) surface brightness can be
used to study the structure of even very dense clouds. We extend our previous
NIR mapping south of the filament. The dust colour temperatures are estimated
using Spitzer 160um and APEX/Laboca 870um maps. The column densities derived
based on the reddening of background stars, NIR surface brightness, and thermal
sub-mm dust emission are compared. A three dimensional toy model of the
filament is used to study the effect of anisotropic illumination on
near-infrared surface brightness and the reliability of dust temperature
determination. Relative to visual extinction, the estimated emissivity at 870um
is kappa(870) = (1.3 +- 0.4) x 10^{-5} 1/mag. This is similar to the values
found in diffuse medium. A significant increase in the sub-millimetre
emissivity seems to be excluded. In spite of saturation, NIR surface brightness
was able to accurately pinpoint, and better than measurements of the colour
excesses of background stars, the exact location of the column density maximum.
Both near- and far-infrared data show that the intensity of the radiation field
is higher south of the filament.Comment: 9 pages, 9 figures, accepted to A&
A Corona Australis cloud filament seen in NIR scattered light I. Comparison with extinction of background stars
With current near-infrared (NIR) instruments the near-infrared light
scattered from interstellar clouds can be mapped over large areas. The surface
brightness carries information on the line-of-sight dust column density.
Therefore, scattered light could provide an important tool to study mass
distribution in quiescent interstellar clouds at a high, even sub-arcsecond
resolution. We wish to confirm the assumption that light scattering dominates
the surface brightness in all NIR bands. Furthermore, we want to show that
scattered light can be used for an accurate estimation of dust column densities
in clouds with Av in the range 1-15mag. We have obtained NIR images of a
quiescent filament in the Corona Australis molecular cloud. The observations
provide maps of diffuse surface brightness in J, H, and Ks bands. Using the
assumption that signal is caused by scattered light we convert surface
brightness data into a map of dust column density. The same observations
provide colour excesses for a large number of background stars. These data are
used to derive an extinction map of the cloud. The two, largely independent
tracers of the cloud structure are compared. Results. In regions below Av=15m
both diffuse surface brightness and background stars lead to similar column
density estimates. The existing differences can be explained as a result of
normal observational errors and bias in the sampling of extinctions provided by
the background stars. There is no indication that thermal dust emission would
have a significant contribution even in the Ks band. The results show that,
below Av=15mag, scattered light does provide a reliable way to map cloud
structure. Compared with the use of background stars it can also in practice
provide a significantly higher spatial resolution.Comment: 14 pages, 15 figures, accepted to A&A, the version includes small
changes in the text and an added appendi
CCN activation and cloud processing in simplified sectional aerosol models with low size resolution
International audienceWe investigate the influence of low size resolution, typical to sectional aerosol models in large scale applications, on cloud droplet activation and cloud processing of aerosol particles. A simplified cloud scheme with five approaches to determine the fraction of activated particles is compared with a detailed reference model under different atmospheric conditions. In general, activation approaches which assume a distribution profile within the critical model size sections predict the cloud droplet concentration most accurately under clean and moderately polluted conditions. In such cases, the deviation from the reference simulations is below 15% except for very low updraft velocities. In highly polluted cases, the concentration of cloud droplets is significantly overestimated due to the inability of the simplified scheme to account for the kinetic limitations of the droplet growth. Of the profiles examined, taking into account the local shape of the particle size distribution is the most accurate although in most cases the shape of the profile has little relevance. While the low resolution cloud model cannot reproduce the details of the out-of-the-cloud aerosol size distribution, it captures well the amount of sulphate produced in aqueous-phase reactions as well as the distribution of the sulphate between the cloud droplets. Overall, the simplified cloud scheme with low size resolution performs well for clean and moderately polluted regions that cover most of the Earth's surface and is therefore suitable for large scale models
Direct Observation of Josephson Capacitance
The effective capacitance has been measured in the split Cooper pair box
(CPB) over its phase-gate bias plane. Our low-frequency reactive measurement
scheme allows to probe purely the capacitive susceptibility due to the CPB band
structure. The data are quantitatively explained using parameters determined
independently by spectroscopic means. In addition, we show in practice that the
method offers an efficient way to do non-demolition readout of the CPB quantum
state.Comment: 4 page
Quantum phase slip phenomenon in superconducting nanowires with low-Ohmic environment
In a number of recent experiments it has been demonstrated that in
ultra-narrow superconducting channels quantum fluctuations of the order
parameter, alternatively called quantum phase slips, are responsible for the
finite resistance well below the critical temperature. The acceptable agreement
between those experiments and the models describing quantum fluctuations in
quasi-one-dimensional superconductors has been established. However the very
concept of the phase slip is justified when these fluctuations are the
relatively rare events, meaning that the effective resistance of the system
should be much smaller than the normal state equivalent. In this paper we study
the limit of the strong quantum fluctuations where the existing models are not
applicable. In particular case of ultra-thin titanium nanowires it is
demonstrated that below the expected critical temperature the resistance does
not demonstrate any trend towards the conventional for a superconductor
zero-resistivity state even at negligibly small measuring currents. Application
of a small magnetic field leads to an unusual negative magnetoresistance, which
becomes more pronounced at lower temperatures. The origin of the negative
magnetoresistance effect is not clear
A phylogeny of members of the family Taeniidae based on the mitochondrial cox1 and nad1 gene data
nad1 gene dat
- …