1,094 research outputs found
Modelling the 3D physical structure of astrophysical sources with GASS
The era of interferometric observations leads to the need of a more and more
precise description of physical structures and dynamics of star-forming
regions, from pre-stellar cores to protoplanetary discs. The molecular emission
can be traced in multiple physical components such as infalling envelopes,
outflows and protoplanetary discs. To compare with the observations, a precise
and complex radiative transfer modelling of these regions is needed. We present
GASS (Generator of Astrophysical Sources Structure), a code that allows us to
generate the three-dimensional (3D) physical structure model of astrophysical
sources. From the GASS graphical interface, the user easily creates different
components such as spherical envelopes, outflows and discs. The physical
properties of these components are modelled thanks to dedicated graphical
interfaces that display various figures in order to help the user and
facilitate the modelling task. For each component, the code randomly generates
points in a 3D grid with a sample probability weighted by the molecular
density. The created models can be used as the physical structure input for 3D
radiative transfer codes to predict the molecular line or continuum emission.
An analysis of the output hyper-spectral cube given by such radiative transfer
code can be made directly in GASS using the various post-treatment options
implemented, such as calculation of moments or convolution with a beam. This
makes GASS well suited to model and analyse both interferometric and
single-dish data. This paper is focused on the results given by the association
of GASS and LIME, a 3D radiative transfer code, and we show that the complex
geometry observed in star-forming regions can be adequately handled by
GASS+LIME
The census of complex organic molecules in the solar type protostar IRAS16293-2422
Complex Organic Molecules (COMs) are considered crucial molecules, since they
are connected with organic chemistry, at the basis of the terrestrial life.
More pragmatically, they are molecules in principle difficult to synthetize in
the harsh interstellar environments and, therefore, a crucial test for
astrochemical models. Current models assume that several COMs are synthesised
on the lukewarm grain surfaces (30-40 K), and released in the gas
phase at dust temperatures 100 K. However, recent detections of COMs
in 20 K gas demonstrate that we still need important pieces to
complete the puzzle of the COMs formation. We present here a complete census of
the oxygen and nitrogen bearing COMs, previously detected in different ISM
regions, towards the solar type protostar IRAS16293-2422. The census was
obtained from the millimeter-submillimeter unbiased spectral survey TIMASSS.
Six COMs, out of the 29 searched for, were detected: methyl cyanide, ketene,
acetaldehyde, formamide, dimethyl ether, and methyl formate. The multifrequency
analysis of the last five COMs provides clear evidence that they are present in
the cold (30 K) envelope of IRAS16293-2422, with abundances 0.03-2
. Our data do not allow to support the hypothesis that the
COMs abundance increases with increasing dust temperature in the cold envelope,
as expected if COMs were predominately formed on the lukewarm grain surfaces.
Finally, when considering also other ISM sources, we find a strong correlation
over five orders of magnitude, between the methyl formate and dimethyl ether
and methyl formate and formamide abundances, which may point to a link between
these two couples of species, in cold and warm gas
Dynamical density-density correlations in the one-dimensional Bose gas
The zero-temperature dynamical structure factor of the one-dimensional Bose
gas with delta-function interaction (Lieb-Liniger model) is computed using a
hybrid theoretical/numerical method based on the exact Bethe Ansatz solution,
which allows to interpolate continuously between the weakly-coupled
Thomas-Fermi and strongly-coupled Tonks-Girardeau regimes. The results should
be experimentally accessible with Bragg spectroscopy.Comment: 4 pages, 3 figures, published versio
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ISO Detection of CO<sup>+</sup> toward the protostar IRAS 16293-2422
In this letter we report the detection of eight high-N rotational transitions of CO+ towards a low mass protostar, IRAS 16293-2422. The source was observed with the Long Wavelength Spectrometer on board the Infrared Space Observatory. This is the first time that CO+ has been detected in a low luminosity source and the first time that high-N lines have been detected in any source. The detection of these lines was not predicted by models and consequently, their interpretation is a challenge. We discuss the possibility that the observed CO+ emission originates in the dense inner regions illuminated by the UV field created in the accretion shock (formed by infalling material), and conclude that this is an improbable explanation. We have also considered the possibility that a strong, dissociative J-shock at ~ 500 AU from the star is the origin of the CO+ emission. This model predicts CO+ column densities in rough agreement with the observations if the magnetic field is ~ 1 mG and the shock velocity is 100 km s-1
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Detection of CO<sup>+</sup> with ISO towards the protostar IRAS16293-242
We observed the low luminosity (and low mass) protostar IRAS16293-2422 with the Long Wavelength Spectrometer on board the Infrared Space Observatory. The observed line spectrum is very reach and shows transitions of several molecules and atoms. Here we report the detection of eight high-N rotational transitions of CO+. This is the first time that CO+ has been detected in a low luminosity source and the first time that high-N lines have been detected in any source. The detection of these lines was not predicted by models and consequently, their interpretation is a challenge. We discuss the possibility that the observed CO+ emission originates in the dense inner regions illuminated by the UV field created in the accretion shock (formed by infalling material), and conclude that this is an improbable explanation. We have also considered the possibility that a strong, dissociative J-shock at ~500 AU from the star is the origin of the CO+ emission. This model predicts CO+ column densities in rough agreement with the observations if the magnetic field is ~1 mG and the shock velocity is 100 km s-1
Correlation functions of the one-dimensional attractive Bose gas
The zero-temperature correlation functions of the one-dimensional attractive
Bose gas with delta-function interaction are calculated analytically for any
value of the interaction parameter and number of particles, directly from the
integrability of the model. We point out a number of interesting features,
including zero recoil energy for large number of particles, analogous to a
M\"ossbauer effect.Comment: 4 pages, 2 figure
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