4,957 research outputs found
Universal behavior of two-dimensional bosonic gases at Berezinskii-Kosterlitz-Thouless transitions
We study the universal critical behavior of two-dimensional (2D) lattice
bosonic gases at the Berezinskii-Kosterlitz-Thouless (BKT) transition, which
separates the low-temperature superfluid phase from the high-temperature normal
phase. For this purpose, we perform quantum Monte Carlo simulations of the
hard-core Bose-Hubbard (BH) model at zero chemical potential. We determine the
critical temperature by using a matching method that relates finite-size data
for the BH model with corresponding data computed in the classical XY model. In
this approach, the neglected scaling corrections decay as inverse powers of the
lattice size L, and not as powers of 1/lnL, as in more standard approaches,
making the estimate of the critical temperature much more reliable. Then, we
consider the BH model in the presence of a trapping harmonic potential, and
verify the universality of the trap-size dependence at the BKT critical point.
This issue is relevant for experiments with quasi-2D trapped cold atoms.Comment: 17 pages, 12 figs, final versio
Near-arcsecond resolution observations of the hot corino of the solar type protostar IRAS 16293-2422
Complex organic molecules have previously been discovered in solar type
protostars, raising the questions of where and how they form in the envelope.
Possible formation mechanisms include grain mantle evaporation, interaction of
the outflow with its surroundings or the impact of UV/X-rays inside the
cavities. In this Letter we present the first interferometric observations of
two complex molecules, CH3CN and HCOOCH3, towards the solar type protostar
IRAS16293-2422. The images show that the emission originates from two compact
regions centered on the two components of the binary system. We discuss how
these results favor the grain mantle evaporation scenario and we investigate
the implications of these observations for the chemical composition and
physical and dynamical state of the two components.Comment: 5 pages (apjemulate), 2 figures; accepted by ApJ
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
First detection of triply-deuterated methanol
We report the first detection of triply-deuterated methanol, with 12 observed
transitions, towards the low-mass protostar IRAS 16293-2422, as well as
multifrequency observations of 13CH3OH, used to derive the column density of
the main isotopomer CH3OH. The derived fractionation ratio [CD3OH]/[CH3OH]
averaged on a 10'' beam is 1.4%. Together with previous CH2DOH and CHD2OH
observations, the present CD3OH observations are consistent with a formation of
methanol on grain surfaces, if the atomic D/H ratio is 0.1 to 0.3 in the
accreting gas. Such a high atomic ratio can be reached in the frame of
gas-phase chemical models including all deuterated isotopomers of H3+.Comment: Accepted by A&
The solar type protostar IRAS16293-2422: new constraints on the physical structure
Context: The low mass protostar IRAS16293-2422 is a prototype Class 0 source
with respect to the studies of the chemical structure during the initial phases
of life of Solar type stars. Aims: In order to derive an accurate chemical
structure, a precise determination of the source physical structure is
required. The scope of the present work is the derivation of the structure of
IRAS16293-2422. Methods: We have re-analyzed all available continuum data
(single dish and interferometric, from millimeter to MIR) to derive accurate
density and dust temperature profiles. Using ISO observations of water, we have
also reconstructed the gas temperature profile. Results: Our analysis shows
that the envelope surrounding IRAS16293-2422 is well described by the Shu
"inside-out" collapsing envelope model or a single power-law density profile
with index equal to 1.8. In contrast to some previous studies, our analysis
does not show evidence of a large (>/- 800 AU in diameter) cavity. Conclusions:
Although IRAS16293-2422 is a multiple system composed by two or three objects,
our reconstruction will be useful to derive the chemical structure of the large
cold envelope surrounding these objects and the warm component, treated here as
a single source, from single-dish observations of molecular emission
Sulphur-bearing species in the star forming region L1689N
We report observations of the expected main S-bearing species (SO, SO2 and
H2S) in the low-mass star forming region L1689N. We obtained large scale
(~300''x200'') maps of several transitions from these molecules with the goal
to study the sulphur chemistry, i.e. how the relative abundances change in the
different physical conditions found in L1689N. We identified eight interesting
regions, where we carried out a quantitative comparative study: the molecular
cloud (as reference position), five shocked regions caused by the interaction
of the molecular outflows with the cloud, and the two protostars IRAS16293-2422
and 16293E. In the cloud we carefully computed the gas temperature and density
by means of a non-LTE LVG code, while in other regions we used previous
results. We hence derived the column density of SO, SO2 and H2S, together with
SiO and H2CO - which were observed previously - and their relevant abundance
ratios. We find that SiO is the molecule that shows the largest abundance
variations in the shocked regions, whereas S-bearing molecules show more
moderate variations. Remarkably, the region of the brightest SiO emission in
L1689N is undetected in SO2, H2S and H2CO and only marginally detected in SO.
In the other weaker SiO shocks, SO2 is enhanced with respect to SO. We propose
a schema in which the different molecular ratios correspond to different ages
of the shocks. Finally, we find that SO, SO2 and H2S have significant abundance
jumps in the inner hot core of IRAS16293-2422 and discuss the implications of
the measured abundances.Comment: Accepted 08/10/0
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