516 research outputs found
Star Formation in the Trifid Nebula
We have obtained maps of the 1.25mm thermal dust emission and the molecular
gas emission over a region of 20' by 10' arcmin around the Trifid Nebula (M20),
with the IRAM 30m and the CSO telescopes as well as in the mid-infrared
wavelength with ISO and SPITZER. Our survey is sensitive to features down to
N(H2) \sim 10^{22} cm-2 in column density. The cloud material is distributed in
fragmented dense gas filaments (n(H2) \sim 1000 cm-3) with sizes ranging from 1
to 10 pc. A massive filament, WF, with properties typical of Infra Red Dark
Clouds, connects M20 to the W28 supernova remnant. These filaments pre-exist
the formation of the Trifid and were originally self-gravitating. The fragments
produced are very massive (100 Msun or more) and are the progenitors of the
cometary globules observed at the border of the HII region. We could identify
33 cores, 16 of which are currently forming stars. They are usually
gravitationally unbound and have low masses of a few Msun. The densest starless
cores (several 10^5 cm-3) may be the site for the next generation of stars. The
physical gas and dust properties of the cometary globules have been studied in
detail and have been found very similar. They all are forming stars. Several
intermediate-mass protostars have been detected in the cometary globules and in
the deeply embedded cores. Evidence of clustering has been found in the shocked
massive cores TC3-TC4-TC5. M20 is a good example of massive-star forming region
in a turbulent, filamentary molecular cloud. Photoionization appears to play a
minor role in the formation of the cores. The observed fragmentation is well
explained by MHD-driven instabilities and is usually not related to M20. We
propose that the nearby supernova remnant W28 could have triggered the
formation of protostellar clusters in nearby dense cores of the Trifid.Comment: 16 pages, 24 figures, 5 Tables To appear in Astronomy and
Astrophysic
Noise Correlations in Three-Terminal Diffusive Superconductor-Normal Metal-Superconductor Nanostructures
We present measurements of current noise and cross-correlations in
three-terminal Superconductor-Normal metal-Superconductor (S-N-S)
nanostructures that are potential solid-state entanglers thanks to Andreev
reflections at the N-S interfaces. The noise correlation measurements spanned
from the regime where electron-electron interactions are relevant to the regime
of Incoherent Multiple Andreev Reflection (IMAR). In the latter regime,
negative cross-correlations are observed in samples with closely-spaced
junctions.Comment: Include Supplemental Materia
Mesoscopic transition in the shot noise of diffusive S/N/S junctions
We experimentally investigated the current noise in diffusive
Superconductor/Normal metal/Superconductor junctions with lengths between the
superconducting coherence length xi_Delta and the phase coherence length L_Phi
of the normal metal (xi_Delta < L < L_Phi). We measured the shot noise over a
large range of energy covering both the regimes of coherent and incoherent
multiple Andreev reflections. The transition between these two regimes occurs
at the Thouless energy where a pronounced minimum in the current noise density
is observed. Above the Thouless energy, in the regime of incoherent multiple
Andreev reflections, the noise is strongly enhanced compared to a normal
junction and grows linearly with the bias voltage. Semi-classical theory
describes the experimental results accurately, when taking into account the
voltage dependence of the resistance which reflects the proximity effect. Below
the Thouless energy, the shot noise diverges with decreasing voltage which may
indicate the coherent transfer of multiple charges.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. B, Rapid
Communicatio
Molecular ions in the protostellar shock L1157-B1
We perform a complete census of molecular ions with an abundance larger than
1e-10 in the protostellar shock L1157-B1 by means of an unbiased
high-sensitivity survey obtained with the IRAM-30m and Herschel/HIFI. By means
of an LVG radiative transfer code the gas physical conditions and fractional
abundances of molecular ions are derived. The latter are compared with
estimates of steady-state abundances in the cloud and their evolution in the
shock calculated with the chemical model Astrochem. We detect emission from
HCO+, H13CO+, N2H+, HCS+, and, for the first time in a shock, from HOCO+, and
SO+. The bulk of the emission peaks at blueshifted velocity, ~ 0.5-3 km/s with
respect to systemic, has a width of ~ 4-8 km/s, and is associated with the
outflow cavities (T_kin ~ 20-70 K, n(H2) ~ 1e5 cm-3). Observed HCO+ and N2H+
abundances are in agreement with steady-state abundances in the cloud and with
their evolution in the compressed and heated gas in the shock for cosmic rays
ionization rate Z = 3e-16 s-1. HOCO+, SO+, and HCS+ observed abundances,
instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the
other hand they are strongly enhanced on timescales shorter than the shock age
(~2000 years) if CO2, S or H2S, and OCS are sputtered off the dust grains in
the shock. The performed analysis indicates that HCO+ and N2H+ are a fossil
record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are
effective shock tracers and can be used to infer the amount of CO2 and
sulphur-bearing species released from dust mantles in the shock. The observed
HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur
carrier on grain mantles. However, the OCS abundance required to fit the
observations is 1-2 orders of magnitude larger than observed. Further studies
are required to fully understand the chemistry of sulphur-bearing species.Comment: 12 pages, 5 figures, accepted by A&
Heavy water around the L1448-mm protostar
Context: L1448-mm is the prototype of a low-mass Class 0 protostar driving a
high-velocity jet. Given its bright H2O spectra observed with ISO, L1448-mm is
an ideal laboratory to observe heavy water (HDO) emission. Aims: Our aim is to
image the HDO emission in the protostar surroundings, the possible occurrence
of HDO emission also investigating off L1448-mm, towards the molecular outflow.
Methods: We carried out observations of L1448-mm in the HDO(1_10-1_11) line at
80.6 GHz, an excellent tracer of HDO column density, with the IRAM Plateau de
Bure Interferometer. Results: We image for the first time HDO emission around
L1448-mm. The HDO structure reveals a main clump at velocities close to the
ambient one towards the the continuum peak that is caused by the dust heated by
the protostar. In addition, the HDO map shows tentative weaker emission at
about 2000 AU from the protostar towards the south, which is possibly
associated with the walls of the outflow cavity opened by the protostellar
wind. Conclusions: Using an LVG code, modelling the density and temperature
profile of the hot-corino, and adopting a gas temperature of 100 K and a
density of 1.5 10^8 cm^-3, we derive a beam diluted HDO column density of about
7 10^13 cm^-2, corresponding to a HDO abundance of about 4 10^-7. In addition,
the present map supports the scenario where HDO can be efficiently produced in
shocked regions and not uniquely in hot corinos heated by the newly born star.Comment: Accepted by A&A as Letter; 5 pages, 3 figure
The L1157-B1 astrochemical laboratory: testing the origin of DCN
L1157-B1 is the brightest shocked region of the large-scale molecular
outflow, considered the prototype of chemically rich outflows, being the ideal
laboratory to study how shocks affect the molecular gas. Several deuterated
molecules have been previously detected with the IRAM 30m, most of them formed
on grain mantles and then released into the gas phase due to the shock. We aim
to observationally investigate the role of the different chemical processes at
work that lead to formation the of DCN and test the predictions of the chemical
models for its formation. We performed high-angular resolution observations
with NOEMA of the DCN(2-1) and H13CN(2-1) lines to compute the deuterated
fraction, Dfrac(HCN). We detected emission of DCN(2-1) and H13CN(2-1) arising
from L1157-B1 shock. Dfrac(HCN) is ~4x10 and given the uncertainties, we
did not find significant variations across the bow-shock. Contrary to HDCO,
whose emission delineates the region of impact between the jet and the ambient
material, DCN is more widespread and not limited to the impact region. This is
consistent with the idea that gas-phase chemistry is playing a major role in
the deuteration of HCN in the head of the bow-shock, where HDCO is undetected
as it is a product of grain-surface chemistry. The spectra of DCN and H13CN
match the spectral signature of the outflow cavity walls, suggesting that their
emission result from shocked gas. The analysis of the time dependent gas-grain
chemical model UCL-CHEM coupled with a C-type shock model shows that the
observed Dfrac(HCN) is reached during the post-shock phase, matching the
dynamical timescale of the shock. Our results indicate that the presence of DCN
in L1157-B1 is a combination of gas-phase chemistry that produces the
widespread DCN emission, dominating in the head of the bow-shock, and
sputtering from grain mantles toward the jet impact region.Comment: Accepted for publication in A&A. 7 pages, 5 Figures, 1 Tabl
Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations
New insights into the formation of interstellar formamide, a species of great
relevance in prebiotic chemistry, are provided by electronic structure and
kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to
what previously suggested, this reaction is essentially barrierless and can,
therefore, occur under the low temperature conditions of interstellar objects
thus providing a facile formation route of formamide. The rate coefficient
parameters for the reaction channel leading to NH2CHO + H have been calculated
to be A = 2.6x10^{-12} cm^3 s^{-1}, beta = -2.1 and gamma = 26.9 K in the range
of temperatures 10-300 K. Including these new kinetic data in a refined
astrochemical model, we show that the proposed mechanism can well reproduce the
abundances of formamide observed in two very different interstellar objects:
the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular
shock L1157-B2. Therefore, the major conclusion of this Letter is that there is
no need to invoke grain-surface chemistry to explain the presence of formamide
provided that its precursors, NH2 and H2CO, are available in the gas-phase.Comment: MNRAS Letters, in pres
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