458 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
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
High spectral resolution observations of HNC3 and HCCNC in the L1544 prestellar core
HCCNC and HNC3 are less commonly found isomers of cyanoacetylene, HC3N, a
molecule that is widely found in diverse astronomical sources. We want to know
if HNC3 is present in sources other than the dark cloud TMC-1 and how its
abundance is relative to that of related molecules. We used the ASAI unbiased
spectral survey at IRAM 30m towards the prototypical prestellar core L1544 to
search for HNC3 and HCCNC which are by-product of the HC3NH+ recombination,
previously detected in this source. We performed a combined analysis of
published HNC3 microwave rest frequencies with thus far unpublished millimeter
data because of issues with available rest frequency predictions. We determined
new spectroscopic parameters for HNC3, produced new predictions and detected it
towards L1544. We used a gas-grain chemical modelling to predict the abundances
of N-species and compare with the observations. The modelled abundances are
consistent with the observations, considering a late stage of the evolution of
the prestellar core. However the calculated abundance of HNC3 was found 5-10
times higher than the observed one. The HC3N, HNC3 and HCCNC versus HC3NH+
ratios are compared in the TMC-1 dark cloud and the L1544 prestellar core.Comment: Accepted in MNRAS letters. 5 pages plus 2 additional pages for the
on-line materia
Molecules with a peptide link in protostellar shocks: a comprehensive study of L1157
Interstellar molecules with a peptide link -NH-C(=O)-, like formamide
(NHCHO), acetamide (NHCOCH) and isocyanic acid (HNCO) are
particularly interesting for their potential role in pre-biotic chemistry. We
have studied their emission in the protostellar shock regions L1157-B1 and
L1157-B2, with the IRAM 30m telescope, as part of the ASAI Large Program.
Analysis of the line profiles shows that the emission arises from the outflow
cavities associated with B1 and B2. Molecular abundance of
and are derived for
formamide and isocyanic acid, respectively, from a simple rotational diagram
analysis. Conversely, NHCOCH was not detected down to a relative
abundance of a few . B1 and B2 appear to be among the richest
Galactic sources of HNCO and NHCHO molecules. A tight linear correlation
between their abundances is observed, suggesting that the two species are
chemically related. Comparison with astrochemical models favours molecule
formation on ice grain mantles, with NHCHO generated from hydrogenation of
HNCO.Comment: 11 pages, 9 figures. Accepted for publication in MNRAS Main Journal.
Accepted 2014 August 19, in original form 2014 July
A global foliation of Einstein-Euler spacetimes with Gowdy-symmetry on T3
We investigate the initial value problem for the Einstein-Euler equations of
general relativity under the assumption of Gowdy symmetry on T3, and we
construct matter spacetimes with low regularity. These spacetimes admit, both,
impulsive gravitational waves in the metric (for instance, Dirac mass curvature
singularities propagating at light speed) and shock waves in the fluid (i.e.,
discontinuities propagating at about the sound speed). Given an initial data
set, we establish the existence of a future development and we provide a global
foliation in terms of a globally and geometrically defined time-function,
closely related to the area of the orbits of the symmetry group. The main
difficulty lies in the low regularity assumed on the initial data set which
requires a distributional formulation of the Einstein-Euler equations.Comment: 24 page
Hot and dense water in the inner 25 AU of SVS13-A
In the context of the ASAI (Astrochemical Surveys At IRAM) project, we
carried out an unbiased spectral survey in the millimeter window towards the
well known low-mass Class I source SVS13-A. The high sensitivity reached (3-12
mK) allowed us to detect at least 6 HDO broad (FWHM ~ 4-5 km/s) emission lines
with upper level energies up to Eu = 837 K. A non-LTE LVG analysis implies the
presence of very hot (150-260 K) and dense (> 3 10^7 cm-3) gas inside a small
radius ( 25 AU) around the star, supporting, for the first time, the
occurrence of a hot corino around a Class I protostar.
The temperature is higher than expected for water molecules are sublimated
from the icy dust mantles (~ 100 K). Although we cannot exclude we are observig
the effects of shocks and/or winds at such small scales, this could imply that
the observed HDO emission is tracing the water abundance jump expected at
temperatures ~ 220-250 K, when the activation barrier of the gas phase
reactions leading to the formation of water can be overcome. We derive X(HDO) ~
3 10-6, and a H2O deuteration > 1.5 10-2, suggesting that water deuteration
does not decrease as the protostar evolves from the Class 0 to the Class I
stage.Comment: MNRAS Letter
Anatomy of HH 111 from CO observations : a bow shock driven molecular outflow
We present millimeter line observations of the HH 111 outflow and its driving
source. The molecular gas emission observed with IRAM 30m and the CSO reveals a
small condensation of cold and dense gas. The low-velocity outflow has been
mapped with the IRAM PdBI interferometer. The cold gas is distributed in a
hollow cylinder surrounding the optical jet. The formation of this cavity and
its kinematics are well accounted for in the frame of outflow gas entrainment
by jet bow shocks. Evidence of gas acceleration is found along the cavity
walls, correlated with the presence of optical bow shocks. The cavity has been
expanding with a mean velocity of 4 km/s on a timescale of 8700 yr, similar to
the dynamical age of the optical jet. The separation of the inner walls reaches
8"-10", which matches the transverse size of the wings in the bow shock. CSO
observations of the J=7-6 line show evidence of a high-velocity and hot gas
component (T=300-1000 K) with a low filling factor, associated with shocked
molecular gas in the jet. [CI] observations are consistent with C-type
non-dissociative shocks. Mapping of the high-velocity molecular bullets B1-B3
located beyond the optical jet, with the PdBI, reveals small structures of 3"
by 7" flattened perpendicular to the flow direction. They are made of cold gas
of moderate density(a few 10^3 cm-3). The bullets appear to expand into the
low-density surrounding medium. We conclude that they are probably shocked gas
knots resulting from past time-variable ejections in the jet.Comment: 21 pages + 4 figures (JPG). To appear in Astrophysical Journa
Ionization fraction and the enhanced sulfur chemistry in Barnard 1
Barnard B1b has revealed as one of the most interesting globules from the
chemical and dynamical point of view. It presents a rich molecular chemistry
characterized by large abundances of deuterated and complex molecules.
Furthermore, it hosts an extremely young Class 0 object and one candidate to
First Hydrostatic Core (FHSC). Our aim was to determine the cosmic ray
ionization rate and the depletion factors in this extremely young star forming
region. We carried out a spectral survey towards Barnard 1b as part of the IRAM
Large program ASAI using the IRAM 30-m telescope at Pico Veleta (Spain). This
provided a very complete inventory of neutral and ionic C-, N- and S- bearing
species with, up to our knowledge, the first secure detections of the
deuterated ions DCS+ and DOCO+. We used a state-of-the-art
pseudo-time-dependent gas-phase chemical model to determine the value of the
cosmic ray ionization rate and the depletion factors. The observational data
were well fitted with between 3E-17 s and 1E-16 s.
Elemental depletions were estimated to be ~10 for C and O, ~1 for N and ~25 for
S. Barnard B1b presents similar depletions of C and O than those measured in
pre-stellar cores. The depletion of sulfur is higher than that of C and O but
not as extreme as in cold cores. In fact, it is similar to the values found in
some bipolar outflows, hot cores and photon-dominated regions. Several
scenarios are discussed to account for these peculiar abundances. We propose
that it is the consequence of the initial conditions (important outflows and
enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that
permits to maintain most S- and N-bearing species in gas phase to great optical
depths. The interaction of the compact outflow associated with B1b-S with the
surrounding material could enhance the abundances of S-bearing molecules, as
well.Comment: Paper accepted in Astronomy and Astrophysics; 28 pags, 21 figure
Zero-temperature spin-glass freezing in self-organized arrays of Co nanoparticles
We study, by means of magnetic susceptibility and magnetic aging experiments,
the nature of the glassy magnetic dynamics in arrays of Co nanoparticles,
self-organized in N layers from N=1 (two-dimensional limit) up to N=20
(three-dimensional limit). We find no qualitative differences between the
magnetic responses measured in these two limits, in spite of the fact that no
spin-glass phase is expected above T=0 in two dimensions. More specifically,
all the phenomena (critical slowing down, flattening of the field-cooled
magnetization below the blocking temperature and the magnetic memory induced by
aging) that are usually associated with this phase look qualitatively the same
for two-dimensional and three-dimensional arrays. The activated scaling law
that is typical of systems undergoing a phase transition at zero temperature
accounts well for the critical slowing down of the dc and ac susceptibilities
of all samples. Our data show also that dynamical magnetic correlations
achieved by aging a nanoparticle array below its superparamagnetic blocking
temperature extend mainly to nearest neighbors. Our experiments suggest that
the glassy magnetic dynamics of these nanoparticle arrays is associated with a
zero-temperature spin-glass transition.Comment: 6 pages 6 figure
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