449 research outputs found
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
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Examining the influence of service additions on manufacturing firms’ bankruptcy likelihood
By evaluating secondary data from 74 bankrupt manufacturers and 199 matched non-bankrupt competitors, this study investigates the relationship of manufacturers' service offerings to their survival. While showing that the number of services offered is not significantly associated with bankruptcy likelihood, the results suggest that greater numbers of product-related and product-unrelated service offerings do reduce bankruptcy likelihood when properly complemented by firm-level contextual factors. Offering more product-related services causes bankruptcy likelihood to decrease for those companies that have a sufficiently diversified product business. In turn, companies with sufficient slack resources can expect bankruptcy likelihood to be reduced from the offering of more product-unrelated services. In contrast, companies should not expect that successful product sales performance will increase their chances of survival by focusing on product-dependent services. In light of these findings, this study challenges the notion from conceptual literature that additional services per se increase the chances of firm survival; it extends prior empirical studies in uncovering critical firm-level context effects; and it proposes portfolio theory as a theoretical foundation to examine manufacturers' service expansions.This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Elsevier
Evaluation of the stiffness tensor of a fractured medium with harmonic experiments
A fractured medium behaves as an anisotropic medium when the wavelength is much larger than the distance between fractures. These are modeled as boundary discontinuities in the displacement and particle velocity. When the set of fractures is plane, the theory predicts that the equivalent medium is transversely isotropic and viscoelastic (TIV). We present a novel procedure to determine the complex and frequency-dependent stiffness components. The methodology amounts to perform numerical compressibility and shear harmonic tests on a representative sample of the medium. These tests are described by a collection of elliptic boundary-value problems formulated in the space-frequency domain, which are solved with a Galerkin finite-element procedure. The examples illustrate the implementation of the tests to determine the set of stiffnesses and the associated phase velocities and quality factors.Fil: Santos, Juan Enrique. Universidad de Buenos Aires. Facultad de Ingeniería. Instituto del Gas y del Petróleo; Argentina. Universidad Nacional de La Plata; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Benedettini, Stefano. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; ItaliaFil: Carcione, José M.. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale; Itali
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ISO-LWS grating spectroscopy: the case of R CrA star forming region
We present the far infrared spectra of the R CrA star forming region obtained with ISO-LWS. We collected a pointed observation on the Herbig Ae star R CrA and a raster scan covering the surrounding region, where HH100 (with its exciting source) and the pre-Main Sequence star T CrA are located. The OI 63μm and the CII 158μm lines have been detected in all the pointed positions, with a ratio consistent with PDR excitation. CO rotational lines (between Jup=14 and Jup=19) are detected on R CrA; from their intensities we derived, using a LVG model, the density and temperature of the emitting region. Other molecular transitions (OH and H2O) have been detected on the investigated objects; the derived cooling of all the molecular species is in agreement with C-shock as the likely excitation mechanism. The continuum emission of R CrA peaks around 100μm (as expected for a Herbig star) while the other sources (T CrA, HH100) show increasing continua up to ~200μm, indicating that they are probably less evolved sources
The clumpy structure of the chemically active L1157 outflow
We present high spatial resolution maps, obtained with the Plateau de Bure
Interferometer, of the blue lobe of the L1157 outflow. We observed four lines
at 3 mm, namely CH3OH (2_K-1_K), HC3N (11-10), HCN (1-0) and OCS (7-6).
Moreover, the bright B1 clump has also been observed at better spatial
resolution in CS (2-1), CH3OH (2_1-1_1)A-, and 34SO (3_2-2_1). These high
spatial resolution observations show a very rich structure in all the tracers,
revealing a clumpy structure of the gas superimposed to an extended emission.
In fact, the three clumps detected by previous IRAM-30m single dish
observations have been resolved into several sub-clumps and new clumps have
been detected in the outflow. The clumps are associated with the two cavities
created by two shock episodes driven by the precessing jet. In particular, the
clumps nearest the protostar are located at the walls of the younger cavity
with a clear arch-shape form while the farthest clumps have slightly different
observational characteristics indicating that they are associated to the older
shock episode. The emission of the observed species peaks in different part of
the lobe: the east clumps are brighter in HC3N (11-10), HCN (1-0) and CS (2-1)
while the west clumps are brighter in CH3OH(2_K-1_K), OCS (7-6) and 34SO
(3_2-2_1). This peak displacement in the line emission suggests a variation of
the physical conditions and/or the chemical composition along the lobe of the
outflow at small scale, likely related to the shock activity and the precession
of the outflow. In particular, we observe the decoupling of the silicon
monoxide and methanol emission, common shock tracers, in the B1 clump located
at the apex of the bow shock produced by the second shock episode.Comment: 11 pages, 8 figures, accepted for publication in the MNRA
The B1 shock in the L1157 outflow as seen at high spatial resolution
We present high spatial resolution (750 AU at 250 pc) maps of the B1 shock in
the blue lobe of the L1157 outflow in four lines: CS (3-2), CH3OH (3_K-2_K),
HC3N (16-15) and p-H2CO (2_02-3_01). The combined analysis of the morphology
and spectral profiles has shown that the highest velocity gas is confined in a
few compact (~ 5 arcsec) bullets while the lowest velocity gas traces the wall
of the gas cavity excavated by the shock expansion. A large velocity gradient
model applied to the CS (3-2) and (2-1) lines provides an upper limit of 10^6
cm^-3 to the averaged gas density in B1 and a range of 5x10^3< n(H2)< 5x10^5
cm^-3 for the density of the high velocity bullets. The origin of the bullets
is still uncertain: they could be the result of local instabilities produced by
the interaction of the jet with the ambient medium or could be clump already
present in the ambient medium that are excited and accelerated by the expanding
outflow. The column densities of the observed species can be reproduced
qualitatively by the presence in B1 of a C-type shock and only models where the
gas reaches temperatures of at least 4000 K can reproduce the observed HC3N
column density.Comment: 13 pages, 12 figure
Broad N2H+ emission towards the protostellar shock L1157-B1
We present the first detection of N2H+ towards a low-mass protostellar
outflow, namely the L1157-B1 shock, at about 0.1 pc from the protostellar
cocoon. The detection was obtained with the IRAM 30-m antenna. We observed
emission at 93 GHz due to the J = 1-0 hyperfine lines. The analysis of the
emission coupled with the HIFI CHESS multiline CO observations leads to the
conclusion that the observed N2H+(1-0) line originates from the dense (> 10^5
cm-3) gas associated with the large (20-25 arcsec) cavities opened by the
protostellar wind. We find a N2H+ column density of few 10^12 cm-2
corresponding to an abundance of (2-8) 10^-9. The N2H+ abundance can be matched
by a model of quiescent gas evolved for more than 10^4 yr, i.e. for more than
the shock kinematical age (about 2000 yr). Modelling of C-shocks confirms that
the abundance of N2H+ is not increased by the passage of the shock. In summary,
N2H+ is a fossil record of the pre-shock gas, formed when the density of the
gas was around 10^4 cm-3, and then further compressed and accelerated by the
shock.Comment: ApJ, in pres
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Strong H<sub>2</sub>O and high-<i>J</i> CO emission towards the Class 0 protostar L1448-mm
The spectrum of the Class 0 source L1448-mm has been measured over the wavelength range extending from 6 to 190 μm with the Long Wavelength Spectrometer (LWS) and the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO). The far infrared spectrum is dominated by strong emission from gaseous H2O and from CO transitions with rotational quantum numbers J ≥ 14; in addition, the H2 pure rotational lines S(3), S(4) and S(5), the OH fundamental line at 119 μm, as well as emission from [O I]63 μm and [C II] 158 μm are also observed. The strong CO and water emission can be consistently explained as originating in a warm gas component at T ~ 700-1400 K and nH2~(3-50) 104cm-3 , which fills about 0.2-2% of the ~ 75" LWS field of view (corresponding, assuming a single emitting region, to a physical size of about (3-12)" or (0.5-2) 10-2 pc at d = 300 pc). We derive an H2O/CO abundance ratio ~ 5, which, assuming a standard CO/H2 abundance of 10-4, corresponds to H2O/H2 ~ 5 10-4. This value implies that water is enhanced by about a factor ~ 103 with respect to its expected abundance in the ambient gas. This is consistent with models of warm shocked regions which predict that most of the free atomic oxygen will be rapidly converted into water once the temperature of the post-shocked gas exceeds ~ 300 K. The relatively high density and compact size inferred for this emission may suggest an origin in the shocked region along the molecular jet traced by SiO and EHV CO millimeter line emission. Further support is given by the fact that the observed enhancement in H2O can be explained by shock conditions similar to those expected to produce the abundant SiO observed in the region. L1448-mm shows the largest water abundance so far observed by ISO amongst young sources displaying outflow activity; we argue that the occurrence of multiple shocks over a relatively short interval of time, like that evidenced in the surroundings of L1448-mm, could have contributed to enrich the molecular jet with a high H2O column density
The CHESS survey of the L1157-B1 bow-shock: high and low excitation water vapor
Molecular outflows powered by young protostars strongly affect the kinematics
and chemistry of the natal molecular cloud through strong shocks resulting in
substantial modifications of the abundance of several species. As part of the
"Chemical Herschel Surveys of Star forming regions" guaranteed time key
program, we aim at investigating the physical and chemical conditions of H20 in
the brightest shock region B1 of the L1157 molecular outflow. We observed
several ortho- and para-H2O transitions using HIFI and PACS instruments on
board Herschel, providing a detailed picture of the kinematics and spatial
distribution of the gas. We performed a LVG analysis to derive the physical
conditions of H2O shocked material, and ultimately obtain its abundance. We
detected 13 H2O lines probing a wide range of excitation conditions. PACS maps
reveal that H2O traces weak and extended emission associated with the outflow
identified also with HIFI in the o-H2O line at 556.9 GHz, and a compact (~10")
bright, higher-excitation region. The LVG analysis of H2O lines in the
bow-shock show the presence of two gas components with different excitation
conditions: a warm (Tkin~200-300 K) and dense (n(H2)~(1-3)x10^6 cm-3) component
with an assumed extent of 10" and a compact (~2"-5") and hot, tenuous
(Tkin~900-1400 K, n(H2)~10^3-10^4 cm-3) gas component, which is needed to
account for the line fluxes of high Eu transitions. The fractional abundance of
the warm and hot H2O gas components is estimated to be (0.7-2)x10^{-6} and
(1-3)x10^{-4}, respectively. Finally, we identified an additional component in
absorption in the HIFI spectra of H2O lines connecting with the ground state
level, probably arising from the photodesorption of icy mantles of a
water-enriched layer at the edges of the cloud.Comment: Accepted for publication in A&A. 12 pages, 9 figures, 4 table
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