4,752 research outputs found
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
The Foggy Disks Surrounding Herbig Ae Stars: a Theoretical Study of the H2O Line Spectra
Water is a key species in many astrophysical environments, but it is
particularly important in proto-planetary disks. So far,observations of water
in these objects have been scarce, but the situation should soon change thanks
to the Herschel satellite. We report here a theoretical study of the water line
spectrum of a proto-planetary disk surrounding Ae stars. We show that several
lines will be observable with the HIFI instrument onboard the Herschel Space
Observatory. We predict that some maser lines could also be observable with
ground telescopes and we discuss how the predictions depend not only on the
adopted physical and chemical model but also on the set of collisional
coefficients used and on the H2 ortho to para ratio through its effect on
collisional excitation. This makes the water lines observations a powerful, but
dangerous -if misused- diagnostic tool.Comment: Accepted for publication in ApJ Letter
First measurements of 15N fractionation in N2H+ toward high-mass star forming cores
We report on the first measurements of the isotopic ratio 14N/15N in N2H+
toward a statistically significant sample of high-mass star forming cores. The
sources belong to the three main evolutionary categories of the high-mass star
formation process: high-mass starless cores, high-mass protostellar objects,
and ultracompact HII regions. Simultaneous measurements of 14N/15N in CN have
been made. The 14N/15N ratios derived from N2H+ show a large spread (from ~180
up to ~1300), while those derived from CN are in between the value measured in
the terrestrial atmosphere (~270) and that of the proto-Solar nebula (~440) for
the large majority of the sources within the errors. However, this different
spread might be due to the fact that the sources detected in the N2H+
isotopologues are more than those detected in the CN ones. The 14N/15N ratio
does not change significantly with the source evolutionary stage, which
indicates that time seems to be irrelevant for the fractionation of nitrogen.
We also find a possible anticorrelation between the 14N/15N (as derived from
N2H+) and the H/D isotopic ratios. This suggests that 15N enrichment could not
be linked to the parameters that cause D enrichment, in agreement with the
prediction by recent chemical models. These models, however, are not able to
reproduce the observed large spread in 14N/15N, pointing out that some
important routes of nitrogen fractionation could be still missing in the
models.Comment: 2 Figures, accepted for publication in ApJ
Hyperentanglement witness
A new criterium to detect the entanglement present in a {\it hyperentangled
state}, based on the evaluation of an entanglement witness, is presented. We
show how some witnesses recently introduced for graph states, measured by only
two local settings, can be used in this case. We also define a new witness
that improves the resistance to noise by increasing the number of local
measurements.Comment: 6 pages, 2 figures, RevTex. v2: new title, minor changes in the
explanation of the witness for hyperentangled states, more comments in the
conclusions sectio
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
HD line emission in Proto-Planetary Disks
%Context: {Previous studies have indicated that the 372.4 GHz ground
transition of ortho-HD might be a powerful probe of Proto-Planetary
Disks. The line could be especially suited for study of the disk mid-plane,
where the bulk of the mass resides and where planet formation takes place.}
%Aims: {Provide detailed theoretical predictions for the line intensity,
profile and maps expected for representative disk models.} %Methods: {We
determine the physical and chemical structure of the disks from the model
developed by Ceccarelli & Dominik (2005). The line emission is computed with
the new radiative transfer method developed recently by Elitzur & Asensio Ramos
(2006).} %Results: {We present intensity maps convolved with the expected ALMA
resolution, which delineate the origin of the HD 372.4 GHz line. In the
disk inner regions, the line probes the conditions in the mid-plane out to
radial distances of a few tens of AU, where Solar-like planetary systems might
form. In the disk outermost regions, the line originates from slightly above
the mid-plane. When the disk is spatially resolved, the variation of line
profile across the image provides important information about the velocity
field. Spectral profiles of the entire disk flux show a double peak shape at
most inclination angles.} %Conclusions: {Our study confirms that the 372.4 GHz
HD line provides powerful diagnostics of the mid-plane of
Proto-Planetary Disks. Current submillimeter telescopes are capable of
observing this line, though with some difficulties. The future ALMA
interferometer will have the sensitivity to observe and even spatially resolve
the HD line emission.}Comment: To appear in A&
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