83 research outputs found
The origin of complex organic molecules in prestellar cores
Complex organic molecules (COMs) have been detected in a variety of
environments, including cold prestellar cores. Given the low temperature of
these objects, these last detections challenge existing models. We report here
new observations towards the prestellar core L1544. They are based on an
unbiased spectral survey of the 3mm band at the IRAM-30m telescope, as part of
the Large Program ASAI. The observations allow us to provide the full census of
the oxygen bearing COMs in this source. We detected tricarbon monoxide,
methanol, acetaldehyde, formic acid, ketene, and propyne with abundances
varying from 5e-11 to 6e-9. The non-LTE analysis of the methanol lines shows
that they are likely emitted at the border of the core, at a radius of ~8000 AU
where T~10 K and nH2~2e4 cm-3. Previous works have shown that water vapour is
enhanced in the same region because of the photodesorption of water ices. We
propose that a non-thermal desorption mechanism is also responsible for the
observed emission of methanol and COMs from the same layer. The desorbed oxygen
and a tiny amount of desorbed methanol and ethene are enough to reproduce the
abundances of tricarbon monoxide, methanol, acetaldehyde and ketene measured in
L1544. These new findings open the possibility that COMs in prestellar cores
originate in a similar outer layer rather than in the dense inner cores, as
previously assumed, and that their formation is driven by the non-thermally
desorbed species.Comment: Accepted in ApJ
Abundance of HOCO+ and CO2 in the outer layers of the L1544 prestellar core
The L1544 prestellar core has been observed as part of the ASAI IRAM Large
Program at 3 mm. These observations led to the detection of many complex
molecules. In this Letter, we report the detection of two lines, at 85.5 GHz
(4,0,4-3,0,3) and 106.9 GHz (5,0,5-4,0,4), respectively, of the protonated
carbon dioxide ion, HOCO+. We also report the tentative detection of the line
at 100.4 GHz (5,0,5-4,0,4) of DOCO+. The non-LTE analysis of the detected lines
shows that the HOCO+ emission originates in the external layer where
non-thermal desorption of other species has previously been observed. Its
abundance is (5 +/- 2) e-11. Modelling of the chemistry involved in the
formation and destruction of HOCO+ provides a gaseous CO2 abundance of 2e-7
(with respect to H2) with an upper limit of 2e-6.Comment: To appear in A&A Letter
Detection of the HCNH and HCNH ions in the L1544 pre-stellar core
The L1544 pre-stellar core was observed as part of the ASAI (Astrochemical
Surveys At IRAM) Large Program. We report the first detection in a pre-stellar
core of the HCNH and HCNH ions. The high spectral resolution of the
observations allows to resolve the hyperfine structure of HCNH. Local
thermodynamic equilibrium analysis leads to derive a column density equal to
(2.00.2)10cm for HCNH and
(1.50.5)10cm for HCNH. We also present
non-LTE analysis of five transitions of HCN, three transitions of
HCN and one transition of HNC, all of them linked to the
chemistry of HCNH and HCNH. We computed for HCN, HCN, and HNC a
column density of (2.00.4)10cm,
(3.60.9)cm, and
(3.01.0)10cm, respectively. We used the gas-grain
chemical code Nautilus to predict the abundances all these species across the
pre-stellar core. Comparison of the observations with the model predictions
suggests that the emission from HCNH and HCNH originates in the
external layer where non-thermal desorption of other species was previously
observed. The observed abundance of both ionic species
([HCNH] and
[HCNH], with respect to H) cannot
be reproduced at the same time by the chemical modelling, within the error bars
of the observations only. We discuss the possible reasons for the discrepancy
and suggest that the current chemical models are not fully accurate or
complete. However, the modelled abundances are within a factor of three
consistent with the observations, considering a late stage of the evolution of
the pre-stellar core, compatible with previous observations.Comment: Accepted for publication in MNRAS, 13 pages, 9 figure
The genealogical tree of ethanol: gas-phase formation of glycolaldehyde, acetic acid and formic acid
Despite the harsh conditions of the interstellar medium, chemistry thrives in
it, especially in star forming regions where several interstellar complex
organic molecules (iCOMs) have been detected. Yet, how these species are
synthesised is a mystery. The majority of current models claim that this
happens on interstellar grain surfaces. Nevertheless, evidence is mounting that
neutral gas-phase chemistry plays an important role. In this article, we
propose a new scheme for the gas-phase synthesis of glycolaldehyde, a species
with a prebiotic potential and for which no gas-phase formation route was
previously known. In the proposed scheme, the ancestor is ethanol and the
glycolaldehyde sister species are acetic acid (another iCOM with unknown
gas-phase formation routes) and formic acid. For the reactions of the new
scheme with no available data, we have performed electronic structure and
kinetics calculations deriving rate coefficients and branching ratios.
Furthermore, after a careful review of the chemistry literature, we revised the
available chemical networks, adding and correcting several reactions related to
glycolaldehyde, acetic acid and formic acid. The new chemical network has been
used in an astrochemical model to predict the abundance of glycolaldehyde,
acetic acid and formic acid. The predicted abundance of glycolaldehyde depends
on the ethanol abundance in the gas phase and is in excellent agreement with
the measured one in hot corinos and shock sites. Our new model overpredicts the
abundance of acetic acid and formic acid by about a factor of ten, which might
imply a yet incomplete reaction network
Infalling-Rotating Motion and Associated Chemical Change in the Envelope of IRAS 16293-2422 Source A Studied with ALMA
We have analyzed rotational spectral line emission of OCS, CH3OH, HCOOCH3,
and H2CS observed toward the low-mass Class 0 protostellar source IRAS
16293-2422 Source A at a sub-arcsecond resolution (~0".6 x 0".5) with ALMA.
Significant chemical differentiation is found at a 50 AU scale. The OCS line is
found to well trace the infalling-rotating envelope in this source. On the
other hand, the CH3OH and HCOOCH3 distributions are found to be concentrated
around the inner part of the infalling-rotating envelope. With a simple
ballistic model of the infalling-rotating envelope, the radius of the
centrifugal barrier (a half of the centrifugal radius) and the protostellar
mass are evaluated from the OCS data to be from 40 to 60 AU and from 0.5 to 1.0
Msun, respectively, assuming the inclination angle of the envelope/disk
structure to be 60 degrees (90 degrees for the edge-on configuration). Although
the protostellar mass is correlated with the inclination angle, the radius of
the centrifugal barrier is not. This is the first indication of the centrifugal
barrier of the infalling-rotating envelope in a hot corino source. CH3OH and
HCOOCH3 may be liberated from ice mantles due to weak accretion shocks around
the centrifugal barrier, and/or due to protostellar heating. The H2CS emission
seems to come from the disk component inside the centrifugal barrier in
addition to the envelope component. The centrifugal barrier plays a central
role not only in the formation of a rotationally-supported disk but also in the
chemical evolution from the envelope to the protoplanetary disk
Extended Far-Infrared CO Emission in the Orion OMC-1 Core
We report on sensitive far-infrared observations of CO pure rotational
transitions in the OMC-1 core of Orion. The lines were observed with the Long
Wavelength Spectrometer (LWS) in the grating mode on board the Infrared Space
Observatory (ISO), covering the 43-197 m wavelength range. The transitions
from up to have been identified across the whole OMC-1
core and lines up to have been detected towards the central
region, KL/IRc2. In addition, we have taken high-quality spectra in the
Fabry-Perot mode of some of the CO lines. In KL/IRc2 the lines are
satisfactorily accounted for by a three-temperature model describing the
plateau and ridge emission. The fluxes detected in the high- transitions
() reveal the presence of a very hot and dense gas component
( K; =2\times 10^{17}\cmmd\rm H_2\geq 80$ K and as high as 150 K at some positions around IRc2,
from a simple Large-Velocity Gradient model.Comment: 10 pages, 3 figure
The Herschel and IRAM CHESS Spectral Surveys of the Protostellar Shock L1157-B1: Fossil Deuteration
We present the first study of deuteration toward the protostellar shock L1157-B1, based on spectral surveys performed with the Herschel-HIFI and IRAM 30 m telescopes. The L1157 outflow is driven by a low-mass Class 0 protostar and is considered the prototype of the so-called chemically active outflows. The young (2000 yr), bright blueshifted bow shock, B1, is an ideal laboratory for studying the gas chemically enriched by the release of dust mantles due to the passage of a shock. A total of 12 emission lines (up to E_u = 63 K) of CH_(2)DOH, HDCO, and DCN are detected. In addition, two lines of NH_(2)D and HDO are tentatively reported. To estimate the deuteration, we also extracted from our spectral survey emission lines of non-deuterated isotopologues (^(13)CH_(3)OH, H_2 ^(13)CO, H^(13)CN, H_2 ^(13)CO, and NH_3). We infer higher deuteration fractions for CH_(3)OH (D/H = 0.2-2 × 10^(–2)) and H_(2)CO (5-8 × 10^(–3)) than for H_(2)O (0.4-2 × 10^(–3)), HCN (~10^(–3)), and ammonia (≤3 × 10^(–2)). The measurement of deuteration of water, formaldehyde, and methanol in L1157-B1 provides a fossil record of the gas before it was shocked by the jet driven by the protostar. A comparison with gas-grain models indicates that the gas passed through a low-density (≤10^3 cm^(–3)) phase, during which the bulk of water ices formed, followed by a phase of increasing density, up to 3 × 10^4 cm^(–3), during which formaldehyde and methanol ices formed
The Photoionization of a Star-Forming Core in the Trifid Nebula
We have carried out a comprehensive multiwavelength study of Bright-Rimmed Globule TC2 in the Trifid Nebula using the IRAM~30m telescope, the VLA centimeter array and the Infrared Space Observatory (ISO). TC2 is one of the very few globules to exhibit signs of active ongoing star formation while being photoevaporated. The study of the kinematics shows that TC2 is currently undergoing an implosion driven by the ionization field. The physical structure of the molecular core, the Photon-Dominated Region and the ionization front are characterized. The properties of the PDR are in good agreement with some recent PDR models. The molecular emission suggests that the star formation process was probably initiated a few 0.1 Myr ago, in the large burst which led to the formation of the nebula. The impact of photoionization on the star formation process appears limited.Peer reviewe
Rotation in the NGC 1333 IRAS 4C Outflow
We report molecular line observations of the NGC 1333 IRAS 4C outflow in the
Perseus Molecular Cloud with the Atacama Large Millimeter/Submillimeter Array.
The CCH and CS emission reveal an outflow cavity structure with clear
signatures of rotation with respect to the outflow axis. The rotation is
detected from about 120 au up to about 1400 au above the envelope/disk
mid-plane. As the distance to the central source increases, the rotation
velocity of the outflow decreases while the outflow radius increases, which
gives a flat specific angular momentum distribution along the outflow. The mean
specific angular momentum of the outflow is about 100 au km/s. Based on
reasonable assumptions on the outward velocity of the outflow and the protostar
mass, we estimate the range of outflow launching radii to be 5-15 au. Such a
launching radius rules out that this outflow is launched as an X-wind, but
rather, it is more consistent to be a slow disk wind launched from relatively
large radii on the disk. The radius of the centrifugal barrier is roughly
estimated, and the role of the centrifugal barrier in the outflow launching is
discussed.Comment: Accepted to ApJ. 29 pages, 8 figure
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