1,122 research outputs found
Our astrochemical heritage
Our Sun and planetary system were born about 4.5 billion years ago. How did
this happen and what is our heritage from these early times? This review tries
to address these questions from an astrochemical point of view. On the one
hand, we have some crucial information from meteorites, comets and other small
bodies of the Solar System. On the other hand, we have the results of studies
on the formation process of Sun-like stars in our Galaxy. These results tell us
that Sun-like stars form in dense regions of molecular clouds and that three
major steps are involved before the planet formation period. They are
represented by the pre-stellar core, protostellar envelope and protoplanetary
disk phases. Simultaneously with the evolution from one phase to the other, the
chemical composition gains increasing complexity.
In this review, we first present the information on the chemical composition
of meteorites, comets and other small bodies of the Solar System, which is
potentially linked to the first phases of the Solar System's formation. Then we
describe the observed chemical composition in the pre-stellar core,
protostellar envelope and protoplanetary disk phases, including the processes
that lead to them. Finally, we draw together pieces from the different objects
and phases to understand whether and how much we inherited chemically from the
time of the Sun's birth.Comment: Invited review to be published in "The Astronomy and Astrophysics
Review
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
The (impossible?) formation of acetaldehyde on the grain surfaces: insights from quantum chemical calculations
Complex Organic Molecules (COMs) have been detected in the interstellar
medium (ISM). However, it is not clear whether their synthesis occurs on the
icy surfaces of interstellar grains or via a series of gas-phase reactions. As
a test case of the COMs synthesis in the ISM, we present new quantum chemical
calculations on the formation of acetaldehyde (CH3CHO) from the coupling of the
HCO and CH3 radicals, both in gas phase and on water ice surfaces. The binding
energies of HCO and CH3 on the amorphous water ice were also computed (2333 and
734 K, respectively). Results indicate that, in gas phase, the products could
be either CH3CHO, CH4 + CO, or CH3OCH, depending on the relative orientation of
the two radicals. However, on the amorphous water ice, only the CH4 + CO
product is possible due to the geometrical constraints imposed by the water ice
surface. Therefore, acetaldehyde cannot be synthesized by the CH3 + HCO
coupling on the icy grains. We discuss the implications of these results and
other cases, such as ethylene glycol and dimethyl ether, in which similar
situations can occur, suggesting that formation of these molecules on the grain
surfaces might be unlikely
We Drink Good 4.5-Billion-Year-Old Water
Water is crucial for the emergence and evolution of life on Earth. Recent
studies of the water content in early forming planetary systems similar to our
own show that water is an abundant and ubiquitous molecule, initially
synthesized on the surfaces of tiny interstellar dust grains by the
hydrogenation of frozen oxygen. Water then enters a cycle of
sublimation/freezing throughout the successive phases of planetary system
formation, namely, hot corinos and protoplanetary disks, eventually to be
incorporated into planets, asteroids, and comets. The amount of heavy water
measured on Earth and in early forming planetary systems suggests that a
substantial fraction of terrestrial water was inherited from the very first
phases of the Solar System formation and is 4.5 billion years old
Rotating Disks and Non-Kinematic Double Peaks
Double-peaked line profiles are commonly considered a hallmark of rotating
disks, with the distance between the peaks a measure of the rotation velocity.
However, double-peaks can arise also from radiative transfer effects in
optically thick non-rotating sources. Utilizing exact solutions of the line
transfer problem we present a detailed study of line emission from
geometrically thin Keplerian disks. We derive the conditions for emergence of
kinematic double peaks in optically thin and thick disks, and find that it is
generally impossible to disentangle the effects of kinematics and line opacity
in observed double-peaked profiles. Unless supplemented by additional
information, a double-peaked profile alone is not a reliable indicator of a
rotating disk. In certain circumstances, triple and quadruple profiles might be
better indicators of rotation in optically thick disks.Comment: MNRAS, to be publishe
A theoretical investigation of the reaction between the amidogen, NH, and the ethyl, C2H5, radicals: a possible gas-phase formation route of interstellar and planetary ethanimine
The reaction between the amidogen, NH, radical and the ethyl, C2H5, radical
has been investigated by performing electronic structure calculations of the
underlying doublet potential energy surface. Rate coefficients and product
branching ratios have also been estimated by combining capture and RRKM
calculations. According to our results, the reaction is very fast, close to the
gas-kinetics limit. However, the main product channel, with a yield of ca.
86-88% in the range of temperatures investigated, is the one leading to
methanimine and the methyl radical. The channels leading to the two E-, Z-
stereoisomers of ethanimine account only for ca. 5-7% each. The resulting ratio
[E-CH3CHNH]/[Z-CH3CHNH] is ca. 1.2, that is a value rather lower than that
determined in the Green Bank Telescope PRIMOS radio astronomy survey spectra of
Sagittarius B2 North (ca. 3). Considering that ice chemistry would produce
essentially only the most stable isomer, a possible conclusion is that the
observed [E-CH3CHNH]/[Z-CH3CHNH] ratio is compatible with a combination of
gas-phase and grain chemistry. More observational and laboratory data are
needed to definitely address this issue
Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant
Cosmic rays are an essential ingredient in the evolution of the interstellar
medium, as they dominate the ionisation of the dense molecular gas, where stars
and planets form. However, since they are efficiently scattered by the galactic
magnetic fields, many questions remain open, such as where exactly they are
accelerated, what is their original energy spectrum, and how they propagate
into molecular clouds. In this work we present new observations and discuss in
detail a method that allows us to measure the cosmic ray ionisation rate
towards the molecular clouds close to the W28 supernova remnant. To perform
these measurements, we use CO, HCO, and DCO millimetre line
observations and compare them with the predictions of radiative transfer and
chemical models away from thermodynamical equilibrium. The CO observations
allow us to constrain the density, temperature, and column density towards each
observed position, while the DCO/HCO abundance ratios provide us with
constraints on the electron fraction and, consequently, on the cosmic ray
ionisation rate. Towards positions located close to the supernova remnant, we
find cosmic ray ionisation rates much larger () than those in
standard galactic clouds. Conversely, towards one position situated at a larger
distance, we derive a standard cosmic ray ionisation rate. Overall, these
observations support the hypothesis that the rays observed in the
region have a hadronic origin. In addition, based on CR diffusion estimates, we
find that the ionisation of the gas is likely due to GeV cosmic rays.
Finally, these observations are also in agreement with the global picture of
cosmic ray diffusion, in which the low-energy tail of the cosmic ray population
diffuses at smaller distances than the high-energy counterpart.Comment: Accepted to A\&
IR Spectral Fingerprint of Carbon Monoxide in Interstellar Water Ice Models
Carbon monoxide (CO) is the second most abundant molecule in the gas-phase of
the interstellar medium. In dense molecular clouds, it is also present in the
solid-phase as a constituent of the mixed water-dominated ices covering dust
grains. Its presence in the solid-phase is inferred from its infrared (IR)
signals. In experimental observations of solid CO/water mixed samples, its IR
frequency splits into two components, giving rise to a blue- and a redshifted
band. However, in astronomical observations, the former has never been
observed. Several attempts have been carried out to explain this peculiar
behaviour, but the question still remains open. In this work, we resorted to
pure quantum mechanical simulations in order to shed some light on this
problem. We adopted different periodic models simulating the CO/HO ice
system, such as single and multiple CO adsorption on water ice surfaces, CO
entrapped into water cages and proper CO:HO mixed ices. We also simulated
pure solid CO. The detailed analysis of our data revealed how the quadrupolar
character of CO and the dispersive forces with water ice determine the
energetic of the CO/HO ice interaction, as well as the CO spectroscopic
behaviour. Our data suggest that the blueshifted peak can be assigned to CO
interacting {\it via} the C atom with dangling H atoms of the water ice, while
the redshifted one can actually be the result of CO involved in different
reciprocal interactions with the water matrix. We also provide a possible
explanation for the lack of the blueshifted peak in astronomical spectra. Our
aim is not to provide a full account of the various interstellar ices, but
rather to elucidate the sensitivity of the CO spectral features to different
water ice environments.Comment: MNRAS, accepte
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