41 research outputs found
C5H9N Isomers: Pointers to Possible Branched Chain Interstellar Molecules
The astronomical observation of isopropyl cyanide further stresses the link
between the chemical composition of the ISM and molecular composition of the
meteorites in which there is a dominance of branched chain amino acids as
compared to the straight. However, observations of more branched chain
molecules in ISM will firmly establish this link. In the light of this, we have
considered C5H9N isomeric group in which the next higher member of the alkyl
cyanide and other branched chain isomers belong. High-level quantum chemical
calculations have been employed in estimating accurate energies of these
isomers. From the results, the only isomer of the group that has been
astronomically searched, n-butyl cyanide is not the most stable isomer and
therefore, which might explain why its search could only yield upper limits of
its column density without a successful detection. Rather, the two most stable
isomers of the group are the branched chain isomers, tert-butylnitrile and
isobutyl cyanide. Based on the rotational constants of these isomers, it is
found that the expected intensity of tert-butylnitrile is the maximum among
this isomeric group. Thus, this is proposed as the most probable candidate for
astronomical observation. A simple LTE (Local thermodynamic equilibrium)
modelling has also been carried out to check the possibility of detecting
tert-butyl cyanide in the millimetre-wave region.Comment: 16 pages, 1 figur
Carbon-Chain Chemistry in the Interstellar Medium
The presence of carbon-chain molecules in the interstellar medium (ISM) has
been known since the early 1970s and such species have been identified
to date, making up of the total of detected ISM molecules. They are
prevalent not only in star-forming regions in our Galaxy, but also in other
galaxies. These molecules provide important information on physical conditions,
gas dynamics, and evolutionary stages of star-forming regions. More complex
species of polycyclic aromatic hydrocarbons (PAHs) and fullerenes (C and
C) have been detected in circumstellar envelopes around carbon-rich
Asymptotic Giant Branch (AGB) stars and planetary nebulae, while PAHs are also
known to be a widespread component of interstellar dust in most galaxies.
Recently, two line survey projects toward the starless core Taurus Molecular
Cloud-1 with large single-dish telescopes have detected many new carbon-chain
species, including molecules containing benzene rings. These new findings raise
fresh questions about carbon-bearing species in the Universe. This article
reviews various aspects of carbon-chain molecules, including observational
studies, chemical simulations, quantum calculations, and laboratory
experiments, and discusses open questions and how they may be answered by
future facilities.Comment: This is a review article submitted to the Publications of the
Astronomical Society of Japan. Comments are welcom
Chemical and radiative transfer modeling of Propylene Oxide
The recent identification of the first complex chiral molecule, propylene
oxide (PrO) in space opens up a new window to further study the origin of
homochirality on the Earth. There are some recent studies to explain the
formation of PrO however additional studies on the formation of this species
are needed for better understanding. We seek to prepare a complete reaction
network to study the formation of propylene oxide in the astrophysically
relevant conditions. Based on our results, a detailed radiative transfer
modeling has been carried out to propose some more transitions which would
potentially be targeted in the millimeter wave domain. Gas-grain chemical
network was used to explain the observed abundance of PrO in a cold shell
surrounding the high-mass star-forming region of Sgr B2. Quantum chemical
calculations were employed to study various reaction parameters and to compute
multiple vibrational frequencies of PrO. To model the formation of PrO in the
observed region, we considered a dark cloud model. Additionally, we used a
model to check the feasibility of forming PrO in the hot core region. Some
potential transitions in the millimeter wave domain are predicted which could
be useful for the future astronomical detection. Radiative transfer modeling
has been utilized to extract the physical condition which might be useful to
know the properties of the source in detail. Moreover, vibrational transitions
of PrO has been provided which could be very useful for the future detection of
PrO by the upcoming James Webb Space Telescope (JWST).Comment: 35 pages, 15 figures, Accepted for the publication in Astronomy and
Astrophysic
Chemical modeling for predicting the abundances of certain aldimines and amines in hot cores
We consider six isomeric groups (CH3N, CH5N, C2H5N, C2H7N, C3H7N and C3H9N)
to review the presence of amines and aldimines within the interstellar medium
(ISM). Each of these groups contains at least one aldimine or amine.
Methanimine (CH2NH) from CH3N and methylamine (CH3NH2) from CH5N isomeric group
were detected a few decades ago. Recently, the presence of ethanimine (CH3CHNH)
from C2H5N isomeric group has been discovered in the ISM. This prompted us to
investigate the possibility of detecting any aldimine or amine from the very
next three isomeric groups in this sequence: C2H7N, C3H7N and C3H9N. We employ
high-level quantum chemical calculations to estimate accurate energies of all
the species. According to enthalpies of formation, optimized energies, and
expected intensity ratio, we found that ethylamine (precursor of glycine) from
C2H7N isomeric group, (1Z)-1-propanimine from C3H7N isomeric group, and
trimethylamine from C3H9N isomeric group are the most viable candidates for the
future astronomical detection. Based on our quantum chemical calculations and
from other approximations (from prevailing similar types of reactions), a
complete set of reaction pathways to the synthesis of ethylamine and
(1Z)-1-propanimine is prepared. Moreover, a large gas-grain chemical model is
employed to study the presence of these species in the ISM. Our modeling
results suggest that ethylamine and (1Z)-1-propanimine could efficiently be
formed in hot-core regions and could be observed with present astronomical
facilities. Radiative transfer modeling is also implemented to additionally aid
their discovery in interstellar space.Comment: 32 pages, 18 Figures, Accepted for publication in the Astrophysical
Journa
Investigating the hot molecular core, G10.47+0.03: A pit of nitrogen-bearing complex organic molecules
Recent observations have shown that Nitrogen-bearing complex organic species
are present in large quantities in star-forming regions. Thus, investigating
the N-bearing species in a hot molecular core, such as G10.47+0.03, is crucial
to understanding the molecular complexity in star-forming regions. They also
allow us to investigate the chemical and physical processes that determine the
many phases during the structural and chemical evolution of the source in
star-forming regions. The aim of this study is to investigate the spatial
distribution and the chemical evolution states of N-bearing complex organic
molecules in the hot core G10.47+0.03. We used the ALMA archival data of the
hot molecular core G10.47+0.03. The extracted spectra were analyzed assuming
LTE. Furthermore, robust methods such as MCMC and rotational diagram methods
are implemented for molecules for which multiple transitions were identified to
constrain the temperature and column density. Finally, we used the Nautilus
gas-grain code to simulate the nitrogen chemistry in the hot molecular core. We
carried out both 0D and 1D simulations of the source and compared with
observational results. We report various transitions of nitrogen-bearing
species (NH2CN, HC3N, HC5N, C2H3CN, C2H5CN, and H2NCH2CN) together with some of
their isotopologues and isomers. Besides this, we also report the
identification of CH3CCH and one of its isotopologues. The emissions
originating from vinyl cyanide, ethyl cyanide, cyanoacetylene, and cyanamide
are compact, which could be explained by our astrochemical modeling. Our 0D
model shows that the chemistry of certain N-bearing molecules can be very
sensitive to initial local conditions such as density or dust temperature. In
our 1D model, simulated higher abundances of species such as HCN, HC3N, and
HC5N toward the inner shells of the source confirm the observational findings.Comment: 40 pages, 30 figure
Effect of Binding Energies on the Encounter Desorption
The abundance of interstellar ice constituents is usually expressed with respect to the water ice because, in denser regions, a significant portion of the interstellar grain surface would be covered by water ice. The binding energy (BE) or adsorption energy of the interstellar species regulates the chemical complexity of the interstellar grain mantle. Due to the high abundance of water ice, the BE of surface species with the water is usually provided and widely used in astrochemical modeling. However, the hydrogen molecules would cover some part of the grain mantle in the denser and colder part of the interstellar medium. Even at around similar to 10 K, few atoms and simple molecules with lower adsorption energies can migrate through the surface. The BE of the surface species with H-2 substrate would be very different from that of a water substrate. However, adequate information regarding these differences is lacking. Here, we employ the quantum chemical calculation to provide the BE of 95 interstellar species with H-2 substrate. These are representative of the BEs of species to a H-2 overlayer on a grain surface. On average, we notice that the BE with the H-2 monomer substrate is almost ten times lower than the BE of these species reported earlier with the H2O c-tetramer configuration. The encounter desorption of H and H-2 was introduced [with E-D (H, H-2) = 45 K and E-D (H-2, H-2) = 23 K] to have a realistic estimation of the abundances of the surface species in the colder and denser region. Our quantum chemical calculations yield higher adsorption energy of H-2 than that of H [E-D (H, H-2) = 23-25 K and E-D (H-2, H-2) = 67-79 K]. We further implement an astrochemical model to study the effect of encounter desorption with the present realistic estimation. The encounter desorption of the N atom [calculations yield E-D (N, H-2) = 83 K] is introduced to study the differences with its inclusion
Chemical evolution of some selected complex organic molecules in low-mass star-forming regions
The destiny of complex organic molecules (COMs) in star-forming regions is
interlinked with various evolutionary phases. Therefore, identifying these
species in diversified environments of identical star-forming regions would
help to comprehend their physical and chemical heritage. We identified multiple
COMs utilizing the Large Program `Astrochemical Surveys At IRAM' (ASAI) data,
dedicated to chemical surveys in Sun-like star-forming regions with the IRAM 30
m telescope. It was an unbiased survey in the millimetre regime, covering the
prestellar core, protostar, outflow region, and protoplanetary disk phase.
Here, we have reported some transitions of seven COMs, namely, methanol
(CH3OH), acetaldehyde (CH3CHO), methyl formate (CH3OCHO), ethanol (C2H5OH),
propynal (HCCCHO), dimethyl ether (CH3OCH3), and methyl cyanide (CH3CN) in some
sources L1544, B1-b, IRAS4A, and SVS13A. We found a trend among these species
from the derived abundances using the rotational diagram method and MCMC fit.
We have found that the abundances of all of the COMs, except for HCCCHO,
increase from the L1544 (prestellar core) and peaks at IRAS16293-2422 (class 0
phase). It is noticed that the abundance of these molecules correlate with the
luminosity of the sources. The obtained trend is also visible from the previous
interferometric observations and considering the beam dilution effect.Comment: 44 pages, 25 figures, and 12 tables. Accepted for the publication in
the Astrophysical Journa
NIR jets from a clustered region of massive star formation: Morphology and composition in the IRAS 18264-1152 region
Context. Massive stars play crucial roles in determining the physical and chemical evolution of galaxies. However, they form deeply embedded in their parental clouds, making it challenging to directly observe these stars and their immediate environments. It is known that accretion and ejection processes are intrinsically related, thus observing the massive protostellar outflows can provide crucial information about the processes governing massive star formation very close to the central engine. Aims. We aim to probe the IRAS 18264-1152 (also known as G19.88-0.53) high-mass star-forming complex in the near infrared (NIR) through its molecular hydrogen (H2) jets to analyse the morphology and composition of the line emitting regions and to compare with other outflow tracers. Methods. We observed the H2 NIR jets via K-band (1.9 2.5 μm) observations obtained with the integral field units VLT/SINFONI and VLT/KMOS. VLT/SINFONI provides the highest NIR angular resolution achieved so far for the central region of IRAS 18264-1152 (∼0.2). We compared the geometry of the NIR outflows with that of the associated molecular outflow, probed by CO (2-1) emission mapped with the Submillimeter Array. Results. We identify nine point sources in the SINFONI and KMOS fields of view. Four of these display a rising continuum in the K-band and are Brγ emitters, revealing that they are young, potentially jet-driving sources. The spectro-imaging analysis focusses on the H2 jets, for which we derived visual extinction, temperature, column density, area, and mass. The intensity, velocity, and excitation maps based on H2 emission strongly support the existence of a protostellar cluster in this region, with at least two (and up to four) different large-scale outflows, found through the NIR and radio observations. We compare our results with those found in the literature and find good agreement in the outflow morphology. This multi-wavelength comparison also allows us to derive a stellar density of ∼4000 stars pc-3. Conclusions. Our study reveals the presence of several outflows driven by young sources from a forming cluster of young, massive stars, demonstrating the utility of such NIR observations for characterising massive star-forming regions. Moreover, the derived stellar number density together with the geometry of the outflows suggest that stars can form in a relatively ordered manner in this cluster