638 research outputs found
Molecular-Cloud-Scale Chemical Composition I: Mapping Spectral Line Survey toward W51 in the 3 mm Band
We have conducted a mapping spectral line survey toward the Galactic giant
molecular cloud W51 in the 3 mm band with the Mopra 22 m telescope in order to
study an averaged chemical composition of the gas extended over a molecular
cloud scale in our Galaxy. We have observed the area of , which
corresponds to 39 pc 47 pc. The frequency ranges of the observation
are 85.1 - 101.1 GHz and 107.0 - 114.9 GHz. In the spectrum spatially averaged
over the observed area, spectral lines of 12 molecular species and 4 additional
isotopologues are identified. An intensity pattern of the spatially-averaged
spectrum is found to be similar to that of the spiral arm in the external
galaxy M51, indicating that these two sources have similar chemical
compositions. The observed area has been classified into 5 sub-regions
according to the integrated intensity of CO() (), and contributions of the fluxes of 11 molecular lines from each
sub-region to the averaged spectrum have been evaluated. For most of molecular
species, 50 % or more of the flux come from the sub-regions with from 25 K km s to 100 K km s, which does not involve
active star forming regions. Therefore, the molecular-cloud-scale spectrum
observed in the 3 mm band hardly represents the chemical composition of star
forming cores, but mainly represents the chemical composition of an extended
quiescent molecular gas. The present result constitutes a sound base for
interpreting the spectra of external galaxies at a resolution of a molecular
cloud scale ( pc) or larger.Comment: Accepted for publication in Ap
Surface Diffusion of Carbon Atoms as a Driver of Interstellar Organic Chemistry
Many interstellar complex organic molecules (COMs) are believed to be
produced on the surfaces of icy grains at low temperatures. Atomic carbon is
considered responsible for the skeletal evolution processes, such as C-C bond
formation, via insertion or addition reactions. Before reactions, C atoms must
diffuse on the surface to encounter reaction partners; therefore, information
on their diffusion process is critically important for evaluating the role of C
atoms in the formation of COMs. In situ detection of C atoms on ice was
achieved by a combination of photostimulated desorption and resonance enhanced
multiphoton ionization methods. We found that C atoms weakly bound to the ice
surface diffused approximately above 30 K and produced C2 molecules. The
activation energy for C-atom surface diffusion was experimentally determined to
be 88 meV (1,020 K), indicating that the diffusive reaction of C atoms is
activated at approximately 22 K on interstellar ice. The facile diffusion of C
at T > 22 K atoms on interstellar ice opens a previously overlooked chemical
regime where the increase in complexity of COMs as driven by C atoms. Carbon
addition chemistry can be an alternative source of chemical complexity in
translucent clouds and protoplanetary disks with crucial implications in our
current understanding on the origin and evolution of organic chemistry in our
Universe.Comment: 33 pages (main + SI), 14 figures, 1 tabl
Depletion of 15N in the center of L1544: Early transition from atomic to molecular nitrogen?
We performed sensitive observations of the N15ND+(1-0) and 15NND+(1-0) lines
toward the prestellar core L1544 using the IRAM 30m telescope. The lines are
not detected down to 3 sigma levels in 0.2 km/s channels of around 6 mK. The
non-detection provides the lower limit of the 14N/15N ratio for N2D+ of
~700-800, which is much higher than the elemental abundance ratio in the local
ISM of ~200-300. The result indicates that N2 is depleted in 15N in the central
part of L1544, because N2D+ preferentially traces the cold dense gas, and
because it is a daughter molecule of N2. In-situ chemistry is unlikely
responsible for the 15N depletion in N2; neither low-temperature gas phase
chemistry nor isotope selective photodissociation of N2 explains the 15N
depletion; the former prefers transferring 15N to N2, while the latter requires
the penetration of interstellar FUV photons into the core center. The most
likely explanation is that 15N is preferentially partitioned into ices compared
to 14N via the combination of isotope selective photodissociation of N2 and
grain surface chemistry in the parent cloud of L1544 or in the outer regions of
L1544 which are not fully shielded from the interstellar FUV radiation. The
mechanism is the most efficient at the chemical transition from atomic to
molecular nitrogen. In other words, our result suggests that the gas in the
central part of L1544 already went trough the transition from atomic to
molecular nitrogen in the earlier evolutionary stage, and that N2 is currently
the primary form of gas-phase nitrogen.Comment: 5 pages, 2 figures, 2 tables, Accepted for publication in A&A Letter
Diffusion activation energy and desorption activation energy for astrochemically relevant species on water ice show no clear relation
The activation energy for desorption (Edes) and that for surface diffusion
(Esd) of adsorbed molecules on dust grains are two of the most important
parameters for the chemistry in the interstellar medium. Although Edes is often
measured by laboratory experiments, the measurement of Esd is sparse. Due to
the lack of data, astrochemical models usually assume a simple scaling
relation, Esd = fEdes, where f is a constant, irrespective of adsorbed species.
Here, we experimentally measure Esd for CH4, H2S, OCS, CH3OH, and CH3CN on
water-ice surfaces using an ultra-high-vacuum transmission electron microscope
(UHV-TEM). Compiling the measured Esd values and Edes values from the
literature, we find that the value of f ranges from ~0.2 to ~0.7, depending on
the species. Unless f (or Esd) for the majority of species is available, a
natural alternative approach for astrochemical models is running multiple
simulations, varying f for each species randomly. In this approach, ranges of
molecular abundances predicted by multiple simulations, rather than abundances
predicted by each simulation, are important. We here run 10,000 simulations of
astrochemical models of molecular clouds and protostellar envelopes, randomly
assigning a value of f for each species. In the former case, we identify
several key species whose Esd most strongly affects the uncertainties of the
model predictions; Esd for those species should be investigated in future
laboratory and quantum chemical studies. In the latter case, uncertainties in
the Esd of many species contribute to the uncertainties in the model
predictions.Comment: Accepted for publication in ApJ
- …