661 research outputs found
Clouds, Clumps, Cores & Comets - a Cosmic Chemical Connection?
We discuss the connection between the chemistry of dense interstellar clouds
and those characteristics of cometary matter that could be remnants of it. The
chemical evolution observed to occur in molecular clouds is summarized and a
model for dense core collapse that can plausibly account for the isotopic
fractionation of hydrogen, nitrogen, oxygen and carbon measured in primitive
solar system materials is presented.Comment: to be published in Advances in Geoscience
Isotopic Anomalies in Primitive Solar System Matter: Spin-state Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds
Organic material found in meteorites and interplanetary dust particles is
enriched in D and 15N. This is consistent with the idea that the functional
groups carrying these isotopic anomalies, nitriles and amines, were formed by
ion-molecule chemistry in the protosolar nebula. Theoretical models of
interstellar fractionation at low temperatures predict large enrichments in
both D and 15N and can account for the largest isotopic enrichments measured in
carbonaceous meteorites. However, more recent measurements have shown that, in
some primitive samples, a large 15N enrichment does not correlate with one in
D, and that some D-enriched primitive material displays little, if any, 15N
enrichment. By considering the spin-state dependence in ion-molecule reactions
involving the ortho and para forms of H2, we show that ammonia and related
molecules can exhibit such a wide range of fractionation for both 15N and D in
dense cloud cores. We also show that while the nitriles, HCN and HNC, contain
the greatest 15N enrichment, this is not expected to correlate with extreme D
enrichment. These calculations therefore support the view that Solar System 15N
and D isotopic anomalies have an interstellar heritage. We also compare our
results to existing astronomical observations and briefly discuss future tests
of this model.Comment: Submitted to ApJ
Observations of Isotope Fractionation in Prestellar Cores: Interstellar Origin of Meteoritic Hot Spot?
Isotopically fractionated material is found in many solar system objects, including meteorites and comets. It is thought, in some cases, to trace interstellar material that was incorporated into the solar system without undergoing significant processing. Here, we show the results of models and observations of the nitrogen and carbon fractionation in proto-stellar cores
Observations of Nitrogen Fractionation in Prestellar Cores: Nitriles Tracing Interstellar Chemistry
Primitive materials provide important clues on the processes that occurred during the formation and early evolution of the Solar System. Space-based and ground-based observations of cometary comae show that comets appear to contain a mixture of the products of both interstellar and nebular chemistries. Significant 15-nitrogen enrichments have been measured in CN and HCN towards a number of comets and may suggest an origin of interstellar chemical fractionation. Additionally, large N-15 enhancements are found in meteorites and has also led to to the view that the N-15 traces material formed in the interstellar medium (ISM), although multiple sources cannot be excluded. Here, we show the results of observations of the nitrogen and carbon fractionation in prestellar cores for various N-bearing species to decipher the origin of primitive material isotopic enrichments
Models for Cometary Comae Containing Negative Ions
The presence of negative ions (anions) in cometary comae is known from Giotto mass spectrometry of IP/Halley. The anions O(-), OH(-), C(-), CH(-) and CN(-) have been detected, as well as unidentified anions with masses 22-65 and 85-110 amu [I]. Organic molecular anions such as C4H(-) and C6H(-) are known to have a significant impact on the charge balance of interstellar clouds and circumstellar envelopes and have been shown to act as catalysts for the gas phase synthesis of larger hydrocarbon molecules in the ISM, but their importance in cometary comae has not yet been fully explored. We present details of our new models for the chemistry of cometary comae that include atomic and molecular anions. We calculate the impact of these anions on the charge balance and examine their importance for cometary coma chemistry
Chemical chronology of the Southern Coalsack
We demonstrate how the observed H2O ice column densities toward three dense
globules in the Southern Coalsack could be used to constrain the ages of these
sources. We derive ages of ~10^5 yr, in agreement with dynamical studies of
these objects. We have modelled the chemical evolution of the globules, and
show how the molecular abundances are controlled by both the gas density and
the initial chemical conditions as the globules formed. Based on our derived
ages, we predict the column densities of several species of interest. These
predictions should be straightforward to test by performing molecular line
observationsComment: 10 pages, 4 figures, in press at MNRA
Ammonia Imaging of the Disks in the NGC 1333 IRAS 4A Protobinary System
The NGC 1333 IRAS 4A protobinary was observed in the ammonia (2, 2) and (3,
3) lines and in the 1.3 cm continuum with a high resolution (about 1.0 arcsec).
The ammonia maps show two compact sources, one for each protostar, and they are
probably protostellar accretion disks. The disk associated with IRAS 4A2 is
seen nearly edge-on and shows an indication of rotation. The A2 disk is
brighter in the ammonia lines but dimmer in the dust continuum than its sibling
disk, with the ammonia-to-dust flux ratios different by about an order of
magnitude. This difference suggests that the twin disks have surprisingly
dissimilar characters, one gas-rich and the other dusty. The A2 disk may be
unusually active or hot, as indicated by its association with water vapor
masers. The existence of two very dissimilar disks in a binary system suggests
that the formation process of multiple systems has a controlling agent lacking
in the isolated star formation process and that stars belonging to a multiple
system do not necessarily evolve in phase with each other
Hydrogen Isocyanide in Comet 73P/Schwassmann-Wachmann (Fragment B)
We present a sensitive 3-sigma upper limit of 1.1% for the HNC/HCN abundance
ratio in comet 73P/Schwassmann-Wachmann (Fragment B), obtained on May 10-11,
2006 using Caltech Submillimeter Observatory (CSO). This limit is a factor of
~7 lower than the values measured previously in moderately active comets at 1
AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile
species, except of HCN. The low HNC/HCN ratio thus argues against HNC
production from polymers produced from HCN. However, thermal degradation of
macromolecules, or polymers, produced from ammonia and carbon compounds, such
as acetylene, methane, or ethane appears a plausible explanation for the
observed variations of the HNC/HCN ratio in moderately active comets, including
the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar
polymers have been invoked previously to explain anomalous 14N/15N ratios
measured in cometary CN.Comment: 6 pages, 5 figures, 2 table
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