464 research outputs found
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
Spin-State-Dependent Ion-Molecule Chemistry as the Origin of N-15 and D Isotopic Anomalies in Primitive Matter.
Many meteoritic and interplanetary dust particle (IDP) samples contain bulk enhancements and hotspots rich in N-15. Similarly low C(14)N/C(15)N ratios have been observed in numerous comets, An almost constant enrichment factor in comets from disti'nct formation zones in the nebular disk (i.e. both Jupiter Family and Oort Cloud comets), strongly suggests that this fractionation is primordial and was set in the protsolar cloud core. Deuterium enrichment is observed in both meteorites and IDP
Measuring molecular abundances in comet C/2014 Q2 (Lovejoy) using the APEX telescope
Comet composition provides critical information on the chemical and physical
processes that took place during the formation of the Solar system. We report
here on millimetre spectroscopic observations of the long-period bright comet
C/2014 Q2 (Lovejoy) using the Atacama Pathfinder Experiment (APEX) band 1
receiver between 2015 January UT 16.948 to 18.120, when the comet was at
heliocentric distance of 1.30 AU and geocentric distance of 0.53 AU. Bright
comets allow for sensitive observations of gaseous volatiles that sublimate in
their coma. These observations allowed us to detect HCN, CH3OH (multiple
transitions), H2CO and CO, and to measure precise molecular production rates.
Additionally, sensitive upper limits were derived on the complex molecules
acetaldehyde (CH3CHO) and formamide (NH2CHO) based on the average of the
strongest lines in the targeted spectral range to improve the signal-to-noise
ratio. Gas production rates are derived using a non-LTE molecular excitation
calculation involving collisions with H2O and radiative pumping that becomes
important in the outer coma due to solar radiation. We find a depletion of CO
in C/2014 Q2 (Lovejoy) with a production rate relative to water of 2 per cent,
and relatively low abundances of Q(HCN)/Q(H2O), 0.1 per cent, and
Q(H2CO)/Q(H2O), 0.2 per cent. In contrast the CH3OH relative abundance
Q(CH3OH)/Q(H2O), 2.2 per cent, is close to the mean value observed in other
comets. The measured production rates are consistent with values derived for
this object from other facilities at similar wavelengths taking into account
the difference in the fields of view. Based on the observed mixing ratios of
organic molecules in four bright comets including C/2014 Q2, we find some
support for atom addition reactions on cold dust being the origin of some of
the molecules.Comment: 10 pages, 7 figures, to be published in MNRA
Nucleobases and other Prebiotic Species from the Ultraviolet Irradiation of Pyrimidine in Astrophysical Ices
Nucleobases are N-heterocycles that are the informational subunits of DNA and RNA, and are divided into two families: pyrimidine bases (uracil, cytosine, and thymine) and purine bases (adenine and guanine). Nucleobases have been detected in meteorites and their extraterrestrial origin confirmed by isotope measurement. Although no Nheterocycles have ever been observed in the ISM, the positions of the 6.2-m interstellar emission features suggest a population of such molecules is likely to be present. In this work we study the formation of pyrimidine-based molecules, including nucleobases, as well as other species of prebiotic interest, from the ultraviolet (UV) irradiation of pyrimidine in combinations of H2O, NH3, CH3OH, and CH4 ices at low temperature, in order to simulate the astrophysical conditions under which prebiotic species may be formed in the interstellar medium and icy bodies of the Solar System. Experimental: Gas mixtures are prepared in a glass mixing line (background pressure approx. 10(exp -6)-10(exp -5) mbar). Relative proportions between mixture components are determined by their partial pressures. Gas mixtures are then deposited on an aluminum foil attached to a cold finger (15-20 K) and simultaneously irradiated with an H2 lamp emitting UV photons (Lyman and a continuum at approx.160 nm). After irradiation samples are warmed to room temperature, at which time the remaining residues are recovered to be analyzed with liquid and gas chromatographies. Results: These experiments showed that the UV irradiation of pyrimidine mixed in these ices at low temperature leads to the formation of several photoproducts derived from pyrimidine, including the nucleobases uracil and cytosine, as well as their precursors 4(3H)-pyrimidone and 4-aminopyrimidine (Fig. 1). Theoretical quantum calculations on the formation of 4(3H)-pyrimidone and uracil from the irradiation of pyrimidine in pure H2O ices are in agreement with their experimental formation pathways. In those residues, other species of prebiotic interest such as urea and the amino acids glycine and alanine could also be identified. However, no pyrimidine derivatives containing CH3 groups, including the third nucleobase thymine, could be identified, suggesting that the addition of methyl groups to pyrimidine is not an efficient process
Formation of Nucleobases from the UV Photo-Irradiation of Pyrimidine in Astrophysical Ice Analogs
Astrochemistry laboratory simulations have shown that complex organic molecules including compounds of astrobiological interest can be formed under interstellarl/circumstellar conditions from the vacuum UV irradiation of astrophysical ice analogs containing H2O, CO, CO2, CH3OH, NH13, etc. Of all prebiotic compounds, the formation of amino acids under such experimental conditions has been the most extensively studied. Although the presence of amino acids in the interstellar medium (ISM) has yet to be confirmed, they have been detected in meteorites, indicating that biomolecules and/or their precursors can be formed under extraterrestrial, abiotic conditions. Nucleobases, the building blocks of DNA and RNA, as well as other 1V-heterocycles, have also been detected in meteorites, but like amino acids, they have yet to be observed in the ISM. In this work, we present an experimental study of the formation of pyrimidine-based compounds from the UV photo-irradiation of pyrimidine in ice mixtures containing H2O, NH3, and/or CH3OH at low temperature and pressure
Resident Corneal Cells Communicate with Neutrophils Leading to the Production of IP-10 during the Primary Inflammatory Response to HSV-1 Infection
In this study we show that murine and human neutrophils are capable of secreting IP-10 in response to communication from the HSV-1 infected cornea and that they do so in a time frame associated with the recruitment of CD8+ T cells and CXCR3-expressing cells. Cellular markers were used to establish that neutrophil influx corresponded in time to peak IP-10 production, and cellular depletion confirmed neutrophils to be a significant source of IP-10 during HSV-1 corneal infection in mice. A novel ex vivo model for human corneal tissue infection with HSV-1 was used to confirm that cells resident in the cornea are also capable of stimulating neutrophils to secrete IP-10. Our results support the hypothesis that neutrophils play a key role in T-cell recruitment and control of viral replication during HSV-1 corneal infection through the production of the T-cell recruiting chemokine IP-10
On the nature of the enigmatic object IRAS 19312+1950: A rare phase of massive star formation?
IRAS 19312+1950 is a peculiar object that has eluded firm characterization
since its discovery, with combined maser properties similar to an evolved star
and a young stellar object (YSO). To help determine its true nature, we
obtained infrared spectra of IRAS 19312+1950 in the range 5-550 m using
the Herschel and Spitzer space observatories. The Herschel PACS maps exhibit a
compact, slightly asymmetric continuum source at 170 m, indicative of a
large, dusty circumstellar envelope. The far-IR CO emission line spectrum
reveals two gas temperature components: of material at
K, and of material at K. The OI 63
m line is detected on-source but no significant emission from atomic ions
was found. The HIFI observations display shocked, high-velocity gas with
outflow speeds up to 90 km s along the line of sight. From Spitzer
spectroscopy, we identify ice absorption bands due to HO at 5.8 m and
CO at 15 m. The spectral energy distribution is consistent with a
massive, luminous () central source surrounded by a
dense, warm circumstellar disk and envelope of total mass
-, with large bipolar outflow cavities. The combination
of distinctive far-IR spectral features suggest that IRAS 19312+1950 should be
classified as an accreting high-mass YSO rather than an evolved star. In light
of this reclassification, IRAS 19312+1950 becomes only the 5th high-mass
protostar known to exhibit SiO maser activity, and demonstrates that 18 cm OH
maser line ratios may not be reliable observational discriminators between
evolved stars and YSOs.Comment: 16 pages. Accepted for publication in Ap
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