| 1. |
- Cordiner, M. A., et al.
(författare)
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On the ubiquity of molecular anions in the dense interstellar medium
- 2013
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 1538-4357 .- 0004-637X. ; 770:1
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Tidskriftsartikel (refereegranskat)abstract
- Results are presented from a survey for molecular anions in seven nearby Galactic star-forming cores and molecular clouds. The hydrocarbon anion C6H- is detected in all seven target sources, including four sources where no anions have been previously detected: L1172, L1389, L1495B, and TMC-1C. The C6H-/C6H column density ratio is greater than or similar to 1.0% in every source, with a mean value of 3.0% (and standard deviation 0.92%). Combined with previous detections, our results show that anions are ubiquitous in dense clouds wherever C6H is present. The C6H-/C6H ratio is found to show a positive correlation with molecular hydrogen number density, and with the apparent age of the cloud. We also report the first detection of C4H- in TMC-1 (at 4.8 sigma confidence), and derive an anion-to-neutral ratio C4H-/C4H = (1.2 +/- 0.4) x 10(-5)(= 0.0012% +/- 0.0004%). Such a low value compared with C6H- highlights the need for a revised radiative electron attachment rate for C4H. Chemical model calculations show that the observed C4H- could be produced as a result of reactions of oxygen atoms with C5H- and C6H-.
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| 2. |
- Cordiner, M. A., et al.
(författare)
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Organic chemistry of low-mass star-forming cores I: 7 mm spectroscopy of Chamaeleon MMS1
- 2012
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 744, s. 131-
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Tidskriftsartikel (refereegranskat)abstract
- Observations are presented of emission lines from organic molecules at frequencies 32-50 GHz in the vicinity of Chamaeleon MMS1. This chemically rich dense cloud core harbors an extremely young, very low luminosity protostellar object and is a candidate first hydrostatic core. Column densities are derived and emission maps are presented for species including polyynes, cyanopolyynes, sulphuretted carbon chains, and methanol. The polyyne emission peak lies about 5000 AU from the protostar, whereas methanol peaks about 15,000 AU away. Averaged over the telescope beam, the molecular hydrogen number density is calculated to be 106 cm–3 and the gas kinetic temperature is in the range 5-7 K. The abundances of long carbon chains are very large and are indicative of a non-equilibrium carbon chemistry; C6H and HC7N column densities are 5.9+2.9 –1.3 × 1011 cm–2 and 3.3+8.0 –1.5 × 1012 cm–2, respectively, which are similar to the values found in the most carbon-chain-rich protostars and prestellar cores known, and are unusually large for star-forming gas. Column density upper limits were obtained for the carbon-chain anions C4H– and C6H–, with anion-to-neutral ratios [C4H–]/[C4H]
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| 3. |
- Meech, K. J., et al.
(författare)
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EPOXI: Comet 103P/Hartley 2 Observations from a Worldwide Campaign
- 2011
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Ingår i: Astrophysical Journal Letters. - London : IOP. - 2041-8213 .- 2041-8205. ; 734:L1, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Earth- and space-based observations provide synergistic information for space mission encounters by providing data over longer timescales, at different wavelengths and using techniques that are impossible with an in situ flyby. We report here such observations in support of the EPOXI spacecraft flyby of comet 103P/Hartley 2. The nucleus is small and dark, and exhibited a very rapidly changing rotation period. Prior to the onset of activity, the period was ~16.4?hr. Starting in 2010 August the period changed from 16.6?hr to near 19?hr in December. With respect to dust composition, most volatiles and carbon and nitrogen isotope ratios, the comet is similar to other Jupiter-family comets. What is unusual is the dominance of CO 2 -driven activity near perihelion, which likely persists out to aphelion. Near perihelion the comet nucleus was surrounded by a large halo of water-ice grains that contributed significantly to the total water production.
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| 4. |
- Wirström, Eva, 1977, et al.
(författare)
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COLD WATER VAPOR IN THE BARNARD 5 MOLECULAR CLOUD
- 2014
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 788:2
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Tidskriftsartikel (refereegranskat)abstract
- After more than 30 yr of investigations, the nature of gas-grain interactions at low temperatures remains an unresolved issue in astrochemistry. Water ice is the dominant ice found in cold molecular clouds; however, there is only one region where cold (similar to 10 K) water vapor has been detected-L1544. This study aims to shed light on ice desorption mechanisms under cold cloud conditions by expanding the sample. The clumpy distribution of methanol in dark clouds testifies to transient desorption processes at work-likely to also disrupt water ice mantles. Therefore, the Herschel HIFI instrument was used to search for cold water in a small sample of prominent methanol emission peaks. We report detections of the ground-state transition of o-H2O (J = 1(10)-1(01)) at 556.9360 GHz toward two positions in the cold molecular cloud, Barnard 5. The relative abundances of methanol and water gas support a desorption mechanism which disrupts the outer ice mantle layers, rather than causing complete mantle removal.
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| 5. |
- Wirström, Eva, 1977, et al.
(författare)
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Depletion Cores - the O2 hideout?
- 2013
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Ingår i: The Universe Explored by Herschel, 15 – 18 October 2013, Noordwijk, The Netherlands.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Molecular oxygen has proven to be the most elusive molecule in theinterstellar medium. Despite the fact that it in theory forms easily in both warm and cold dense gas, extensive searches with SWAS, Odin and Herschel have only resulted in detections in a handful of sources. In addition, upper limits in various astronomical environments are at levels of 1000 times less abundant than predicted by chemical models. This situation requires either for atomic carbon to be abundant enough to suppress the O2 by CO formation, or for atomic oxygen to accrete onto grains and remain bound there. However, the binding energies of atoms to grains are highly uncertain and high abundances of OI in depleted gas have both been directly observed and inferred from observations of other molecules. A possible explanation is that OI is bound to grains by fixing (get hydrogenated to form ices) rather than sticking (van der Waals bonding to the surface potential), which will become less efficient in high density gas. We will present a stochastic gas-grain model including the kinetics of OI fixing - demonstrating the possibility of elevated O2 abundances at times when CO is significantly depleted - as well as results from searches for O2 emission in a small sample of starless depletion cores using Herschel HIFI.
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| 6. |
- Wirström, Eva, 1977, et al.
(författare)
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ISOTOPIC ANOMALIES IN PRIMITIVE SOLAR SYSTEM MATTER: SPIN-STATE-DEPENDENT FRACTIONATION OF NITROGEN AND DEUTERIUM IN INTERSTELLAR CLOUDS
- 2012
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Ingår i: Astrophysical Journal. - 1538-4357 .- 0004-637X. ; 757:1, s. L11-
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Tidskriftsartikel (refereegranskat)abstract
- Organic material found in meteorites and interplanetary dust particles is enriched in D and 15N. This is consistentwith the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed byion–molecule chemistry in the protosolar nebula. Theoretical models of interstellar fractionation at lowtemperaturespredict large enrichments in both D and 15N and can account for the largest isotopic enrichments measured incarbonaceous meteorites. However, more recent measurements have shown that, in some primitive samples, a large15N enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, ifany, 15N enrichment. By considering the spin-state dependence in ion–molecule reactions involving the ortho andpara forms of H2, we show that ammonia and related molecules can exhibit such a wide range of fractionation forboth 15N and D in dense cloud cores.We also show that while the nitriles, HCN and HNC, contain the greatest 15Nenrichment, this is not expected to correlate with extreme D enrichment. These calculations therefore support theview that solar system 15N and D isotopic anomalies have an interstellar heritage. We also compare our results toexisting astronomical observations and briefly discuss future tests of this model.
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| 7. |
- Wirström, Eva, 1977, et al.
(författare)
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Observations of Carbon Isotopic Fractionation in Interstellar Formaldehyde
- 2012
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Ingår i: 43rd Lunar and Planetary Science Conference. ; 43, s. 1611-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- While solar system organics exhibit small fluctuations in δ^1^3C as compared to δ^1^5N and δD, an interstellar origin cannot be excluded. This study off ^1^3C fractionation in the cold, dense ISM reveals an enrichment in H_2CO that remains to be explained.
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| 8. |
- Wirström, Eva, 1977, et al.
(författare)
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Using Methanol Beacons to Find Water in the Dark
- 2013
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Ingår i: The Universe Explored by Herschel, 15 – 18 October 2013, Noordwijk, The Netherlands.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Interstellar methanol is only formed efficiently from hydrogenation of COmolecules accreted onto grains, and icy grain mantles are observed to consist of 1-30% methanol relative to water. In regions of both low and high mass star formation gas-phase methanol abundances are consistent with partial or complete removal of the ices, either by thermal evaporation or by shock-induced sputtering in outflows. However, the widespread presence of gas-phase methanol in molecular clouds attests to some non-thermal desorption process at work. In particular, distinct peaks of methanol emission at positions significantly offset from protostellar activity implies a transient desorption process, such as clump-clump collisions, rather than a continuous one like photodesorption. Such processes are likely to disrupt a major part of the ice mantles and lead to high gas-phase water abundances clearly distinguishable from what is expected from photodesorption or steady-state gas-phase chemistry.We will report on the first detection of gas-phase water in a cold dark cloud - well offset from protostellar activity - resulting from a small scale survey with Herschel HIFI towards methanol peaks. Physical properties of the sources as well as implications for mantle desorption mechanisms and chemistry in dark clouds will be discussed and compared to those of active star formation.
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