3,421 research outputs found

    Classical and Quantum-Mechanical Calculations of HCO^+ + e ‚Üí CO(v) + H

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    It has been suggested that interstellar CO produced in the dissociative recombination between HCO+ and electrons can be detected via infrared emission from excited vibrational levels. Using classical and quantum mechanical equations of motion, we have calculated the distribution of CO product vibrational states for the dissociative recombination reaction between HCO+ and electrons. We have assumed the dissociation to occur along a purely linear geometry. The CO vibrational distribution has been calculated for two electronic states: (1) the ground X1ő£^+ state and (2) the excited a ^3ő† state. For the X state, we have used a newly calculated ab initio potential surface for the dissociation of HCO. Results for the a state can be compared with the results of experimental work by Adams and collaborators. Our results show that in general vibrational excitation of the CO product is not large, so that detection of infrared emission from interstellar clouds will not be facile

    Theoretical studies of photoexcitation and ionization in H_2O

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    Theoretical studies are reported of the complete dipole excitation and ionization spectrum in H_2O employing Franck‚ÄďCondon and static‚Äźexchange approximations. Large Cartesian Gaussian basis sets are used to represent the required discrete and continuum electronic eigenfunctions at the ground‚Äźstate equilibrium geometry, and previously devised moment‚Äźtheory techniques are employed in constructing the continuum oscillator‚Äźstrength densities from the calculated spectra. Detailed comparisons are made of the calculated excitation and ionization profiles with recent experimental photoabsorption studies and corresponding spectral assignments, electron impact‚Äďexcitation cross sections, and dipole (e,‚ÄČ2e)/(e,‚ÄČe+ion) and synchrotron‚Äźradiation studies of partial‚Äźchannel photoionization cross sections. The various calculated excitation series in the outer‚Äźvalence (1b(^‚ąí1)_1, 3a(^‚ąí1)_1, 1b(^‚ąí1)_2) region are found to include contributions from valence‚Äźlike 2b_2‚ÄČ(ŌÉ*) and 4a_1(ő≥*) virtual orbitals, as well as appropriate nsa_1, npa_1, nda_1, npb_1, npb_2, ndb_1, ndb_2, and nda_2 Rydberg states. Transition energies and intensities in the ‚ąľ7 to 19 eV interval obtained from the present studies are seen to be in excellent agreement with the measured photoabsorption cross section, and to provide a basis for detailed spectral assignments. The calculated (1b(^‚ąí1)_1)X(^‚ÄČ2)B_1, (3a_1(^‚ąí1))^2A_1, and (1b_2(^‚ąí1))(^2)B_2 partial‚Äźchannel cross sections are found to be largely atomic‚Äźlike and dominated by 2p‚Üíkd components, although the 2b_2(ŌÉ*) orbital gives rise to resonance‚Äźlike contributions just above threshold in the 3a_1‚Üíkb_2 and 1b_2‚Üíkb_2 channels. It is suggested that the latter transition couples with the underlying 1b_1‚Üíkb_1 channel, accounting for a prominent feature in the recent high‚Äźresolution synchrotron‚Äźradiation measurements. When this feature is taken into account, the calculations of the three outer‚Äźvalence channels are in excellent accord with recent synchrotron‚Äźradiation and dipole (e,‚ÄČ2e) photoionization cross‚Äźsectional measurements. The calculated inner‚Äźvalence (2a_1(^‚ąí1)) cross section is also in excellent agreement with corresponding measured values, although proper account must be taken of the appropriate final‚Äźstate configuration‚Äźmixing effects that give rise to a modest failure of the Koopmans approximation, and to the observed broad PES band, in this case. Finally, the origins of the various spectral features present in the measured 1a_1 oxygen K‚Äźedge electron energy‚Äźloss profile in H_2O are seen to be clarified fully by the present calculations

    Vortex nucleation in rotating BEC: the role of the boundary condition for the order parameter

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    We study the process of vortex nucleation in rotating two-dimensional BEC confined in a harmonic trap. We show that, within the Gross-Pitaevskii theory with the boundary condition of vanishing of the order parameter at infinity, topological defects nucleation occurs via the creation of vortex-antivortex pairs far from the cloud center, where the modulus of the order parameter is small. Then, vortices move towards the center of the cloud and antivortices move in the opposite direction but never disappear. We also discuss the role of surface modes in this process.Comment: 6 pages, 2 figure

    Antecedent lake conditions shape resistance and resilience of a shallow lake ecosystem following extreme wind storms

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    Extreme wind storms can strongly influence short-term variation in lake ecosystem functioning. Climate change is affecting storms by altering their frequency, duration, and intensity, which may have consequences for lake ecosystem resistance and resilience. However, catchment and lake processes are simultaneously affecting antecedent lake conditions which may shape the resistance and resilience landscape prior to storm exposure. To determine whether storm characteristics or antecedent lake conditions are more important for explaining variation in lake ecosystem resistance and resilience, we analyzed the effects of 25 extreme wind storms on various biological and physiochemical variables in a shallow lake. Using boosted regression trees to model observed variation in resistance and resilience, we found that antecedent lake conditions were more important (relative importance = 67%) than storm characteristics (relative importance = 33%) in explaining variation in lake ecosystem resistance and resilience. The most important antecedent lake conditions were turbidity, Schmidt stability, %O2 saturation, light conditions, and soluble reactive silica concentrations. We found that storm characteristics were all similar in their relative importance and results suggest that resistance and resilience decrease with increasing duration, mean precipitation, shear stress intensity, and time between storms. In addition, we found that antagonistic or opposing effects between the biological and physiochemical variables influence the overall resistance and resilience of the lake ecosystem under specific lake and storm conditions. The extent to which these results apply to the resistance and resilience of different lake ecosystems remains an important area for inquiry

    Spatially-Resolved Spectra of the "Teacup" AGN: Tracing the History of a Dying Quasar

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    The Sloan Digital Sky Survey (SDSS) Galaxy Zoo project has revealed a number of spectacular galaxies possessing Extended Emission-Line Regions (EELRs), the most famous being Hanny's Voorwerp galaxy. We present another EELR object discovered in the SDSS endeavor: the Teacup Active Galactic Nucleus (AGN), nicknamed for its EELR, which has a handle like structure protruding 15 kpc into the northeast quadrant of the galaxy. We analyze physical conditions of this galaxy with long-slit ground based spectroscopy from Lowell, Lick, and KPNO observatories. With the Lowell 1.8m Perkin's telescope we took multiple observations at different offset positions, allowing us to recover spatially resolved spectra across the galaxy. Line diagnostics indicate the ionized gas is photoionized primarily by the AGN. Additionally we are able to derive the hydrogen density from the [S II] 6716/6731 ratio. We generated two-component photoionization models for each spatially resolved Lowell spectrum. These models allow us to calculate the AGN bolometric luminosity seen by the gas at different radii from the nuclear center of the Teacup. Our results show a drop in bolometric luminosity by more than two orders of magnitude from the EELR to the nucleus, suggesting that the AGN has decreased in luminosity by this amount in a continuous fashion over 46,000 years, supporting the case for a dying AGN in this galaxy independent of any IR based evidence. We demonstrate that spatially resolved photoionization modeling could be applied to EELRs to investigate long time scale variability.Comment: 38 pages, 11 figures, accepted for publication in the Astrophysical Journa

    On The Reddening in X-ray Absorbed Seyfert 1 Galaxies

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    There are several Seyfert galaxies for which there is a discrepancy between the small column of neutral hydrogen deduced from X-ray observations and the much greater column derived from the reddening of the optical/UV emission lines and continuum. The standard paradigm has the dust within the highly ionized gas which produces O~VII and O~VIII absorption edges (i.e., a ``dusty warm absorber''). We present an alternative model in which the dust exists in a component of gas in which hydrogen has been stripped, but which is at too low an ionization state to possess significant columns of O~VII and O~VIII (i.e, a ``lukewarm absorber''). The lukewarm absorber is at sufficient radial distance to encompass much of the narrow emission-line region, and thus accounts for the narrow-line reddening, unlike the dusty warm absorber. We test the model by using a combination of photoionization models and absorption edge fits to analyze the combined ROSAT/ASCA dataset for the Seyfert 1.5 galaxy, NGC 3227. We show that the data are well fit by a combination of the lukewarm absorber and a more highly ionized component similar to that suggested in earlier studies. We predict that the lukewarm absorber will produce strong UV absorption lines of N V, C IV, Si IV and Mg II. Finally, these results illustrate that singly ionized helium is an important, and often overlooked, source of opacity in the soft X-ray band (100 - 500 eV).Comment: 17 pages, Latex, includes 1 figure (encapsulated postscript), one additional table in Latex (landscape format), to appear in the Astrophysical Journa

    Propagation of defects in doped magnetic materials of different dimensionality

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    Defects intentionally introduced into magnetic materials often have a profound effect on the physical properties. Specifically tailored neutron spectroscopic experiments can provide detailed information on both the local exchange interactions and the local distances between the magnetic atoms around the defects. This is demonstrated for manganese dimer excitations observed for the magnetically diluted three- and two-dimensional compounds KMn(x)Zn(1-x)F(3) and K(2)Mn(x)Zn(1-x)F(4), respectively. The resulting local exchange interactions deviate up to 10% from the average, and the local Mn-Mn distances are found to vary stepwise with increasing internal pressure due to the Mn/Zn substitution. Our analysis qualitatively supports the theoretically predicted decay of atomic displacements according to 1/r**2, 1/r, and constant (for three-, two-, and one-dimensional compounds, respectively) where r denotes the distance of the displaced atoms from the defect.Comment: 21 pages, 4 figures, 3 table

    VLA OH and H I Zeeman Observations of the NGC 6334 Complex

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    We present OH and H I Zeeman observations of the NGC 6334 complex taken with the Very Large Array. The OH absorption profiles associated with the complex are relatively narrow (del-v_FWHM ~ 3 km s^1) and single-peaked over most of the sources. The H I absorption profiles contain several blended velocity components. One of the compact continuum sources in the complex (source A) has a bipolar morphology. The OH absorption profiles toward this source display a gradient in velocity from the northern continuum lobe to the southern continuum lobe; this velocity gradient likely indicates a bipolar outflow of molecular gas from the central regions to the northern and southern lobes. Magnetic fields of the order of 200 microG have been detected toward three discrete continuum sources in the complex. Virial estimates suggest that the detected magnetic fields in these sources are of the same order as the critical magnetic fields required to support the molecular clouds associated with the sources against gravitational collapse.Comment: 14 pages, 9 postscript figures, accepted for publication in the Astrophysical Journal (ApJ), tentatively scheduled for vol. 533, Apr. 20, 2000; also available at http://www.pa.uky.edu/~sarma/RESEARCH/aps_research.htm
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