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    ABSTRACT: Measurements on the dissociative recombination (DR) of protonated acrylonitrile, CH 2 CHCNH + , have been performed at the heavy ion storage ring CRYRING located in the Manne Siegbahn Laboratory in Stockholm, Sweden. It has been found that at ∼2 meV relative kinetic energy about 50% of the DR events involve only ruptures of X–H bonds (where X = C or N) while the rest leads to the production of a pair of fragments each containing two heavy atoms (alongside H and/or H 2). The absolute DR cross section has been investigated for relative kinetic energies ranging from ∼1 meV to 1 eV. The thermal rate coefficient has been determined to follow the expression k(T) = 1.78 × 10 −6 (T/300) −0.80 cm 3 s −1 for electron temperatures ranging from ∼10 to 1000 K. Gas-phase models of the nitrile chemistry in the dark molecular cloud TMC-1 have been run and results are compared with observations. Also, implications of the present results for the nitrile chemistry of Titan's upper atmosphere are discussed.
    The Astrophysical Journal 10/2015; 695(1):317-324. DOI:10.1088/0004-637X/695/1/317
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    ABSTRACT: Energy levels and radiative rates are reported for transitions in Cl-like W LVIII. Configuration interaction (CI) has been included among 44 configurations (generating 4978 levels) over a wide energy range up to 363 Ryd, and the general-purpose relativistic atomic structure package ({\sc grasp}) adopted for the calculations. Since no other results of comparable complexity are available, calculations have also been performed with the flexible atomic code ({\sc fac}), which help in assessing the accuracy of our results. Energies are listed for the lowest 400 levels (with energies up to $\sim$ 98 Ryd), which mainly belong to the 3s$^2$3p$^5$, 3s3p$^6$, 3s$^2$3p$^4$3d, 3s$^2$3p$^3$3d$^2$, 3s3p$^4$3d$^2$, 3s$^2$3p$^2$3d$^3$, and 3p$^6$3d configurations, and radiative rates are provided for four types of transitions, i.e. E1, E2, M1, and M2. Our energy levels are assessed to be accurate to better than 0.5%, whereas radiative rates (and lifetimes) should be accurate to better than 20% for a majority of the strong transitions.
    Atomic Data and Nuclear Data Tables 06/2014; 100(6). DOI:10.1016/j.adt.2014.02.006
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    ABSTRACT: Super-luminous supernovae that radiate more than 10(44) ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of (56)Ni are synthesized; this isotope decays to (56)Fe via (56)Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10(-6) times that of the core-collapse rate.
    Nature 10/2013; 502(7471):346-9. DOI:10.1038/nature12569
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    ABSTRACT: The discovery of two superluminous supernovae at large distances from Earth pushes the frontier of supernova studies to just 1.5 billion years after the Big Bang, and suggests that they may be common in the young Universe. See Letter p.228
    Nature 10/2012; DOI:10.1038/nature11643
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    ABSTRACT: Building on previous work, a new search of the SuperWASP archive was carried out to identify eclipsing binary systems near the short-period limit. 143 candidate objects were detected with orbital periods between 16000 and 20000 s, of which 97 are new discoveries. Period changes significant at 1 sigma or more were detected in 74 of these objects, and in 38 the changes were significant at 3 sigma or more. The significant period changes observed followed an approximately normal distribution with a half-width at half-maximum of ~0.1 s/yr. There was no apparent relationship between period length and magnitude or direction of period change. Amongst several interesting individual objects studied, 1SWASP J093010.78+533859.5 is presented as a new doubly eclipsing quadruple system, consisting of a contact binary with a 19674.575 s period and an Algol-type binary with a 112799.109 s period, separated by 66.1 AU, being the sixth known system of this type.
    Astronomy and Astrophysics 10/2012; 549. DOI:10.1051/0004-6361/201220562
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    ABSTRACT: We report calculations of energy levels, radiative rates, and electron impact excitation rates for transitions in Li-like ions with 12 <= Z <= 20. The GRASP (general-purpose relativistic atomic structure package) is adopted for calculating energy levels and radiative rates, while for determining the collision strengths and subsequently the excitation rates, the Dirac atomic R-matrix code DARC is used. Oscillator strengths, radiative rates, and line strengths are reported for all E1, E2, M1, and M2 transitions among the lowest 24 levels of the Li-like ions considered. Collision strengths have been averaged over a Maxwellian velocity distribution, and the effective collision strengths obtained are reported over a wide temperature range up to 10$^{7.4}$ K. Additionally, lifetimes are also listed for all calculated levels of the above ions. Finally, extensive comparisons are made with available results in the literature, as well as with our parallel calculations for all parameters with the Flexible Atomic Code FAC in order to assess the accuracy of the reported results.
    Atomic Data and Nuclear Data Tables 07/2012; 99(2). DOI:10.1016/j.adt.2012.03.001
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    ABSTRACT: The core collapse supernova (CC SN) rate provides a strong lower limit for the star formation rate (SFR). Progress in using it as a cosmic SFR tracer requires some confidence that it is consistent with more conventional SFR diagnostics. We compare standard SFR measurements based on Halpha , Far Ultraviolet (FUV) and Total Infrared (TIR) galaxy luminosities with the observed CC SN rate in the same galaxy sample. The comparison can be viewed from two perspectives. Firstly, by adopting an estimate of the minimum stellar mass to produce a CC SN one can determine a SFR from SN numbers. Secondly, the radiative SFRs can be assumed to be robust and then the SN statistics provides a constraint on the minimum stellar mass for CC SN progenitors. We exploit the multi-wavelength data set from 11HUGS, a volume-limited survey designed to provide a census of SFR in the local Volume. There are 14 SNe discovered in this sample of galaxies within the last 13 years. Assuming a lower limit for CC SN progenitor of 8 M⊙, the CC SN rate matches the SFR from the FUV luminosity. However, the SFR based on mbox {M_{B}}ox{H\alpha} luminosity is lower than these two estimates by a factor of about 2. If we assume that the FUV or mbox {M_{B}}ox{H\alpha} based luminosities are a true reflection of the SFR, we find that the minimum mass for CC SN progenitors is 8 ±1 M⊙, and 6± 1 M⊙, respectively.
    01/2012;
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    ABSTRACT: We present observations of comet-like main-belt object P/2010 R2 (La Sagra) obtained by Pan-STARRS 1 and the Faulkes Telescope-North on Haleakala in Hawaii, the University of Hawaii 2.2 m, Gemini-North, and Keck I telescopes on Mauna Kea, the Danish 1.54 m telescope at La Silla, and the Isaac Newton Telescope on La Palma. An antisolar dust tail is observed from August 2010 through February 2011, while a dust trail aligned with the object's orbit plane is also observed from December 2010 through August 2011. Assuming typical phase darkening behavior, P/La Sagra is seen to increase in brightness by >1 mag between August 2010 and December 2010, suggesting that dust production is ongoing over this period. These results strongly suggest that the observed activity is cometary in nature (i.e., driven by the sublimation of volatile material), and that P/La Sagra is therefore the most recent main-belt comet to be discovered. We find an approximate absolute magnitude for the nucleus of H_R=17.9+/-0.2 mag, corresponding to a nucleus radius of ~0.7 km, assuming an albedo of p=0.05. Using optical spectroscopy, we find no evidence of sublimation products (i.e., gas emission), finding an upper limit CN production rate of Q_CN<6x10^23 mol/s, from which we infer an H2O production rate of Q_H2O<10^26 mol/s. Numerical simulations indicate that P/La Sagra is dynamically stable for >100 Myr, suggesting that it is likely native to its current location and that its composition is likely representative of other objects in the same region of the main belt, though the relatively close proximity of the 13:6 mean-motion resonance with Jupiter and the (3,-2,-1) three-body mean-motion resonance with Jupiter and Saturn mean that dynamical instability on larger timescales cannot be ruled out.
    The Astronomical Journal 09/2011; 143. DOI:10.1088/0004-6256/143/5/104
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    ABSTRACT: Using direct numerical magneto-hydrodynamic (MHD) simulations, we demonstrate the evidence of two physically different types of vortex motions in the solar pho-tosphere. Baroclinic motions of plasma in non-magnetic granules are the primary source of vorticity in granular re-gions of the solar photosphere, however, there is a signifi-cantly more efficient mechanism of vorticity production in strongly magnetised intergranular lanes. These swirly mo-tions of plasma in intergranular magnetic field concentrations could be responsible for the generation of different types of MHD wave modes, for example, kink, sausage and tor-sional Alfvén waves. These waves could transport a relevant amount of energy from the lower solar atmosphere and con-tribute to coronal plasma heating.
    Annales Geophysicae 01/2011; 29(5):883-887. DOI:10.5194/angeo-29-883-2011
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    ABSTRACT: In this paper, we discuss simulations of MHD wave generation and propagation through a three-dimensional open magnetic flux tube in the lower solar at-mosphere. By using self-similar analytical solutions for modelling the magnetic field in Cartesian coordinate system, we have constructed a 3-D magnetohydrostatic configuration which is used as the initial condition for non-linear MHD wave simulations. For a driver we have implemented a high-frequency vortex-type motion at the footpoint region of the open magnetic flux tube. It is found that the implemented swirly source is able to excite different types of wave modes, i.e. sausage, kink and torsional Alfvén modes. Analysing these waves by magneto-seismology tools could provide in-sight into the magnetic structure of the lower solar atmo-sphere.
    Annales Geophysicae 01/2011; 29(6):1029-1035. DOI:10.5194/angeo-29-1029-2011
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